Obtaining reliable winding paths for non-axisymmetric shapes with current filament winding technologies is still a challenge. In this study, an algorithm was developed for generating geodesic and non-geodesic paths that are slippage- and bridging-free and can be applied to axisymmetric as well as non-axisymmetric mandrel models represented by triangular meshes. By performing a stability analysis on the winding path on a curved surface, the non-slipping and non-bridging conditions on a triangular mesh are deduced. Then, according to the inverse process of stability analysis, the next path point that satisfies the stability conditions is determined. In this method, the surface normal vector is calculated by a geodesic rather than via the weighted average method. Consequently, a stroke of the winding path is constructed by adding the next path point recursively. In addition, strategies for generating stable paths on a mandrel surface that includes concave regions are presented to avoid bridging.
A novel variational model is developed that can predict the stress transfer between plies in cracked general cross-ply laminates subject to general in-plane (Nxx, Nyy, Nxy) and out-of-plane bending (Mxx, Myy, Mxy) loading. The effects of thermal residual stresses are taken into account. Admissible stress systems, which satisfy equilibrium and all boundary and interface conditions, are constructed and the principle of minimum complementary energy is employed to find the optimal solution. The approach yields rigorous lower bounds for stiffness matrices. A methodology based on Levin's theorem is developed to evaluate the effective thermal expansion coefficients of non-symmetric cracked laminates. A ply-refinement technique is used in order that through-thickness variations of the stress components can be precisely taken into account. It is found that the developed method, when used in conjunction with ply refinement technique, results in stress fields and thermo-mechanical properties comparable in accuracy to refined finite and boundary element solutions.
Composite prepared by mixing of different forms of carbon or other reinforcing fillers with polymer, is one of the possible ways to enhance the performance of polymeric materials. The present work focuses on the study of viscoelastic, thermal, electrical and mechanical properties of graphite flake-reinforced high-density polyethylene composites. The composites are processed by melt mixing using vertical twin-screw micro-compounder followed by final sample preparation via micro injection moulding. The reinforcing filler, graphite flake, is varied from 0 to 50 vol.% with respect to the polymer matrix. Dynamic mechanical thermal analysis reveals an increase in the storage modulus (E') as well as loss modulus (E'') throughout the temperature range; however, damping (tan ) shows a mixed behaviour. There is 550% and 479% increase of E' and E'' in the rubbery region. Degree of entanglement, reinforcement efficiency and C factor are also calculated and correlated with the mechanical properties. On comparison, high-density polyethylene /graphite flake composite having 50 vol.% graphite flake with pure high-bcdensity polyethylene shows 52% increase in melt viscosity, whereas bulk density increases by 38%. This graphite flake is also responsible for the increase in the thermal stability (shift in the onset degradation temperature of ~7℃ and the degradation temperature is more than 400℃), thermal conductivity (175% improvement) and electrical conductivity (~6125% improvement, as the conductivity of pristine high-density polyethylene is ~9.67 E-08 S/m). Mechanical properties determined by tensile and flexural tests show an initial increase and then a slight decrease in the tensile and flexural strength. Therefore, the graphite flake-reinforced high-density polyethylene composite with improved thermal conductivity, electrical conductivity, heat stability, viscoelastic behaviour and flexural modulus can be a promising as well as suitable composite material for making of various electronic and electrical accessories including bipolar plate for fuel cell applications.
Nylon 6/6 nanofibers of diameter 80–100 nm were electrospun on bidirectional E-glass fabric. The fabric with nanofibers on one surface was used to fabricate glass/epoxy structural composites, through resin film infusion. Mechanical properties of composites with interleaved nylon 6/6 nanofibers were found to be substantially improved from that of the control specimens fabricated under identical conditions, but without nanofibers. Compressive strength of composites showed over 30% increase, while interlaminar shear strength improved by 17% with nylon nanofibers of an areal density as low as 0.4 gsm. Residual compressive strength of laminates after a low-velocity impact event also showed a promising improvement with interleaved electrospun nanofibers.
Light-weight polymeric foams are frequently used in composite sandwich construction in which foam core material properties could significantly influence the overall performance of the sandwich structure. Foam mechanical properties usually depend on a number of factors, including foam density, cell microstructure, and properties of foam–matrix polymer. Although the properties of foam–matrix polymer are determined mainly by the properties of the foam base (parent) polymer, they are also affected by other factors such as foam processing conditions. With the large number of material and microstructure parameters that influence foam properties, modeling mechanical behavior of polymeric foams could be quite involved, especially if foam behavior is anisotropic. This paper describes an effort to predict static elastic stiffness of closed-cell PVC foams. PVC foams are modeled as transversely isotropic materials with properties in the foam rise direction different from those in the planar (plane of isotropy) directions. An engineering approach, based on fibrous composites, is developed to predict in-plane and out-of-plane stiffness of PVC foams. The validity of the engineering model for the PVC foam stiffness is first demonstrated through comparison with test results on DIAB H80 foam obtained from a systematic in-house test program. Comparison of the predictions with the stiffness properties reported by a PVC foam manufacturer for various other density foams is also carried out. Good agreements are obtained for the cases studied. Comparison of stiffness predictions obtained in the paper with predictions from other published models of isotropic foam behavior is presented.
Nanoparticle assembly through a novel photothermal dewetting was demonstrated on a macro-scale carbon nanotube (CNT) film. Intense pulsed Xe-light (IPL) was applied to transform a gold (Au) thin-film on CNT into nanoparticles (NPs). Au films measuring 3, 6, and 9 nm were completely dewetted by 10, 20, and 35 J/cm2 of IPL intensities, respectively. The means of NP diameters after dewetting were 7.25 nm (standard deviation, = 2.23 nm), 13.07 nm ( = 2.38 nm), and 21.02 nm ( = 5.86 nm) for the 3, 6, and 9 nm of Au films, respectively. On the other hand, the means of Au NPs formed by furnace annealing were 13.16 nm ( = 1.78 nm) and 20.98 nm ( = 15.60 nm) for 6 and 9 nm of Au films, respectively. The 6 and 9 nm of Au films on CNTs were annealed in a furnace at 300 and 400℃, respectively. The distributions of NPs induced by IPL were not significantly different from the result of conventional furnace annealing (p values = 0.45 and 0.96 for 6 and 9 nm Au films, respectively). Finally, thermodynamic stability of IPL dewetted NPs was evaluated by comparing the samples treated with multiple IPL up to five times and with extended thermal annealing up to 10 h.
The study presents a novel method for the protection of Grubbs’ catalyst, by incorporation in polystyrene fibres via electrospinning technique. Epoxy-glass fibre composite with embedded self-healing agents (polystyrene fibres with Grubbs’ and microcapsules with dicyclopentadiene) was processed. Fibres retained pale purple colour during processing, revealing that fibres provided good protection of the catalyst from the amine hardener. The influence of self-healing agents’ content and thermal treatment on self-healing efficiency was investigated. Fourier transform infrared spectroscopy revealed that a polydicyclopentadiene formed at the healed interface. Thermal analysis revealed that ‘bleed’ at the healing sites from different samples had similar concentration of polydicyclopentadiene, indicating that the same amount of the catalyst has been provided to dicyclopentadiene for polymerization. This finding lead to assumption that electrospun polymer fibres enabled good dispersion of the catalyst in the composites. The low energy impact tests of the samples showed a recovery of 90% after 24 h at room temperature and up to 111% after repeated heating cycles.
The discrete damage modeling method is extended for progressive failure analysis in laminated composites under fatigue loading. Discrete damage modeling uses the regularized extended finite element method for the simulation of matrix cracking at initially unknown locations and directions independent of the mesh orientation. A material history variable in each integration point is introduced and updated after each loading increment, corresponding to certain load amplitude and number of cycles. The accumulation of the material history variable is governed by Palmgren-Miner’s rule. Cohesive zones associated with mesh-independent cracks are inserted when the material history parameter reaches the value of 1. Cohesive zone model consistently describing crack initiation and propagation under fatigue loading without any assumption of initial crack size is proposed. The fatigue properties required for matrix failure prediction include shear and tensile S-N curves as well as Mode I and II Paris law parameters. Tensile fiber failure is assumed unaffected by fatigue. All input data required for model application are directly measured by ASTM tests except tensile fiber scaling parameter and compression fiber failure fracture toughness, which were taken from literature sources. The model contains no internal calibration parameters. Fatigue damage extent, stiffness degradation and residual tensile and compressive strength of IM7/977-3 laminates have been evaluated. Three different layups, [0/45/90/-45]2S, [30/60/90/-60/-30]2S and [60/0/-60]3S, were modeled and tested. The predictions captured most experimental trends and showed good agreement with X-ray CT damage assessment; however, significant further work is required to develop reliable methodology for quantitative composite durability prediction.
Composite structures currently used in the automotive industry must meet strict requirements for safety reasons. They need to maintain strength under varied temperatures and strain rates, including impact. It is therefore critical to fully understand the impact behaviour of composites. This work presents experimental results regarding the influence of a range of temperature and strain rates on the fracture energy in mode I, GIC, of carbon fibre reinforced plastic plates. To determine GIC as a function of temperature and strain rate, double cantilever beam specimens were tested at 20, 80 and –30℃, with strain rates of 0.2 and 11 s–1. A complementary numerical study was performed with the aim of predicting strength using the measured values. This work has demonstrated a significant influence of the strain rate and temperature on GIC of the composite materials, with higher strain rates and lower temperatures causing a decrease in the GIC values.
To investigate the effect of fabric architectures and weaving parameters on thermal conductivities of three-dimensional woven composites, the 2.5D angle-interlock woven composites, 2.5D angle-interlock (with warp reinforcement) woven composites, and 3D orthogonal woven composites were prepared. The thermal conductivities of these woven composites were measured by using transient hot-wire method in this study. It was indicated that the thermal conductivities of three-dimensional woven composites showed significant differences due to the distribution of continuous fibers in three-dimensional woven composites with different structures, which could influence the sum fiber content on the cross section of heat flow. More importantly, compared with 2.5D angle-interlock woven composites and 2.5D angle-interlock (with warp reinforcement) woven composites, 3D orthogonal woven composites exhibited better performance in thermal conductivity. Overall, it was concluded that the thermal conductivities of three-dimensional woven composites were influenced by the fabric architectures and weaving parameters, such as the volume fraction, density, and types of fibers. Furthermore, the volume fraction of fibers on the cross section of heat flow was the dominant factor for thermal conductivities of different woven composites.
In this study, polyoxymethylene/graphene nanocomposites were fabricated by the spray layer-by-layer method. Mechanical properties such as ultimate tensile strength, Young’s modulus and Poisson’s ratio were measured. The addition of 2.4 wt.% graphene increased the ultimate tensile strength by 64% and the Young’s modulus by 103%. The tensile data are compared with those predicted according to the Halpin-Tsai equation. Dynamic mechanical thermal analyzer results showed an increase in storage modulus with increasing graphene weight percentage. Morphologies of the fracture surface of polyoxymethylene/graphene nanocomposite were observed by scanning electron microscopy.
Single layer graphene sheets and carbon nanotubes have resulted in the development of new materials for a variety of applications. Though there are a large number of experimental and numerical studies related to these nanofillers, still there is a lack of understanding of the effect of geometrical characteristics of these nanofillers on their mechanical properties. In this study, molecular dynamics simulation has been used to assess this issue. Two different computational models, single layer graphene sheets–copper and carbon nanotube–copper composites have been examined to study the effect of nanofiller geometry on Young’s modulus and thermal conductivity of these nanocomposites. Effect of increase in temperature on Young’s modulus has also been predicted using molecular dynamics. The effect of nanofiller volume fraction (Vf) on Young’s modulus and thermal conductivity has also been studied. Results of thermal conductivity obtained using molecular dynamics have been compared with theoretical models. Results show that with increase in Vf the Young’s modulus as well as thermal conductivity of single layer graphene sheets–Cu composites increases at a faster rate than that for carbon nanotube–Cu composite. For the same Vf, the Young’s modulus of single layer graphene sheets–Cu composite is higher than carbon nanotube–Cu composite.
The behavior of a woven E-glass–Vinylester composite in dry and seawater-saturated conditions was investigated for strain rates in the range of 0.001–100 s – 1. Reliable data were obtained by conducting servo-hydraulic tests at high speed with DIC strain measurement. Issues related to inertial effects of the test system and controlling vibrational stress waves were resolved through a unique experimental setup and special attention to specimen geometry and fixturing. In addition, a rate-dependent constitutive model was adapted to characterize the elastic–viscoplastic response at various strain rates and validated by comparison to test data. The subject investigation demonstrates a viable methodology to identify strain rate sensitivities in high speed experiments and develop a representative material model for analysis of high rate phenomena.
In this work, three types of 3D woven fabric (orthogonal, angle interlock, and layer-to-layer) were used to study the effect of weaving architecture on processing and mechanical properties. In order to characterize the fabrics for liquid composite molding processes, the compaction and permeability characteristics of the reinforcements were measured as function of fiber volume fraction. High compaction pressures were required to achieve a target fiber volume fraction of 0.65, due to presence of through-thickness binder yarns that restricts fiber nesting. In-plane permeability experiments were completed and flow front patterns were obtained to understand the anisotropy in the laminates. The resin transfer molding process was then used to manufacture panels that were then tested under quasi-static flexure and low-velocity impact conditions. It was found that the flexural strength and modulus were higher along the weft direction, where high in-plane permeability of the reinforcement was observed, due to fiber alignment. Impact tests on composite plates based on the three types of fabric indicated that the orthogonal system offered a slightly higher perforation resistance and lower levels of damage at any given energy.
This experimental study investigates the degradation mechanisms of a glass fiber-reinforced plastic material commonly used in civil engineering applications. A substantial reduction in tensile, shear, and compression properties was observed after 100 days of freeze–thaw cycling in saline environment (–20℃ to 20℃). Non-destructive inspection techniques were progressively conducted on unexposed (ambient condition) and exposed (conditioned) specimens. The dynamic mechanical analysis showed permanent decrease in storage modulus that was attributed to physical degradation of the polymer and/or fiber–matrix interface. This indicated the formation of internal cracks inside the exposed glass fiber-reinforced plastic laminate. The 3D X-ray tomography identified preferred damage sites related to intralaminar and interlaminar cracks. The ultrasonic C-scan and optical microscopy showed the nature of the damage and fibers fracture. The thermal cycling events degraded the matrix binding the warp and fill fibers, thus impairing the structural integrity of the cross-ply laminate. The result of this work could benefit a multi-scale durability and damage tolerance model to predict the material state of composite structures under typical service environments.
The topographical features of fractured tensile, flexural, K1C, and impact specimens of 1.0 wt% multi-layered graphene/nanoclay-epoxy nanocomposites have been investigated. The topographical features studied include maximum roughness height (Rmax or Rz), root mean square value (Rq), roughness average (Ra), and waviness (Wa). Due to deflection and bifurcation of the cracks by nanofillers, specific fracture patterns are observed. Although these fracture patterns seem aesthetically appealing, however, if delved deeper, they can further be used to estimate the influence of nanofiller on the mechanical properties. By a meticulous examination of topographical features of fractured patterns, various important aspects related to fillers can be approximated such as dispersion state, interfacial interactions, presence of agglomerates, and overall influence of the incorporation of filler on the mechanical properties of nanocomposites. In addition, treating the nanocomposites with surfaces of specific topography can help improve the mechanical properties of nanocomposites.
In the present work, Taguchi method was used for the optimization of geometric parameters for double pin joint configurations. The orthogonal array, the signal-to-noise ratio, and analysis of variance were employed to study the effect of geometric parameters on the bearing strength of the joints. Geometric parameters, i.e. the distance from the free edge of the specimen to the diameter of the first hole (E/D) ratio, width of the specimen to the diameter of the hole (W/D) ratio, the distance between the two holes to the diameter of the hole (P/D) ratio and side width to the diameter of the hole (K/D) ratio were investigated for the serial and parallel hole configurations. The results demonstrate that the E/D ratio is the most significant parameter to increase the bearing strength in both serial and parallel pin joint configurations. Its percentage contribution is about 84.5% and 64.23% in serial and parallel pin joint configurations, respectively. Characteristic curve with Tsai–Wu failure criterion was used for the prediction of the bearing strength in the joints numerically. A good agreement was obtained between experimental results and numerical predictions.
This paper presents a micromechanical analysis of the influence of fiber–matrix interface fracture properties on the transverse tensile response of fiber-reinforced composite. The method combines three-dimensional (3D) computational micromechanics and augmented finite element method to provide high-fidelity results of damage initiation and propagation. Random arrangement of fibers and normal distribution of interface toughness and strength are considered in representative volume elements to capture the stochastic behavior of the composite under loading. Sensitivity analysis with respect to the interface properties distribution, and shape and size of fibers on the representative volume element’s strength are performed. The effect of fiber volume fractions on the strength and elastic modulus of the composite is investigated. Failure path in different representative volume elements are compared. The results show that the response of a representative volume element with identical interface properties overestimates the composite’s transverse strength. It is also shown that the damage initiation and propagation locations are affected by the distributions of fracture properties, and the shape and size of fibers within the representative volume element.
The increasing use of composite in the aircraft industry has raised the interest for a better understanding of the failure process in these materials, which can be also influenced by the manufacturing process of the laminate. Some materials used in vacuum assisted resin transfer molding process have been studied in the open literature but very few data have been published for resin transfer molding-6 epoxy based laminates, in particular studies showing the influence of the temperature on the interlaminar fracture behavior of this type of laminates. The aim of this article is to investigate the interlaminar fracture behavior of resin transfer molding-6 based carbon composite laminates manufactured by vacuum assisted resin transfer molding subjected to Modes I and II at 25℃ and 80℃. The results show the influence of the temperature on the interlaminar fracture toughness of composites and provide a database to design composite aerostructures subjected to temperatures commonly experienced in civil aviation. The fracture aspects of the tested laminates were also investigated and directly related to the trend in results found for the fracture toughness values.
The aim of this study is to diagnose and classify the failure modes for two serial fastened sandwich composite plates using data mining techniques. The composite material used in the study was manufactured using glass fiber reinforced layer and aluminum sheets. Obtained results of previous experimental study for sandwich composite plates, which were mechanically fastened with two serial pins or bolts were used for classification of failure modes. Furthermore, experimental data from previous study consists of different geometrical parameters for various applied preload moments as 0 (pinned), 2, 3, 4, and 5 Nm (bolted). In this study, data mining methods were applied by using these geometrical parameters and pinned/bolted joint configurations. Therefore, three geometrical parameters and 100 test data were used for classification by utilizing support vector machine, Naive Bayes, K-Nearest Neighbors, Logistic Regression, and Random Forest methods. According to experiments, Random Forest method achieved better results than others and it was appropriate for diagnosing and classification of the failure modes. Performances of all data mining methods used were discussed in terms of accuracy and error ratios.
Influence of topography on the variation in mechanical performance of 1.0 wt% multi-layer graphene (MLG)/nanoclay-epoxy samples has been investigated. Three different systems were produced: 1.0 wt% MLG-EP, 1.0 wt% nanoclay-EP, and 0.5 wt% MLG-0.5 wt% nanoclay-EP. The influence of synergistic effect on mechanical performance in case of hybrid nanocomposites is also studied. Various topography parameters studied include maximum roughness height (Rz or Rmax), root mean square value (Rq), roughness average (Ra), and surface waviness (Wa). The Rz of as-cast 1.0 wt% multi-layer graphene, nanoclay, and 0.5 wt% MLG-0.5 wt% nanoclay-EP nanocomposites were 41.43 µm, 43.54 µm, and 40.28 µm, respectively. The 1200P abrasive paper and the velvet cloth decreased the Rz value of samples compared with as-cast samples. In contrary, the 60P and 320 P abrasive papers increased the Rz values. Due to the removal of material from the samples by erosion, the dimensions of samples decreased. The weight loss due to erosion was commensurate with the coarseness of abrasive papers. It was recorded that multi-layer graphene is more influential in enhancing the mechanical performance of epoxy nanocomposites than nanoclay. Additionally, it was observed that mechanical performance of hybrid nanocomposites did not show a marked difference suggesting that synergistic effects are not strong enough in multi-layer graphene and nanoclay.
The ferromagnetic shape memory alloy polyurethane, Ni-Mn-Ga-PU, polymer composites absorb more mechanical energy than the conventional materials used for vibration damping applications. The vibration damping has been investigated using the custom-made experimental setup. The computed resonance peak values are in agreement with the experimental data. The dilatational wave decreases at high frequencies which is consistent with the theory of resonance frequency. The previously free end of the system increases the amplitude stress for an appreciable static load but loss appears to be very small. The 20% ferromagnetic shape memory alloy in polyurethane absorbs much more energy than the pure polyurethane due to the movement of twin boundaries present in Ni-Mn-Ga. The Ni-Mn-Ga-PU polymer composites of this nature can be a suitable candidate for acoustic attenuation applications.
Tensile experiments of three-dimensional needled C/C-SiC composite from room temperature to 1800℃ were performed to investigate tensile behavior. The damage characteristics and macroscopic mechanical behavior of the composite are relevant to the testing temperature and off-axis angles of the tensile loading. The tensile strength increased while the modulus decreased with the increase of temperature. A high-temperature nonlinear constitutive model was established to analyze the nonlinear stress–strain relationship of the composite. Plastic strain accumulation and stiffness degeneration were described by the plasticity and damage theories. The effect of temperature on the tensile behavior of the composite was particularly considered in this model by introducing a thermal damage variable. The proposed constitutive model can predict the stress–strain behavior of the material subjected to different off-axis tensile load, and at different temperatures. Fairly good agreement was achieved between the predicted and experimental results.
This paper provides overall comparisons of the static results of an Air Force Research Laboratory exploration into the state of the art of existing technology in composite progressive damage analysis. In this study, blind and re-calibration bench-marking exercises were performed using nine different composite progressive damage analysis codes on unnotched and notched (open-hole) composite coupons under both static and fatigue loading. This paper summarizes the results of the static portion of this program. Comparisons are made herein of specimen stiffness and strength predictions against each other and the test data. Overall percent error data is presented, as well as a list of observations and lessons learned during this year-long effort.
The linking of microstructural uncertainty with the random variation in the response of heterogeneous structures at the macroscale is particularly important in the framework of the stochastic finite element method. In this work, the effect of uncertainty in the constituent material properties and the geometry of the microstructure, on the macroscopic properties of composite materials is assessed through computational homogenization. Based on Hill–Mandel homogeneity condition, the homogenization procedure utilizes the excellent synergy of the extended finite element method and the Monte Carlo simulation. In this way, the computation of the statistical characteristics of the homogenized elasticity tensor of random composite materials reinforced with arbitrarily shaped inclusions is performed in a computationally efficient manner. The effect of stochastic variation in the elastic properties of the constituents as well as the effect of inclusion shape on the statistical characteristics of the homogenized elasticity tensor is assessed through probabilistic sensitivity analysis. A comparison is performed with regard to the relative influence of material and geometrical uncertainty which are considered separately. More realistic results are obtained by considering simultaneously material and geometrical uncertainty in the microstructural modeling of composite materials. The results can be further exploited in the stochastic finite element analysis of composite structures where material properties with random characteristics obtained by the presented multiscale homogenization procedure will be assigned to each finite element.
Organic and inorganic materials are usually added to polymers in order to achieve some benefits such as reducing the product cost, as well as achieving higher modulus and strength. Addition of these materials would change polymers’ behavior. Adding nano-materials to polymers on the other hand is a new challenge in the field of polymer composites where previous studies were unable to achieve good correlation with nano-composites at higher particle volume fractions. In this research, Yamamoto network theory is developed to investigate the behavior of highly nano-filled systems. For this purpose, five different types of sub-chain and two types of junctions are considered and the effect of particle size, concentration, and the model parameters in association with the behavior of the junctions are studied. Moreover, some experiments are performed on polystyrene filled with nano-silica at different particle size and concentration values in frequency mod in the linear region. At last, we compared the results of our final model with the experiments in order to evaluate its accuracy, which confirmed a very good agreement.
This article aims to find the relation between the multiscale mechanical structure of natural fibre reinforced plastic composites and the analysis scales in the topographic signals of machined surfaces as induced by profile milling process. Bamboo, sisal and miscanthus fibres reinforced polypropylene composites were considered in this study. The multiscale process signature of natural fibre reinforced plastic machined surfaces based on wavelet decomposition was determined. Then, the impact of wavelet function was inspected by testing different wavelet shapes. Finally, the analysis of variance was carried out to exhibit the contribution rate of fibre stiffness and tool feed on the machined surface roughness at each analysis scale. Results demonstrate that studying the machining of natural fibre reinforced plastic requires the selection of the relevant scales. They show also the insignificance of the wavelet choice. This study proves that the contribution rate of fibre stiffness and tool feed on machined surface roughness is significantly dependent on the analysis scales, which are directly related to the mechanical properties of natural fibres structure inside the composite.
This study is concerned with the influence of four metallic reinforcements on aluminum-silicon (AlSi) composites, with respect to wear behavior. AlSi-Ti; AlSi-Ti6Al4V; AlSi-1.4301 stainless steel and AlSi-Ni particulate reinforced composites were produced by a hot-pressing route. Microstructural characterization showed a uniform distribution of the reinforcing particles in the AlSi matrix. Reciprocating pin-on-plate wear tests were performed for AlSi and AlSi-based composites against gray cast iron plates. In order to compare the effect of different metallic particulates on the AlSi-based composites/gray cast iron tribopair wear performance, besides the pin, the counterface was also analyzed. The particle/matrix interface is analyzed in order to understand its influence on the tribopair behavior and on the controlling wear mechanisms. It was shown that the better compromise between both sliding surfaces performance was attained by AlSi-Ni/gray cast iron tribopair.
The surface of a one-dimensional silver nanowire was covered with amphiphilic materials, N-(2-aminoethyl)-3a-hydroxy-5b-cholan-24-amide, and patched with two-dimensional graphene to form individually controlled hybrid. Graphene was prepared from ultrasonic in o-dichlorobenzene without any additives. As N-(2-aminoethyl)-3a-hydroxy-5b-cholan-24-amide between silver nanowires and graphene tightly held each other, silver nanowire was individually covered with graphene without introducing ultrasonic power, the necessary process to evenly mix silver nanowires and graphene but lead to damage and oxidize silver nanowires. Although the quality of graphene was inferior, the properties of hybrid were superior compared with pristine silver nanowire/graphene except introducing N-(2-aminoethyl)-3a-hydroxy-5b-cholan-24-amide. All processes to form the hybrid were carried out in solution. Therefore, this makes the processes less expensive and more useful and opens up opportunities for the mass production for conductive materials.
This article presents the results of tensile tests performed on E-glass/Epoxy composite coupons. Eighteen GFRP panels were manufactured and cured in laboratory environment, out of which specimens in size of 25 mm x 250 mm specimens were cut. The specimens were categorised into three groups; the first group was post-cured at 60℃ oven for 16 h while the second and the third groups were placed under seawater at 60℃ for about 60 days and 4 months to represent saturation condition and ageing, respectively. The mechanical parameters such as elastic modulus (E), ultimate tensile strain (u) and shear modulus (G) which are reported here were measured and were compared to one another showing that these are influenced by different conditioning regimes to a great extent.
In this paper, thermoviscoelastic behavior of a hollow cylinder made of short fiber-reinforced polymer considering porosity is investigated by an analytical method. Material properties, except the Poisson’s ratio and coefficient of thermal expansion, are assumed to be changed with the volume of constituents and porosity. Utilizing the finite Hankle integral transform and Laplace transform, analytical solutions for thermoviscoelastic behaviors of short fiber-reinforced polymer hollow cylinders under thermal and mechanical loads are obtained. Numerical examples show the influences of thermal load, mechanical load, and material porosity on the thermoviscoelastic behaviors of short fiber-reinforced polymer cylindrical structures.
During composites manufacturing with partially pre-impregnated fibers (i.e. "prepregs") in Out-of-Autoclave processes, non-impregnated fabric cross-sections serve as air pathways to evacuate entrapped bubbles of air, moisture, or volatiles. The bubbles trapped within a laminate during processing lead to decreased structural performance. In this work, the motion of resin and bubbles during the processing of a characteristic prepreg is directly visualized in situ. This is performed utilizing a previously developed flow visualization technique under known pressure and temperature conditions. This study investigates the processing conditions under which a bubble succeeds or fails to meet and coalesce with available air pathways in order to escape the laminate. A key finding of this study is that tunable process parameters, such as pressure and temperature, are less important for successful bubble removal as compared to the initial state of resin impregnation in the prepreg. Prepregs with initially high states of resin impregnation will often fail to draw bubbles into air pathways through the center of fiber tow cross sections, whereas prepregs with initially low states of resin impregnation have clear pathways for bubbles to meet local resin flow fronts, coalesce, and escape. The relevant literature on the motion of bubbles in confined spaces is discussed. It is observed that small Capillary number theory (i.e. Ca < 0.01) under predicts the relative velocity of bubbles, and the faster than expected bubble transport is likely due to effects given by the bubble aspect ratio via the fibrous micro-channel geometry.
The physical, mechanical, thermal, and flammability properties of high-density polyethylene/old corrugated container composites reinforced with carbon nanotubes are presented in this study. High-density polyethylene/old corrugated container composites with different loadings of carbon nanotube (0, 1, 3, and 5 phc) were prepared by melt compounding followed by injection molding. Results indicated that the incorporation of carbon nanotube into high-density polyethylene, significantly improved the mechanical properties of the composites. The tensile and flexural properties achieved the maximum values when 3 phc carbon nanotube was added. Meanwhile, the impact strength of the composites progressively decreased with increasing carbon nanotube content. Furthermore, the water absorption and thickness swelling of the samples remarkably reduced with the addition of carbon nanotube. From thermogravimetric analysis data, the presence of carbon nanotube could enhance the thermal stability of the composites, especially the maximum weight loss rate temperature and also the better char residual was obtained at high loading level of carbon nanotube. Simultaneous differential scanning calorimetry thermograms revealed that the thermal degradation temperatures for the samples with carbon nanotube were much higher than those made without carbon nanotube. Moreover, it was found that the addition of carbon nanotube results in a significant enhancement in flame retardancy of the composites. Morphological observations showed that the nanoparticles were predominantly dispersed uniformly within the high-density polyethylene matrix.
Rheological behavior of polypropylene/graphite nanoplatelet composites of varying content, temperature, and filler shape was investigated by capillary and rotational rheometers. Scanning electron microscope images were taken in order to examine the filler shape and interaction between fillers and polymer matrix. Viscosity measurements of polypropylene/graphite composites showed shear thinning behavior like neat polypropylene. Filler inclusion resulted in increase in shear viscosity and shear thinning behavior of composites. The effect of filler concentration on viscosity is more appreciable in the low shear rate region. PP/graphite nanoplatelet composites with larger interface between filler and polymer matrix were of greater shear viscosity values through the entire shear rate range. However, filler morphology did not affect shear viscosity in high shear rate region remarkably. Composite viscosity as a function of volume fraction was modeled by Maron–Pierce equation. As temperature increased, shear viscosities of polypropylene/graphite composites and neat PP melt decreased. Temperature has less effect on composite viscosity than on neat PP viscosity due to the restricting effect of fillers on polymer molecules.
Poly(lactic acid)/ethylene vinyl acetate blends and poly(lactic acid)/ethylene vinyl acetate/sugarcane bagasse composites were prepared by melt mixing. The lower viscosity of poly(lactic acid), the lower interfacial tension between poly(lactic acid) and sugarcane bagasse, and the wetting coefficient of poly(lactic acid)/sugarcane bagasse being larger than one, all suggested that sugarcane bagasse would preferably be in contact with poly(lactic acid). A fairly good dispersion of sugarcane bagasse was observed in the composites. Exposed fibre ends were observed in the composite micrographs, which were believed to add to the efficiency of metal adsorption. The impact properties depended more on the poly(lactic acid):ethylene vinyl acetate ratio than on the presence of sugarcane bagasse. The poly(lactic acid)/ethylene vinyl acetate blends showed two melting peaks at approximately the same temperatures as those of the neat polymers, which confirms the complete immiscibility of poly(lactic acid) and ethylene vinyl acetate at all the investigated compositions. Sugarcane bagasse-related weight loss occurred at higher temperatures for sugarcane bagasse in the composites, which could have been the result of the sugarcane bagasse being protected by the polymers, or a delay in the diffusion of the sugarcane bagasse decomposition products out of the sample. Water absorption increased with an increase in sugarcane bagasse loading in the composites. More lead was adsorbed than one would expect if the partial coverage of the fibre by the polymer is taken into account, and therefore it may be assumed that some of the lead was trapped inside the cavities in the composites and that the polymers may also have played a role in the metal complexation process, since both polymers have functional groups that could interact with the lead ions. The metal impurities underwent monolayer adsorption.
Vacuum-bag-only curing is an attractive out-of-autoclave method as an alternative to conventional autoclave curing. Previous extensive researches provided great insight into void formation during the vacuum-bag-only method and these findings are reflected in current vacuum-bag-only cure cycles to minimize void content. Cure process can be further improved by taking into consideration cure-induced residual stress/strain. The present paper proposed a residual stress/strain reduction method and evaluated its effectiveness using a commercially available vacuum-bag-only material by fiber-optic-based in-situ strain monitoring and tensile tests. First, cure process monitoring and tensile tests were conducted for the manufacturer’s recommended cure cycle. Cure process monitoring showed that the material vitrifies during post-cure temperature dwell. Furthermore, the tensile test revealed that the vacuum-bag-only material has lower strength than conventional autoclave materials, suggesting the importance of the effect of cure-induced residual stress/strain. Then, two cure cycles were proposed based on the findings from the manufacturer’s recommended cure cycle tests and a cure kinetics model. In the proposed cycles, resin vitrifies at a lower temperature than the manufacturer’s recommended cure cycle, leading to reduced residual stress/strain. Cure process monitoring and tensile test results for the new cycles showed that the residual strain was reduced by 12–18%, and the strength was increased by 26% in the best case. Moreover, void content was not significantly affected by changing the cure cycle. Although vacuum-bag-only material was used in this research, the proposed concept can be widely applied for autoclave cures and other types of vacuum-bag-only processes with slight modification.
The mechanical properties of carbon fiber-reinforced plastics used in aerospace are vulnerable to degradation under thermo-oxidative aging conditions. However, it is hard to establish a mechanical property prediction model for carbon fiber-reinforced plastics from thermo-oxidative aging mechanism point of view since the thermo-oxidative aging degradation processes are very complex. A mathematical model was proposed based on the theory of stochastic processes for predicting mechanical property degradation of carbon fiber-reinforced plastics under thermo-oxidative aging conditions in the present work. However, the predicted values calculated by the "random process model" were not in good agreement with experimental data. And then a "modified random process model" (namely a wider random process model) was established through Box–Cox transformation for random process model. The verification of the evaluation model showed that the modified random process model can nicely describe the mechanical performance degradation of carbon fiber-reinforced plastics with the increasing of aging time under certain aging temperatures. As the modified random process model was established without limiting the reinforced structure of carbon fiber-reinforced plastics, the described method provides an opportunity to rapidly predict the mechanical properties and the lifetime of any carbon fiber-reinforced plastics by testing the mechanical properties of carbon fiber-reinforced plastics before and after aging for a short period of time.
A transient-dynamics model based on the approximate Riemann algorithm is proposed for the failure analysis of a frangible composite canister cover. The frangible cover, manufactured with a traditional manual lay-up method, is designed to conduct a simulated missile launch test using a specially developed test device. Deformation of the cover’s centre is determined using a transient-dynamics finite element model; failure pressure for the frangible cover is obtained based on a failure criterion and compared with simulated experimental results. Weak-zone position of the frangible cover has a significant effect on failure pressure compared to that of deformation of the cover’s centre. With the same structure of the weak-zone, an increase in its height can first raise and then reduce the level of failure pressure of the frangible cover. Close agreements between the experimental and numerical results are observed.
Relaxation is a key factor that controls the application of prestressing fiber-reinforced polymer tendons. This paper focuses on the evaluation of the relaxation behavior of newly developed basalt fiber-reinforced polymer tendons through an approach considering anchorage slippage. A series of relaxation tests on basalt fiber-reinforced polymer tendons subjected to three levels of initial stresses (0.4fu, 0.5fu, and 0.6fu, where fu = ultimate strength) were conducted using a specially designed test setup that eliminates the impact of slippage at the anchor zone. An additional group of tests was conducted to validate the enhancement effect of pretension on the relaxation behavior. The relaxation rates at one million hours were predicted based on experimental fitting. Finally, the relaxation rates at 1000 h were predicted using the correlation between the relaxation and creep and were validated with the experimental relaxation rates. The results demonstrate the effectiveness of the proposed setup in measuring the relaxation loss of specimens and reveal that the relaxation rates of untreated basalt fiber-reinforced polymer tendons at 1000 h are 4.2%, 5.3%, and 6.4% at 0.4fu, 0.5fu, and 0.6fu, respectively. Pretension treatment performs effective in relaxation loss controlling. BFRP tendons are recommended to be applied at an initial stress of 0.5fu after pretension treatment, with one-million-hour relaxation rate equal to 6.7%. Furthermore, the relaxation rate at 1000 h can be predicted accurately based on the creep behavior. The conclusions of this study can provide guidance for the prestressing applications of basalt fiber-reinforced polymer tendons.
Effect of industrial grade multi-walled carbon nanotubes on mechanical, decay, and thermal properties of wood polymer nanocomposites was investigated. To meet this objective, pine wood flour, polypropylene with and without coupling agent (maleic anhydride grafted polypropylene), and multi-walled carbon nanotube (0, 1, 3, 5 wt%) were compounded in a twin screw co-rotating extruder. The mass ratio of the wood flour to polypropylene was 50/50 (w/w) in all compounds. Test specimens were produced using injection molding machine from the pellets. The flexural and tensile properties, biological durability, and thermal analysis (thermogravimetric analysis and differential scanning calorimetry) of the nanocomposites were investigated. The flexural and tensile properties of the wood polymer nanocomposites increased with increasing content of the industrial grade multi-walled carbon nanotubes (from 1 to 5 wt%) and maleic anhydride grafted polypropylene (3 wt%). The mass loss rates of the wood polymer nanocomposites decreased with increasing amounts of the maleic anhydride grafted polypropylene and industrial grade multi-walled carbon nanotube. The differential scanning calorimetry analysis showed that the melt crystallization enthalpies of the wood polymer nanocomposites increased with increasing amount of the industrial grade multi-walled carbon nanotubes. The increase in the Tc indicated that the industrial grade multi-walled carbon nanotubes were the efficient nucleating agent for the wood polymer nanocomposites.
Carbon black particles surrounded by copper nanoparticles (Cu NPs) were synthesized using electroless plating method. Palladium chloride was adsorbed onto carbon black, followed by the reduction of palladium chloride for catalyzing the reduction of Cu ions on the surface of carbon black particles. After that, carbon black particles doped by palladium catalyst were dispersed and stirred in Cu plating bath. Cu ions being reduced, Cu NPs surrounded the surface of carbon black particles (Cu@CB). The ratios of Cu to carbon black were controlled through variation of weight of Cu ions in Cu plating bath from 1:1 to 1:7. Cu@CB was applied to electrically conductive substrates with ethyl cellulose binder. Electrical properties and morphology were measured and compared with different weight ratio of Cu and carbon black. It was found that when weight ratio of Cu to carbon black was above three, resistivity of conductive substrates fabricated decreased dramatically. Lowest resistivity was 5.93 x 10–4 cm, confirming the advantages of Cu@CB which has possibility of lowering weight percentage of metal in conductive substrates through simple process.
The microstructure, mechanical properties, thermal stability and tensile fracture of two hot-rolled Al-15 vol.% B4C composite sheets (S40 with 0.4 wt.% Sc and SZ40 with 0.4 wt.% Sc and 0.24 wt.% Zr) were investigated. During multi-pass hot rolling, coarse Al3Sc or Al3(Sc, Zr) precipitations appeared and resulted in the loss of most of their hardening effect. In an appropriate post-rolling heat treatment, the hot-rolled sheets regained a significant precipitation hardening because of the precipitation of fine nanoscale Al3Sc and Al3(Sc,Zr) that uniformly distributed in the aluminum matrix. After the peak aging, the ultimate tensile strength at ambient temperature of the S40 and SZ40 sheets can reach 198 MPa and 215 MPa, respectively. During 2000 h of annealing at 300℃, the strengths at ambient temperature of both S40 and SZ40 composite sheets slowly decreased with increasing annealing time. However, the tensile strengths at 300℃ of both S40 and SZ40 composite sheets remained nearly unchanged and were less sensitive to the annealing time and more tolerable for precipitate coarsening, which demonstrated an excellent long-term thermal stability of both materials at elevated temperature. The tensile fracture at ambient temperature of both S40 and SZ40 composite sheets was dominated by the brittle B4C particle fracture, whereas the interfacial decohesion of B4C particles became the prominent characteristic of the fracture at 300℃.
High electrical performances of polytetrafluoroethylene composites containing few-layer graphenes are established by solid-state processing. Polytetrafluoroethylene and FLG powders are mechanically mixed without solvents at room temperature, and hot-pressed. Few-layer graphenes are attached to the polytetrafluoroethylene powder, and gradually wrap the powder surface during milling with a low milling speed. The few-layer graphene-wrapped polytetrafluoroethylene powders readily facilitate the formation of a continuous few-layer graphene network due to the contact between adjacent few-layer graphene-wrapped powders. The final composites using few-layer graphene-wrapped polytetrafluoroethylene powders include a three-dimensional conducting network. Eventually, the wrapping morphology of the polytetrafluoroethylene/few-layer graphene powder results in a remarkable electrical conductivity of 7353 Sm–1 at 30 vol. %. few-layer graphene loading.
The effects of the shallow angle on the static strength and the fatigue life of the multi-directional glass fiber-reinforced plastics for wind turbine blades were presented based on experimental results and predictions. The static tests and the tension–tension fatigue tests under cyclic fatigue loads with a stress ratio of 0.1 were performed on bi-axial (BX, [±]), tri-axial 1 (TA, [0/±2]), and tri-axial 2 (TX, [02/±]) laminates with ply angles of 25°, 35°, and 45°. A multiscale approach was applied to predict the static tensile and compressive strengths and the S–N curves of BX, TA, and TX laminates based on the constituents: fiber, matrix, and interface. Three ply-based failure criteria (Hashin, Puck, and Tsai–Wu) were also employed to predict the static strength and compare with the experimental results. The predictions and the experimental results show that the tensile strength increases as becomes shallower, while laminates with a shallow ply angle of 35° showed similar or even lower compressive strengths, especially for TA and TX laminates. The laminate fatigue life increases as becomes shallower. The shallow angle effect on strength and fatigue life is greater for BX than TA and TX laminates since the ply angle plays a more important role in BX. By using the multiscale approach, the shallow angle effect on the laminate static and fatigue behaviors were also explained based on the ply stresses as well as the constitutive micro stresses.
The paper reports an experimental investigation on the mechanical and thermal properties of multifunctional composite laminates integrated with microencapsulated phase change materials. The different microstructures were created by incorporating microencapsulated phase change materials in glass–epoxy composites at weight fraction between 0 and 20 wt.%. To characterise the mechanical properties, tension, compression and flexural tests were conducted. The scanning electron microscope studies were used to investigate the damage mechanisms associated with these loading conditions. Thermal storage capability of the multifunctional composites was characterised using heat flux meters. The apparent heat capacity of the composites was linearly proportional to the concentration of microencapsulated phase change materials. Alternative design analysis resulted in an optimised laminate configuration with high thermal storage capability coupled with excellent mechanical properties.
In the present investigation, Al–Cu composites with SiC particulates were fabricated via mechanical alloying process. The aim of this study was to evaluate the effect of milling time (8, 12, 16 and 32 h), particle size (30 nm and 15 µm) and volume fractions (5, 10 and 15 wt.%) of SiC particles on the metallurgical and mechanical properties. Scanning electron microscopy equipped with X-ray diffraction method was used to investigate the microstructural evolution and morphological changes created during mechanical alloying. Microstructural study indicated that SiC particles were well distributed after the mechanical alloying process. A homogenous distribution of the particles was obtained by 15 wt.% of SiC particles in the aluminum matrix. The results revealed that the SiC particle size also affected the distribution and size of the powders in the matrix and it improved as particle size decreased from 15 µm to 30 nm. The study of mechanical properties clearly showed that a reduction in hardness of composite occurs which is attributed to positive effect of reinforcement particles in resistance to the movement of dislocations. Furthermore, it was found that the wear weight loss of Al–Cu/SiC composite decreases monotonically with increasing SiC content and more uniform particle size distribution. The excellent wear rate was primarily attributed to uniform distribution of the SiC particles.
This study focuses on understanding and prediction of short-term thermal degradation of polymer matrix composites. One sided irradiation of two commercial composites (HexPly® 8552/IM7 and M18-1/G939) is carried out on specimens of various thickness (2, 4, 6 mm) at different heat fluxes (50 and 80 kW/m2) for various exposure times prior to ignition. The aim is to correlate the amount of the applied thermal energy with the heat damage and the residual mechanical strength. Among the two primary components of each matrix the epoxy resin is observed to degrade faster than the thermoplastic under thermal load, as measured by IR spectroscopy. A correlation is achieved between the interlaminar shear strengths and the relative amount of the residual matrix components. The interlaminar shear strengths and degradation processes are assessed in dependence of the applied energy per volume. The derived relationships and a chemometric analysis of IR spectra, can be used to rapidly estimate mechanical properties, as well as other properties of specimens with unknown thermal preload. Degradation processes are discussed in detail.
Composite scarf repairs were cured using heat generated by passing an electrical current through a woven graphite-epoxy prepreg embedded in the bondline. Resistance heating using the embedded prepreg resulted in a more uniform temperature distribution in the bondline while preventing any potential thermal damage to the surface of the scarf repairs. In contrast, conventional surface heating methods such as heat blankets or heat lamps lead to large through thickness thermal gradient that causes non-uniform temperature in the bondline and overheating the outer surface adjacent to the heater. Composite scarf repair specimens were created using the proposed embedded heating approach and through the use of a heat blanket for circular and rectangular scarf configurations. Tensile tests were performed for rectangular scarf specimens, and it was shown that the bond strengths of all specimens were found to be comparable. The proposed embedded curing technique results in bond strengths that equal or exceed those achieved with external heating and avoids overheating the surface of the scarf repairs.
Nanofibrillated cellulose from eucalyptus pulp, produced by high-pressure homogenization, was used as cement partial replacement for cement paste at a content ranging from 0% to 0.5% by weight of cement. The effect of the content of nanofibrillated cellulose on porosity, thermal properties, compressive strength and degree of cement hydration was investigated. Results have shown an improvement in the compressive strength by more than 50% with 0.3 wt% of added nanofibrillated cellulose. The porosity was reduced by nanofibrillated cellulose addition, and the greatest result was achieved with mixture incorporating 0.3 wt% nanofibrillated cellulose. The coefficient of thermal expansion and the thermal conductivity measurements, relative to nanofibrillated cellulose-reinforced cement pastes, have pointed out the reinforcement effectiveness of nanofibrillated cellulose. The degree of cement hydration has increased with nanofibrillated cellulose content. This trend was confirmed by X-ray diffraction and Fourier Transform Infrared spectroscopy. These analyses have revealed that the presence of nanofibrillated cellulose promoted the hydration of cement, by producing more portlandite and calcium silicate gel, which is likely the main reason accounting for the strong enhancement in the compressive strength.
In this article, a continuum-based constitutive model is developed to predict the mechanical behavior of 5052 resin epoxy reinforced by multiwalled carbon nanotubes (MWCNTs) based on experimentally generated data. For this purpose, MWCNTs/epoxy specimens with various percentages of functionalized and nonfunctionalized MWCNTs are prepared. The SEM graphs indicate that functionalization leads to a better bound between epoxy and MWCNTs and a higher level of dispersion. The specimens are then tested under standard ASTM D638-02 a procedure and their true plastic stress–strain curves are extracted. Investigations on experimentally generated data reveal that a wt% dependent equation which is obtained using any two series of these data can be successfully implemented for others. The equation is then implemented into a finite element software using a developed user material subroutine in which is utilized based on a particular solution algorithm. In order to verify the accuracy of the model some tensile as well as load-unload-reload tension tests are performed according to standard conditions and acceptable agreement between the numerical and experimental results are observed. Results also indicate that the proposed empirical model can precisely predict the stress–strain behavior of 5052 resin epoxy containing arbitrary wt% of MWCNTs in the range 0–1 wt%.
Carbon nanotubes and graphene are considered effective reinforcement materials for various polymers because of their superior properties. However, they are expensive and difficult to separate and incorporate individually into matrix systems because of their tendency to exist in clustered form. In this work, carbon nanoparticles produced from graphitic carbon-rich fly ash by high-energy ball milling are evaluated as a reinforcement in a high-performance epoxy matrix system. They were used in various weight fractions ranging from 0.1 to 2 wt.%. The obtained carbon nanoparticles have an average particle size of around 20 nm, while XPS spectrum shows active carbonyl groups on their surfaces. The mechanical tensile properties of the carbon nanoparticles/epoxy nanocomposite, including their Young's modulus, stiffness, and load at fracture, were investigated. Moreover, the effect of ethanol as a dispersion medium was studied. The obtained results indicate that the Young's modulus and load at fracture changed only slightly upon the addition of carbon nanoparticles to the epoxy matrix system. On the other hand, the stiffness was improved by 60% over that of the pure epoxy matrix system. This improvement was obtained at 0.6 wt.% carbon nanoparticle content. The test results indicate that ethanol is effective in modifying the nanocomposite mechanical properties. Additionally, results show that low-cost CNPs might be useful as a reinforcement material for high-stiffness products.
Carbon fiber-reinforced polymer composites have been widely used in the aerospace industry. However, they are extremely sensitive to crack initiation, propagation and interlaminar delamination which severely reduce their service life. This paper demonstrated that the Mode-I interlaminar fracture toughness could be significantly improved in carbon fiber/bismaleimide composites using a microwave curing process. An increase of about 53.5% in critical load and an increase of approximately 133.5% and 61.2% in fracture toughness and fracture resistance have been achieved, respectively. The microwave manufacturing cycle for composites was cut to 44% of the thermal processing cycle. Dynamic mechanical thermal analysis was performed to investigate the enhanced interfacial strength in microwave-cured composites. The improvement in fracture toughness was attributed to a better interfacial adhesion between resin and fiber, which was investigated by the observation of fracture surfaces with optical microscopes.
This paper reports the influence of temperature and braided angle on compressive behaviors of 3D braided carbon fiber–epoxy composites. The compressive behaviors of the 3D braided with three braided angles (26°, 35° and 48°) were tested at various temperatures (–100℃, –50℃, 0℃, 20℃). The compressive damage morphologies were observed with SEM photographs. It was observed that the temperature and the braided angle have significant effect on the longitudinal compressive behaviors of 3D braided composites. The overall effect of braided angle on the 3D braided composites was greater than the temperature. The influence of the braided angle on the compressive behaviors is from the yarn orientation angle, while the influence of the temperature is from the temperature-dependent behaviors of the epoxy resin. Under low temperatures, the 3D braided composite behaved as brittle material and the compressive damage was easier than that of room temperature. The changes of yarn trajectory also led to generate the damage zone, especially in the edge and surface regions of the 3D braided composites.
A geometrical modelling scheme is presented to produce representative architectures for discontinuous fibre composites, enabling downstream modelling of mechanical properties. The model generates realistic random fibre architectures containing high filament count bundles (>3k) and high (~50%) fibre volume fractions. Fibre bundles are modelled as thin shells using a multidimensional modelling strategy, in which fibre bundles are distributed and compacted to simulate pressure being applied from a matched mould tool. Finite element simulations are performed to benchmark the in-plane mechanical properties obtained from the numerical model against experimental data, with a detailed study presented to evaluate the tensile properties at various fibre volume fractions and specimen thicknesses. Tensile modulus predictions are in close agreement (less than 5% error) with experimental data at volume fractions below 45%. Ultimate tensile strength predictions are within 4.2% of the experimental data at volume fractions between 40 and 55%. This is a significant improvement over existing 2D modelling approaches, as the current model offers increased levels of fidelity, capturing dominant failure mechanisms and the influence of out-of-plane fibres.
This article aims to investigate the flexural creep behaviour as a function of temperature of long glass fibre polyamide 6.6 taking into account the thermal-oxidative degradation occurring during the test. The mould geometry has been chosen so as to reproduce some geometrical accidents (e.g. sharp frontal and tangential steps) occurring on industrial moulds. The nominal fibre content (10, 40 and 55 wt%), initial fibre length (short glass fibre, long glass fibre), load rate (up to 70%) and creep temperature (23℃, 100℃ and 130℃) have been considered to estimate the Findley’s model coefficients. A first investigation on the polyamide 6.6 degradation under thermo-oxidative environment has been led to understand the mechanisms of thermal-degradation of the polyamide 6.6 composites. The pure polyamide 6.6 matrix has shown a 20% increase of flexural modulus during the first period of ageing attributed to a combined chain scissions and cross-linking reactions. Then, a decrease of properties attributed to predominant chain scission mechanism was noticed after 1000 h of thermal exposure reaching up to 30% after 5000 h. In case of reinforced polyamide 6.6, the flexural properties tend to increase (+6.5%) up to 2000 h of exposure. The least square method has then permitted to evaluate the material coefficients from the experimental data; the instantaneous creep strain has been estimated from a power law representation. In any cases, the calculations are in a good accordance with experimental measurements.
The increasing use of high-value carbon fibre in composites is linked with increasing waste generation: from dry fibre and prepreg offcuts during manufacturing to end-of-life parts. In this work, a novel thermoplastic tape was produced from 60 wt.% manufacturing waste carbon fibres (60 mm long) and 40 wt.% polyester fibres using a thermal consolidation technique. The thin (0.2 mm) and narrow (20 mm wide) tapes were then used to fabricate laminated composite panels in two 0/90 tape architectures: cross-ply and woven ply. Various mechanical properties, including tensile, flexural, compression and impact were evaluated. It was found that cross-ply performed better than woven ply laminates, with failure in the latter materials typically initiating at the tape interlacement points.
In this paper, a reliable and reproducible experimental procedure for the study of the through-thickness flow induced by the compaction of a saturated porous media is presented. Experimental fluid pressure data are exploited in the validation of a fully coupled fluid-mechanical model and the verification of the related material parameters. The experimental results show overall good agreement with the numerical solution, for all three configurations tested. In addition, up-scaling rules have been identified, which relate the consolidation time with the fluid viscosity and the number of layers.
Kevlar® 49 fabrics have excellent performances such as high elastic modulus and high impact resistance, which are widely used in ballistic systems, aerospace, fabric reinforced composite materials and other fields. The present work studied the multi-scale mechanical behaviors of Kevlar® 49 in the forms of fiber, yarn and fabric subjected to uniaxial tension. The experimental results showed that the material mechanical properties are dependent on structural size scale and gage length of samples. The tensile strengths decrease with increasing gage length and structural size scale from fiber to yarn, and to fabric, and follow Weibull distribution by conducting statistical analysis, which is used to quantify the degree of variability in the tensile strengths of fiber and yarn with different gage lengths. At last, user-defined subroutines (UMAT) in ANSYS were implemented to simulate the tensile behaviors of single yarn and fabric by using the constitutive models of fiber and yarn, respectively, which considered their Weibull distribution in tensile strength. This probabilistic approach can simulate the multi-scale tensile behaviors of Kevlar® 49 accurately and reveal the mechanisms of deformation and failure process based on the various size scales. This approach is also applicable to study the multi-scale behaviors of other fabrics once their properties and Weibull parameters are determined.
Compaction-induced deformations affect the properties of fibre reinforced composites. In order to account for this, accurate models of the compacted textile geometry are required. This paper presents the development of an efficient simulation approach to capture the compaction-induced deformations of fibre reinforced textiles. The simulations are carried out using the Abaqus dynamic explicit solver, whereby two flat plates are used to compact the textile model mesh to the desired cavity thickness. Isotropic elastic material models are used. The transverse elastic modulus of the tows and a Poisson’s ratio are determined by matching single-tow deformation simulations to deformations obtained experimentally. The compacted unit cell geometries are verified by comparing with measurements obtained from images of compacted textile samples. Using this simplified simulation setup and material definitions enables the compacted geometry of textiles to be predicted with minimal input data that is simple to obtain.
This paper analyses the influence of graphite reinforcement, load and sliding speed with constant sliding distance on tribological behavior of A356 aluminum matrix composites reinforced with 10 wt.% silicon carbide and graphite using the Taguchi design. Hybrid composites were produced in the compo-casting process. Tribological tests were performed on a block-on-disc tribometer where the weight percentage of graphite has three variations (0, 3, and 5), as well as load (10 N, 20 N, and 30 N) and sliding speed (0.25 m/s, 0.5 m/s, and 1 m/s), with sliding distance of 300 m. The wear of the composite is investigated under dry sliding condition. The specific wear rate was analyzed using Taguchi method with the aim of finding the optimal parameters. By applying analysis of variance, it was determined that the best tribological properties has A356/10SiC/3Gr hybrid composite. It was also found that the greatest impact on specific wear rate has load with the percentage effect of 69.163%, then sliding speed with 14.426% and the interaction between wt.% graphite and load. The dominant wear mechanism is adhesive wear as confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS).
Aluminium 6061 alloy matrix composite materials reinforced with carbon nanotubes (CNTs) and silicon carbide nanoparticles (nSiCs) were prepared by high-energy ball milling and hot pressing. In addition to inducing fine particle strengthening, nSiCs were also used as a solid mixing agent to improve the dispersion of the CNTs in the Al matrix powder. The dependence of the densification and mechanical strength of the composites reinforced with the dual nanoparticles on the milling time is discussed. The crystallite sizes of Al in the composites were also investigated. Moreover, the relative defect ratios of the CNTs in the composites were calculated from the intensities of the D and G peaks of the Raman spectra. With this new approach to composite fabrication, a hardness and tensile strength of 334 HV and 293 MPa, respectively, were achieved. The high-energy ball milling time significantly affected the microstructure and mechanical properties of the composites; however, the dual nanoparticle reinforcement can potentially be used in a variety of industrial component materials with precisely controlled material properties.
The influences of weight percentage of different reinforcement particles such as SiC particles, waste uncarbonized eggshell particles, carbonized eggshell particles, and CaCO3 powder were compared in the processing of aluminium-based metal matrix composite. The results revealed that by the addition of SiC particles up to 10 wt.% and waste eggshell particles up to 12.5 wt.% in AA2014 matrix alloy, the tensile strength, hardness, and fatigue strength increased. Toughness and ductility decreased by the addition of SiC and eggshell particles in AA2014 matrix alloy. Corrosion rate decreased by the addition of SiC particle up to 7.5 wt.% and eggshell particles up to 12.5 wt.%. Results showed that hardness and heat-treatable properties are improved after the addition of SiC reinforcement particles in AA2014 aluminium alloy as compared to eggshell particles. However, porosity and overall cost increased after addition of SiC particles in AA2014 alloy. Corrosion rate increased after the heat treatment for all reinforced metal matrix composite. These results showed that using the carbonized eggshell as reinforcement in the AA2014 alloy gave better physical properties at lower cost as compared to SiC particles. Proper wettability was observed between matrix and reinforcement material for both carbonized eggshell particles and SiC particles. No wettability was observed between AA2014 alloy and CaCO3 reinforcement particles. Poor wettability reduced the mechanical properties of AA2014/CaCO3 metal matrix composite.
Post-industrial trimmings and off-cuts of carbon fiber/polyether ether ketone composite were successfully recycled into new composite products. The original composites were thermally characterized by dynamic thermomechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. Melt-bonding and thermoset adhesives were used to bond the carbon fiber/polyether ether ketone. Performance of the bond was evaluated through double lap-shear tests. The carbon fiber/polyether ether ketone scraps were mechanically refined to a variety of elemental sizes, subsequently subjected to high-temperature hot pressing to form panel composites. The influences of element size and processing temperature were evaluated through mechanical testing.
Modern bridge structures need light decks with long durability and promising technical parameters. Glass fiber-reinforced polymer orthotropic bridge deck creates unconventional possibilities in bridge designing. Parallel identification of glass fiber-reinforced polymer deck panel by differential thermal analysis, spectroscopy analysis, scanning and optical microscope monitoring, dynamic mechanical analysis and differential scanning calorimetry analysis, tensile and flexural tests will be presented in the paper. Differential thermal analysis was carried out for estimation of the physical and chemical transformation of glass fiber. The differential scanning calorimetry experiments were performed in the glass fiber-reinforced polymer–bridge deck material for determining the mass variation and the energy changes suffered by the materials, as a function of temperature and time. Dynamic mechanical analysis was allowed to detect thermal effects based on the changes in the modulus or damping behavior. Tensile and flexural tests allowed the observation of the decomposition process and information about the basic stress parameters of glass fiber-reinforced polymer material used in bridge applications was taken. Aforementioned analyses are necessary to examine the durability description of the composite element.
Rotorcraft drivelines typically utilize a multi-segmented metallic system to transmit power between the engine and tail rotor. The typical arrangement of metal driveshaft segments, hanger bearings, and flexible couplers contribute to a significant logistical footprint, maintenance downtime, and life-cycle costs. Thus, an innovative flexible matrix composite driveshaft design alternative is presented in this paper, intended to simultaneously reduce the number of couplers and bearings, as well as, provide high fatigue strain capacity. Through reduction in number of parts, the maintenance cost and time as well as weight of the system are reduced. Composite driveshafts, representing those used in utility helicopters, were designed using an optimization process that considers: (1) damping-induced self-heating, (2) whirling stability, (3) torsional buckling stability, and (4) lamina strength. The paper provides a ballistic comparison study between a baseline carbon/epoxy composite and flexible carbon/polyurethane composite driveshaft segments. One driveshaft of each material was torsionally loaded to failure without ballistic impact. Additionally, two driveshafts were impacted obliquely at zero torque with 7.62 and 12.7 mm armor piercing/incendiary (API) rounds. After impact, the driveshafts were loaded in torsion to failure. Residual torsional strengths were 17–21% and 13% of un-impacted strengths for the 7.62 mm and 12.7 mm rounds, respectively. For the small sample size, flexible driveshafts had a marginally higher residual strength compared to the carbon/epoxy counterpart. Residual torsional stiffness values were 83–86% and 52–59% for the 7.62 mm and 12.7 mm rounds, respectively.
In this study, circular disk model and cylinder theory for two dimension (2D) and three dimension (3D), respectively, have been used to determine residual stresses in three-phase representative volume element. The representative volume element is consisting of three phases: carbon fiber, carbon nanotubes, and polymer matrix, that carbon fiber is reinforced by carbon nanotube using electrophoresis method. Initially, the residual stresses analysis of two-phase representative volume element has been implemented. The two-phase representative volume element has been divided to carbon fiber and matrix phases with different volume fractions. In the three-phase representative volume element, although the volume fraction of carbon fiber is constant and equal to 60%, the volume fractions of carbon nanotubes for various cases are different as 0%, 1%, 2%, 3%, 4%, and 5%. Also, there are two different methods to reinforce the fiber according to different coefficients of thermal expansion of the carbon fiber and carbon nanotube in two longitudinal and transverse directions; carbon nanotubes are placed on carbon fiber either parallel or around it like a ring. Subsequently, finite element method and circular disk model have been used for analyzing micromechanic of the residual stresses for 2D and then the results of stress invariant obtained by the finite element method have been compared with the circular disk model. Moreover, for 3D model, the finite element method and cylinder theory have been utilized for micromechanical analysis of the residual stresses and the results of stress invariant obtained by them, have been compared with each other. Results of the finite element method and analytical model have good agreement in 2D and 3D models.
A finite element procedure is developed for the computation of the thermoelastic properties of textile composites with complex and compact two- and three-dimensional woven reinforcement architectures. The purpose of the method is to provide estimates of the properties of the composite with minimum geometrical modeling effort. The software TexGen is used to model simplified representations of complex textiles. This results in severe yarn penetrations, which prevent conventional meshing. A non-conformal meshing strategy is adopted, where the mesh is refined at material interfaces. Penetrations are mitigated by using an original local correction of the material properties of the yarns to account for the true fiber content. The method is compared to more sophisticated textile modeling approaches and successfully assessed towards experimental data selected from the literature.
This paper aims to develop a numerical nonlinear progressive damage model for laminated composite materials considering in-plane and out-of-plane shear stresses in combination with cohesive interface elements to predict the structural response and the failure mechanisms of laminated composite materials. For this purpose, the constitutive models for intralaminar and interlaminar damage modes have been developed as a numerical code by a UMAT subroutine and implemented in commercial finite element software. This model, which is based on the continuum damage mechanics approach, enables to predict the gradual degradation of material properties with five distinct damage parameters for different failure modes; three of these damage factors apply the shear damage contribution as a separate damage mode by a separate damage factor into the model and characterize it by shear damage dissipation energy, and two parameters for fiber and matrix in transverse directions. Also, a series of experiments have been performed to characterize and validate the nonlinear behavior of glass/epoxy laminate. This model is used to predict the behavior and the final strength of open-hole tension specimens. A reasonably good agreement was also achieved between numerical predictions and experimental observations in terms of shapes, orientations and sizes of individual intraply damages induced around the notch and also the final strength of open-hole tension specimen.
Process-induced total spring-in of corner-shaped composite parts manufactured via autoclave-forming technique using unidirectional prepreg is studied both numerically and experimentally. In the numerical study, a three-dimensional finite element model which takes into account the cure shrinkage of the resin, anisotropic material properties of the composite part and the tool-part interaction is developed. The outcome of the numerical model is verified experimentally. For this purpose, U-shaped composite parts are manufactured via autoclave-forming technique. Process-induced total spring-in, due to the combined effect of material anisotropy and tool-part interaction, at different sections of the U-shaped parts are measured with use of the combination of the three-dimensional optical scanning technique and the generative shape design. Total spring-in determined by the numerical model is found to be in good agreement with the average total spring-in measured experimentally. The effect of tool-part interaction mechanism on the total spring-in is studied separately to ascertain its effect on the total spring-in behavior clearly. It is shown that with the proper modeling of the tool-part interaction, numerically determined total spring-in approaches the experimentally determined total spring-in.
The cutting edge of the polycrystalline diamond tool easily blunts in high-speed milling of carbon-fiber-reinforced plastic with the tool deterioration. It aggravates the burrs damage due to the change in the tool–material interaction. Therefore, this paper analyzes the tool–material interaction in milling of carbon-fiber-reinforced plastic based on the material-removal mechanism to investigate the tool deterioration mechanism. It reveals that there are two main reasons for the tool deterioration: the extreme crashing and ploughing of the uncut fibers on the tool, and the serious impact of fibers strongly supported on the cutting edge. An indirect measure method is proposed to quantify the tool deterioration including the ploughing-caused wear and impact-caused microchipping. Furthermore, the milling tests are performed to evaluate the tool deterioration under different cutting speeds in the range of 7.33–9.42 m/s. Meanwhile, a modified mathematical model of tool life is proposed based on a strict burr specification in milling of the carbon-fiber-reinforced plastics. Polycrystalline diamond tool has the longest life with the run-in wear and the quasi-steady-state wear for 7.33 m/s cutting speed, and the life rapidly decreases with the increase in the cutting speed in this range. For the cutting speed larger than 8.37 m/s, the wear resistance of polycrystalline diamond tool is very low, because the accelerated state wear occurs instead of the quasi-steady-state wear. Thus, the optimization of the tool geometry and the assisted lubrication should be applied for its improvement.
The Air Force Research Laboratory led a research effort to benchmark the accuracy of static and fatigue predictions of several emerging composite progressive damage analysis techniques. The static portion of this technical effort is described in detail in a previous special issue of the Journal of Composite Materials. This paper provides the details of the fatigue experiments that were conducted to calibrate and validate the computational models. Initially, in-plane and out-of-plane S–N curves were generated through coupon tests that were performed on unidirectional laminae. The challenges experienced during fatigue testing of in-plane, matrix-dominated unidirectional coupon specimens are presented in detail. The higher fidelity test data from the fiber-dominated and out-of-plane experiments are also included in this paper. Following the calibration experiments, a series of tension–tension fatigue validation tests were conducted on open-hole coupons with three different stacking sequences. Each specimen was cycled to a pre-determined number of fatigue cycles, followed by static residual strength tests in both tension and compression. This paper provides the stress–strain responses of these validation tests as well as high-resolution X-ray computed tomography images of the subsurface damage as a function of cycles. Seven analysis teams used these test results to calibrate their models and to benchmark the accuracy of their predictions of damage and residual mechanical properties.
Chemical functionalization of carboxylated multiwalled carbon nanotubes with vitamin B1 was carried out under ultrasonic irradiation. The functionalized nanotubes were embedded in a chiral and biodegradable poly(ester-imide) to prepare multiwalled carbon nanotubes reinforced polymer nanocomposites. Optically active poly(ester-imide) was synthesized by step-growth polymerization of aromatic diol and amino acid based diacid. The vitamin B1 functionalized multiwalled carbon nanotubes and the resulting nanocomposites were examined using Fourier-transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. Thermogravimetric analysis results indicated that temperature at 10% weight loss was increased from 409℃ for pure PEI to 419℃, 427℃, and 430℃ for nanocomposites containing 5%, 10%, and 15% functionalized multiwalled carbon nanotubes, respectively. The Fourier-transform scanning electron microscopy and transmission electron microscopy images exhibited that the functionalized multiwalled carbon nanotubes were separated individually and enwrapped by polymer chains.
Natural diatom frustules composing nanometer size silica particles were heat-treated at temperatures between 600 and 1200℃ for 2 h and used as filler/reinforcing agent (15 wt%) in an epoxy resin. The opal structure of as-received natural diatom frustules was transformed into cristobalite after the heat-treatment above 900℃. The epoxy resin test samples reinforced with heat-treated and as-received frustules and neat epoxy test samples were compression tested at the quasi-static strain rate of 7 x 10–3 s–1. The results showed that the inclusion of the frustules heat-treated at 1000℃ increased the compressive yield strength of the resin by 50%, while the addition of the diatom frustules heat-treated above and below 1000℃ and the as-received frustules increased the strength by ~25% and 16%, respectively. The heat treatment above 1000℃ decreased the surface area of the frustules from 8.23 m2 g–1 to 3.46 m2 g–1. The cristobalite grains of the frustules heat-treated at 1000℃ was smaller than 100 nm, while the grain size increased to ~500 nm at 1200℃. The increased compressive stresses of the resin at the specific heat treatment temperature (1000℃) were ascribed to nano size crystalline cristobalite grains. The relatively lower compressive stresses of the epoxy resin filled with frustules heat-treated above 1000℃ were attributed to the micro-cracking of the frustules that might be resulted from higher density of the cristobalite than that of the opal and accompanying reduction of the surface area and the surface pore sizes that might impair the resin-frustule interlocking and intrusion.
Carbon compounds have high dielectric losses, which means that these materials are heated efficiently by microwave irradiation. Carbon materials can be used as microwave absorbers in polymeric materials that are transparent to microwave irradiation. In this paper, carbon-reinforced polypropylene composites were exposed to microwave irradiation and then their dynamic mechanical thermal properties, electromagnetic shielding, and surface morphology were investigated. The test results showed that mechanical and physical properties of carbon–polypropylene composites improved following microwave exposure. The dynamic mechanical thermal analysis results showed that their storage and loss modulus were improved following microwave treatment. It is postulated that microwave irradiation heats carbon materials, which is likely to melt neighboring matrix thus improving interfacial adhesion and structural defects.
In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process that leads to stress concentrations in the interface, matrix and neighboring fibers that can propagate at high speed over long distances. To gain better understanding of this event, a fiber-level finite element model of a two-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Moreover, dynamic interfacial failure is predicted where debonding initiates, propagates and arrests at longer distances than predicted by models that assume quasi-static fiber breakage. In the case of higher strength fibers breaks, unstable debond growth is predicted. A stability criterion to define the threshold fiber break strength is derived based on an energy balance between the release of fiber elastic energy and energy absorption associated with interfacial debonding. A contour map of peak dynamic stress concentrations is generated at various break stresses to quantify the zone-of-influence of dynamic failure. The dynamic results are shown to envelop a much larger volume of the microstructure than the quasi-static results. The implications of dynamic fiber fracture on damage evolution in the composite are discussed.
A coupled, transversely isotropic, deformation and damage fatigue model has been implemented within the finite element method and utilized, along with a static progressive damage model, to predict the fatigue life, stiffness degradation as a function of number of cycles, and post-fatigue tension and compression response of notched, multidirectional laminates. The material parameters for the fatigue model were obtained utilizing ply-level classical lamination theory simulations and the provided [0], [90] and [±45] experimental composite laminate S–N data. Within the fatigue damage model, the transverse and shear stiffness properties of the plies were degraded with an isotropic scalar damage variable. The stiffness damage in the longitudinal (fiber) ply direction was suppressed, and instead the strength of the fiber was degraded as a function of fatigue cycles. A maximum strain criterion was used to capture the failure in each element, and once this criterion was satisfied, the longitudinal stiffness of the element was eliminated. The resulting, degraded properties were then used to calculate the new stress state. This procedure was repeated until final failure of the composite laminate was achieved or a specified number of cycles was reached. For post-fatigue tension and compression behavior, four internal state variables were used to control the damage and failure. The predictive capability of the above-mentioned approach was assessed by performing blind predictions of notched multidirectional IM7/977-3 composite laminate response under fatigue and post-fatigue tensile and compressive loading, followed by a recalibration phase. Tabulated data along with detailed results (i.e. stress–strain curves as well as damage evolution states at given number of cycles compared to experimental data) for all laminates are presented.
Control and reduction of microorganism infections in high-risk environments is up to date a challenge. Traditional techniques imply several limitations including development of antibiotics resistance and ecotoxicity. Then, polymers functionalized with photocatalyts arise as a promising solution against a broad spectrum of microorganisms found at, e.g. sanitary, food, and medical environments. Here, we present silicone rubber–TiO2 composites as novel antibacterial polymers. Four different types of composites with different TiO2 contents were produced and analyzed under UV irradiation and dark conditions in terms of particle distribution, chemical composition, photocatalytic activity, wettability, and antibacterial efficacy against Escherichia coli. Under UV irradiation, antibacterial sensitivity assay showed a 1000 times reduction of colony forming units after 2 h of light exposure so that the antibacterial ability of silicone–TiO2 composites was proved. Photocatalytic activity assessment suggested that reactive oxygen species induced by photocatalytic reaction at TiO2 particles are the main cause of the observed antibacterial effect. Scanning electron microscopy indicated no topographical damage after UV exposure. In addition, chemical analysis through Raman and X-Ray photoelectron spectroscopies demonstrated the stability of the silicone matrix under UV irradiation. Hence, the current work presents silicone–TiO2 composites as stable nonspecific antibacterial polymers for prevention of infections at multiple high-risk environments.
Carbon nanotube-reinforced polyurethane elastomer composites were prepared by melt-mixing. Nitric acid oxidation and silanization were applied to carbon nanotube surfaces to achieve better interfacial interactions with polyurethane elastomer. Tensile and hardness tests, differential scanning calorimetry, melt flow index test, dielectric measurements, and morphological studies of composites were reported. The best results were obtained for surface-modified carbon nanotubes containing composites with lower loading levels. Addition of carbon nanotubes leads to almost two-fold increase in strain and modulus compared to pristine polyurethane elastomer. Tensile strength of composites was also improved by inclusion of carbon nanotubes. However, strength values drop down with increasing carbon nanotube content. Shore hardness increased with the inclusion of modified carbon nanotube to polyurethane elastomer while pristine carbon nanotube caused remarkable decrease in hardness of polyurethane elastomer. Relatively higher melting points and slightly lower glass transition temperatures were observed for carbon nanotube-loaded composites compared to polyurethane elastomer because of plasticizing effect of carbon nanotube. Incorparation of carbon nanotube to polyurethane elastomer matrix caused reduction in melt flow index values due to formation of agglomarates, and n the contrary, surface modifications of carbon nanotube exhibited increase in melt flow index thanks to enhanced interfacial interactions between carbon nanotube and polyurethane elastomer. Significant increase in dielectric constant of composites was observed. Better dispersion of surface modified carbon nanotubes into polyurethane elastomer was also concluded from SEM micrographs of composites.
The properties of polypropylene/low-density polyethylene and maleic anhydride grafted polypropylene/low-density polyethylene blends, and their wood powder composites are compared in this study. The blends contained equal amounts of polymers, and the wood powder was added into the blends to form polypropylene/low-density polyethylene/wood powder and maleic anhydride grafted polypropylene/low-density polyethylene/wood powder ternary systems. The Fourier-transform infrared analysis of the blends and composites did not provide any evidence of significant interactions between the different components, although the rest of the results clearly showed that maleic anhydride grafted polypropylene and wood powder significantly interacted, and that there was some interaction between maleic anhydride grafted polypropylene and low-density polyethylene. The differential scanning calorimetry and dynamic mechanical analysis results confirmed the immiscibility of polypropylene and low-density polyethylene, and polypropylene and maleic anhydride grafted polypropylene, and indicated that wood powder was distributed in both the low-density polyethylene and polypropylene phases in the polypropylene/low-density polyethylene blend, but most probably only in the maleic anhydride grafted polypropylene phase in the maleic anhydride grafted polypropylene/low-density polyethylene blend. The polypropylene/low-density polyethylene and maleic anhydride grafted polypropylene/low-density polyethylene blends were found to be more thermally stable than the neat polymers, while the presence of wood powder in both polymer blends further increased the thermal stability of the polymers. The blends and composites with maleic anhydride grafted polypropylene showed higher tensile modulus values and lower elongation at break values than the composites with polypropylene, while the stress at break values of the two sets of samples were comparable.
This paper reports on a modified pultrusion process where the conventional resin bath was replaced with a custom-designed, enclosed resin impregnation unit. A feature of this modified production process is that the rovings were spread, prior to impregnation, using a compact fibre spreading unit. The resin impregnator was designed to accommodate 60 rovings of 2400 tex E-glass. The design features enabled specified modes of impregnation to be enacted including, resin-injection, pin-impregnation, capillary-impregnation and compaction. The impregnator was designed to accept pre-mixed resin from a pneumatically activated pressure-pot or a custom-designed resin delivery system. Pultrusion trials were conducted on a commercial machine using a conventional resin bath, the pressure-pot and the impregnation unit. The physical, mechanical and thermo-mechanical properties of the composites pultruded using the modified technique were marginally superior to those manufactured using the conventional resin bath. However, the environmental benefits of the modified pultrusion process were demonstrated to be significant.
For industrial application of carbon nanotube/polymer composites, it is critical to produce composites in an efficient way. In this study, large size carbon nanotube buckypaper, which is directly produced by floating catalyst chemical vapor deposition method, was used to reinforce epoxy through conventional resin transfer molding process. The infiltration behavior of resin was analyzed, indicating that efficient carbon nanotube-epoxy interface has been formed near the surface of the composites. With 26.87% weight fraction of carbon nanotubes, the composites show a great enhancement in mechanical properties, that the failure strength and Young’s moduli increase from 45 MPa and 2.5 GPa to 254.39 MPa and 6.69 GPa with a large failure strain over 15%. In addition, the composite film is electrically conductive with a conductivity of 220 S/cm. It also shows linear piezoresistive effect with a gage factor tested to be 2.68, indicating a great potential in multifunctional smart structure applications.
Composite materials are sensible to temperature variations which can lead to the development of internal stresses. The induced stresses may be large enough to damage the material. The objective of this study is to evaluate the effects of extreme temperature cycles on three cyanate ester laminates and one sandwich panel. Thermal cycles going from –170℃ to 145℃ were conducted up to 360 cycles. Microscopic observations of the edges and the middle section of the specimens were performed to evaluate damage growth. Three types of damage were observed in the laminates: transverse microcracks, debonding between the fibers and the matrix and to a limited extent delamination. However, debonding between the fibers and the matrix were only visible on the edges of the laminates. The effect of the observed damages on the mechanical properties of the laminates was studied. Results show that properties influenced by matrix behavior were affected by thermal cycling.
This paper reports the dielectric relaxation studies of carbon nanotubes loaded in polyester polymer matrix. The study was carried out in the frequency range between 100 Hz and 1 MHz at constant temperature, T = 300 K. The frequency dependence of the electrical data was treated in the frameworks of the impedance Havriliak-Negami formalism and by using the universal Jonscher power law. The imaginary and real parts of the dielectric permittivity change with concentration of the carbon nanotubes. This work consists in studying the influence of these nanoparticles on the dielectric properties, describing the electrical relaxation and the conduction mechanisms.
A finite element-based model was developed to predict progressive damage evolution within a plain weave textile composite subjected to various combinations of in-plane tension and shear. Cracking in the tows, matrix, and interfaces was accounted for through cohesive zone modeling. Shear damage in the tows was accounted for through a continuum damage model. The damage behavior in the tows was stochastic in nature with properties determined from prior investigations of composite microstructures that included randomness in fiber positions. The predicted progressive damage evolution was found to qualitatively match well with experimental observations performed on similar material systems. The effect of temperature change, which modifies the thermally induced stresses in the tows as well as the apparent strength of the tows (due to changes in thermally induced microstresses at the fiber–matrix scale) was examined. Finally, the progressive failure responses under different loadings were compared to identify common characteristic behaviors. The effect of these characteristic behaviors on the textile’s effective response was investigated along with approaches to incorporate the behaviors into a structural scale progressive failure model.
With environmental concerns, there is a strong need to develop posterior restorative dental material as a true alternative to dental amalgam, while one of the serious environmental concerns is how to utilize marble waste such as marble dust produced during machining and cutting of stones. Therefore, the present study proposed a material formulation as a solution for both the issues and investigated the performance of silane-treated marble dust-filled dental composite. Fourier Transform Infrared spectroscopy analysis, Raman Spectroscopy analysis, and Thermo-gravimetric analysis were used to characterize treated and untreated filler. The physical and mechanical characterizations such as void content test, water sorption test, micro-hardness, compressive strength test, dynamic mechanical analysis, and thermo-gravimetric analysis were performed. The result of the study indicated that addition of 0–9 wt.% of marble dust powder increased the hardness, compressive strength, and flexural strength in the range of 69–96 HV, 175–296 MPa, and 55–80 MPa, respectively. However, the void content and water sorption were also increased in the range of 0.31–1.41% and 1.05–2.15%, respectively. Further, it was also revealed that dynamic properties and thermal degradation temperature were increased with the increase of marble dust.
This manuscript presents the blind prediction of fatigue life performance in three laminated carbon fiber reinforced polymer composite layups using a reduced-order space-time homogenization model. To bridge the spatial scales, the modeling approach relies on the Eigendeformation-based reduced order homogenization method. To bridge the time scales associated with a single load cycle and the overall life of the composite, a homogenization-based accelerated multiple-time-scale integrator with adaptive time stepping capability is employed. The proposed multiscale modeling approach was used to predict the evolution of composite stiffness and progressive damage accumulation as a function of loading cycles, as well as residual strength after fatigue in tension and compression, for three layups ([0,45,90,–45]2s, [30,60,90,–60,–30]2s, and [60,0,–60]3s). Following blind prediction, the experimental data from the blind prediction specimens were employed to better understand the failure mechanisms and recalibrate the model. This study was performed as a part of the Air Force Research Laboratory's "Damage Tolerant Design Principles" Program.
Filler dispersion is critical in the nanocomposite feature determination. Physical methods such as sonication are usually employed to disperse carbon nanotubes inside a thermoset polymeric matrix. Those methods often use strong forces to disperse the filler but they could damage it, compromising its reinforcing action. In this paper, we have employed acetone solvent during the sonication process of carbon nanotubes and polyester resin. The solvent helps the carbon nanotube bundle dissolution and favors its homogeneous distribution inside the matrix, thus reducing the action of ultrasounds. Moreover, the carbon nanotubes employed were both pristine and properly oxidized, to favor the opening of carbon nanotube bundles. Solvent was then removed although traces remained in the mixture. We have analyzed the role of solvent during the mixing and the following polymeric network growth. The experimental analyses highlighted as the solvent interacts with the carbon nanotubes during the mixing, thus hindering the right network development. Styrene fragments remain entrapped within the network of polyester resin, softening and improving the adhesive properties. Instead, without solvent, the carbon nanotubes improve the material stiffness in the order CNTox > CNTp.
Bio-ferroelectric composites represent an inexpensive and environmentally friendly electronic alternative for electrical applications such as capacitors, transistors, and actuators. The present research relates to the development of a biocomposite made of a chitosan–cellulose polymeric layer and bearing ferroelectric nanoparticles. The variables considered included the volume percentage of cellulose (15 v% and 25 v%) in the matrix and the amount of ferroelectric nanoparticles (0 wt.%, 10 wt.%, and 20 wt.%). Upon electrical characterization, the results indicated that the addition of the nanoparticles raised the capacitance and resistivity of the composite while the addition of cellulose lessened both electrical properties. The measured capacitance of the composites diminished as the applied voltage increased when contrasted with commercial capacitors where under similar testing conditions, as expected, the said capacity remained constant. Additionally, higher current flows were obtained for those capacitors than for a capacitor made with the nanocomposite. In general, it is proposed that capacitors made of this biopolymer reinforced with ferroelectric particles be suitable for radio frequency and microwave applications in which high electrical tunability and low dielectric loss are required.
In this study, weathering effect on untreated textile fiber-reinforced polymer composites and the effect of different chemical treatments for better interfacial adhesion on the outdoor performance were investigated. Degradation of physical, mechanical, and chemical properties of textile fiber-reinforced polymer composites was evaluated through common chemical treatments such as maleated coupling, alkaline treatment, silane treatment, and alkali–silane treatment. Untreated and chemically treated textile fiber-reinforced polymer composites were subjected to water uptake and UV exposure up to 1000 h. Tensile and impact properties were mechanically examined, and the changes on the physical properties due to water uptake, swelling, and color change were investigated. In addition, Fourier transform infrared spectrum analysis was performed in order to evaluate the chemical changes after exposure.
Three-dimensional spacer fabrics due to their specific structural features have achieved significant importance to be used as highly effective technical textiles especially in composite manufacturing. Increasing applications of three-dimensional fabrics in technical fields as well as the high ability of knitting process in production of highly structured-knitted preforms made the researchers to be focused on finding novel structures with high potential application as the composite reinforcements in order to achieve some desired properties. Along with others researches, in this paper an especially structured three-dimensional spacer weft-knitted fabrics was produced on an electronic flat knitting machine, using 1680 denier high-tenacity polyester yarn. The fabrics samples were employed in thermoset composite manufacturing process using the epoxy resin. In order to investigate the mechanical behavior of the composite samples in term of their flexural performance, a three-point bending test was performed. For precise investigation, numerical simulation based on finite element method was also applied. Good correlation between the experimental and theoretical results showed such reasonable validity of the proposed model for simulating the mechanical behavior of the spacer-knitted reinforced composite under external bending loads.
This paper presents a methodology and research study that characterises toughened materials, as is needed for optimisation of composite manufacturing processes. The specific challenge is to cover all of the stages of advanced composite manufacturing: fibre deposition by automatic fibre placement machines, hot or room temperature debulking, and consolidation in an autoclave. In these processes the material experiences a wide range of processing parameters: pressure, load rate, temperatures, and boundary constraints. In these conditions, toughened prepregs exhibit complex rheological behaviour, with diverse flow and deformation mechanisms at various structural scales. Here a series of experimental results are presented in order to describe the temperature, viscosity, flow mechanisms, and scale-effects of simple uncured prepreg stacks. The driver for this study is to obtain a further understanding of flow mechanisms throughout the consolidation phase of composites manufacture since fibre path defects are most likely to occur during compaction, prior to vitrification.
In this paper, the mechanical performance of resin transfer moulded nonwoven kenaf fibre/epoxy composites in the fibre volume fraction (V f ) range of 0–0.42 was investigated. The effect of the needle-punching direction on the tensile properties of the composites was also investigated. The highest tensile, flexural and fracture properties were attained at a V f of 0.42. The nonwoven kenaf fibre/epoxy composites were proven to exhibit tensile isotropy. The typical load versus displacement graph and scanning electron microscopy micrographs of the epoxy and nonwoven kenaf fibre/epoxy composites revealed that the energy absorbing events caused by the fibres led to improvements in the fracture toughness. Meanwhile, the micromechanical parameters of the composites were determined by a micromechanics analysis using the Cox–Krenchel model. The analysis proved the applicability of the model for nonwoven kenaf fibre/epoxy composites as the calculated efficiency factors were comparable to the values from previous literatures.
A system of pultruded carbon fibre-reinforced plastics micro-tubes is used for self-healing simulation in laminated polymer composite. The system consists of a package of micro-tubes, placed in the symmetry plane of the GFR/epoxy laminate stack. Healing agent is a mixture of the epoxy resin and hardener. The healing agent releases and penetrates into the cracks after the composite is damaged by the quasi-static indentation. The specimens are healed at 30℃ for 24 h. Rectangular specimens notched under ASTM D2733 have been subjected to tensile test to determine interlaminar shear strength. Shear strength of specimens has been compared in three states (virgin, damaged and healed) for various ways of healing. After the most effective self-healing, the interlaminar shear strength has been recovered to 70 ± 15% of those for virgin specimens that almost twice exceeds the residual strength of the damaged specimens.
The mechanical properties of cellulose nanofiber-reinforced polyvinyl alcohol composite were studied. Neat polyvinyl alcohol films, cellulose nanofiber sheets, and their nanocomposites containing cellulose nanofiber weight ratios of 5, 15, 30, 40, 45, 50 and 80 wt% were fabricated. Heat treatment by hot pressing at 180℃ was conducted on the specimens to study its effect to the mechanical properties and the results were compared with the non heat-treated specimens. Morphology of the composites was studied by scanning electron microscopy and the mechanical properties were evaluated by means of tensile tests. The results showed that increase of cellulose nanofiber content from 5 wt% to 80 wt% has increased the tensile strength of the composites up to 180 MPa, with cellulose nanofiber content higher than 40 wt% yielding higher tensile strength. The heat-treated specimens exhibited higher tensile strength compared to those of untreated specimens.
In this investigation, the thermal and mechanical properties of cellulose fibers from sugarcane bagasse reinforced with high density polyethylene composites were evaluated. Cellulose fibers were modified with hydrous Zr oxide to clean the fiber surface and improve the fibers–matrix adhesion. Composites were manufactured using a thermokinetic mixer process and the fiber content was responsible for 5, 10, 20, 30, and 40wt% in the composition. The chemical modification of the cellulose fibers with zirconium oxide was verified by FTIR analysis and the fibers’ morphological aspects by SEM. After the chemical modification, the FTIR results showed reduction of OH bonds. SEM characterization showed that the modification changed the morphology of fibers. The results show that composites reinforced with modified cellulose fibers have an improvement in the thermal and mechanical properties, when compared to the non-cellulose fibers. In addition, an enhancement on the mechanical properties of composites was found, i.e. a gain of 122.4% compared to neat polymer at 40wt.% fiber loading in Young’s modulus. The thermal properties show a slight decrease with increase of modified cellulose.
Multiscale Designer, developed by Altair, has been studied for its suitability for fatigue life prediction of advanced composite aircraft structures made of polymer matrix composites. The extensive experimental data provided by the Air Force Research Laboratory have been utilized to characterize the linear, non-linear, monotonic, and cyclic loading properties of micro-constituents comprising the polymer matrix composite system. The characterized properties have been then utilized to predict fatigue life and residual strength and stiffness of the aerospace grade polymer matrix composites.
A melt mixing method was used for preparation of immiscible isobutylene–isoprene rubber/ethylene propylene diene monomer blends and their polymer nanocomposites containing cloisite 15A nanoparticles. The morphology of nanocomposites was characterized by Fourier transform infrared spectroscopy and scanning electron microscope techniques. The dispersion of nanoclay in isobutylene–isoprene rubber/ethylene propylene diene monomer blends was studied by transmission electron microscopy and XRD analyses. Curing data of the prepared nanocomposites showed a decrease in scorch time (t5) and optimum cure time (t90) with increasing nanoclay content. Intercalation of rubber chains into the organoclay silicate layers was specified by d-spacing amount estimated according to the XRD results. Based on XRD results, a leftward shift towards lower diffraction angles in the organoclay characteristic peak was observed, indicating an increase in the d-spacing values in comparison with the pure organoclay. It confirms the intercalation of elastomer chains into the nanoclay galleries. The mechanical properties of the polymer nanocomposite specimens including hardness, fatigue strength, elongation at break, modulus and tensile strength were found to be better with increasing the nanoclay content. Also, resilience of the polymer nanocomposites decreased.
In an attempt to develop new materials that combine structural (mechanical) and functional (electrical and magnetic) properties, a copper composite alloy reinforced through the dispersion of fine Nd2Fe14B intermetallic particles has been synthesized by the powder metallurgy route. Composite master alloy was prepared by blending copper and 8 wt% of intermetallic NdFeB in a planetary ball mill working at 250 r/min under argon atmosphere for 10 h. Resulting composite powders were encapsulated in Cu cans and then consolidated by extrusion at 1023 K. Microstructure features of blended powders and consolidated materials were characterized by means of X-ray diffraction, scanning electron microscopy, and electron probe micro analysis. Fitting of the X-ray diffraction patterns with the Rietveld method revealed that during processing, some NdFeB particles reacted with copper and oxygen to form Nd2CuO4. The lower Nb content on the Nd2Fe14B intermetallic phase due to this oxidation causes the dispersion of Fe and Fe2B particles, which also have soft magnetic properties but a higher moment compared to Nd2Fe14B. The extruded alloys showed enhanced mechanical properties (with yield strength ≥ 600 MPa, ultimate tensile strength ≥700 MPa, and 5% elongation to failure) with satisfactory electrical conductivity (46% IACS) and high values of the coercive field (≥30,000 A/m).
Hybrid composites have been concerning in the wide variety of applications especially in the aircraft industry. Therefore, it is fantastic to achieve a hybrid composite with a high mechanical performance. For this purpose, it has been decided to imbed secondary nanoscale reinforcement into matrix of glass/carbon/epoxy composite where amino multi-walled carbon nanotubes and hybridization of amino multi-walled carbon nanotube and Nanoclay (Cloisite 30B) have been utilized. The tensile, flexural and impact properties of hybrid composites have been evaluated and a comparative study between hybrid composite reinforced with amino-MWCNTs and simultaneous amino-MWCNTs and Nanoclay has been conducted. The fractured surfaces of tensile testing and bending testing specimens were characterized with a high precise field emission scanning electron microscopy. The results of the tensile test revealed that incorporation of amino-MWCNTs reduced the ultimate strength of hybrid composite while the elastic modulus of composite with combination of amino-MWCNTs and Nanoclay increased. It was demonstrated that incorporation of nanotubes (MWCNTs) exclusively the same as hybridization of amino-MWCNTs/Nanoclay enhanced the flexural properties of conventional composites with highest increment of 10.5% and 22%, simultaneously. Simultaneous presence of nano-fillers resulted in 50% flexural modulus and strength respectively for hybrid composite reinforced with combination of amino-MWCNTs and Nanoclay. Morphological characterization of composites indicated to strengthen interfacial interaction of fabrics to epoxy when matrix reinforced with nano-fillers especially in combination of both nanotubes and nanoclays.
The aim of this study is to investigate the effects of debonding length on the fatigue and vibration behaviour of sandwich material. The sandwich material used in this study is constructed with glass fibre laminates as skins and with PVC closed-cell foams as core. The tests were conducted using impulse frequency response technique and cyclic fatigue loading in three-point bending with various debonding lengths. The vibration test was used to study the effects of debonding length on modal parameters (frequency, loss factor). The effects of debonding lengths on the stiffness, hysteresis loops and damping were studied during fatigue tests. The different relative change in frequency, loss factor and maximum load is determined. These parameters are compared for vibration and fatigue tests.
Non-destructive test technique for monitoring delamination failure under complex load and environments is still not mature until now. The purpose of this paper is to study mixed-mode delamination failure properties of carbon fiber/epoxy composite laminates under hygrothermal environment using acoustic emission. Different water-saturated composite specimens with initial intralaminar and interlaminar defects are tested. Two loading modes including single-leg and over-leg three-point bending are applied under hygrothermal environment. By analyzing the responses of acoustic emission parameters including amplitude and energy, the effects of the hygrothermal environment, layup pattern and initial defect on the delamination behaviors of composite specimens are studied. Besides, different failure modes are observed through scanning electron microscope. Quantitative acoustic emission results show hygrothermal environment and load mode affect the delamination properties of composites remarkably.
The performance of a composite material system depends critically on the interfacial characteristics of the reinforcement and the matrix material. In this study, the interfacial adhesion was tailored by the creation of textures on the glass fiber surface using inorganic-organic silane blends. A single-fiber microdroplet test was conducted to assess the interfacial properties between the textured glass surface and an epoxy matrix. The load–displacement curves from microdroplet tests were analyzed. The stress-based and energy-based micromechanic models of interfacial debonding and corresponding adhesional parameters (apparent and ultimate interfacial shear strength, friction stress, critical energy release rate, work of adhesion, and adhesional pressure) were applied for theoretical calculations. The results showed a clear trend for the impact of different silane blends on the interfacial properties. The specimens containing 75:25 and 50:50 of inorganic–organic silane blends show the most effective improvement in the interfacial adhesion properties between glass fiber and epoxy resin. Scanning electron microscopy was used to visualize the failure surface of the specimen after the microdroplet test. The scanning electron microscopic images indicated that the failure in the blend sized treated glass fiber–epoxy matrix specimen runs predominantly along the interphase and combines both cohesive failure in resin (the presence of some resin fragments) and adhesive failure (some bare fiber surfaces can be seen).
This study investigates the influence of cooling rate on the residual strain of the carbon fibre/polyphenylenesulfide unidirectional laminates. Three different cooling rates (–300℃/min, –100℃/min and –10℃/min) were applied to simulate a wide range of cooling conditions. The crystallisation behaviour which depends on the cooling rate was evaluated using differential scanning calorimetry. A process monitoring test was then conducted using embedded fibre Bragg grating sensors. In-plane transverse strain of carbon fibre/polyphenylenesulfide unidirectional laminates was measured and the results were presented based on the crystallisation behaviour determined by differential scanning calorimetry. Furthermore, residual strain change after subsequent annealing was examined. This study successfully demonstrates the effectiveness of in situ process monitoring using fibre Bragg grating sensors for evaluating material behaviour of thermoplastic composites during rapid heating and cooling under realistic processing conditions.
Mechanical behavior and reliability of composites are driven significantly by microstructural variability. Such variability can be present in the form of both morphological and constituent property variability. To understand the effect of this variability on macroscopic mechanical behavior, many statistically equivalent microstructures must be evaluated. This requires the ability to generate such microstructures. In this work, morphological variability was quantified by image analysis of actual microstructures. To reproduce this variability, a methodology was developed in which random microstructures are generated and subsequently adjusted to simultaneously match both short- and long-range statistics of actual microstructures. Synthetic microstructures were generated at a length scale of 70 µm, corresponding to the length scale at which fiber volume fractions of adjacent microstructures are uncorrelated. The utility of this methodology was also demonstrated for larger microstructures containing defects such as alignment fibers, voids and resin seams.
Fiber-reinforced composites are a well-recognized option for repair and rehabilitation of the pipelines for the oil and gas industry. Infilled composite sleeve system provides an effective rehabilitation solution, where the sleeve acts as prime reinforcement without any direct contact with steel. However, the long-term performance of the repair is dependent, in part, on the effect of hygrothermal ageing of the composites. In this publication, glass transition temperature and mechanical properties are compared for glass-fiber reinforced vinyl ester composite, both as-manufactured and after hot-wet conditioning at 80℃. The tensile and shear strength reduced substantially during conditioning, whilst the elastic modulus was relatively stable. The average glass transition temperature of the composite dropped from the as-manufactured value of 110℃ to 97℃ and 101℃, after 1000 and 3000 h of conditioning, respectively, indicating that it is stable and that the composite is suitable for use as a pipeline repair material operating at 80℃. The results indicate that a 1000 h conditioning period, specified as a minimum period in ISO/TS 24817 is suitable for representing long-term properties for stiffness-based designs for the composite material and conditioning temperature investigated.
This paper presents an experimental and statistical study of the fatigue behavior of unidirectional glass fiber-reinforced epoxy composite rods manufactured using pultrusion technique and modified with nanoparticles of alumina (Al2O3) and silica (SiO2) at four different weight fractions (0.5, 1.0, 2.0 and 3.0 wt.%). Tensile test was performed to investigate the influence of nanoparticles. Addition of alumina nanoparticles up to 3 wt.% increases the tensile strength by 54.76% over the pure glass fiber-reinforced epoxy specimen. For silica nanoparticles, there is an increase in the tensile strength of 31.29% for the content of 0.5 wt.% over the pure glass fiber-reinforced epoxy specimen. As the silica nanoparticles’ content increases over 0.5 wt.%, there is a decrease in the tensile strength. Rotating bending fatigue tests have been conducted at five different stress levels. Fatigue life of glass fiber-reinforced epoxy composite rods modified with alumina nanoparticles increases as the content of the nanoparticles increases. The effect of adding silica nanoparticles on the fatigue life of glass fiber-reinforced epoxy composite rods is relatively insignificant with a small improvement in the content of 0.5 wt.% silica above the pure glass fiber-reinforced epoxy specimens. Two-parameter Weibull distribution function was used to statistically analyze the fatigue life data.
Properties of resin and composite, especially anisotropic coefficients of thermal expansion, are very crucial to precisely determine residual stress generated in a composite part. No comprehensive study is available in the literature to determine these properties for woven composites and then its application to model residual stress in woven carbon epoxy composite parts. In the present article, experimental results on thermal coefficients of RTM6 epoxy resin as well carbon/epoxy woven composites obtained using different experimental techniques are compared with homogenised coefficients of thermal expansion results. Evolution of spring-in angle of L-shaped carbon/epoxy woven composite (during and after cure) with three different thicknesses is modelled by simultaneously solving the thermal-kinetics and thermal-chemical-mechanics coupling by using finite element code COMSOL Multiphysics. Objective was to quantify the contribution of curing and cooling to the formation of residual stress. Anisotropic properties of composite, during and after cure, required for numerical simulation are obtained using an analytical method. Variation in properties with degree of cure and thermal gradients induced in the part during fabrication are considered while modelling. Modelled properties of cured composites were compared with experimental values and were found in agreement. The spring-in angle values obtained by numerical simulation are compared with the results of the analytical model as well as experiments. Effect of variation of fibre volume fraction and presence of thermal gradients on spring-in was studied as well.
Nano-microhybrid reinforced metal matrix composites are the novel combination of composite system which enhanced the mechanical properties of the metal matrix composites. The aim of this study is to determine the nano- and macromechanical properties of aluminium (A356)-based hybrid composites reinforced with multiwall carbon nanotubes and alumina short fibers (Al2O3sf). Hybrid preforms were developed initially, by a combination of multiwall carbon nanotubes and Al2O3sf with total volume fractions of 10%, 15% and 20% and by varying the weight percentage of multiwall carbon nanotubes such as 1%, 2% and 3%. The fabricated hybrid preforms were then infiltrated with aluminium alloy (A356), and the microstructure and mechanical properties of the composites were evaluated. The distribution of multiwall carbon nanotubes within the array of the Al2O3sf network which exists in clusters was found to be relatively good. The mechanical properties such as the hardness and tensile strength of Al-based hybrid metal matrix composites were found to be improved by up to 2 wt% of multiwall carbon nanotubes. The causative reason for this is attributed to a combined effect of both multiwall carbon nanotubes and Al2O3sf, which contributed to better load sharing between the fibers and the Al matrix, and also accounted for the resistance of dislocation movements caused by the presence of the multiwall carbon nanotubes. In addition, the continuous stiffness measurement method was also used to evaluate the nanomechanical properties of the composites. The results showed that the influence of multiwall carbon nanotubes highlighted the properties on a nanoscale.
This article focuses on the evaluation of stress concentration factors for composite materials with varying hole diameters. An innovative method is presented for measuring the stress concentration around a hole using the digital image correlation method. This method can provide a quicker and cheaper testing technique. The digital image correlation method is utilized to evaluate the strain field and the displacement at the edge of the hole and obtain information about fracture mechanisms in the studied materials. The effect of the ratio of the hole diameter to its width (a/w ratio) on both gross and net stress concentration factors around the hole is investigated. In addition, there is a comparison between the net and the gross stress concentration factors uses of the digital image correlation method, the analytical solutions, and the finite-element analysis for both composite and steel materials.
The interference-fit joint of composite laminates is widely used in assembly of thin-walled components in aviation product, but the interference percentage has a significant effect on squeezed damage which may reduce structural reliability. An investigation is conducted into the in-plane stress distribution and initial damage mechanism of symmetrical carbon fiber reinforced plastics laminates during the interference-fit bolt installation process. Considering the elastic deformation of the bolt, a general stress distribution model around the interference-fit joint is established with complex potential method. The initial damage mechanism of carbon fiber reinforced plastics laminates is characterized and critical interference percentages without damage are obtained with the mixed damage criteria. The effects of ply orientation and interference percentage on damage mechanism of each individual layer are discussed. Then, extensive finite element models with USDFLD subroutine of interference fit process are used to analyze and simulate the stress distribution and squeezed damage which are validated by strain measurement and micrographs by experiments subsequently. It is observed that theoretical solutions fall within the finite element results. The matrix tensile damage occurs first, and the critical interference percentages decrease from 1.10% to 0.85% with bolt diameters varying from 4 to 10 mm.
The encapsulation of dye solar cells in translucent, structural and lightweight glass fiber-reinforced polymer laminates was investigated with a view to designing multifunctional envelopes for daylit buildings. Small and large integrating sphere experiments and solar radiation experiments were performed to determine the light transmittance of the laminates and the electrical efficiency of the encapsulated cells. An overall cell efficiency of 3.9% (before encapsulation) only decreased to 3.4% after encapsulation below laminates of around 3-mm thickness. Thermal cycle experiments and finite element analysis allowed the thermal performance of the encapsulation for two types of cell substrates (glass and acrylic polymer) to be evaluated. Contrary to glass substrates, no delaminations were observed on acrylic substrates after 300 h of cycles +60/–20℃. A design for integrating dye solar cells into multifunctional sandwich building envelopes is proposed. A light transmittance of around 0.35 was estimated through a sandwich envelope with cell modules occupying 50% of the external face sheet. Research on the manufacturability of cells on polymeric substrates is encouraged.
This study contributes to the understanding of the mechanism behind process-induced distortions and stresses related to the Resin Transfer Moulding manufacturing process. The objective is to comprehend the phenomena and to identify related parameters. During the manufacturing process, engineering constants of the matrix are changing and are influenced by the existence of a large number of effects. A viscoelastic material model has been derived. This developed material model integrates a dependency of the time–temperature–polymerisation and fibre volume content on the relaxation behaviour of residual stresses in a transversally isotropic reinforced material. The model is validated using a test case on the coupon level and results / limitations are discussed.
Wood–plastic composites were prepared from poplar wood flour and high-density polyethylene (HDPE) by melt blending and injection molding techniques, using polyethylene-grafted glycidylmethacrylate (HDPE-g-GMA) as compatibilizer and -aminopropyltriethoxysilane as coupling agent. The scanning electron microscopy results showed that a stronger interfacial adhesion was formed between the wood flour and HDPE matrices during the combined use of -aminopropyltriethoxysilane and HDPE-g-GMA, while the X-ray photoelectron spectroscopy results showed that more HDPE chains are linked to the surface of poplar wood flour through the formation of chemical bonding in the presence of -aminopropyltriethoxysilane and HDPE-g-GMA. So, HDPE-g-GMA and -aminopropyltriethoxysilane showed a synergistic effect on the improvement of compatibility between the poplar wood flour and HDPE matrices and better mechanical properties of wood–plastic composites could be obtained. Furthermore, the thermogravimetric analysis results also indicated the synergistic effects to some extent. The synergistic mechanism of -aminopropyltriethoxysilane and HDPE-g-GMA was proposed on the basis of investigation results.
The main objective of this article is to exploit a phantom paired element based discrete crack network toolkit for predicting the damage progression and residual strength of laminated composites without and with a hole under tension and compression. Both intra-ply matrix cracking and inter-ply delamination are considered under a co-simulation framework in the discrete crack network toolkit. A mesh-independent kinematic description of discrete matrix cracks is accomplished via user-defined phantom paired solid elements to capture the initiation and evolution of fiber orientation dependent matrix cracking. In-ply matrix crack initiation is realized by inserting a crack along the fiber direction when a matrix driven failure criterion is satisfied and a cohesive injection along the matrix crack interface is applied to account for energy dissipation during matrix crack opening. The delamination failure mode is characterized by applying Abaqus’ cohesive interaction at ply interfaces. The non-linear shear behavior is introduced by employing a power law based curve-fit model and the fiber failure is described using a continuum damage mechanics based model. Both the blind and recalibrated predictions are performed for specimens of three different layups under the Air Force Tech Scout 1 program. The predicted damage progression and the load displacement curves are compared with the testing results provided by the Air Force Research Laboratory.
The degradation of mechanical properties in halloysite nanoclay–polyester nanocomposites was studied after an exposure of 24 h in diluted methanol system by clamping test specimens across steel templates. The glass transition temperature (Tg) and storage modulus increased steadily with the increase of halloysite nanoclays before and after diluted methanol exposure. The addition of nano-fillers was found to reduce liquid uptake by 0.6% in case of 1 wt% reinforcement compared to monolithic polyester. The mechanical properties of polyester-based nanocomposites were found to decrease as a result of diluted methanol absorption. After diluted methanol exposure, the maximum microhardness, tensile, flexural and impact toughness values were observed at 1 wt% of halloysite nanoclay. The microhardness increased from 203 to 294 HV (45% increase). The Young’s modulus increased from 0.49 to 0.83 GPa (70% increase) and the tensile strength increased from 23 to 27 MPa (17.4% increase). The impact toughness increased from 0.19 to 0.54 kJ/m2 in diluted methanol system (184% increase). Surprisingly, the fracture toughness of all types of nanocomposites was found to increase after exposing to diluted methanol due to plasticization effect. Scanning electron microscope images of the fractured surfaces of tensile specimens revealed that the methanol increased the ductility of the matrix and reduced the mechanical properties of the nanocomposites.
It is critical to study the mass transfer of supercritical fluid degradation for carbon fiber composites to investigate their degradation mechanism, design the reactor, and develop recycling processes. The mass transfer process of supercritical fluid degradation was described from two aspects: mass diffusion from outside to inside, and from inside to outside. Mass transfer model of supercritical fluid degradation was established based on a proposed concentric cylindrical representative volume element. The reaction kinetic parameters were incorporated into the mass transfer equation, and the concentration distribution of supercritical fluid, mass transfer rate, reaction order, and reaction rate constant during the carbon fiber composites degradation process were calculated. Relaxation time was incorporated into the mass transfer process, and the supercritical fluid concentration calculation method considering non-Fick effect was proposed. Finally, two pretreatment methods were adopted to speed up the mass transfer process.
The strength prediction of open-hole fibre-reinforced composite laminate under compression is very important in the design of composite structures. The modelling of fibre, matrix damage and delamination plays an important role in the understanding of the damage mechanics of laminate under open-hole compression. In this article, a progressive damage model for open-hole compression that is based on continuum shell elements and cohesive elements is established to model in-plane damage and delamination, respectively. The damage mechanics of sublaminate-scaled laminates with ply sequence [45/0/–45/90]ms and ply-level-scaled laminates with ply sequence [45n/0n/–45n/90n]s are investigated by our proposed model. The Tsai-Wu and Hoffman failure criteria are employed for the determination of matrix damage initiation. Compared with the experiments, the numerical results using the Tsai-Wu criterion exhibit better accuracy regarding open-hole compression strength prediction and failure modes simulation.
In this article, static and dynamic responses of cross-ply bi-stable composite plates were studied. To accurately predict the natural frequencies and snap-through load, a set of higher order shape functions were proposed. In static analysis, the stable configurations, the deflection of corners, and the midpoint of the plate were calculated. For dynamic analysis, Hamilton’s principle is used to provide approximate solutions to the vibration problem under study. The responses of the plate under ramp and harmonic applied forces were determined, the effect of shape functions on the prediction of the first natural frequency of the plate and the required force for snap-through were investigated. A finite element model is also developed to study the static and vibration characteristics of bi-stable composite plate. The qualitative and quantitative comparisons between the finite element method results and those obtained from the present analysis are generally good and satisfactory. The developed analytical model can also be used for parametric study and further design modification.
The Air Force Research Laboratory directed a research program to evaluate nine different composite progressive damage analysis methods under both quasi-static and fatigue loading. This paper describes the coupon tests that were performed at the Air Force Research Laboratory for calibration and validation of the methods under quasi-static conditions. The basic elastic and failure properties of unidirectional IM7/977-3 graphite/epoxy were first determined in order to properly calibrate the models. Validation tests were then performed on unnotched and open-hole coupons with three different laminate stacking sequences under both tension and compression loading conditions. This paper summarizes these experimental results and provides X-ray computed tomography images at subcritical load levels.
Recently, a new manufacturing process for the production of metallic matrix composite materials reinforced with carbon nanotubes, known as sandwich technique has been proposed. This technique produces a material comprised of a metallic matrix and a banded structures-layers of multi-walled carbon nanotubes. However, among other issues, the matrix-reinforcement interface and the reinforcement dispersion degree are still open questions. The present study uses field emission scanning electron microscopy and high resolution transmission electron microscopy to probe that the method is capable to achieve a good dispersion of the multi-walled carbon nanotubes with no evidence of carbon nanotubes’ damage. The mechanical properties were measured by tensile and nanoindentation tests; improvements in the elastic modulus, yield and ultimate strengths were found, with respect to the unreinforced material.
The foundations for a multiscale numerical framework for composites, that can assist in the design of composites structures, have been laid in this study. Fibre tensile and shear properties were investigated experimentally, and subsequently modelled using the laminate theory. The interface properties of a flax/polypropylene and a flax/epoxy system were determined using microbond tests combined with micromechanical models, and properties from transverse tensile tests. These properties were used in numerical models of the microbond test, and good agreement with experiments was obtained for both systems. When the interface and fibre properties were used in numerical models of single flax yarns impregnated with polypropylene or epoxy resin, the stiffness behaviour showed good agreement with experiments for flax/polypropylene and flax/epoxy systems. The peak stress prediction from the flax/polypropylene numerical model was only 6.2% lower than the experimental average of 24.33 MPa. The flax/epoxy model, however, predicted a peak stress value of 85.6 MPa which was 20.6% higher than the experimental value of 70.9 MPa. The properties obtained from this study, along with the methodology, can be used to estimate macro-scale strengths of composite members in structures for different layups and loading conditions.
This investigation highlights the influence of plasma modified carbon nano fiber (CNF) on the various properties of poly ether ketone (PEK). CNFs were modified with oxygen plasma under low pressure to enhance the interfacial adhesion between the reinforcement and matrix. Significant changes are evident in the elemental composition of oxygen and carbon on the plasma modified CNFs as observed by X-ray photo electron spectroscopy. Based on results from compression and tensile strength, significant change in the mechanical properties of the composites is observed. Dynamic mechanical thermal analysis (DMTA) reveals that the storage modulus increases on reinforcing modified CNF in PEK. The increase in modulus is noticeable only up to 1.5% wt reinforcement of CNF, while higher percentage of reinforcement leads to decline in properties. DMTA studies also clearly shows that the dispersion of CNF is not uniform after 1.5% of CNF reinforcement. However, differential scanning calorimeter and thermo gravimetric analysis studies reveal that the thermal properties of the CNF reinforced composite do not vary significantly. Thermal conductivity results show a substantial increase in the thermal conductivity of polymeric composites on increasing the reinforcements. Transmission electron microscopy (TEM) analysis reveals that there is uniform dispersion of CNF in PEK. TEM also clearly shows that higher percentage of CNF leads to agglomeration. Physico-chemical analysis indicates that the contact angle increases on increasing the reinforcements. These findings would be highly useful to make way for PEK composites for high temperature and high strength application.
Aluminum (Al)-based metal matrix composites reinforced with multiwalled carbon nanotubes were developed by powder metallurgy route. The Al and multiwalled carbon nanotubes powder mixtures were consolidated under a load of 565 MPa followed by sintering at 550℃ for 2 h in inert atmosphere. Al–1, 2, and 3 wt.% multiwalled carbon nanotube composites were developed. In the present study, the microstructure, mechanical properties, sliding wear behavior, and crystallographic texture of various Al–multiwalled carbon nanotube composites were investigated. The multiwalled carbon nanotubes produced by low-pressure chemical vapor deposition technique and the various sintered composites were characterized using scanning electron microscope, high-resolution transmission electron microscope, X-ray diffraction, differential scanning calorimetry and thermogravimetric analysis, Raman spectroscopy, and Fourier transform infrared spectroscopy. A significant improvement in relative density, Vickers microhardness, and wear resistance of the composites up to addition of 2 wt.% of multiwalled carbon nanotubes was observed. The deterioration in these properties beyond 2 wt.% of multiwalled carbon nanotubes was possibly due to the agglomeration of multiwalled carbon nanotubes in the Al matrix. The tensile strength of Al–multiwalled carbon nanotube composites continuously decreases with the addition of multiwalled carbon nanotubes. The decrease in tensile strength can be attributed to the detrimental effect of Al4C3 formed at the interface of the Al matrix and the multiwalled carbon nanotubes which will cause premature failure of the composite. The addition of multiwalled carbon nanotubes altered the crystallographic texture of the composites. The residual stresses in the various composites were found to be compressive in nature and also show improvement up to addition of 2 wt.% multiwalled carbon nanotubes in the Al matrix.
Sisal fiber (SF) reinforced recycled polypropylene biocomposites were prepared by melt blending technique. Biocomposites prepared with the incorporation of 40 wt% untreated sisal fiber loading showed a marginal improvement in mechanical properties as compared with matrix recycled polypropylene. SF surface was mercerized and maleic anhydride grafted polypropylene was used as a coupling agent for better fiber matrix interfacial bonding. Mercerized sisal fiber reinforced biocomposites prepared with compatibilizer (maleic anhydride grafted polypropylene) shows significant improvement in tensile and flexural strength. Damage tolerance of recycled polypropylene matrix and its biocomposites were evaluated in monotonic and cyclic tensile test. Untreated sisal fiber reinforced biocomposites prepared with maleic anhydride grafted polypropylene shows improvement in damage tolerance compared with untreated sisal fiber biocomposites. Impact fractured morphology of biocomposites revealed better interfacial bonding between fiber, maleic anhydride grafted polypropylene, and recycled polypropylene matrix.
Photonic crystal structures were fabricated by the formation of periodic arrays of poly(ferrocenylsilane) (PFS) derivations and silica spheres. Here, we present the primary steps of a simulation-driven design of photonic crystal based on PFS as a potentially stimuli-responsive polymer, which can present predesigned color when subjected to the visible light. The poly(ferrocenylmethylvinylsilane) (PFMVS) was synthesized by photolytic anionic ring opening polymerization of methylvinylsila[1]ferrocenophane monomer under controlled condition and monodisperse silica spheres were synthesized by the Stober process. Bare silica opal was prepared by the convective self-assembly process of different sizes of silica spheres on glass substrate followed by infiltration of PFMVS to obtain PFMVS/silica opal and finally removal of the silica spheres by acid washing to attain the inverse PFMVS opal. The particle size of silica spheres along with the refractive indices contrast of different components forming photonic crystal played an important role in obtaining the maximum reflectance wavelength of the desired opal. Photonic crystal prepared by 210, 270, and 330 nm monodisperse silica spheres represents the blue, green, and red color, respectively. Simulation analyses of these opal structures were investigated by using the plane-wave expansion (PWE) method to assess the photonic crystal behavior versus the visible light. According to the simulation results, the photonic crystal structure was designed. The experimental reflectance outcomes of the three opal structures were in good agreement with the simulation analysis.
In this study, the effects of filler characteristics and composite preparation methods on the morphology, mechanical property, electrical conductivity, and electromagnetic interference shielding effectiveness of the polypropylene/polycarbonate (70/30, wt%)/hybrid conductive filler composites were investigated. Nickel-coated carbon fiber (NCCF) was used as main filler and TiO2, multi-walled carbon nanotube, and graphite were used as second fillers in the composites. The pultruded NCCF/polypropylene composite was used in the preparation of the polypropylene/polycarbonate/NCCF/second filler composites. The electrical conductivity and electromagnetic interference shielding effectiveness of the polypropylene/polycarbonate/NCCF/second filler composites were compared with the type of second filler. The superior value of electromagnetic interference shielding effectiveness was observed to be 51.6 dB (decibel) when the hybrid fillers such as NCCF (5.2 vol% or 20 wt%) and TiO2 (1.2 vol% or 5 wt%) were added in the polypropylene/polycarbonate (70/30) composite. The electrical properties of the polypropylene/polycarbonate (70/30)/NCCF/TiO2 composites was compared with the composite preparation methods, which were injection molding and screw extrusion. The results suggested that fiber length of the NCCF affected significantly to the electrical conductivity and electromagnetic interference shielding effectiveness of the polypropylene/polycarbonate (70/30)/NCCF/TiO2 composites.
Metallic nanoparticle inks (nanoinks) have attracted great interest in the manufacturing of printed flexible electronics. However, sintering pure nanoinks in ambient conditions results in micro-cracks and pores within the sintered film, which deteriorate the mechanical and electrical characteristics of the sintered nanoinks. To alleviate these problems, we demonstrate the use of very long carbon nanofiber (average length 200 µm) to reinforce the sintered nanoink films. In this study, different weight fractions of carbon nanofiber are dispersed into the Cu nanoink to improve the mechanical bending characteristics. Scanning electron micrographs show improved dispersion of oxidized carbon nanofiber in the nanoink compared to the as-received carbon nanofiber. The composite nanoinks are stencil printed on polyethylene terephthalate film and sintered by intense pulsed light using Xe-flash. The electrical measurements show 90%, 65%, and 66% improved electrical conductivity in the composite nanoink film (0.7% of oxidized carbon nanofiber) compared to the pure Cu nanoink under the 7.5 cm, 5.0 cm, and 2.5 cm of bending radii, respectively.
A methodology for the creation of 304LSS-CNT metal matrix composites using the mechanical alloying approach is presented. Planetary ball milled powders were both melted and hot pressed and achieved up to 96% theoretical density. High resolution scanning electron microscopy, Scanning Transmission Electron Microscopy, X-ray diffraction, energy dispersive spectroscopy, thermal diffusivity measurements, and Vickers microhardness measurements are used to characterize as processed and heat treated composites. Melted and solidified samples show highly anisotropic austenite/martensite microstructures with the presence of large dendritic carbon agglomerations, while hot-pressed samples show equiaxed austenite/martensite grains with a large number density of carbide precipitates. Grain size and thermal diffusivity decrease while microhardness increases up to 36% with up to 2% carbon nanotube addition for hot-pressed samples. Thus, mechanical alloying has been shown to be a potential option for the production of homogeneous 304LSS-CNT metal matrix composites for applications requiring increased strength.
Integrated steel plants in general produce large amounts of solid wastes during iron and steel making process. LD sludge or Linz–Donawitz sludge (LDS) are the fine solid particles recovered after wet cleaning of the gas emerging from LD convertors during steel making. In general 8 kg Linz–Donawitz sludge is generated per one ton of crude steel production. This solid waste can have many valuable product-oriented applications if processed economically. The common reuse methods for Linz–Donawitz sludge are recovery of metal values, recycling in sinter plant, and production of value-added products. But its use as filler in polymer composites is not yet established. This work includes the processing and characterization study of epoxy resin filled with micro-sized Linz–Donawitz sludge. Mechanical properties of the composites are evaluated under standard test conditions. Sliding wear tests are conducted using Taguchi’s L25 orthogonal arrays over a range of sliding velocities (36–315 cm/s), normal loads (5–25 N), sliding distances (500–2500 m), and Linz–Donawitz sludge contents (0–20 wt.%). A theoretical model is developed to estimate the wear rate of these composites under different test conditions. The results obtained from the proposed theoretical model are found to be in good agreement with the experimental values under similar test conditions. Taguchi’s analysis suggests that the composition of the composite and the sliding velocity are the most significant factors affecting the specific wear rate. This study further reveals that wear resistance of neat epoxy is enhanced by incorporation of micro-sized LD sludge particles.
The fatigue hysteresis behavior in fiber-reinforced ceramic–matrix composites under multiple loading stress levels has been investigated. Based on the fatigue damage mechanism of fiber slipping relative to matrix in the interface debonded region upon unloading/reloading, the fatigue hysteresis loops models corresponding to different interface slip cases under multiple loading stress levels have been derived. The fatigue hysteresis loss energy and interface slip corresponding to single/multiple loading stress levels and different loading sequences have been investigated. The fatigue hysteresis loops of unidirectional SiC/CAS–II composite under multiple loading stress levels have been predicted.
The potential of using of sub-microalumina/titanium particles as a reinforcement that can produce multifunctional polymer composites was explored. Novel multifunctional composites have been developed by incorporating sub-micro-alumina/titanium particles into polyamide6. The composites were investigated for their thermal, viscoelastic, water uptake and mechanical properties, as a function of alumina/titanium concentration. A detailed study of the morphological observation by scanning electron microscope was used to correlate the microstructures to the mechanical properties. Flexural testing shows that the flexural modulus and strength of the composite are improved by 22%, and 15%, respectively, with incorporating 10 wt% alumina/titanium. In addition, the impact strength was improved by about 19%. Furthermore, 10 wt% alumina/titanium increases the interfacial shear strength of polyamide6 by about 23%.
The purpose of this paper is to investigate the mechanical behavior of fiber-reinforced incompressible nonlinearly elastic solids under large simple shear deformations. Two different rubberlike materials, with distinct properties of adhesion, were reinforced by a single family of parallel fibers of nylon. Fibers of nylon 6 monofilament fishing line with diameters of 0.25, 0.45, and 0.80 mm were used. Fiber-reinforced specimens were tested under monotonic load at constant temperature, and values of amount of shear were obtained by the digital image correlation method. A phenomenological constitutive model is proposed to predict the mechanical behavior of the transversely isotropic materials. The proposed model takes into account the shear and stretch in the fiber, and fiber-matrix iterations. These iterations are related to the quality of the fiber-matrix bonding and fibers pullout. The obtained results can be useful in understanding the mechanical behavior of fiber-reinforced rubberlike solids and fibrous soft tissues.
We have designed and demonstrated a complementary metal-oxide-semiconductor compatible process for fabricating high capacitance micro-capacitors based on vertically grown silver nanowires on silicon substrates. Array of silver nanowires with high-aspect ratio were electrochemically grown in the pores of anodized aluminum oxide film, which was pre-formed through anodization of aluminum thin film deposited on titanium/silicon oxide/silicon substrates. High dielectric bismuth ferric oxide layer was electrodeposited to fill the gap between silver nanowires after anodized aluminum oxide film was removed. It was found that the micro-capacitor based on the silver nanowires/bismuth ferric oxide composite film possessed higher capacitance by approximately one order of magnitude from the COMSOL simulation results from the flat Ag thin-film capacitor and the silver nanowire capacitor.
This study had as its main objective to evaluate the flexural properties (strength and modulus) and degree of conversion of a dimethacrylate resin containing different amounts of nanoparticulated clay Montomorillonite (MMT) as filler. A series of composites containing similar amounts (in volume) of barium glass particles was also tested as control data. Eight formulations with polymeric matrix-based BisGMA/TEGDMA (Bisphenol A Bis(2-hydroxy-3 methacryloxypropyl)Ether/Triethyleneglycol Dimethacrylate), four added with MMT and four added with barium glass in the volume concentration of 20, 30, 40 and 50 vol% were studied. The degree of conversion was determined using near-IR spectroscopy. Elastic modulus and flexural strength were determined by the three-point bending test. The dispersion of MMT nanoparticles was determined by means of X-ray diffraction and transmission electron microscopy analysis. The fillers montomorillonite and barium glass interacted with polymer matrix-based BisGMA/TEGDMA in a distinct manner. Although the addition of montomorillonite nanoparticles resulted in similar degree of conversion and higher elastic modulus values at all concentrations tested, only at the 20 vol% the flexural strength was statistically higher, compared to the control groups filled with barium glass. This could be related to the need of concentration optimization of montomorillonite for each type of polymer matrix in order to adjust or improve mechanical properties. The addition of low concentrations (<l 20% vol) of montomorillonite nanoparticles in dental composites resins – such as additive or hybrid filler – should be studied, aiming to the reduction of polymerization shrinkage, better mechanical properties and improvement of a new technology for future applications.
The main objective of this work was to explore for the first time the potential of the co-grinding process using a high-energy vibrated ball mill to prepare poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/wheat straw fibers biocomposites. Grinding conditions of virgin PHBV pellets were examined by focusing on the evolution of particle size, morphology, crystallinity, and molecular weight. Temperature and grinding duration were demonstrated to be the key parameters affecting PHBV milling. In a second step, mechanical properties of biocomposites prepared by cryo-co-grinding were discussed in relation to the processing conditions and the resulting structure of materials. Comparing to virgin PHBV, the reinforcing effect of wheat straw fibers was very poor, regardless of the good dispersion of fibers within the polymer matrix induced by co-grinding. The increased brittleness and decreased toughness of biocomposites were attributed to (a) a poor interfacial compatibility between wheat straw fibers and PHBV and (b) the degradation of PHBV during processing, as revealed by the decrease in molecular weight.
Experimental and numerical analyses of a woven composite were performed in order to assess the effect of yarn path and layer shift variability on properties of the composite. Analysis of the geometry of a 12 K carbon fibre 2 x 2 twill weave at the meso- and macro-scales showed the prevalence of the yarn path variations at the macro-scale over the meso-scale variations. Numerical analysis of yarn path variability showed that it is responsible for a Young’s modulus reduction of 0.5% and CoV of 1% which makes this type of variability in the selected reinforcement almost insignificant for an elastic analysis. Finite element analysis of damage propagation in laminates with layer shift showed good agreement with the experiments. Both numerical analysis and experiments showed that layer shift has a strong effect on the shape of the stress–strain curve. In particular, laminates with no layer shift tend to exhibit a kink in the stress–strain curve which was attributed solely to the layer configuration.
There is an emerging interest in the aerospace industry to manufacture components with intricate geometries using discontinuous-fibre carbon/polyether-ether-ketone moulding systems (obtained by cutting unidirectional tape into strands). Great formability and high modulus can be achieved with this type of composites, but the high variability of measured properties can have a detrimental effect on the design allowables. When it comes to prediction of mechanical properties, it is important to capture the average strength and modulus as well as their statistical variability. This article proposes a stochastic finite element technique that uses the concept of randomly oriented strands to model variability, and the application of Hashin’s failure criteria and fracture energies to estimate strength. Overall, the model matches the trends observed during experiments and shows that strength of randomly oriented strand composites is significantly lower than that of continuous-fibre laminates due to the ‘weakest-link’ principle.
Out-of-autoclave prepreg processing requires evacuation of volatiles in the early stages of processing to achieve an acceptable final void content. In this study, single prepreg plies were laid-up onto a glass tool to simulate a ply–ply interface, to gain an understanding of initial air entrapment and eventual removal mechanisms. The contact was recorded during processing with various edge breathing configurations to identify the relationship between evacuation pathways and contact evolution. The existence of preferential flow channels along the fibre direction of the material was demonstrated by characterizing the prepreg surface. Gas evacuation in those channels prevented contact during an extended ambient temperature vacuum hold. The contact between the prepreg and glass tool equilibrated around 80% during the ambient vacuum hold, and reached full contact at elevated temperature after a brief loss in contact due to moisture vaporization, when the resin pressure decreased to below the water vapour pressure.
The thermal property of textile structures plays an important role in the understanding of thermal behaviour of the clothing. In this work, user-friendly GUI plug-ins have been developed to generate both microscopic and mesoscopic scale models for finite element analysis. The plug-ins were developed by using Abaqus/CAE as a platform. The GUI Plug-ins enable automatic model generation and prediction of the effective thermal conductivity of woven composite and microencapsulated Phase Change Materials composites via finite element analysis by applying boundary conditions. The predicted effective thermal conductivities from plug-ins have been compared with the results obtained from published experimental research work based on an established mathematical model. They are correlated well. Moreover, the influence of phase change materials on heat transfer behaviour of microencapsulated Phase Change Materials composites was further analysed.
A multiscale-modeling framework is presented to understand damage and failure response in carbon nanotube reinforced nanocomposites. A damage model is developed using the framework of continuum damage mechanics with a physical damage evolution equation inspired by molecular dynamics simulations. This damage formulation is applied to randomly dispersed carbon nanotube reinforced nanocomposite unit cells with periodic boundary conditions to investigate preferred sites and the tendency towards damage. The continuum model is seen as successfully capturing much of the unique nonlinear trends observed in the molecular dynamics simulations in a volume 1000 times greater than the molecular dynamics unit cell. Additionally, application of the damage model to the continuum unit cell revealed insights into the failure of carbon nanotube reinforced nanocomposites at the sub-microscale.
Automated fiber placement (AFP) process provides high potential to repeatability and flexibility required for manufacturing of complex parts in many industries. Performance of such parts can be influenced by AFP manufacturing induced defects such as gaps and overlaps. In this work, the effect of gaps on fatigue behavior of unidirectional carbon/epoxy laminates was investigated. Tension–tension fatigue tests were conducted on defected samples and compared to reference samples free from defects. Infrared thermography technique was used for monitoring of damage propagation during fatigue loading. Moreover, a fatigue progressive damage model (FPDM) was developed and applied to laminates containing gaps to predict fatigue damage progression and failure. The experimental results revealed that the effect of gaps depends on the maximum applied stress during fatigue. The higher is the applied stress, the higher is the reduction in fatigue life. Good agreement was found between the results of fatigue life prediction from the FPDM and the experimental results for defected specimens.
The work describes the manufacturing and testing of novel hybrid epoxy/carbon fibre composites with silica micro and poly-diallyldimethylammonium chloride-functionalised nanoparticles. A specific chemical dispersion procedure was applied using the poly-diallyldimethylammonium chloride to avoid clustering of the silica nanoparticles. The influence of the various manufacturing parameters, particles loading, and mechanical properties of the different phases has been investigated with a rigorous Design of Experiment technique based on a full factorial design (2131). Poly-diallyldimethylammonium chloride-functionalised silica nanoparticles were able to provide a homogenous dispersion, with a decrease of the apparent density and enhancement of the mechanical properties in the hybrid carbon fibre composites. Compared to undispersed carbon fibre composite laminates, the use of 2 wt% functionalised nanoparticles permitted to increase the flexural modulus by 47% and the flexural strength by 15%. The hybrid carbon fibre composites showed also an increase of the tensile modulus (9%) and tensile strength (5.6%).
The main purpose of this paper is to present the basic fatigue properties of metal matrix syntactic foams. The investigated syntactic foams consisting of expanded perlite and A356 aluminum matrix were produced using an inert gas pressure infiltration technique. The obtained foams were subjected to cyclic compressive loading in order to investigate their fatigue properties. The standard procedure for cyclic fatigue testing was slightly modified to account for the variation of porosity and strength which is typical for metallic foam samples. This approach allows the direct comparison of the fatigue test results between all investigated samples. Depending on the applied load level, two different failure mechanisms were identified that resulted in characteristic deformation – loading cycle curves. The failure mechanisms were further investigated on the microstructural scale: traces of fatigue beachmarks and extensive plastic deformation were found. Furthermore, Wöhler-like deformation – lifetime diagrams were created in order to predict the expected lifetime of the properties of metal matrix syntactic foams .
The aim of this article is to examine for the first time the morphological, physical, chemical, mechanical, and thermal properties of a new kind of fibers, extracted from the leaves of a plant of the Asparagaceae family, to make it possible to use them as potential reinforcement for composite structures. The fibers were extracted from the leaves of Sansevieria zeylanica by decortication process. The presence of mechanical fibers and ribbon fibers were identified through the anatomy of Sansevieria zeylanica leaves. The hierarchical cell structure of these fibers was analyzed through polarized optical microscopy and scanning electron microscopy. It consists of primary cell wall, secondary cell wall, fiber lumen, and middle lamellae. The chemical composition of the natural fibers, in terms of cellulose 76.12%, hemicelluloses 9.32%, lignin 4.28%, and ash content 1.36%, was analyzed by using standard test methods and compared with other natural fibers. The fiber density and fineness were found to be 0.945 ± 0.004 g/cm3 and 8.35 tex, respectively. The thermal behavior of the fiber was investigated through thermogravimetric analysis/differential thermogravimetric analysis. The initial degradation temperature of the cellulose component is 304℃. The results obtained through Fourier transform infrared spectroscopy and X-ray diffraction showed the presence of cellulose with the crystallinity index of 66.67%. Finally, single fiber tensile tests have been performed to assess the mechanical properties. Tensile test of Sansevieria zeylanica fibers showed the tensile strength of 359 MPa and Youngs modulus of 8 GPa.
Low-velocity impact response of glass/epoxy composite plates and fiber metal laminates with and without holes is investigated. The critical parameters that affect the delamination characteristics of laminates are impact energy, holes separation distance, type and directionality of fibers. An experimental investigation has been conducted to evaluate the effect of the presence of holes and the incorporation of aluminum layers in the extent of delamination. The extent of damage introduced during the impact event was observed on images obtained from C-scan non-destructive ultrasonic technique. Results indicate that fiber metal laminate made with aluminum layers exhibits an improved dynamic response in comparison with that of conventional laminates. The beneficial effect of using aluminum layers to reduce the extent of delamination produced by impact loading especially on laminates with holes is demonstrated. Furthermore, fiber metal laminates show better load carrying capability than conventional composite plates. The better response of fiber metal laminate with multidirectional fabric in comparison with fiber metal laminate with woven fabric is also examined. These results may be useful to better design the location of holes in composite structures.
In this work, we investigate the effect of the matrix on the mechanical performance of woven carbon fibre composites. More specifically, composites with the same 5-harness satin carbon fabric reinforcement and different thermoplastic matrices, PPS and PEEK, are compared in various mechanical tests (tensile, interlaminar fracture toughness and compression-after-impact tests). The results of tension tests show the influence of the matrix type on the development of cracks in yarns. The cracks in carbon fabric/PEEK composite appear later than in carbon fabric/PPS composite. Their density is also lower. A correlation between cumulative acoustic emission energy and transverse crack appearance in tensile tests is shown. The most evident difference is demonstrated for the Double Cantilever Beam tests and End Notch Flexure tests. The interlaminar fracture toughness for both mode I and mode II is more than 1.5 times higher for carbon fabric/PEEK laminates as compared to carbon fabric/PPS ones. The higher fracture toughness of carbon fabric/PEEK results in its higher residual compressive strength after impact (~25%). Thus, the study concludes that the performance of textile composites is highly sensitive to the performance of the matrix. Matrices that have higher strength, ductility and fracture toughness lead to structural composites with lower crack densities, better performance in the bias direction, higher resistance to delaminations and higher residual strength after impact.
The generalized method of cells (GMC) is demonstrated to be a viable micromechanics tool for predicting the deformation and failure response of laminated composites with and without notches subjected to tensile and compressive static loading. Given the axial [0], transverse [90], and shear [+45/–45] response of a carbon/epoxy (IM7/977-3) system, the unnotched and notched behavior of three multidirectional layups (1) Layup 1: [0,45,90,–45]2S, (2) layup 2: [60,0,-60]3S, (3) layup 3: [30,60,90,–30,–60]2S) are predicted under both tensile and compressive static loading. Matrix nonlinearity is modeled in two ways. The first assumes all nonlinearity is due to anisotropic progressive damage of the matrix only, which is modeled, using the multiaxial mixed mode continuum damage model (MMCDM) within GMC. The second utilizes matrix plasticity coupled with brittle final failure based on the maximum principle strain criteria to account for matrix nonlinearity and failure within NASA's multiscale framework (FEAMAC). Both MMCDM and plasticity models incorporate brittle strain and stress based failure criteria for the fiber. Upon satisfaction of this criterion, the fiber properties are immediately reduced to a nominal value. The constitutive response for each constituent (fiber/matrix) is characterized using a combination of vendor data and the axial, transverse and shear response of unnotched laminates. Then, the capability of the multiscale methodology is assessed, by performing blind predictions of the mentioned notched and unnotched composite laminates response under tensile and compressive loading. Tabulated data along with the detailed results (i.e. stress–strain curves as well as damage evolution states at various ratios of strain to failure) for all laminates are presented.
A multiscale design software (Multiscale Designer), developed by Altair, has been studied for its suitability in analysis of advanced composite aircraft structures made of polymer matrix composites. The extensive experimental data provided by the Air Force Research Laboratory (AFRL) have been utilized to characterize the linear, non-linear, monotonic, and cyclic loading properties of micro-constituents comprising the PMC system. The characterized properties have been then utilized to predict progressive damage, residual strength, and damage mechanisms in notched and unnotched plates subjected to tensile and compressive loading. The validation program consisted of two phases: (i) the blind prediction phase and (ii) the recalibration phase. While simulation results in the blind prediction phase were reasonably good, considerable improvement has been observed in the recalibration phase.
Finite element simulations of three laminates in open-hole and unnotched configurations subjected to tension and compression quasi-static loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. The coupons are made from unidirectional IM7/977-3 plies, which are a composite material composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Blind simulations of coupon stiffness, nominal coupon stress at failure and damage evolution are benchmarked against experimental measurements and X-rays. The blind simulations are followed by a second round of simulations where the modeling strategy is modified to improve agreement between the simulations and experiments. In the present article, the commercial software Autodesk Helius PFA is used to model the non-linear response of the composite material. Within Helius PFA, failure is evaluated at the constituent level by extracting the fiber and matrix volume average stress state from the homogenized composite stress state. The relationships between the composite and constituents are developed using multicontinuum theory and a high-fidelity micromechanics model.
The onset and growth of damage in fiber/matrix composites under transverse loads were modelled using cohesive elements and representative volume elements of randomly arranged fibers. Switching between iterative schemes, using an appropriate tolerance and load increment size, and using an extrapolated solution as an initial guess for load increments led to over an order of magnitude reduction in the solution time. The effect of several model parameters on the failure properties for the next larger scale was studied. The crack path did exhibit a dependence on the mesh, but the RVE strength and amount of dissipated energy in the representative volume element did not vary more than 4% for any of the mesh refinements considered. Periodic boundary conditions minimally interfered with the localization of damage when the localized band of damage did not extend across the entire RVE or when the damage naturally localized parallel to a boundary or diagonal of the representative volume element. A local method for quantifying the energy dissipated within the representative volume element was proposed, which provides an improved accuracy and flexibility. An approach to precisely define the dominant crack was given, which allowed the energy dissipate diffusely and along the dominant crack to be separated. It was shown that the predicted critical strain energy release rate for the representative volume element was sensitive to the representative volume element unless the diffusely dissipated energy was accounted for separately. The proposed technique for calculating failure properties within a multiscale framework has the potential to be applied to other damage models.
A multiaxial quasi-brittle failure criterion for notched orthotropic composites is developed with an emphasis on establishment of an analytical formula to predict their anisotropic notched strength for any notch size under any multiaxial proportional loading. It is formulated by replacing the principal unnotched strengths with the principal notched strengths in the framework of the Tsai–Hill static failure criterion for orthotropic composites. The effects of notch size and specimen width on the principal notched strengths are described by means of the Suo-Ho-Gong model that can consider notch ductile-to-brittle transition. From the proposed multiaxial quasi-brittle failure criterion, an analytical formula is derived to predict the notched strength of finite orthotropic composite plates under multiaxial proportional loading at any angle with the principal directions of material anisotropy. The notched strength prediction formula involves a generalized notch sensitivity parameter that can be defined for any multiaxial state of stress. The multiaxial notch sensitivity parameter allows uniquely defining an intrinsic equivalent mode-I fracture toughness that is independent of notch size as well as of specimen width for any multiaxial proportional loading. Furthermore, an anisotropic size effect law for apparent equivalent mode-I fracture toughness that considers not only the effect of notch size but also the effect of specimen width is derived from the failure criterion. Finally, a quasi-brittle failure criterion for notched interface is briefly discussed as a particular case of the proposed quasi-brittle failure criterion for notched orthotropic composites.
This paper presents the results from the authors’ participation in the Air Force Research Laboratory’s Damage Tolerance Design Principles Program. The Eigendeformation-based reduced order homogenization method was employed to predict the mechanical response of a suite of open hole and unnotched IM7/977-3 composite laminates under static tension and compression. Damage accumulation, effective stiffness, and ultimate strength blind predictions are included in addition to the results of the recalibration study. In blind predictions, the proposed multiscale model produced predictions with an average error of 13.1% compared to the experiments for static ultimate strength and 13.6% for stiffness. After recalibration, the average prediction error was improved to 8.7% for static ultimate strength and 4.4% for stiffness. Details of the blind predictions and the recalibration are discussed.
This study introduces a unit cell-based finite element micromechanical model that accounts for correct post cure fabric geometry, in situ material properties and void content within the composite to accurately predict the effective elastic orthotropic properties of 8-harness satin weave glass fiber-reinforced phenolic composites. The micromechanical model utilizes a correct post cure internal architecture of weave, which was obtained through X-ray microtomography tests. Moreover, it utilizes an analytical expression to update the input material properties to account for in situ effects of resin distribution within yarn (the yarn volume fraction) and void content on yarn and matrix properties. This is generally not considered in modeling approaches available in literature and in particular, it has not been demonstrated before for finite element micromechanics models of 8-harness satin weave composites. The unit cell method is used to obtain the effective responses by applying periodic boundary conditions. The outcome of the analysis based on the proposed model is validated through experiments. After validation, the micromechanical model was further utilized to predict the unknown effective properties of the same composite.
In the present article, the variational energy principle is used to derive the expression for energy release rate in buckled composite laminate containing through-the-width delamination, subjected to in-plane strains. Boundary conditions are clamped at both edges. Buckling and post-buckling solutions are obtained and expressions for critical buckling load and post-buckling deflection have been developed. A through-the-width delamination model has been considered and formulations are based on higher order shear deformation theory. The effects of considering the higher order shear deformation theory on equivalent bending rigidity, buckling load, and energy release rate have been investigated. Finally, the results of current study have been compared with the results of finite element method analysis by Abaqus/CAE and those available in the literature.
The microstructure and mechanical properties of AZ91–SiC nanocomposites processed by cyclic closed-die forging were investigated. The results showed that much finer grain size and more homogeneous distribution of Mg17Al12 phase and SiC nanoparticles were obtained along with significant improvement in strength and elongation after five passes. During cyclic closed-die forging processing, the agglomeration of nanoparticles disintegrated through kneading effect induced by intense matrix flow, and the nano-sized SiC particles were dispersed uniformly into the matrix. However, a few SiC clusters still existed due to the high surface energy of nanoparticles. Properties enhancement of the composites was mainly attributed to Hall–Petch effect and Orowan strengthening.
Beyond a certain loading threshold, glass-fiber-reinforced polymer composites with vinylester matrix can show considerably nonlinear deformation behavior due to damage of the matrix material. To model such nonlinear deformation properties, we follow the approach of a fully anisotropic damage model suggested by Govindjee et al. (Govindjee S, Kay GJ and Simo JC. Anisotropic modelling and numerical simulation of brittle damage in concrete. International Journal for Numerical Methods in Engineering 1995; 38(21): 3611–3633).
In addition, the inter-fiber fracture criterion introduced by Puck (Puck A. Festigkeitsanalyse von Faser-Matrix-Laminaten: Modelle für die Praxis. München, Germany: Hanser Fachbuchverlag, 1996) is used for the damage function that defines the damage initiation and the material strength. A particular procedure is chosen to identify the material properties to validate the model: three glass-fiber-reinforced polymer laminates with different layups are tested in tension under different loading orientations, assuming laminate theory to be reasonably well fulfilled. Finally, further experiments are compared with corresponding simulation results to demonstrate the performance of the model.
Mechanical properties of glass fibre reinforced polymers are dependent on the manufacturing curing cycles. During the laminate manufacturing process, each thickness position experiences a different local curing cycle. Therefore, it can be expected that mechanical properties vary through the thickness, particularly for thick laminates. To study the through-thickness variation of static and fatigue mechanical properties, thick laminates were divided into sub-laminates and these sub-laminates were separately tested. The present work reports temperature profiles through the thickness recorded during the manufacturing of thick laminates, as well as experimental data from static and fatigue tests (S–N curves) of sub-laminates obtained at different thickness positions. The variation of the mechanical properties through the thickness is discussed and related to the local curing temperatures experienced by each sub-laminate.
Carbon nanotubes have been proposed as an ideal reinforcement for the fabrication of nanocomposites. However, because of their chemical inertness, carbon nanotubes have to be functionalized in order to acquire superior properties. In the present paper, we examine the effect of functionalization of single-, double-, and triple-walled carbon nanotubes with ethylene-di-amine, analyzing their elastic properties. Condensed-phase optimized molecular potentials for atomistic simulations studies force field is used to model the interatomic interactions for armchair (5,5), (9,0), and (10,10) configuration carbon nanotubes. Molecular dynamics simulations for carbon nanotubes with various densities of the attached ethylene-di-amine molecules have been performed. This study quantitatively investigates the effect of amine functionalization (up to 12 numbers of ethylene-di-amine groups) on the Young's, bulk, and shear moduli and tensile strengths of different carbon nanotube structures.
Vacuum infusion experiments with and without distribution medium were carried out in woven E-glass fabric preforms. The experimental set-up was carefully designed to obtain a unidimensional in-plane flow between the inlet and outlet gate, which was combined with through-the-thickness infiltration in the tests carried out with a distribution medium. The variation of the fabric thickness during the infusion experiments was measured by means of digital image correlation, whereas the fluid pressure along the strip was obtained by means of pressure gages. Finally, the in-plane fabric permeability as a function of the fiber volume fraction and the fabric compactability were measured by means of independent tests. The experimental results provided a very detailed picture of the interaction between the fluid infiltration mechanisms and the preform deformation during in-plane and through-the-thickness flow, which is very useful to understand the complex interaction between permeability and compaction during infiltration.
The multiaxial quasi-brittle failure criterion for notched orthotropic composites that was formulated in the first part of this study is evaluated on the basis of the off-axis notched strength of a unidirectional carbon/epoxy composite. Static tension tests are first carried out on center circular hole specimens of different hole diameters and fiber orientations to examine the notch size effect as well as the fiber orientation effect on the off-axis notched strength of the unidirectional composite. Experimental results show that the tensile strength in the fiber direction is more notch-sensitive than that in the transverse direction. The notch sensitivity in off-axis strength increases as the off-axis angle increases from the 0° fiber direction to about 10° off-axis direction, but it turns to decrease monotonically as the off-axis angle increases further in the range up to 90°. Then, the analytical formulas to predict the off-axis notched strength, off-axis notch sensitivity, off-axis intrinsic equivalent mode-I fracture toughness, and off-axis apparent equivalent mode-I fracture toughness of unidirectional composites are established on the basis of the multiaxial quasi-brittle failure criterion. These analytical formulas succeed in efficiently predicting both the effects of notch size and fiber orientation on the off-axis notched strength of the unidirectional composite. It is also shown that the intrinsic and apparent off-axis equivalent mode-I fracture toughness values predicted for brittle failure and quasi-brittle failure, respectively, correlate well with the experimental results.
A SD-effect-incorporated nonlinear model for investigating the tension-compression asymmetry in stress–strain curves of fibrous composites was developed in Part I as a new application of the one-parameter plasticity model. In Part II of this investigation, the model is further developed to three-dimensional finite element models and is implemented in the finite element code ABAQUS; in addition, the model is validated against experimental results of stress–strain curves for unidirectional and angle-ply laminates under off-axis tension and compression. Last, parametric studies for a composite cantilever beam with and without SD effect are discussed to illustrate the features of the implemented model that is proposed herein.
To predict the dynamic response of shock absorbers based on magnetorheological elastomers and investigate the contributions of various possible energy dissipation mechanisms, a modified four-parameter model of magnetorheological elastomers was proposed, which includes the viscoelastic characteristics of rubber matrix, the variable stiffness and damping property, and the interfacial bond conditions of magnetorheological elastomers under the applied magnetic field. The constitutive equations of magnetorheological elastomers were derived and all parameters were identified based on a published literature. It is theoretically demonstrated that the maximum response force under an impulse input could be attenuated approximately 30% when the magnetic field with 0.57 T is applied. Using the proposed theoretical model, it is shown that the energy dissipation mechanisms mainly come from the interfacial friction between particles and matrix, and the increment on stiffness and dynamic viscosity of the rubber matrix provides reverse contributions to the shock mitigation, while the interfacial bond stiffness has little influence on the response force amplitude. Hence, when magnetorheological elastomers are utilized in shock absorbers, it is suggested to take advantage of the interfacial friction energy.
Production of high-performance nanocomposite materials obtained from unsaturated polyester resin, based on products of the waste poly(ethylene terephthalate) recycling, and modified multi-walled carbon nanotubes is presented. Di-hydroxy functional glycolysates, synthesized by catalytic depolymerization of poly(ethylene terephthalate) with propylene glycol, were used for the unsaturated polyester resin synthesis. The structure of the obtained glycolysis product and unsaturated polyester resin were characterized by using FTIR and NMR spectroscopy, and by acid, iodine, and hydroxyl value. Nanofillers were prepared by direct and two-step amidation of oxidized multi-walled carbon nanotubes. Direct amidation with diallylamine produced multi-walled carbon nanotube-diallylamine reactive nanofiller. Two-step modification with diamines: hexamethylenediamine and p-phenylenediamine gave multi-walled carbon nanotube-hexamethylenediamine and multi-walled carbon nanotube-p-phenylenediamine nanofiller, respectively, whose amidation with methyl ester of linseed oil fatty acids gave multi-walled carbon nanotube-hexamethylenediamine/methyl ester of linseed oil fatty acid and multi-walled carbon nanotube-p-phenylenediamine/methyl ester of linseed oil fatty acid nanofiller, respectively. Influences of vinyl functionalities on mechanical properties of nanocomposite were analyzed from tensile strength (b), elongation (b) and Young’s modulus (E) determination. An increase of 97.4, 119 and 139% of b was obtained for nanocomposites with addition of 0.25 wt.% of diallylamine, p-phenylenediamine/methyl ester of linseed oil fatty acid and hexamethylenediamine/methyl ester of linseed oil fatty acid multi-walled carbon nanotubes, respectively. Short techno-economic analysis, performed on the basis of fixed and variable unsaturated polyester resin production costs, showed satisfactory potential profit, which could be realized by the implementation of the presented technology.
The effective electrical conductivity of multi-walled carbon nanotube/polydimethylsiloxane composites with chain-structured ferromagnetic particles has been investigated by experiments and micromechanics-based modeling. A multi-scale modeling approach is used to consider different size of fillers of multi-walled carbon nanotubes and particles as well as their distribution in the matrix. At nanoscale, for multi-walled carbon nanotube/polydimethylsiloxane composite, eight-chain model and influence of waviness of multi-walled carbon nanotube are considered to render an effective electrical conductivity. At microscale, ferromagnetic particles are aligned in the matrix made of the multi-walled carbon nanotube/polydimethylsiloxane composite, and an analytical model is established based on representative volume element. The influence of inter-particle distance is evaluated. The proposed analytic results agree well with the experimental results. The present model can be a useful tool for design and analysis of these composites for sensing applications considering their percolation threshold and overall electrical conductivity.
The determination of the mechanical properties of thermoset resin and their evolution during transformation still represents a scientific issue in the composite materials community. A homemade apparatus named PvTα has recently been adapted to the measurement of neat resin bulk modulus evolution during cure and has been presented in a previous study. Several assumptions were used to directly obtain this value but they cannot be checked in situ. A multi-physic modelling of the system is proposed for this purpose in this paper. It accounts for the thermal, chemical and mechanical behaviours of the different components of the apparatus as well as their interactions during an experiment. This model is thoroughly validated thanks to several comparisons with experimental results. This study shows that the early assumptions are not verified during the whole cure in the case of RTM6 resin. It leads to 40% error in the bulk modulus estimation, thus making impossible a direct measurement of the resin bulk modulus using the simple protocol proposed by Nawab et al. A new procedure, which is based on the developed model, is proposed to improve the analysis accuracy.
The main goal of this investigation is to characterize the damage in laminated composites under low-velocity impact tests using a new cost-effective approach. To this aim, a quasi-static test was first carried out to obtain initial information about impact tests. Low-velocity impact tests were then applied in unidirectional glass/epoxy composite specimens, and acoustic emission signals were captured during impact events. Next, acoustic emission signals were analyzed using wavelet approach to distinguish released energy related to each distinct damage mechanism. Besides, an approach was provided to estimate threshold impact energy from the quasi-static test, beyond which damage significantly extends. As a final point, the acoustic emission-based procedure using wavelet transform method was proposed to predict the total damage area. Finally, it was found that this acoustic emission methodology can be a capable approach in damage characterization under impact loads in composite structures.
In this paper, a progressive intra- and inter-laminar damage model is proposed to study the effect of out-of-plane confinement on the pin-bearing behaviour of laminated composite. The constitutive law for fibre direction is formulated at the ply scale and is related to classical models dealing with matrix and inter-laminar damage. The influence of shear and transverse stresses on fibre damage is represented through a modified damage evolution law. Specific issues resulting from the use of a softening law in a structural analysis are discussed from a physical point of view in order to select a relevant modelling strategy whose limitations are well understood. The experimental procedure is based on an extensive set of tests in order to identify separately most of the material constants. Pin-bearing tests on configurations with and without confinement are used to evaluate the model performances. Results show the model’s ability to predict the damage scenario and the loss of stiffness implied by first damage stages.
The classical continuum models used for the woven fabrics do not fully describe the whole set of phenomena that occur during the testing of those materials. This incompleteness is partially due to the absence of energy terms related to some microstructural properties of the fabric and, in particular, to the bending stiffness of the yarns. To account for the most fundamental microstructure-related deformation mechanisms occurring in unbalanced interlocks, a second-gradient, hyperelastic, initially orthotropic continuum model is proposed. A constitutive expression for the strain energy density is introduced to account for (a) in-plane shear deformations, (b) highly different bending stiffnesses in the warp and weft directions, and (c) fictive elongations in the warp and weft directions which eventually describe the relative sliding of the yarns. Numerical simulations which are able to reproduce the experimental behavior of unbalanced carbon interlocks subjected to a bias extension test are presented. In particular, the proposed model captures the macroscopic asymmetric S-shaped deformation of the specimen, as well as the main features of the associated deformation patterns of the yarns at the mesoscopic scale.
This paper focuses on the study of the effect of fiber content and alkali treatment on the thermal properties of wheat straw epoxy composite. Four levels of fiber loading (10, 20, 30, and 40 wt%) of wheat straw and three levels of alkali treatment (1, 3, and 5%) were considered and merged into epoxy composites. The composites were prepared by hand layup technique. The thermal stability of the components was studied by thermogravimetric analysis and differential scanning calorimetry, as well as by the differential thermogravimetric. The experimental results show that the thermal stability of the composites prepared from 3% alkali-treated fibers is superior as compared to the untreated and another level of alkali-treated fiber composite. This is mainly due to the efficient fiber–matrix adhesion in the alkali-treated wheat straw epoxy composites. Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR) studies were carried out to evaluate the microstructure and composition of wheat straw fiber/epoxy composites, respectively.
Characterization of the damage induced by machining of fibre-reinforced composites is usually performed by measuring surface roughness. Contact-based surface profilometers are the most used equipment in industry; however, it has been found that there are performance limitations which may result when used to measure machined heterogeneous composite surfaces. In this research, surface roughness is characterised using a commercial non-contact optical method, and compared with a conventional stylus profilometer. Unidirectional and multidirectional carbon fibre laminates were edge trimmed and slot milled. The variation in surface roughness was compared using different tool types, fibre orientations and cutting parameters. Surface damage and cutting mechanisms were assessed by using scanning electron microscope images, and the suitability of roughness parameters were also analysed including: Sa, Skewness and Kurtosis. Using the optical system allowed accurate roughness calculation of individual plies on a multidirectional laminate with different fibre orientations. The research has also shown that the optical system, including the use of areal roughness parameters, can increase the accuracy of roughness measurement for machined fibrous composite surfaces and is less sensitive to measurement position than the stylus.
Carbon fiber-reinforced plastic composites have been intensively used for many applications due to their attractive properties. The increasing demand of carbon fiber-reinforced plastic composites is driving novel manufacturing processes to be in short manufacturing cycle time and low production cost, which is difficult to realize during carbon fiber-reinforced plastic composites fabrication in common molding processes. Fused deposition modeling, as one of the additive manufacturing techniques, has been reported for fabricating carbon fiber-reinforced plastic composites. The process parameters used in fused deposition modeling of carbon fiber-reinforced plastic composites follow those in fused deposition modeling of pure plastic materials. After adding fiber reinforcements, it is crucial to investigate proper fused deposition modeling process parameters to ensure the quality of the carbon fiber-reinforced plastic parts fabricated by fused deposition modeling. However, there are no reported investigations on the effects of fused deposition modeling process parameters on the mechanical properties of carbon fiber-reinforced plastic composites. In the experimental investigations of this paper, carbon fiber-reinforced plastic composite parts are fabricated using a fused deposition modeling machine. Tensile tests are conducted to obtain the tensile properties. The effects of fused deposition modeling process parameters on the tensile properties of fused deposition modeling-fabricated carbon fiber-reinforced plastic composite parts are investigated. The fracture interfaces of the parts after tensile testing are observed by a scanning electron microscope to explain material failure modes and reasons.
An energy-based continuum damage mechanics model was employed to investigate the effect of drilling-induced delamination on tensile strength of carbon fiber-reinforced polymer laminates and the residual strength was predicted. The decrease in tensile strength caused by delamination was investigated in detail by a finite element method (FEM) model. Experiments were also conducted to validate the numerical results. The drilling-induced delamination was measured with ultrasonic C-scan technique and modeled as deletions of corresponding cohesive elements in the FEM model. Different sizes of delamination zones were investigated and the predicted strengths were compared with experimental results. The FEM model showed that the decrease in strength is mainly caused by early failures of outer plies. Good correlation between numerical simulations and experimental results was also obtained.
Herein, a one-parameter plasticity model proposed by Sun and Chen [Sun CT and Chen JL. A simple flow rule for characterizing nonlinear behavior of fiber composites. J Compos Mater 1989; 23: 1009–1020] demonstrates features that make it highly attractive for characterizing non-linear responses of fibrous composites. However, a detailed exploitation of the model’s potential has been halted by unresolved problems that include tension–compression asymmetry in stress–strain curves, FEM implementation as well as optimal parameters determination, which are addressed in this investigation as well as proposed solutions are presented. The major focus in Part I of this three-part study was devoted to developing a simple model for predicting the tension–compression asymmetry in stress–strain curves for fibrous composites, which was based on Sun and Chen’s one-parameter plasticity model. A generalized Hill yield criterion was proposed from combinations of the Drucker–Prager yield criterion that considers the effect of hydrostatic pressure for isotropic materials and the Hill yield criterion suitable for anisotropic materials. By incorporating the yield strength-differential effect on the plastic flow rule in composite laminates, the one-parameter plasticity model was extended to a strength-differential effect-incorporated model. The improved model has been calibrated and validated by off-axis tension and compression tests on unidirectional carbon/epoxy (IM600/Q133) composite laminates. Results verified that the proposed model captured the complex tension–compression asymmetry in observed non-linear responses of stress–strain curves.
The paper investigates new composites fully based on wastes of polyethylene terephthalate, rubber, high-density polyethylene, and wood, aiming at multifunctional, environmental-friendly materials, for indoor and outdoor applications. The rubber: polyethylene terephthalate: high-density polyethylene: wood ratio and compression molding temperatures are optimized considering the output mechanical properties, focusing on increasing the waste polyethylene terephthalate content. To investigate the durability in the working conditions, the water-stable composites, with good tensile and compression strengths were exposed to surfactant systems, saline aerosols, and ultraviolet radiations. The results prove that surfactant immersion improves the interfaces and the mechanical properties and a pre-conditioning step involving the dodecyltrimethylammonium bromide surfactant is recommended, prior application. The interfaces and the bulk composites were investigated by X-ray diffraction, Fourier-transform infrared, differential scanning calorimetry, contact angle measurements, scanning electron microscopy, atomic force microscopy, to identify the properties that influence the mechanical behavior and durability. The composites containing 30% of polyethylene terephthalate, obtained at 160℃ and 190℃ have a good combination of mechanical properties and durability that is enhanced by the plasticizing effect of water and surfactants. The compressive strength of the composite processed at 190℃ was 51.2 MPa and the value increased to 58.4 MPa after water immersion. The ultraviolet and saline exposure slightly diminished this effect; however, long time testing (120 h) ended up with values higher than those corresponding to the pristine composite: 55.3 MPa after ultraviolet and 57.1 MPa after saline exposure.
Technique to fabricate magnetoelectric piezoelectric/magnetostrictive ferrite composites with 1-3, 3-1, 1-1 connectivities through binding uniformed by size and package ceramic elements was developed. Advantage of this technique is the use of piezoceramic which was previously poled under optimal conditions; this is important, because piezoelectric phase embedded in magnetoelectric composite is difficult to pole due to the high electrical conductivity of adjacent ferrite phase. The effect of piezoelectric material type, volume ratio of phases, and linear size of repeating fragment on the dielectric, piezoelectric, and magnetoelectric properties of PZT + (1–) NiCo0.02Cu0.02Mn0.1Fe1.8O4– (v – volume fraction of piezoelectric, PZT – commercial grades of lead–zirconate–titanate such as PZT-36, PZTNB-1, PZTST-2, PZTTBS-2, PZT-19) composites is studied. It is shown that, with an equal volume ratio of phases, composites based on piezoceramics with high piezoelectric voltage coefficient gij (PZT-36, PZTNB-1) exhibit the most prominent magnetoelectric coupling efficiency. Decrease in the linear size of repeating fragment l also contributes to an increase in E/H coefficient. Given other conditions being equal, 1-1 type composites commonly exhibit the highest values of magnetoelectric coefficient. Maximal values of magnetoelectric coefficient E/H for 0.5 PZT-36 + 0.5 NiCo0.02Cu0.02Mn0.1Fe1.8O4– magnetoelectric heterostructures reach ~500 mV/(cm·Oe) at a frequency of 1 kHz.
In this investigation, a mathematical model based on the dispersed phase model and multiphase flow principle was introduced, and applied to simulate the mold filling and particulate flow of particulate reinforced metal matrix composites casting. Experiments of spiral-square shaped indirect squeeze castings of A356/50 µm SiCp were conducted to validate the established model. The SiCp fractions in the casting different locations were quantitatively measured with micro digital image analysis system and were compared with the simulation results. It was found that particulate fractions and distribution patterns at different locations were quite different along the filling distance. The experimental and simulated results about the SiCp distribution in the castings show acceptable agreement. Further, the effect of fluid flow and particulate trajectories on the particulate distribution was analyzed and discussed. The research has shown the validity of application of the model for practical particulate reinforced Al-based composites casting.
This research investigates the effect of elevated temperature on behavior of reinforced concrete (RC) circular columns strengthened with different fiber reinforced polymer (FRP) systems. For this purpose, 32 column specimens were prepared. The test matrix comprised: 14 unstrengthened columns, 14 columns strengthened with a single layer of CFRP sheet, and 4 specimens strengthened with a single layer of GFRP sheet. Out of the 14 CFRP-wrapped specimens, 4 columns were thermally insulated with commercially available fire-protection mortar. In addition to control specimens at room temperature, some other columns were subjected to high temperature regimes of 100℃, 200℃, 300℃, 400℃, 500℃, and 800℃ for a period of 3 h. After cooling down, the columns were tested under axial compression until failure. It was indicated that exposure to elevated temperature adversely affected the residual strength, stiffness, and axial/lateral stress–strain response of unstrengthened columns. FRP composites were found effective in enhancing the axial load capacity of exposed columns provided that the temperature at the FRP level does not exceed the decomposition limit of the epoxy resin. The degradation in strength and stiffness was higher in CFRP-strengthened columns compared with GFRP-strengthened columns when exposed to the same temperature level. The used insulation material was found efficient in preventing heat induced damage to CFRP-strengthened columns up to temperature of 800℃ for 3 h duration. Besides this study, the experimental data of 48 uninsulated FRP-strengthened circular concrete specimens subjected to different heating regimes were collected from the literature. The dataset of 55 uninsulated FRP-strengthened specimens was then employed to evaluate the ACI 440.2R-08 model used for assessing compressive strength of FRP-confined concrete. This model was found non-conservative for 48.6% of the data and thus it was revised by the inclusion of an FRP strength reduction factor due to heating, which can be utilized in the design of FRP-strengthened RC columns exposed to elevated temperature.
The mechanical properties of polypropylene-bentonite nanocomposites were studied in this work. In this study, stearic acid was used as both a new surface modifier of the nano bentonite and a new interface modifier during the compounding of the nanocomposites with a twin-screw extruder. Three different weight concentrations (1.5 wt.%, 2.5 wt.%, and 5.0 wt.%.) were chosen for each type of nanobentonite compounds. Fourier transform infrared spectroscopy allowed us the possibility to discard any chemical interaction between the nanobentonite and stearic acid. Nevertheless, the physical interactions between both components favored the mechanical properties, resulting in around 150% improvement in the elongation of the nanocomposites containing stearic acid as surface and interface modifier. This can be due to the good intercalation of the nanobentonite platelets as found by wide angle X-ray diffraction and this was further confirmed by scanning electron microscopy, where the fracture surface analyses of these nanocomposites showed the best dispersion and wetting of the nanoplatelets by the polymer matrix. Crystallization behavior was also modified by stearic acid incorporation and the nanocomposites with better dispersion exhibited crystallization temperatures similar to pure polypropylene.
The aims of this study are to design, fabricate and investigate the mechanical properties of new hybrid composite laminates made from polypropylene-glass unidirectional fibers and epoxy matrix. Specimens were fabricated following the hand lay-up technique in intraply and inter-intraply configurations. Results are presented regarding the tensile, flexural, in-plane shear and interlaminar shear behaviors of fabricated composites with particular consideration of the effects of the plies stacking sequence and hybrid configuration. The experimental results reveal that the mechanical properties of polypropylene/epoxy composite can be effectively improved by the incorporation of glass fiber through the formation of either intraply or inter-intraply hybrid composites. With a proper choice of the hybrid configuration and the plies stacking sequence, the fabricated hybrid composites achieved property profiles close to those of homogeneous glass reinforced laminate in terms of specific properties. Resistance of the intraply hybrid composite to tensile and flexural loadings is higher than inter-intraply hybrid composites. On the other hand, the highest in-plane and interlaminar shear strengths are associated with the inter-intraply hybrid composite with glass fiber core. Additionally, an analytical analysis was also introduced to provide a good correlation with the experimental data, which give an insight on the ideal plies stacking sequence to achieve the required properties.
In this work, the evaluation of a post-industrial residue of coffee industry (coffee husk) as organic filler in functionalised linear low-density polyethylene matrix was investigated. The properties of these composites were compared with more widely used inorganic fillers, such as calcium carbonate and SiO2. All the composites were prepared by grafting of linear low-density polyethylene maleic anhydride and then by melt mixing of fillers. The rheological behaviour of the compounds during processing was established to evaluate the effects of a nanosized (SiO2), micron sized calcium carbonate and coffee husk fillers. Additionally, thermal, morphological and mechanical properties of the polymer composites were evaluated. The results indicated an increase in tensile modulus and tensile strength resulting from incorporation of fillers in the polymer matrix. Some relevant modifications of melting temperature and crystallinity degree for coffee husk composites were observed, but not observed for inorganic fillers composites. The properties of the composites prepared with coffee husk were comparable to those obtained with inorganic fillers, demonstrating that this coffee residue can be used as filler for obtaining composites for many possible applications.
Polyphenylene sulfide-based nanocomposites filled with unmodified multiwalled carbon nanotubes from 0.5 wt% to 8.0 wt% have been prepared by melt mixing technique with a single-screw extruder and hot press. Transmission electronic microscopy and scanning electron microscopy analysis were carried out in order to assess the multiwalled carbon nanotubes dispersion throughout the polyphenylene sulfide matrix. Electrical conductivity of the polymer was dramatically enhanced by about 11 decades between 2.0 wt% and 3.0 wt% of nanotubes, suggesting the formation of three-dimensional conductive network within the polymeric matrix. The storage modulus (G') of neat polyphenylene sulfide presented an increase by two orders of magnitude when 2.0 wt% of pristine multiwalled carbon nanotubes was considered, with the formation of an interconnected nanotube structure, indicative of "pseudo-solid-like" behavior. In addition, percolation networks were formed when the loading levels achieve up to 1.5 wt% for multiwalled carbon nanotubes/polyphenylene sulfide composites.
The aim of this work is to evaluate the effect of sonication and clay content on the crosslinking and curing characteristics and the final properties of unsaturated polyester/montmorillonite nanocomposites. The Cloisite 30B clay (1, 3 or 5 wt.%) was dispersed in the resin by mechanical stirring and sonication at different ultrasonic amplitudes (20% and 30%). Amplitude is one of the most important parameter when reproducing sonication results. An increase in viscosity with clay incorporation, at 20% amplitude, was related to the better dispersion, corroborating differential scanning calorimetry results. The sequential diffractograms confirmed the influence of the clay on resin crosslinking and the curing process. An intercalated structure was observed for samples up to 3 wt.% of clay and an amplitude of 20% during sonication, corroborating the increase in flexural strength and lower values of coefficient of thermal expansion. Modulus increased with the incorporation of clay, whereas impact strength and linear burning rate declined.
Co3O4 is a promising candidate as an anode material for the next generation lithium ion batteries because of its high theoretical storage capacity and energy density. However, the disadvantages of poor capacity retention caused by large volume changes during cycling and low rate capability due to its poor electronic conductivity frustrate its practical applications. We have developed a binary nanocomposite based on Co3O4 and porous carbon nanofibers synthesized via an electrospinning method followed by thermal treatment. As an anode for lithium ion batteries, the Co3O4/ porous carbon nanofibers composite exhibits a remarkably improved electrochemical performance in terms of lithium storage capacity (869.5 mAh g–1 at 0.1 C), high-initial Coulombic efficiency (73.8%), cycling stability (94.9% capacity retention at 50 cycles), and rate capability (403.6 mAh g–1 at 2 C at 25 cycles) compared to pure Co3O4. This improvement is attributed to the introduction of porous carbon nanofibers which could improve electrical conductivity of material and accommodate the volume expansion/contraction of Co3O4 nanoparticles during cycling.
In this work, A357/0.5 wt.% SiC nanocomposites were fabricated with a combination of ultrasonic processing and a nanoparticle feeding mechanism that involves the introduction of a closed end aluminium tube filled with the ball-milled SiC nanoparticles (20–30 nm) and aluminium powders (<75 µm) into the melt for complete deagglomeration and uniform dispersion of nanoparticles through the matrix. The microstructural and mechanical properties of the fabricated nanocomposites were investigated. The microstructural studies conducted with optical and advanced electron microscopes indicate that relatively effective deagglomeration and uniform dispersion of SiC nanoparticles into the molten alloy were achieved. The hardness and tensile properties of the nanocomposites were notably improved compared to those of the ultrasonically processed A357 alloy without reinforcement, showing the strengthening potency of nanoparticles and the good bonding obtained at the particle-reinforcement interface.
A novel method of characterizing moisture effect on mechanical performance of epoxy resin is presented in this paper. A 50-µm-thick layer of cured epoxy resin was fabricated and cut into strips of 4 mm wide and 30 mm long as specimens to be tested on a dynamic mechanical analyzer equipped with thin-film tension clamp. Static tension and force-controlled tension–tension fatigue tests were first carried out using thin-film specimens made from Momentive 135/137 and BASF 5400/5440 epoxy resin systems without applying moisture, and results were compared with those obtained using conventional dog-bone specimens to validate the proposed testing method. Another batch of thin-film specimens were then immersed into deionized water, and the weight gain was recorded regularly until full saturation to obtain the absorption curve. Static and fatigue tests were performed using thin-film specimens made from BASF 5400/5440 with 55% and 100% saturation of moisture respectively, to evaluate moisture-induced material degradation. The aging effect on BASF 5400/5440 caused by cyclic water immersion and drying process was also assessed by performing static and fatigue tests using fully dried thin-film specimens after aging. It was concluded that the combination of thin-film specimen and dynamic mechanical analyzer would yield as good measurements of tensile strength and fatigue life as conventional dog-bone specimen does, and the small thickness of thin-film specimen would greatly reduce the time to reach a certain level of moisture content, facilitating further studies on effect of moisture ingression on polymeric matrix composites using multi-scale approaches.
Structural response under combined bending (M) and torsion (T) of pultruded Glass Fiber Reinforced Polymer composite hollow circular and square sections has been investigated, using unique experimental facility for combined loads. Prior to determining combined response, bending and torsional strengths at failure were found separately for all test sections. To evaluate structural response, strain gages were attached at 6 to 12 critical locations of each specimen. For each cross-section, three samples were tested under combined load combinations, i.e. (0.25 Mmax, TFailure), (0.50 Mmax, TFailure) and (0.75 Mmax, TFailure). The results have been presented as M vs , T vs , and
A simple model was developed to characterize the constrained polymer chains at the interphase of amorphous/semicrystalline polymer nanocomposites based on ethylene vinyl acetate copolymer and some nanosheets such as expanded graphite, graphene and organo-modified montmorillonite. It was found that this method is a useful tool to describe the reinforcement efficiency of nanoparticles. Models were developed using dynamical mechanical thermal analysis data to identify the interphase properties. The volume fraction of constrained polymer chains shows power law relationship with filler content. Since the total volume fraction consists of the confined polymer chains at the surface of nanoparticles and in crystal lamellas, the contribution of nanosheet interphase was evaluated separately. Moreover, the thickness of constructed interphase between polymer chains and nanosheets were predicted using the filler characteristics in the polymer nanocomposites. This implies that the dispersion state of nanofiller in polymeric nanocomposites can be obtained by using this simple model.
Matrix as well as interlayer regions of laminated polymer composites have been reinforced with carbon nanotubes, additionally to shape memory alloy wires, in order to further enhance the overall material toughness and introduce the improved impact resistance mechanisms through micro- and nano-engineering. In this work, we examine carbon fiber reinforced polymer composites with constant carbon fiber volume fraction, further reinforced with carbon nanotube and shape memory alloy wires, under controlled impact. Single-type as well as multiple-type impact tests have been carried out, demonstrating that the energy absorption and damage development are similar in both impact tests for the same material. When the carbon nanotube and shape memory alloy wires reinforcements are compared separately, shape memory alloy-reinforced carbon fiber reinforced polymers present higher energy absorption than the carbon nanotube-reinforced carbon fiber reinforced polymers. When they are combined, although the carbon nanotube + shape memory alloy-reinforced carbon fiber reinforced polymers present similar energy absorption improvement to shape memory alloy-only carbon fiber reinforced polymers, the carbon nanotube addition increases toughness, resulting in damage initiation at higher depths of impact penetration.
The coefficients of friction and wear rates of thermoset polyester matrix composites with plain woven polyester fabric reinforcement were studied during reciprocating sliding under adhesive line contact at 50 to 200 N normal loads and 0.3 to 1.2 m/s sliding velocities. The samples were prepared along different orientations of the fabric relatively to the sliding surface and sliding directions, including 0°, 30°, 45°, 60° and 90° in-plane directions and thickness z direction. The coefficients of friction and wear rates were maximum along 90° and minimum along 45° for pure and polytetrafluoroethylene-filled composites. For the latter, overload conditions were minimized, and friction and wear became low for perpendicular fabric orientations. Although sliding is controlled by solid lubrication of polytetrafluoroethylene, the fiber orientations remain dominating the friction under mild sliding conditions. The differences between friction properties along weft and warp orientations could be correlated with mechanical properties along those directions. However, a systematic study in parallel with interpretation of the thermal heating could provide better insights in dry sliding properties. By considering the effects of thermal heating, thermo-mechanical sliding conditions in the interface controlled self-lubricating properties of polytetrafluoroethylene. The wear mechanisms along 0° and 90° directions were mainly determined by the fabric reinforcement, and the sliding along off-axis 45° and thickness directions was mainly controlled by the matrix and polytetrafluoroethylene deposits.
Short-carbon-fibre-reinforced polyether ether ketones are materials of great interest for the aeronautical industry. In this study, a design of experiment was carried out to understand the effect of process parameters on micro- and macro-scale properties of injection-moulded short-carbon-fibre-reinforced polyether ether ketone (90HMF40). Mould temperature was found to be the most significant parameter; it had a positive effect, essentially on failure stress and strain. Once the damage and plasticity scenarios were understood, a micromechanical model based on Mori–Tanaka homogenization theory was developed, featuring micro-damage and coupling with macro-plasticity. This model gave good predictions for quasi-static tensile tests.
Glass fiber reinforced plastic structures are mostly used in mid-sized marine vessels due to high strength and stiffness to weight ratio, corrosion resistance, and total life cost reductions. Mechanical joints using metallic bolts, screws, and pins are commonly used for joining thick glass fiber reinforced plastic laminates. Interference-fit pin connections provide beneficial effects such as fatigue enhancement and/or prevention of moisture intrusion to the fiber reinforced composites. This numerical and experimental study aims to investigate the effect of interference-fit on the bearing stiffness and strength of pin joined glass fiber reinforced plastic. The stress and strain distributions have been investigated for bearing loading through experiments as well as a nonlinear three dimensional finite element analysis. The quasi-static properties of the pin-loaded composites with interference-fit (0.6% and 1%) are compared with the samples with transition-fit (0% of interference-fit). The radial and the tangential strains on the vicinity of the hole obtained from the FE simulation were verified with the experimental results. The radial strains on the interference-fit pin joined glass fiber reinforced plastic coupons are lower than those on the transition-fit pin joined glass fiber reinforced plastic coupons at the consistent pin displacement, resulting in enhancement of the joint stiffness per unit bearing area by interference-fit.
The successful formation of composite parts without defects remains a challenging issue due to the complexity of the forming process. A better understanding of the factors that cause these flaws is necessary to optimize the operation. The present work investigates the in-plane shear behavior of out-of-autoclave carbon epoxy thermoset prepregs OOA and its effect on wrinkling using the picture frame test. The deformability of OOA at the real processing conditions helps to understand the applicability of such material for forming processes such as the double-diaphragm forming technique with aims to minimize overall manufacturing time and cost. Tests were performed at varying temperatures and displacement rates in order to determine their contribution to the fabric deformability. Digital image correlation was used to take sequential images at various stages of deformation and capture the onset of wrinkling. It was found that the processing temperature (resin viscosity), displacement rate, and layer counts (layer interactions) are the three most important parameters that influence the wrinkling. Presence of resin between the layers makes them interact with each other and therefore has an impact on the each layer shear angle. These parameters were then analyzed using the Taguchi and analysis of variance techniques to determine which factor has the most significant influence on the wrinkling.
The influence of multi-layered nanostructured graphene as reinforcement on thermal and mechanical properties of epoxy-based nanocomposites has been studied. The maximum improvement in mechanical properties was observed at 0.1 wt%. The Young’s and flexural moduli increased from 610 MPa to 766 MPa (26% increase) and 598.3 MPa to 732.8 MPa (23% increase), respectively. The tensile and flexural strengths increased from 46 MPa to 65 MPa (43% increase) and 74 MPa to 111 MPa (49% increase), respectively. The mode-1 fracture toughness (K1C) and critical strain energy release rate (G1C) increased from 0.85 MPa.m1/2 to 1.2 MPa.m1/2 (41% increase) and from 631 J/m2 to 685 J/m2 (9% increase), respectively. The increase in fracture toughness is attributed to the obstruction of cracks by graphene layers. The reinforcing effect of nanostructured graphene was also manifested in dynamic mechanical properties. The storage modulus and alpha-relaxation temperature values significantly increased indicating the fine integration of NSG in epoxy chains. The thermal properties of nanocomposites were simulated which showed that graphene is very efficient in significantly increasing the scattering and dissipation of thermal flux.
This paper aims at investigating the resistance-to-vitrification of glass fiber-reinforced plastics when milling. A three-dimensional thermal model using volumetric heat source with Gaussian distributed cylindrical flux was developed as DFLUX subroutine and implemented into Abaqus/Standard code. The wheel feed was simulated by the source motion at 250 mm min–1 with spindle speeds of 11,460, 15,280, and 19,100 rpm. Milling tests using abrasive wheel with 10 mm in diameter were conducted on the composite specimens of dimensions 100 x 25 x 4 mm3 with fibers oriented both parallel and perpendicular to the milling direction. Four equidistant thermocouples were embedded within the medium plane of the specimen in order to measure the temperature histories. Each series of tests was repeated four times under identical conditions. Predictions confronted to measurements demonstrated the validity of the proposed model. Cutting perpendicular to fibers was found favoring in-depth heat dissipation. However, the fibers acted as thermal barriers so as to limit the heat propagation within the composite plate with fibers oriented parallel to the milling direction. The pure thermal analysis was found sufficient to predict the heat-affected zone in the glass fiber-reinforced plastics specimens, which, in fact, is a function of both wheel spindle speed and fiber orientation.
The results of experimental and numerical studies on temperature dependence of carbon fiber/bismaleimides composites subjected to transverse tensile load at –120℃, 25℃, 150℃, 170℃, 200℃ are summarized. The scanning electron microscopic fractographs showed that fibers were coated by a small amount of resin along with split resin at –120℃, melted resin attached to fibers is found in the view at 200℃ and naked fibers were observed at room temperature. It is concluded that the interfacial strength reduced with the increase of temperature. Experimental stress versus strain curves showed that modulus decreased with the increase of the temperature, and the obviously nonlinear tendency was observed at 200℃. Employed Mohr–Coulomb criterion to characterize plastic behavior of bismaleimides matrix, representative volume element based on random sequential expansion algorithm was modeled to simulate the entire damage progress with thermomechanical load. The analytical results showed that high stress concentration occurred in the matrix band between closely arranged fibers and became more severe with temperature rising. The percentage of interfacial debonding was larger at room temperature than those at higher and lower temperature. The experimental and analytical results showed that transverse failure modes at different temperature are related to thermal residual stress and the Young’s module of matrix.
In this study, the preforms of 3D woven composite materials were made by a flexible oriented 3D composite woven process. The vacuum-assisted resin infusion (VARI) process was used to impregnate the preforms. The short-beam shear test, the compression test, and SEM were used to investigate the interlaminar shear performance and the compression behavior of the 3D woven composite with guide sleeves, and the effect of the guide sleeves on the above properties. It is indicated that the interlaminar shear behavior of 3D woven composites with guide sleeves showed the typical fracture characteristics of a pseudoplastic material. And the fracture modes of interlaminar shear mainly include interlaminar shear fracture and tensile fracture of fibers at the bottom. The interlaminar shear strength of materials increased with the diameter and interval of guide sleeves decreasing. Furthermore, the loss of in-plane compression properties of the materials brought by guide sleeves could be effectively avoided by reasonable control of the diameter and the volume fracture of guide sleeves.
The mechanical properties of Kevlar® 29 single filaments and yarns with different gage lengths were investigated by utilizing an MTI miniature tester and an MTS load frame. Single yarns of 25 mm were also tested over four different strain rates using a drop-weight impact system. The experimental results showed that the mechanical properties of Kevlar® 29 are sensitive to gage length, structural size scale, and strain rate. The tensile strength decreased with increasing gage length and the structural scale from fiber to yarn, and increased with increasing strain rate. Weibull analysis was conducted to quantify the degree of variability in tensile strength. The obtained Weibull parameters were then used in an analytical model to simulate the stress–strain response of single yarn. Finally, Weibull parameters of single filaments with other gage lengths and strain rates were also obtained by fitting the stress–strain curves of single yarns with corresponding testing conditions.
The use of carbon fiber/epoxy matrix composite is widely developed to store hydrogen at high pressure because of its low weight and its good specific mechanical properties. In order to secure this type of storage, it is necessary to tackle the thermal degradation and the influence of a fire or a heating source on the residual mechanical behavior of such materials. In the present study, carbon/epoxy composite samples with different fiber orientations are considered. The thermal aggression (representative of a fire) is performed by using a cone calorimeter apparatus (ISO 5660). The fire exposure is stopped at different time in order to study the influence of the thermal energy (different heat fluxes and exposure durations) on the residual mechanical tensile properties. The results obtained show that the residue thickness (char) of the samples is proportional to the incident energy. Strength and stiffness reduction can be observed even without ignition (i.e., without combustion flame) when the mechanical properties are controlled only by the resin (fiber perpendicular to the loading axis). When the fibers are mechanically loaded (quasi-isotropic samples or ± 45° samples), a very little strength decrease is observed before ignition and accelerated after ignition. A proportional relationship between the ultimate stress of the exposed sample and the non-charred thickness is also observed.
In this work, a surfactant-aided TiO2–MgO nanocomposite (Ti-M-S) was successfully synthesized by a sol-gel process with the aid of sodium dodecyl sulfate as a structure-directing anionic surfactant. The results demonstrated the effectiveness of this approach for controlling both the amount and the distribution of MgO nanoparticles within the TiO2 framework after calcination. The photocatalytic activity of the synthesized nanocomposite for degradation of methyl orange (MO) and methylene blue (MB) dyes used as a model wastewater contaminant was investigated under visible light irradiation. The synthesized nanocomposite was systematically characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis. The decolorization results revealed that the Ti-M-S1 (with an anionic surfactant: sodium dodecyl sulfate) and Ti-M-S2 (with a non-ionic surfactant: Triton X-100) nanocomposites showed much more photocatalytic activity than the pure TiO2 did under visible light. MB and MO dye removal efficiencies of 82.4% and 77.8 %, respectively, were achieved by Ti-M-S1 (1%) within about 120 min and no further changes in the uptake were observed up to 24 h. This confirmed the suitability of the synthesized nanocomposite for use as a photocatalyst under visible light with the added advantage of increasing the versatility of potential applications for TiO2 photocatalysts.
Polybutylene/starch/nanoclay composite blends were prepared by melt extrusion technique. Maleic anhydride grafted polybutylene was used as a compatibilizer. The nanocomposites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, rheological, and mechanical analysis. Addition of compatibilizer to the polybutylene/starch/nanoclay showed dispersion and nucleation related to the nanoclay in the polybutylene matrix. An increase in the mechanical properties like modulus and tensile strength at break and a decrease in the elongation at break were observed on the addition of compatibilizer to the matrix compared to that of uncompatibilized matrix. The biodegradability of the nanocomposites was studied using the landfill burial test. The blends subjected to the burial test were evaluated for their tensile properties. The results revealed that the tensile strength and elongation at break of the compatibilized nanocomposites decreased after 80 days of land burial test compared to the initial period.
In the present work, the thermal stability, changes in chemical structure during thermal degradation, and the kinetics of thermal degradation of a phenolic foam were studied. An 8.5 wt% of Pinus radiata wood flour reinforcement was added to the phenolic foam. A commercial phenolic resol was used as the matrix for the foam. The wood flour-reinforced foam showed a structure similar to the phenolic foam according to the Fourier transform infrared spectroscopy results. The wood flour increased the thermal stability of the phenolic foam in the first stage of thermal degradation (T 5% ), decreased it in the second step (T 25% ), and negligibly influenced the final stage. The activation energies of the degradation processes of the studied materials were obtained by the Kissinger-Akahira-Sunose and Flynn-Wall-Ozawa model-free kinetic methods and a 2-Gaussian distributed activation energy model. The values of the activation energies obtained by the model-free kinetic methods for the first degradation stage of the phenolic foams were in a range between 110 and 170 kJ mol–1, whereas for the wood flour it was 162 kJ mol–1 for almost all of the conversion range of its main degradation stage. The applied models showed good fits for all the materials, and the activation energies calculated were in agreement with the values found in the literature.
This paper focuses on the development and the validation of flexural modulus and flexural strength predictive models of long glass fibre reinforced polyamide 6.6 (PA66). Based on previous injection moulding optimization of 40 wt% long glass fibre PA66, a microstructure analysis was investigated on glass fibre reinforced PA66 by varying the parameters of the material (fibre length, fibre content, fibre diameter). In a first phase, analytical models established within the framework of the processing condition limits previously determined have been elaborated. These models lead to a good experimental/calculation correlation but remain limited to a mould and part design. In a second phase the flexural modulus and maximal flexural stress have been then estimated from structural models based on a five layer morphological description of the composites (local residual fibre length, local fibre content and fibre orientations). The long glass fibre PA66 composites were characterized in terms of fibre content distribution model and fibre orientation model through the part thickness. The experimental/model correlation was achieved whatever the process variability is (mould, material and processing conditions) both for the flexural modulus or flexural strength. The models have been then validated with an industrial part. Finally, a correlation between the two studied properties has been revealed depending on the nature of the composite matrix (PA66, PA6 or PP).
This article reviews the literature studies based on improving the mechanical properties of fibre-reinforced composites using fibre-prestressing method. The idea is characterized by pretensioning the fibres either elastically or viscoelastically prior matrix curing. The beginnings of the studies in this field were focused on reducing fibre waviness and breaking the weaker fibres by pretensioning the fibres to a relatively high stress level prior moulding process. In the last three decades, the concept of fibre prestressing had been developed to include its ability to reduce the effect of undesired residual stresses existence accompanying manufacturing process of fibre-reinforced composites. The main advantage of fibre prestressing method is to generate a desired and controlled residual stress state within the matrix in order to obstruct the initiation and propagation of cracks. Various techniques of fibre prestressing have been reviewed to show their scope of applications, developments and limitations. Therefore, the findings drawn from this review can be used for further studies in the field of fibre prestressed composites in order to select the most suitable methodology and develop it to fit the manufacturing process requirements towards a production of high-performance composites without a considerable additional cost.
In this study, aluminum samples with various microsurface roughness values were produced by sandblasting to investigate the effect of the Ra (Surface roughness) value on the samples’ mechanical properties. Toward this end, a carbon fiber reinforced plastic/Al5052 hybrid sample was produced, and its mechanical properties were investigated through a tensile test, three-point bending test, and shear lap test. The theoretical and experimental tensile strength values of the hybrid composite were compared. During the bending test, CFRP and AI5052 separated in untreated specimens. A side-view examination revealed that the adhesion was best when the surface roughness was greatest (Ra = 1.2 µm). Furthermore, shear load increased with the surface roughness. Therefore, the surface treatment was a crucial factor in making the specimen surface even and in increasing the roughness and therefore improving adhesion.
In this study, crystalline cellulose was prepared through hydrolysis of jute fiber and was used as reinforcement of gelatin-based biocomposite film. The effects of crystalline celluloses loading on the morphology, mechanical properties and water sensitivity of the biocomposite were investigated by means of Scanning electron microscopy, tensile strength testing and water absorption testing. The developed biocomposite film showed homogeneous dispersion of crystalline celluloses within the gelatin matrix and strong interfacial adherence between matrix and reinforcement. A significant increase in tensile strength and E Modulus was also found (tensile strength was 25.4 MPa for pure gelatin and 48.2 MPa for 2% crystalline celluloses/gelatin film at 45% relative humidity), which was further induced by gamma radiation. The resulting biocomposite film also showed a higher water resistance and excellent biocompatibility. Therefore, crystalline celluloses played an important role in improving the mechanical properties as well as water resistance of the biocomposite film.
Composite structure design experience has demonstrated that use of the finite element method during the first stage of the design process is unfounded and that analytical methods to determine the stress–strain state are needed for more accurate calculations. Therefore, an analytical model of the stress–strain state of multilayer composite plates under the influence of temperature, technological, and power loads with different boundary conditions around four boundaries of a rectangular plate was developed. This model enables the solution of more than 240 different boundary value problems with a combination of the following boundary conditions: fixed, moving, hinged, and free edge. In the derivation of this mathematical analytic model, the Kirchhoff hypothesis was applied to the entire body of the anisotropic medium for the interconnected deflection and bending in the plate plane. The resulting equation is an octic linear partial differential equation to express the generalized function of movements.
A freeze casting technique was processed to fabricate porous mullite/alumina-layered composite with a gradient in porosity and pore size. In this work, a camphene/coal fly ash slurry system with an appropriate addition of Al2O3 was used. The pore channels with circular-shaped cross-sections were aligned along the solidification direction of molten camphene and connected with each other. The pore morphology was influenced by the starting solids loading and sintering temperature. The compressive strength of monolithic specimen greatly decreased when the porosity increased. However, the layered composites with a graded porosity exhibited improved compressive strength with minor decrease in porosity, probably due to the formation of residual compressive stress and relatively dense glass phase in the outer layer.
In this study, silver nitrate was added to polyacrylonitrile filament structure and chemical reduction was applied to composite filaments in order to develop multifunctional polyacrylonitrile filaments with electrostatic dissipative and antibacterial properties. Composite filaments of polyacrylonitrile and silver nitrate were characterized and evaluated in terms of morphology, chemical structure, tensile properties, crystallinity, conductivity, thermal properties, silver ion release behaviour and antibacterial activity. Additionally, ultraviolet-visible spectroscopy was used to confirm the formation of nanoparticles and the variation in the concentration of the nanoparticles with the application of the chemical reduction process. Scanning electron microscope images and ultraviolet-visible spectroscopy results confirmed the formation of nanoparticles in the filament structure. Breaking strength and breaking elongation increased at silver nitrate content of 1%. Composite filaments displayed improved thermal stability and their conductivities were in the semiconductive range. Atomic absorption spectroscopy confirmed that necessary amounts of silver release for antibacterial activity occurred, while the antibacterial activity analysis showed that the composite filaments have excellent antibacterial activity. The results obtained were promising and showed that the composite filaments could be used in electrostatic dissipative and antibacterial applications.
The effect of carbon nanotubes introduced by electrophoretic deposition on the tensile behavior of carbon fiber tow was investigated. As-received and heat-treated carbon fiber tows that deposited with carbon nanotubes and their counterparts were tested under monotonous tensile loading. Morphology and presence state of carbon nanotubes on the surface of carbon fiber tows were observed for determining the role of carbon nanotubes. Results showed that tensile properties of carbon fiber tow were not damaged by the electrophoretic deposition process. Due to the uniformly distributed carbon nanotubes network on the surface of carbon fiber tow, the fracture of filaments were obviously constrained by carbon nanotubes instead of instantaneous separation. What’s more, carbon nanotubes exhibited a stronger confining ability for heat-treated carbon fiber tow and improved their tensile properties significantly.
The flexural performance of polyvinyl alcohol-steel hybrid fiber reinforced engineered cementitious composite with characteristics of low drying shrinkage special focus on impacts of steel fiber content and matrix strength has been investigated in both experimental and theoretical aspects in this paper. Four matrix types with water to binder ratio of 0.25, 0.35, 0.45, and 0.55 and three additional steel fiber contents in the composite with polyvinyl alcohol fiber content of 1.7% in volume were used in the test program. The experimental results show that cracking and flexural strength of the composites are increased with the addition of steel fiber. This enhancement becomes more and more pronounced with decreasing of water to binder ratio of the composites. Meanwhile, fracture mechanics-based flexural model is used to simulate the flexure performance of the polyvinyl alcohol -steel hybrid fiber reinforced engineered cementitious composite with characteristics of low drying shrinkage. The model results show that a double peak load is expected of the composites under bending load. The first peak is controlled by the fracture toughness of matrix or cracking strength of matrix, and the second peak is governed by the fiber bridging. The effect of addition of steel fiber in engineered cementitious composite with characteristics of low drying shrinkage on the first peak is unapparent. The impact of steel fiber on the second peak is significant. This enhancement of additional steel fiber gradually decreases with the decrease of water to binder ratio of the matrix, which coincides well with the experimental findings. The test results are compared to the model and reasonable agreement is found.
In this study, a fishing line artificial muscle reinforced syntactic foam composite was investigated. About 3.5 vol.% of polymer artificial muscle was woven into a two-dimensional grid skeleton and embedded into a syntactic foam matrix. The grid-stiffened syntactic foam composite was designed to be able to repeatedly heal cracks on-demand. Short thermoplastic fibers were also dispersed into the foam matrix both as reinforcement and as a healing agent. The composite panel was repeatedly impacted, bending fractured, and healed as per the biomimetic close-then-heal strategy. It is found that the composite panel responds to impact quasi-statically. The impact- and bending-induced macroscopic cracks can be repeatedly healed with high healing efficiency, under both free and clamped boundary conditions. Contrary to most healing systems, the healing efficiency within the damage–healing cycles in this study increases as the damage-healing cycle increases. Because the artificial muscle is made of low-cost and high-strength fishing line, it is envisioned that the composite panel developed in this study may be a viable alternative core for healable lightweight composite sandwich structures at competitive cost.
Orthophosphates are bioactive crystals with similar structure, in terms of elemental composition and crystal nature, to human bone. In this work, biocomposite materials were prepared with poly(lactic acid) (PLA) as matrix, and beta-tricalcium phosphate (β-TCP) as osteoconductive filler by extrusion-compounding followed by conventional injection molding. The β-TCP load content was varied in the 10–40 wt% range and the influence of the β-TCP load on mechanical performance of PLA/β-TCP composites was evaluated. Mechanical properties of composites were obtained by standardized tensile, flexural, impact, and hardness tests. Thermal analysis of composites was carried out by means of differential scanning calorimetry; degradation at high temperatures was studied by thermogravimetric analysis; and the effect of the β-TCP load on dynamical response of composites was studied by mechanical thermal analysis in torsion mode. The best-balanced properties were obtained for PLA composites containing 30 wt% β-TCP with a remarkable increase in the Young’s modulus. These materials offer interesting properties to be used as base materials for medical applications such as interference screws due to high stiffness and mechanical resistance.
In this article, the authors presented a unified solution for the dynamic analysis of laminated composite annular, circular, and sector plate with general boundary conditions. The first-order shear deformation theory is employed to formulate the theoretical model. Regardless of the shapes of the plates and the types of boundary conditions, each displacement and rotation component of the elements is expanded as an improved Fourier series expansion which is composed of a double Fourier cosine series and several auxiliary functions introduced to eliminate all the relevant discontinuities with the displacement and its derivatives at the boundaries and to accelerate the convergence of series representations. Since the displacement fields are constructed adequately smooth throughout the entire solution domain, an exact solution is obtained based on the Rayleigh–Ritz procedure by the energy functions of the plates. The accuracy, reliability, and versatility of the current solution is fully demonstrated and verified through numerical examples involving plates with various shapes and boundary conditions. Some new results of free vibration analysis for composite laminated annular sector plate, circular sector plate, annular plate, and circular plate are presented, which may be served as benchmark solution for future computational methods. The effects of the sector angles, layer numbers, and boundary spring stiffness on vibration characteristics of the plates are reported. In addition, the force vibration analysis of the plates is also studied. The influence of the boundary spring stiffness, layer number, orthotropic stiffness ration, and fiber orientation angle on dynamic characteristics of the plates is investigated.
An approach to predict static and fatigue failure of composite laminates with holes is presented. Static failure is predicted when the stress averaged over a characteristic distance is equal to the un-notched failure strength. This averaging distance is determined analytically without the use of additional testing or need for extra material parameters. During fatigue loading, the size of the damage region next to the hole is calculated and the strains at the hole edge are determined. These are used along with the stresses just outside the damage region to determine whether failure starts at the hole edge or the edge of the damage region extends. A previously developed fatigue model based on the cycle-by-cycle probability of failure is used to calculate the number of cycles needed for the residual strain at hole edge or the residual strength at the edge of the damage region to fall below the corresponding applied values. The procedure is repeated until laminate failure. The method is also used to predict cycles to failure for impacted specimens. The predictions are in very good agreement with test results.
Under environmental conditions, triaxial braided composites exhibit complex behavior and damage mechanisms. This paper investigates the damage mechanisms of these complex composites under varying environmental conditions. Tensile, compressive, and shear specimens of triaxial braided composite material were tested at room, hot (100℃), and hot/wet conditions (60℃/90% relative humidity). The strain field was studied using a digital image correlation system and the effect that the specimens’ edges have on the strain field was quantified. For the tension specimens, the environmental conditions caused reductions in the elastic and failure properties, whereas the compression specimens exhibited degradation exclusively in the failure properties. An increase in temperature rather than humidity was found to be a driving factor for the degradation of the mechanical properties. A non-destructive, flash thermography technique was used to characterize surface/subsurface damage in the specimens. Scanning electron microscopy was conducted to determine the microstructural modes of failure.
In this paper, high strain rate compression properties of aramid and ultrahigh molecular weight polyethylene composites in the out-of-plane direction are tested at room temperature on a Split Hopkinson Pressure Bar apparatus. Tests were conducted on composites reinforced with woven or Uni-Directional (UD) fabrics made from aramid or ultrahigh molecular weight polyethylene as well as on composites reinforced with hybrid reinforcement. The strain rate is varied in the tests by changing the projectile shooting pressure. Four different pressures 2, 4, 6 and 8 bar were selected to change the strain rate. Stress–strain and energy absorption behaviour of eight type of samples were noted. Hybrid samples showed better performance in the energy absorption compared with other samples.
Current state of the art within textile truss structures requires a variety of production, assembly and joining processes to conclude in a fully integrated truss configuration. This approach sees the joining and bonding of separate struts to node parts. The node is the connecting area which accommodates the strut-to-strut intersections. A production process of separate truss components (struts and nodes) inherently has constraints, such as increased labour, bonding issues and longevity of product. In the development of a fully integrated textile truss, the utilisation of conventional weaving technology and production principles allowed the development of the three-dimensional woven nodal truss structure. The three-dimensional woven nodal truss structure’s node and nodal segmentation, defined by boundary lines provided defined areas within the weaving width, length and depth for the assignment of weave architectures. The commonalities within the production of varying strut-to-strut intersections and strut-to-strut variable dimensions within a T-shaped and K-shaped nodal configuration provide the foundations for the development of elementary nodes for other three-dimensional woven nodal truss structures. The development of the generic procedure and application of the three-dimensional-to-two-dimensional-to-three-dimensional nodal structure production process and elementary nodes will be presented within this article.
The aim of this study is to investigate the effect of surface geometry for low-velocity impact applications. To achieve this purpose, aramid fiber-reinforced laminated polyester composite with various geometries such as cylindrical, elliptical, and spherical were prepared, and low-velocity impact properties were investigated numerically and experimentally. All properties such as orientation, fiber volume fraction, matrix material, and average thickness are the same in all samples. Experimental low-velocity impact behaviors of structure were determined by drop weight tester at low velocity 2.012 m/s. Simulations were carried out by LS-Prepost 4.2 and LS-Dyna v971 software. By this way, results of impact tests were verified and modeled with finite element method. Results of the impact tests showed that the elliptical samples have the highest energy absorption capability due to effective stress transfer capacity. According to experimental results, maximum energy absorption rate difference is 17% between elliptical 10 mm and cylindrical 5 mm geometries.
Cement mortars with different contents of nano silica (NS) were fabricated and tested. Their compressive and flexural strengths showed significant increases. Theoretical calculation and thermogravimetry (TG) analysis, scanning electron microscope (SEM) and X-Ray powder diffraction (XRD), and electrical resistivity test were used to analyze the reinforcing mechanisms of NS. Theoretically, consumed calcium hydroxide (CH) increases with NS content, which indicates that NS has huge potential to react with CH. According to the results of TG, the amount of consumed CH increases and agrees with theoretical calculation when the content of NS is less than 1.5%. However, a plateau is achieved for the mass of consumed CH in results of TG when the content of NS exceeds 1.5%. SEM shows that NS can make matrix dense and also reduce the size of CH in matrix beside interfacial transition zone (ITZ). The results of XRD prove that NS can change the tendency of crystal of CH in cement matrix. However, the change degree of tendency of crystal of CH in cement matrix is lower than that in ITZ. The change trends of electrical resistivity with increasing NS content and curing age are similar with those of flexural and compressive strengths. This indicates that electrical resistivity can reflect strength and structural compactness of cement matrix.
Two experimental set-ups used to characterise the in-plane and through-thickness permeabilities of reinforcing textiles have been developed and are presented. Both the experimental testing and data processing techniques used have been selected to ensure that the characterisation is completed in an efficient and robust method, increasing the repeatability of tests while minimising user induced errors as well as the time and resources needed. A number of key results and outputs obtained are presented from tests carried out on a plain woven reinforcing textile with a range of number of layers and at different fibre volume fractions.
A two-step finite element framework is presented that examines the effect of microscale thermal residual stress on the nanoindentation properties of fibre-reinforced composites. Firstly, micromechanical modelling is used to determine the residual stress state following thermal cooldown of a carbon-fibre composite material from cure temperature. A three-dimensional finite element nanoindentation model is then used to characterise the effects of residual stress on material properties determined by nanoindentation theory. The results show that the hardness of the matrix pockets decreases following thermal cooldown due to the existence of equibiaxial tensile residual stresses. The hardness property is also found to decrease for the majority of interfacial region stress states, while the microstructural areas where the effects of the residual stress are nullified are determined. The indentation modulus property is relatively insensitive to the microstructural residual stress, and thus is the recommended indentation property to be determined when carrying out a comparative parametric analysis between microstructural regions. The property changes are shown to be insensitive to any errors associated with contact area estimation using the Oliver and Pharr method.
Expressions for transverse matrix strain magnification and fibre strain reduction are derived for square and hexagonal fibre array reinforced composites. Respective transverse matrix and fibre strain magnification and reduction, for the square arrays are shown to be higher for all reinforcing fibre volume fractions than those for the hexagonal arrays. The respective magnification and reduction of the transverse matrix and fibre strains are shown to decrease with increasing values of the ratio of elastic modulus (Em/Ef) for both reinforcing fibre arrays. The magnified transverse matrix strains in axially loaded longitudinally aligned continuous fibre-reinforced composites are shown to be higher than the applied longitudinal strains for all square array reinforcing fibre volume fractions and for all hexagonal array reinforcing fibre volumes fractions above 31%. This raises possibilities of longitudinal matrix splitting before interfacial bond failure and transverse matrix failure, in a strain based rather than stress-based failure mode.
To establish the correlation between grain size, dislocations, dispersed particles (size and vol.%) along with their solid solution strengthening effects in alloy and combined effect of all on the strengthening of advanced composite materials (Al3Zrmp + ZrB2np)/AA5052 hybrid composites have been selected for the investigation. (Al3Zrmp + ZrB2np)/AA5052 hybrid composites have been synthesized by the direct melt reaction of AA5052 alloy and inorganic salts (K2ZrF6 and KBF4). These composites have been characterised by X-ray diffractometer, optical microscopy, scanning-electron microscopy with energy-dispersive spectroscopy, transmission-electron microscopy, tensile and hardness test. Results indicate the successful formation of second phase reinforcement particles namely Al3Zr and ZrB2 in the AA5052 alloy matrix. Al3Zr particles exhibit rectangular and polyhedron morphology within an average of micron size while ZrB2 show hexagonal and rectangular within an average of nano size. Grain refinement of Al-rich phase observed in the composites, increases with increasing vol.% of reinforcement particles. TEM observation shows the presence of dislocations in the composite matrix which help to improve the strength parameters. Tensile results show the improvement in strength parameters which improve with the increasing amount of particles whereas percentage elongation also improves up to certain vol.% of particles and beyond that decrease. However, bulk hardness shows an increasing trend continuously with vol.% of particles. The strengthening mechanisms, namely dislocation, Orowan, grain-refined and solid solution are quantified for the hybrid (Al3Zrmp + ZrB2np)/AA5052 composites and the total of above are in agreement with experimental results. Solid-solution and Orowan are the predominant strengthening mechanisms in the composites.
A two-step masterbatch mixing technique was studied for preparation of carbon nanotube-filled ethylene–propylene diene elastomer compounds, and compared to conventional one-step mixing process. In the two-step process, a masterbatch compound with carbon nanotube content of 50 parts per hundred was prepared by melt-mixing ethylene–propylene diene elastomer. This material was then compounded with pristine ethylene–propylene diene elastomer and composites with different carbon nanotube concentrations were compared. The aim of this study is to compare the efficiency of two different mixing processes on the dispersion of carbon nanotubes and to facilitate the handling of carbon nanotubes, as the masterbatch can be prepared in a controlled way and used for further dilution without the problems related to carbon nanotube processing. The compound properties were studied with emphasis on mechanical characterization and dynamic mechanical thermal analysis. Masterbatch mixing resulted in the similar mechanical properties of the composites compared to the direct mixing method. At the relatively low loadings of carbon nanotubes, the considerable improvements of the mechanical properties were observed. The aspect ratio of the carbon nanotubes determined by transmission electron microscope was found to be similar to the one calculated from the Guth equation. It showed a considerable reduction in aspect ratio independent of the used mixing method.
The vinylester resins were physically modified by 0.05 and 0.1 wt% nano polyvinyl alcohol fibers with about 80 nm in diameter prepared by electrospinning. Addition of nano polyvinyl alcohol fiber with 0.05–0.1 wt% improved the fracture toughness of vinylester resin slightly by ~14.3%, while the carbon fiber/vinylester resin adhesion was almost unchanged. Then, carbon fiber/vinylester resin composites were fabricated by using modified vinylester resin as matrix and carbon fiber plain-woven fabrics as reinforcement. Low and high-cycle fatigue tests were conducted under the laboratory condition as well as static tensile tests. The tensile strength of carbon fiber/vinylester resin/polyvinyl alcohol composites increased slightly due to increased resistance to damage propagation at transverse bundles and resin-rich region. The fatigue properties of carbon fiber/vinylester resin composites were improved significantly by 3–50 times with the addition of no more than 0.1 wt% nano polyvinyl alcohol fibers. Due to the incorporation of nano filler, damage degree in carbon fiber/vinylester resin/polyvinyl alcohol composite was reduced at the fatigue initial stage based on thermoelastic stress damage analysis. Furthermore, a delay of delamination growth was found as well at the fatigue middle stage according to CT-scan and scanning electron microscopy investigation. The resistance to fatigue damage accumulation due to energy absorption by the addition of nano fiber contributed to fatigue life extension.
A variety of perfluoroacyl fluorides such as perfluoro-3-(1-piperidinyl)propionyl fluoride, perfluoro-2-methyl-3-morpholinopropionyl fluoride, perfluoro-3-di-n-propylaminopropionyl fluoride, and perfluoro-3-di-n-butylaminopropionyl fluoride were applied to the nanocomposite reactions with calcium carbonate nanoparticles under alkaline conditions to afford the corresponding perfluorocarboxamides/calcium carbonate/calcium fluoride nanocomposites. The modified glass surfaces treated with the fluorinated nanocomposites, which were prepared by using perfluoro-2-methyl-3-morpholinopropionyl fluoride and perfluoro-3-(1-piperidinyl)propionyl fluoride, afforded an oleophobic–superhydrophilic characteristic. The fluorinated nanocomposites, of whose each preparative feed ratio of perfluoro-3-di-n-propylaminopropionyl fluoride (or PF-DBAPF) and CaCO3 is 0.50 (g/g) (2.50 (mol/mol) ), can provide a superamphiphobic characteristic on the modified surfaces; however, the lower feed ratios: 0.25–0.17 (g/g) (1.25–0.83 (mol/mol)) of the acyl fluorides based on CaCO3 can afford a superoleophobic–superhydrophilic characteristic on the modified surfaces. The similar wettability was observed on the modified filter paper and polyethylene terephthalate fabric surfaces by using the fluorinated nanocomposites, which were prepared with perfluoro-3-di-n-butylaminopropionyl fluoride. Interestingly, these modified filter paper and polyethylene terephthalate fabric swatch were applied to the separation membranes for not only the mixture of oil and water but also the oil-in-water emulsion. The modified filter paper treated with the fluorinated nanocomposites possessing a superamphiphobic characteristic was also applied to the separation membrane for the mixture of fluorocarbon and hydrocarbon.
Resin infusion (RI) process has been widely used for manufacturing composite parts. The variation of preform thickness brings great difficulty to the three-dimensional simulation of the filling stage. To accurately simulate the preform thickness change and resin flow during resin infusion, precise preform compaction models and dynamically changing geometry models need to be adopted. At present, resin flow is usually considered as two-dimensional and simple compaction models are employed to simplify the simulation, which degrades the prediction accuracy seriously. In this paper, general equations to describe the resin flow in the changing thickness cavity are developed, and the viscoelastic model is adopted which can fully express the dynamic characteristics of the preform compaction. To avoid solving the coupled resin flow/preform deformation equations directly, the volume of fluid method and the dynamic mesh model are employed to implement the tracking of the flow front and updating of cavity geometry model. The resin storage and release induced by porosity variations are adjusted by a master-slave element method to ensure mass conservation. Two simulation examples are carried out to demonstrate the capability of the above approach. The applicability of the approach on arbitrary complex domains and sequential injection strategy is also verified.
Novel high-density aluminum (Al)-tungsten (W) fiber composites in the tubular shape with highly ordered tungsten fibers in axial and hoop directions were processed in the solid state using the combination of cold isostatic pressing and hot isostatic pressing. Half of the specimens were additionally heat treated after hot isostatic pressing to regain the properties of aluminum 6061-T6. The strength of both types of samples was investigated under quasistatic compression. Samples after additional heat treatment had the higher microhardness of matrix and compressive strength. No significant reaction between tungsten fibers and aluminum matrix was detected. The micromechanism of samples failure under compression was revealed by removing the aluminum matrix after tests with acid-etching demonstrating that tungsten fibers oriented in the axial direction were deformed by microbuckling and kinking. The sample bulging due to plastic flow of aluminum matrix resulted in the cooperative fracture of tungsten fibers in the hoop direction.
Static and parametric stability of thin symmetrically laminated composite super-elliptical plates resting on Winkler-type foundation and subjected to uniform in-plane harmonic loads, under clamped, simply supported and free boundary conditions, are investigated based on the classical laminated plate theory. The governing equations are obtained from a variational approach and then the classical Ritz method is used to reduce the problem into a set of coupled Mathieu–Hill equations. Hsu’s technique is utilized to determine the dynamic instability regions of principal and combination resonance frequencies. Extensive numerical data are provided to examine the effects of plate aspect ratio, super-ellipticity power, foundation stiffness parameter, stacking sequence, and fiber orientation on the vibrational, static, and parametric stability characteristics of symmetrically laminated super-elliptical composite plates. Furthermore, three-dimensional buckling mode shapes are illustrated. The accuracy of formulation is checked by performing convergence studies and the validity of results is established by comparison with the existing results in the literature, exact results obtained from the analytical approach, and as well as from FEM results.
Time-dependent behavior and factors affecting preload relaxation in a carbon/epoxy composite bolted joint under resonance were studied. The effect of viscoelasticity of composite material on bolt relaxation was studied quantitatively through modal analysis from the perspective of energy dissipation and stiffness degradation. Damping ratio and resonance frequency were utilized to characterize the effects of preload relaxation on structural dynamic response. The loss of preload was found to decrease with increasing initial preloads over a 10 h vibration fatigue. However, an increase in preload loss occurred as exciting frequency increases. Vibration fatigue damage was found to result in decaying stiffness and amplitude responses of the bolted joints, along with an increase in damping ratio. As a proof-of-concept study, a beam-like specimens with and without bolted joints were comparatively excited to ascertain their respective dynamic responses; results revealed that relaxation in bolted joints could be attributed to the conjunct mechanisms between viscoelastic behavior of polymer matrix composites and interface friction for different contact surfaces, where such relaxation behavior was mainly due to viscoelasticity of the joint materials.
Tourmaline/graphene oxide (TGOx, x = 5, 10, 15, and 20) compound with high thermal conductivity and high far infrared emissivity was prepared by a refluxing method. Transmission electron microscopy results confirmed that graphene oxide with a few layers was fabricated, and tourmaline nanoparticles were supported by graphene oxide layers. Thermal interface materials were prepared by adding tourmaline/graphene oxidex compound into epoxy resin. Far infrared emissivity of TGOx and thermal conductivity of thermal interface materials were increased with the weight ratio of graphene oxide in compound, but the corresponding electrical conductivity was slightly decreased. In particular, the tourmaline/graphene oxide15 (tourmaline/graphene oxide ratio of 85:15) showed an enhancement of 4% in far infrared emission than that of tourmaline. The thermal conductivity of thermal interface materials with 5 wt% tourmaline/graphene oxide15 was improved by 380% compared with that of pure epoxy resin, and the electrical conductivity of tourmaline/graphene oxide/epoxy was decreased slightly compared to that of graphene oxide/epoxy.
In this study, composite nanofibers of polyacrylonitrile (PAN) and polyaniline (PANI) were successfully produced by electrospinning technique and the effects of different dopants such as camphorsulfonic acid (CSA), dodecylbenzene sulfonic acid (DBSA) and dodecylbenzene sulfonic acid sodium salt (DBSANa+), and different solvents such as dimethylsulfoxide (DMSO) and N,N'-dimethylformamide (DMF) on the properties of PAN/PANI composite nanofiber webs have been investigated. It has been observed that nanofibers produced from DMSO generally had larger fiber diameters and higher breaking strength than nanofibers produced from DMF. CSA could dope better than DBSA(iso) and DBSANa+. CSA resulted in the highest conductivity when DMSO was used while it resulted in lower conductivity in DMF. The insulator PAN became a semiconductive material with the incorporation of CSA-doped PANI. The highest electrical conductivity obtained was 10–6 S/cm which is in the range suitable for electrostatic discharge applications.
This paper describes a method for characterizing composite materials subjected to mode III delamination fracture using a custom-designed testing device and test equipment which allows loads or displacements to be applied to the test specimen in two directions, one axial and the other torsional. To verify the method’s functionality experimentally, a composite material made up of an epoxy matrix and unidirectional carbon-fiber reinforcement was used in conjunction with an image analysis device for the purpose of determining the displacement field in the crack front of a double cantilever beam test specimen. According to the results, this test method permits almost pure mode III fracture tests to be carried out, as the mode II component is practically negligible. Another feature of the method is the improvement in the quality and ease of inserting the specimen in the device, thus permitting more repetitive results to be obtained with less dispersion.
Rapid re-launch aerospace vehicles require materials with high specific strength to withstand thermal shock associated with repeated re-entry. Glass fiber reinforced polyester (GRP) composites have rapidly become preferred for high value structural components requiring high specific strength. Their ability to sustain high tensile and impact loads has allowed them to be used as light-transmitting panels and fuselages. Due to service conditions, heat flux strongly alters mechanical properties with exposure time. The effect of including a thermal-barrier coating, in the form of a carbon nanopaper, on the monotonic flexural properties of a GRP composite is analyzed. A series of three-point bend experiments was performed on specimen-sized samples of composites subjected to various levels of heat fluxes across numerous exposure times. Analysis of these experiments reveals trends in the deformation mechanisms of these materials near failure. Correlations of flexural modulus and critical load are used to develop associations to strength.
This paper reports transverse impact behaviors of 3D braided composites T-beam at elevated temperatures. A modified split Hopkinson pressure bar apparatus combined with a self-designed heating device was used. The impact load–displacement curves and impact damage morphologies have been obtained. The impact peak load and total energy absorption of the T-beam under impact loading have been calculated from the curves. It was found that both the elevated temperature and transverse impact velocity influence the transverse impact responses of the T-beam. The responses were more sensitive to the transverse impact velocity than to the elevated temperatures. The results also showed that the failure mechanism changes from brittle failure to ductile failure as the temperature increases. The matrix crack and fragmentation are often occurred in front surface of the T-beam. The fiber breakage in the rib position and the flange were the failure modes in the rear surface.
The properties of steel substrates coated with soft polymers were characterized, in order to assess their connection to ballistic properties. An impact-induced viscoelastic phase change of the polymer effects large energy dissipation, while also spreading the force over a wider area, which reduces the impact pressure. Both effects enhance the performance, as directly measured and seen from strain measurements on the substrate taken during ballistic tests. The contribution of the front-surface polymer to impact performance is increased for harder substrates, indicating a coupling of the layers related to impedance mismatching. Since this effect is very local, the phenomenon can be exploited by surface-hardening of the steel.
The temperature and time-dependent behaviors of thin-walled high-strain composites including structural nonlinearity are experimentally and numerically investigated. The high-strain composites using woven fabric fibers with heat-softenable resin of shape memory polymers are fabricated. The numerical model based on the elastic properties of the woven fabric fiber considering its actual fabric geometry and the thermoviscoelastic properties of the shape memory polymer resin is developed. Experimental tests of the high-strain composite specimen are analyzed to acquire the parameters needed for its model. A thin-walled high-strain composite structure, which can be flexibly folded into an arbitrary configuration by heating, is fabricated. Furthermore, its folding and unfolding viscoelastic behaviors at elevated temperature are investigated for application in deployable space structures.
Poly(lactic acid)/halloysite nanotube nanocomposites containing epoxidized natural rubber were prepared using melt compounding, followed by compression molding. The mechanical properties of the nanocomposites were determined by tensile, flexural, and Charpy impact test. The addition of 15 wt.% epoxidized natural rubber into poly(lactic acid)/halloysite nanocomposites increased the impact strength to about 340%. However, the tensile modulus, flexural modulus, tensile strength, flexural strength, and elongation at break of poly(lactic acid)/halloysite nanotube were decreased in the presence of epoxidized natural rubber. Water absorption tests were performed at three immersion temperatures (i.e. 30, 40, 50℃). The equilibrium water absorption (Mm), diffusion coefficient (D), and activation energy (Ea) of water diffusion of the poly(lactic acid)/halloysite nanotube/epoxidized natural rubber nanocomposites were determined. The activation energy of poly(lactic acid)/halloysite nanotube was increased from 14.7 to 31.8 kJ/mol by the addition of epoxidized natural rubber. The percentage retention of impact strength of poly(lactic acid)/halloysite nanotube/epoxidized natural rubber nanocomposites after exposure to water absorption is higher than 80% for the one containing 5 and 10 wt.% epoxidized natural rubber loading.
A three-dimensional finite element model of the induction welding of carbon fiber/polyphenylene sulfide thermoplastic composites is developed. The model takes into account a stainless steel mesh heating element located at the interface of the two composite adherends to be welded. This heating element serves to localize the heating where it is needed most, i.e. at the weld interface. The magnetic, electrical, and thermal properties of the carbon fiber/polyphenylene sulfide composite and other materials are identified experimentally or estimated and implemented in the model. The model predicts the temperature–time curves during the heating of the composite and is used to define processing parameters leading to high-quality welded joints. The effect of the heating element size and input current on the thermal behavior is investigated, both experimentally and using the developed model. The welds quality is assessed through microscopic observations of the weld interfaces, mechanical testing, and observations of the fracture surfaces. A comparison with two other welding processes, namely resistance welding and ultrasonic welding is finally conducted.
This paper reports the compressive behaviors of three-dimensional four-directional and three-dimensional five-directional circular braided composite tubes subjected to quasi-static and impact compressions along longitudinal direction. The compression tests of the three-dimensional four-directional and three-dimensional five-directional carbon fiber/epoxy circular braided composite tubes were tested under strain rates ranging from 0.001 to 884 s–1. The compression stress–strain curves were obtained and the damage morphologies were observed to analyze the damage behaviors. A microstructure model of the braided preform and the braided composite tube was established to calculate the compressive deformation and damage mechanisms with finite element method. The stress–strain curves, specific energy absorption, deformations, and damage morphologies were sensitive to the strain rate and the braiding structures. The three-dimensional five-directional braided composite tubes have higher compressive strength and specific energy absorptions than the three-dimensional four-directional braided composite tubes.
A 3D finite element model has been developed for predicting manufacturing distortions of fibre-reinforced thermosetting composite parts. The total curing process is divided into three steps that correspond to the states that resin passes through during curing: viscous, rubbery, and glassy. Tool–part interaction properties were calibrated by modelling the distortion of a single ply part. For comparison, composite parts of various geometries (L-section and U-section), stacking sequences, thicknesses, and bagging conditions were manufactured. The full field thickness profile and full field distortion pattern were obtained using a 3D laser scanner, which reveals higher and lower resin bleeding and corner thickening locations. The effect of stacking sequence is also examined with the full field distortion pattern. It was found that the parts manufactured under the bleeding condition give higher spring-in and warpage values. The spring-in predictions were well matched to measurements of the manufactured parts.
Composite laminates are being increasingly used in a wide variety of industrial applications, but there are difficulties in applying these materials in ways that exploit their full potential, in particular under multi-axial loading. The objective of the present study is to determine by experiments the biaxial failure data for composite laminates produced by Fokker Aerostructures based on the thermoplastic UD carbon reinforced material AS4D/PEKK-FC. A test machine and accompanying cruciform specimens for in-plane biaxial failure tests have been developed. A coupon-level biaxial test program covering various biaxial load combinations in tension-tension, tension-compression and compression–compression has been successfully executed and biaxial failure values for the thermoplastic laminate have been determined. Besides, the experimental biaxial test program, also finite element models and analyses have been used to predict the global outcomes of the biaxial tests and to interpret the test results. Both plain (un-notched) and open-hole (notched) specimens of the thermoplastic laminate have been tested. The biaxial failure data have been collected and further processed in biaxial failure criteria. From the experiments, the failure strains, stresses and loads are determined and a failure envelope is created for both plain and open-hole specimens. Good agreement is found between the theoretically predicted envelopes and the test data. From the findings for biaxial failure criteria from this study, it is expected that structural weight saving can be achieved in the design of multi-axially loaded composite parts as compared to the design with the previous uni-axially based failure criteria.
Microstructure evolution of 15 wt% boron carbide particle reinforced aluminum matrix composites (B4C/Al composites) with titanium addition during liquid-stirring process was dynamically characterized in this paper. B4C particles were rapidly dispersed under the mechanical stirring. Many B4C clusters were formed in the melt before 20 min, but gradually scattered in matrix beyond 20 min, owing to further reactive wetting through interface reaction in addition to stirring. After rapid improvement, distribution uniformity slowly approached to completely uniform distribution during 20–55 min, even better than random distribution at 55 min. Interface reaction produced Al3BC, TiB2, and AlB2 by B4C erosion and Al3Ti decomposition; however, AlB2 only precipitated in matrix after long time stirring. The growth of TiB2 transformed from a fine layer to discretely coarse crystals on the B4C surface. Reaction mechanism and relationship between reactive wetting and particle dispersion were discussed.
In this paper, the effect of addition of nanosilica on mechanical properties of pure epoxy and epoxy/fiberglass composite has been investigated. The epoxy/nanosilica composites and epoxy/fiberglass/nanosilica hybrid composites have been fabricated, and the Young’s modulus, tensile strength, yield stress and elongation at break have been determined by simple extension tests. The results show that by addition of 1 wt% of nanosilica in both types of composites, Young’s modulus, yield stress and tensile strength decrease and elongation at break increases. By increasing the nanosilica content, the Young’s modulus, yield stress and tensile strength increase and elongation at break decreases. Also, imperialist competitive algorithm is employed to model the mechanical properties as fourth degree polynomial functions. The accuracy of polynomial is maximized and coefficients are obtained. The results show 25.66%, 56.87% and 45.84% improvement in Young’s modulus, yield stress and tensile strength of pure epoxy, respectively. Also, 12.9%, 24.83% and 12.85% improvement in Young’s modulus, yield stress and tensile strength of epoxy/fiberglass composites, has been observed, respectively.
The effect of clay-organo modifier on the thermal and mechanical properties and fracture behaviors of pure polycaprolactone (PCL) and 5 wt% PCL/clay nanocomposites were studied. The different materials were prepared by melt intercalation. It was demonstrated by X-ray diffractometry, differential scanning calorimetry, tensile, and fracture tests that the addition of modified nanoclays affected significantly the final properties of the materials. The optimal combination of properties was achieved with the PCL reinforced with 5 wt% of C30B obtaining improvements of 17% in the Young’s modulus and 1500% in the specific essential fracture work.
An experimental and numerical study on low-velocity impact responses on [Ti/0/90] s hybrid titanium composite laminates (HTCLs) is presented. Different energy levels from 10 to 40 J are investigated using a drop-weight instrument and post-impact inspection. An explicit finite element implementation provides a detailed analysis of impact response in composite and titanium layers, respectively. It accounts for interfacial debonding, progressive failure in composite plies and elastic–plastic deformation in titanium. The main failure modes are experimentally and numerically found to be debonding between titanium and composite, matrix cracking and interlaminar delamination. The principal energy-absorbing mechanism is plastic dissipation of the two titanium sheets. The low cost numerical model is able to effectively predict the overall impact response and major failure modes with good accuracy.
The mechanism of mass loss and endothermic properties of silica fiber-reinforced phenolic composites during ablation were investigated in this paper. A theoretical prediction model combining the surface ablation theory and heat transfer theory of heat shield was developed to study the surface ablation behavior. In the formulation of the mathematical model, the effect of the moving boundary on the thermal response was considered, which results from the surface recession of the material in the thickness direction during ablation. The surface ablation recession rate and wall temperature of silica fiber-reinforced phenolic composite specimen were measured using an oxyacetylene torch experimental platform. Then, the efficiency of the model was verified by comparing calculation and experimental results. According to the principles of energy conservation on the ablated surface of the material, the proportion formulas of the heat absorption induced by individual endothermic mechanisms and the total heat absorption were derived. Similarly, the proportions of the mass loss caused by individual mass loss mechanisms were also given. Finally, variations of the ablation properties of the silica fiber-reinforced phenolic composites versus thermal exposure time were calculated and analyzed.
In this study, the effects of improvement methods on the mechanical and thermal properties of textile fiber reinforced (T-FRP) composites were investigated. Five different chemical methods namely, silane treatment, alkaline treatment, alkali–silane treatment, maleic anhydride, and alkali–maleic anhydride coupling agents were applied to evaluate the suitable process parameters (concentration, soaking time, ratio by weight) for the enhanced properties of T-FRP composites. Tensile, three-point flexural and impact tests were performed on both untreated and treated composites for comparison purpose. Treated and untreated T-FRP composites were characterized using scanning electron microscopy, differential scanning calorimetry, and Fourier-transform infrared spectroscopy to evaluate thermomechanical properties of composites. Results show that a significant improvement up to 60–70% can be seen on the mechanical properties of T-FRP composites via improving the interfacial adhesion and compatibility between fiber and matrix.
The research investigated the thermal and mechanical properties of graphene/epoxy nanocomposites. Pristine graphene and functionalized graphene were used as nano-reinforcement in the nanocomposites. The graphene loadings employed in the nanocomposites were 0.1, 0.3, 0.5, and 1.0 wt%. The functional groups grafted on the functionalized graphene were characterized through Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Results indicated that two kinds of functional groups are grafted on the functionalized graphene surfaces: one contains only COOH group and the other contains both COOH and NH2 groups. Moreover, from mechanical and thermal testing, it was found that the nanocomposites with functionalized graphene demonstrate better mechanical and thermal properties than those with pristine graphene. The graphene containing NH2 and COOH functional groups exhibits superior mechanical and thermal properties than the graphene with only COOH functional group. In addition, Young’s modulus and thermal conductivity of the nanocomposites increase as the graphene loading increases. However, the fracture toughness and tensile strength of the nanocomposites attain peak values when the functionalized graphene loading is 0.1 wt%. The effects of the functional groups on the mechanical and thermal properties of nanocomposites were elaborated using molecular dynamics (MD) simulation. It was revealed that the interfacial thermal conductance and normalized interaction energy increase between the functionalized graphene and epoxy matrix, which may be responsible for the enhanced mechanical properties in the functionalized graphene/epoxy nanocomposites.
This paper presents the effect of geometric parameters on the behavior of bolted glass fibre reinforced polymer (GFRP) pultruded plates for civil engineering applications. After a literature review, results of an experimental analysis investigating the behavior of GFRP-to-steel single-lap bolted connections are presented. Then, a finite element analysis validated by experimental data is used to evaluate the effects of the end-distance, side-distance, pitch, and plate properties on the strength and failure mode of the connection. A critical examination of geometric recommendations proposed in design references is presented. Bearing failure caused by contact of the bolt on the GFRP plate is usually defined as the preferred failure mode. With highly orthotropic plate, this type of failure was found to be less likely to occur when loading is applied in the pultruded direction. The investigation showed that the minimum end-distance and pitch-distance recommended by design references usually produce a connection with the maximum capacity. However, it was found that the minimum side-distance recommended by these references does not necessarily lead to the maximum capacity for one bolt and for two bolt in a column connections.
Generally, in thin laminates, residual stresses are studied in two scales; micro mechanics and macro-mechanics. So far, no study has included both the micro- and macro-scale analysis for predicting the thermal residual stresses of composite laminates. In other words, no literature model is proper for both the micro and macro scale at the same time. This paper presents a procedure to integrate micro and macro thermal residual stresses in composite laminates. The combination of micromechanics-based analytical results and hole-drilling calibration factors leads to predict the macro thermal residual stresses of a unidirectional laminate. Classical lamination theory (CLT) is then employed to predict macroscopic thermal residual stress of each layer in laminated composites (with different orientations). Comparison of longitudinal, transverse and in-plane shear stresses between the proposed approach and published experimental results is made for glass/epoxy laminates in cross-ply and quasi-isotropic configurations. Available experimental data from incremental hole-drilling method show that the present combination procedure yields more precise residual stresses predictions in comparison with the state that the CLT is used only. In other words, the presented procedure which adds the micro residual scale results to the CLT, gives rise to a better prediction of residual stress fields. Consequently, ignoring the micro-mechanical effects may result in some uncertainties in thermal residual stress evaluation of thin composite laminates. Furthermore, while the CLT is not able to take into account the in-plane shear residual stresses for the cross-ply composites, the present procedure can provide an estimation of shear residual stress field.
The metallic airplane structure fuselage design is characterized by skin, frames, stiffeners, and attachments. In most airplanes, the attachments between these components are made by rivets. The influence of the attachments in the panel behavior under diagonal tension can be verified in the metallic Wagner beam. For stiffened composite panels, like metallic Wagner beams, there is insufficient data about attachment design. In order to design and build lightweight composite structures, the analyst must consider different ways in which the skin is connected to the stiffeners and frames. Therefore, the objective of this paper is to investigate different conceptions of a real-reinforced composite panel used in the aeronautical industry. Experimental and numerical results for strains showed good agreement. The finite element model and the criteria used in the failure analysis are also presented. Comparisons between different panel configurations are made, and conclusions are drawn about attachment efficiency.
The aim of the work was to achieve assumed gradation of hard coal particles distribution in epoxy matrix and thus gradation of properties. It was done by proper selection of compounds composition and conditions of the gradation formation. The graded composites were produced using centrifugal casting. Two types of epoxy resins as a matrix and two types of hard coal of various granulation as a filler were used. Samples in the form of bushings with radial carbon filler gradation were produced by changing filler type, its volumetric content and parameters of centrifugal casting. The first part of the paper presents results of graded materials structure investigations. The microscopic observations of the structure of composites prepared according to elaborated experimental program show that the composites are characterized by continuous change of the filler particles content in radial direction, so they can be classified as graded materials. Results also show that gradation of particles content and thus gradation of properties may be planned and foreseen when mechanisms of gradation formation are known.
The objective of this study is to examine the mechanical, thermal, and physical properties of industrially produced nano-CaCO3 filled high-density polyethylene nanocomposites. For this purpose, 1.0, 3.0, 5.0, 10.0, and 15.0 wt.% loading of nano-CaCO3 filled high-density polyethylene nanocomposites were prepared by the melt mixing method using a compounder system, which consist of industrial banbury mixer, single screw extruder, and granule cutting. The effect of nano-CaCO3 on mechanical, thermal, and physical properties of nano-CaCO3/HDPE nanocomposites was investigated. As a result of all experiments, the tensile strength of nano-CaCO3 filled high-density polyethylene nanocomposite increased about 5% with addition of 1.0 wt.% nano-CaCO3. But did not increase further as more nano-CaCO3 was added. The flexural strength of nano-CaCO3 filled high-density polyethylene nanocomposite increased about 4.5% with addition of 15.0 wt.% nano-CaCO3.Then increased slightly as the nano-CaCO3 content increased to 15.0 wt.%. The tensile and flexural modulus of high-density polyethylene were significantly improved after (from 1.0 wt.% up to 15.0 wt.%) addition of nano-CaCO3. The tensile elongation at break and shore D hardness was consistently decreased with the addition of nano-CaCO3. The nano-CaCO3 filled high-density polyethylene nanocomposites were determined to have lower impact energy level than neat high-density polyethylene. The occurred fracture areas with the impact were detected by scanning electron microscopy examination. It is understood that fracture surface morphology changes when nano-CaCO3 ratio increases. The fracture surface changes were examined to determine the fracture mechanism of nano-CaCO3 filled high-density polyethylene nanocomposites. Density, melting flow index, differential scanning colorimetry, and vicat softening temperature were used to characterize the physical and thermal properties of the nanocomposites. The X-ray diffraction, the fourier transform infrared spectrophotometry, the transmission electron microscopy, and the scanning electron microscopy were used to analyze the structural characteristics of the nanocomposites. It is concluded that the addition of the nano-CaCO3 in high-density polyethylene has significantly influenced the mechanical, thermal, and physical properties of the nanocomposites.
This paper presents an experimental and numerical investigation on low velocity impact response of S2 glass/epoxy and aramid/epoxy composite plates. Two different impact energy levels, 20 J and 30 J, were considered for impact tests. The commercial software called LS-DYNA was used in order to perform numerical simulations. The experimental and numerical results were found to be in good agreement.
The goal set for this paper was to investigate the relationship between gradation of filler particles content in the epoxy matrix and the gradation of electric properties. In order to characterize the electric properties, the surface resistivity was selected, because it determined the surface ability to electrostatic charging, important in expected applications. In particular, the influence of conditions of centrifugal casting technique and microstructure parameters on electric resistivity was searched. Experimental models were elaborated that juxtapose the dependence of surface resistivity on the volumetric content of the filler, as well as casting rotational velocity. Investigations results confirmed that achieved graded composites were characterized by the reduced ability of electrostatic charging on one side and good insulating properties on the other side. Measured surface resistivities of order 109–1011 allow to classify them as surfaces with reduced susceptibility to static electricity accumulation. These composites may be applied in areas where antistatic properties of the surface, with preservation of insulating properties in the remaining volume of the composite, are beneficial.
In this study, failure behavior of fiber-reinforced composites under four-point bending is investigated. First, the tests are modeled analytically using the classical lamination theory (CLT). The maximum allowable moment resultants of [12]T off-axis laminate as well as balanced and symmetric angle-ply [3/–3]s composite laminates as a function of fiber orientation angle, , are obtained using Tsai-Wu, maximum stress, maximum strain, Hashin, Tsai-Hill, Hoffman, quadric surfaces, modified quadric surfaces, and Norris failure criteria. Second, the same tests are simulated using the finite element method (FEM). Thermal residual stresses are calculated and accounted for in the failure analysis. An analysis is conducted for optimal positioning of the supports so as to ensure that intralaminar failure modes dominate interlaminar (delamination) failure mode. A test setup is then constructed accordingly and experiments are conducted. The correlation of the predicted failure loads and the experimental results is discussed. The quadric surfaces criterion is found to correlate better with the experimental results among the chosen failure criteria for the selected configurations.
This work comprises an experimental investigation of thermal intensification in polymeric nanocomposites. Polyester and epoxy resins were employed as continuous phases and alumina spherical nanoparticles with different diameters (30–40, 27–43, 150, and 200 nm) were used as fillers, in volumetric concentrations up to 10%. The thermal conductivities of the fabricated samples were measured using a guarded heat flow meter and the influence of filler size was investigated. Density measurements were performed for verifying the volume concentrations of particles in batches containing distinct sorts of nanoparticles. In addition, scanning electron micrographs of all samples were obtained. Results show that larger particles are responsible for larger agglomerates and greater thermal conductivity augmentation of the polymeric matrices, indicating that interfacial properties play a significant role in the effective thermal conductivity of nanocomposites.
In this paper, the effects of fiber shape, fiber distribution, fiber size and fiber hybridization on the tensile properties of short sisal fiber-reinforced polypropylene composites were studied by the generalized method of cells and laminate analogy approaches. It is found that the fiber distribution has a large effect on the axial Young’s modulus. The fiber shape and fiber size have stronger influences on the transverse Young’s modulus and axial Poisson’s ratio than the fiber distribution. The fiber type hybridization and fiber shape are shown to be important factors in affecting the axial shear modulus. Hybridization with glass fiber is found to be an efficient method of creating high-performance composites.
In this work, an oxide-based ceramic matrix composite (CMC) consisting of a zirconia toughened alumina (ZTA) matrix and reinforced with mullite whiskers is produced with the purpose of providing more oxidation resistance and cost-effective alternative to covalent discontinuously reinforced ceramics. ZTA has enhanced toughness, strength and creep resistance over single-phase alumina or zirconia. ZTA can further be strengthened by the inclusion of SiC whiskers; however, these whiskers are prone to oxidation at temperatures above 1000℃ leading to loss of properties. In this work, mullite whiskers are used as the reinforcement due to its stability in oxidizing atmospheres are high temperatures. Mullite whiskers are grown through the molten salt method and incorporated into the ZTA matrix using a colloidal processing route. The microstructure and room temperature properties have been reported in an earlier paper. Whisker additions have been shown to improve the flexural strength of ZTA at 1200℃ by 59.31%. There is some concern over the stability of large diameter whiskers at high temperatures, especially in environments with excessive moisture content or residual alkali contamination. Further work will be carried out to address these concerns as well as to develop a statistical analysis of the results presented.
An experimental study on the underwater collapse of composite tubes with polymeric coatings is conducted in an attempt to mitigate the implosion pressure pulse released. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure during collapse. Filament-wound carbon-fiber/epoxy tubes are studied with polyurea coatings of different thicknesses on the interior and exterior of the tube to explore the effects of these configurations on implosion pulse mitigation. 3-D Digital Image Correlation (DIC) technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulses. Local pressure data and DIC results are used to obtain a measure of normalized energy released during implosion. Results show that thick interior coatings significantly reduce the energy released in the pressure pulse by slowing the collapse and softening the initial wall-to-wall contact. In contrast, thick exterior coatings increase this energy by suppressing damage, thereby reducing the energy absorption capacity of the structure.
In the present investigation, a comparative study using X-ray mapping analysis and Field emission gun scanning electron micrographs is performed to understand the distribution and the mechanical properties of aluminum nano metal matrix composites. Parameters considered for comparison have two different forms for adding nanoreinforcement into metal melt. One form is produced by the addition of mechanically alloyed powders with an increasing launching vehicle weight percentage (L-1, L-3 and L-5), and the other form is produced by pellets of mechanically alloyed powders (PL-1). Micrographs reveal uniform distribution of nanoreinforcements, while X-ray mapping observations show Iron (Fe) contamination due to the addition of pellets in some areas unlike the mechanically alloyed powders. L-5 is observed to attain the highest tensile strength of 202 MPa for the Al-Cu/1.5 wt. % Al2O3 composite. The results illustrate an increase in composites strength with increase in launching vehicle content but on the expense of nanoreinforcement particle rejection from the melt.
To fabricate large-scale multi-walled carbon nanotube-reinforced aluminum matrix composites, in this study, nanocomposites powder is successfully consolidated with simple processing methods. Ball milling is used to disperse multi-walled carbon nanotubes in the aluminum alloy 2024 matrix, and sequential cold press, hot press, and sintering techniques are selected for fabricating fully dense samples. As a result, an aluminum alloy 2024 composite with 4 vol.% multi-walled carbon nanotubes shows a yield strength of ~700 MPa and an elongation to failure of 1.5% with Young’s modulus of 86.4 GPa. The large-scale composite can be used for automobile parts such as swash plates or piston heads.
In this study, ballistic behavior of multi-layered ceramic armors under high velocity impact is studied numerically. The model consists of 2D-axisymmetric Lagrangian approach with Johnson–Holmquist constitutive model for alumina ceramic tiles, Mie–Gruneisen equation of state for polymeric interlayers, and Johnson–Cook constitutive relation for Armor Piercing (AP) projectile. The finite element results obtained from various armor layups show the potentiality of multi-layered ceramic armor in extending conoid fracture through the ceramic layers, thus leading to the increase in the armor performance. It is illustrated that besides ceramic armor layup, interlayers of ceramic tiles have a significant role in increasing the armor performance. Finally, the effect of different polymeric interlayers on ballistic performance of multi-layered ceramic armors against AP projectile is investigated. In order to measure the ballistic performance of the armors, various criteria are introduced. Depth of penetration of the projectile in the armor, residual velocity of the projectile, time duration in which the projectile is engaged inside the armor, projectile tip erosion during impact, and interaction volume ratio are some of these criteria. The study indicates that armor layup with thin front ceramic tiles backed by thicker tiles shows better conoid fracture extension and often better ballistic performance. Furthermore, among the polymeric interlayers used between the ceramic tiles, polystyrene causes the best and nylon causes the worst ballistic performance in the armor.
The combination of carbon fiber and organo-modified montmorillonite nanolayers with epoxy matrix can produce a hybrid composite that is competitive to carbon/epoxy composites. In this work, carbon fabrics and organo-modified montmorillonite nanolayers (1.5, 3, and 5 wt%) were used to produce hybrid carbon/epoxy composites using hand lay-up technique followed by autoclave curing, aiming to evaluate their static mechanical and dynamic mechanical properties. Higher organo-modified montmorillonite content in carbon/epoxy yielded slight decrease in the weight and lower voids in carbon/epoxy composite. Transmission electron micrographs showed that the organo-modification improved the dispersion and interfacial bonding of organo-modified montmorillonite with an epoxy at loadings of 1.5 and 5 wt%. The flexural strength, interlaminar shear strength, and impact strength and modulus of the composites were improved with increasing organo-modified montmorillonite content. Carbon/epoxy composite with 5 wt% organo-modified montmorillonite had the greatest increase in mechanical properties, with the flexural modulus and strength increasing by about 33% and 27%, respectively. Although the flexural properties were improved for hybrid composites, the glass transition temperature decreased for lower organo-modified montmorillonite content up to 3 wt% and increased for 5 wt%. Dynamic mechanical analysis results revealed that the storage modulus of carbon/epoxy composite was increased significantly for 5 wt% organo-modified montmorillonite loading. However, the loss modulus was decreased for 1.5 and 3 wt% organo-modified montmorillonite loading. Also, tan has increased for 1.5 wt% and later decreased for 3 and 5 wt% organo-modified montmorillonite loading in carbon/epoxy hybrid composite.
Push-out delamination is a serious concern in the drilling of fibre-reinforced composite materials. This damage occurs as the drill reaches the exit side of the material and can reduce the strength and stiffness of the structure. In this paper, a three-point bending test is performed on glass/epoxy-laminated composites to simulate the push-out delamination induced by thrust force during drilling. Cohesive zone modelling and acoustic emission monitoring are utilized to investigate the push-out delamination. Initially, double cantilever beam and end-notched flexure tests were performed to calibrate the cohesive zone modelling model. Following that, the actual loading condition is simulated using cohesive zone modelling-based finite element modelling. Energy of the acoustic emission signals is also used to detect the initiation of the delamination. The results obtained from cohesive zone modelling and acoustic emission showed that the applied methods can be used to understand and predict the initiation of push-out delamination and its progression. Finally, it is concluded that the proposed cohesive zone modelling and acoustic emission techniques can be used in the design stage as well as during the drilling process of laminated composite structures to avoid delamination.
This paper presents the results of a combined experimental and analytical study of the pull-out behavior of natural fiber (grass straw) from an earth-based matrix. A single fiber pull-out approach was used to measure interfacial properties that are significant to toughening brittle materials via fiber reinforcement. This was used to study the interfacial shear strengths of straw fiber-reinforced earth-based composites with a matrix that consists of 60 vol. % laterite, 20 vol. % clay and 20 vol. % cement. The composites that were used in the pull-out tests included composites reinforced with 0, 5, 10 and 20 vol. % of straw fibers. The toughening behavior of fiber-reinforced earth-based matrix was analyzed in terms of their interfacial shear strengths and bridging zones, immediately behind the crack tip. This approach is consistent with microscopic observations that reveal intact bridging fibers behind the crack tip, as a result of debonding of the fiber–matrix interface. Analytical models were used to study the debonding of fiber from the matrix materials, as well as the toughening due to crack-tip shielding via bridging. The results show that increasing the fiber embedment length and the fiber volume fraction (in the earth/cement matrix) increases the peak pull-out load. The debonding process was also found to be associated with a constant friction stress. The combined effects of multiple toughening mechanisms (debonding and crack bridging) are elucidated along with the implications of the results for the design of earth-based composites for potential applications in robust building materials for sustainable eco-friendly homes.
In this paper, optimization of volume fraction distribution in a thick hollow heterogeneous cylinder subjected to impulsive internal pressure is considered. Dynamic behavior and wave propagation are considered in radial and axial directions. Volume fractions of constituent materials on a finite number of design points are taken as design variables and the volume fractions at any arbitrary point in the cylinder are obtained via cubic spline interpolation functions. The objectives are to minimize the amplitude of stress waves propagating through the structure during a specified time interval, while the total mass of the structure is also minimized. Minimizing the displacement amplitude of the outer surface of the cylinder will also be considered as another objective function. Multi-objective Genetic Algorithm jointed with interior penalty-function method for implementing constraints is effectively employed to find the global solution of the optimization problem. Obtained results indicates that by using the mentioned objective functions, considerably more efficient usage of materials can be achieved compared with the common power law volume fraction distribution. Based on our results, the proposed methodology provides a framework for designing functionally graded structures with optimum material tailoring.
This article presents a critical review of papers dealing with solid particle erosion characteristics of polymer matrix composites, metal matrix composites, and ceramic matrix composites. In addition, the solid particle erosion characteristics of coatings for composite materials are also reviewed. Attention was paid to type of a reinforcement material (fiber/filler), amount of fiber/filler, fiber orientation, and interfacial strength between fiber/filler and matrix, which affect the solid particle erosion in addition to the variables affecting the solid particle erosion of monolithic materials, that is, impact angle, particle velocity, temperature, particle flux, and erodent properties such as shape, size, hardness, and so on. General characteristics of solid particle erosion for composites are extracted from the review of the papers.
This paper addresses a novel method for structural damage assessment based on the electromechanical impedance (EMI) technique applied to composite structures. This method is based on a high-frequency excitation range in order to overcome the difficulties caused when the low vibration modes are excited. A structure made up of unidirectional carbon fiber pre-impregnated with resin-epoxy is excited using a chirp signal which varies the frequency in a wide range through the PZT (Lead Zirconate Titanate) patches. From the structural response signal, the Euclidean Distance (ED) is computed from each structural condition considering the pristine condition as a reference (baseline). The ED is used as input to the Simplified Fuzzy ARTMAP Network (SFAN) which is responsible for identifying various different structural conditions. It is shown that the robustness provided by SFAN allows the evaluation of the progress of structural damage even under critical conditions. The structural damage was simulated by loosening bolts in a structure. Furthermore, the repaired condition was also considered by retightening bolts. The paper discusses the advantages and drawbacks of the approach in light of the experimental results.
The effectiveness of m-isopropenyl-α,α-dimethylbenzyl isocyanate-grafted-polypropylene (m-TMI-g-PP) as a coupling agent was evaluated for wood flour filled polypropylene composites. The performance was compared with the conventionally used maleic anhydride grafted polypropylene (MAPP) coupling agent in terms of mechanical properties. Rubberwood flour filled polypropylene composites were prepared at varying proportion (1 wt% to 9 wt% of wood fibers) of m-TMI-g-PP and MAPP coupling agents. Composites with m-TMI-g-PP coupling agent exhibited consistently superior strength properties as compared to MAPP at all concentrations of coupling agents inferring that m-TMI-g-PP is a better coupling agent. About 6–9 wt% coupling agent with respect to wood fiber was found to be the optimal level of coupling agent. The effect of filler content and fiber dimension on the properties of composites with m-TMI-g-PP coupling agent was also studied. Tensile strength, flexural strength and tensile modulus increased with increasing filler content in the composites with the coupling agent. At 50% filler loading, composites with coupling agent exhibited about 50%, 78% and 270% improvement in tensile strength, flexural strength and tensile modulus, respectively, over virgin polypropylene. Fiber aspect ratio was found to have a significant effect on the tensile strength of the composites.
Composite materials are used in areas that have varying environmental conditions due to their advantages such as generally higher stiffness- and strength-to-weight ratio, and corrosion resistance compared to metallic alloys. This experimental study is carried out to investigate the bearing strengths and failure modes of woven glass–epoxy composite pinned joints subjected to rainwater. The specimens were immersed in rainwater in a closed plastic container indoors for 20 month periods at room temperature. The ratio of edge-distance-to-hole diameter (E/D) and the ratio of the specimen width-to-hole-diameter (W/D) were selected as parameters. Failure modes were determined by observing the failure regions on the specimens. Damage of immersed and unimmersed specimens was examined using scanning electron misroscopy for the same failure load. Experimental results showed that the bearing load values obtained from the specimens immersed in rainwater decreased in comparison to unimmersed specimens.
A study is conducted with the aim of evaluating the transverse compressive behaviors of three-dimensional five-directional braided composites based on the meso-scale representative volume cells. Finite element models with three kinds of interior braiding angles, 20°, 30°, and 45°, are established on the basis of the realistic cross-section shape of each yarn. A damage mechanics model, which is implemented by user-defined material subroutines in ABAQUS/Standard, is used to investigate the mechanical responses and the damage initiation/evolution throughout the transverse compression. For the verification of numerical results, a series of transverse compression tests are conducted. Numerical results indicate that the transverse compression strength of three-dimensional five-directional braiding composite is strongly dependent on the matrix properties, and is not very sensitive to the braiding angle. The predicted results show good agreement with the relevant experimental data, demonstrating the applicability of meso-scale finite element model.
A stochastic cure simulation approach is developed to investigate the variability of the cure process during resin infusion related to thermal effects. Boundary condition uncertainty is quantified experimentally and appropriate stochastic processes are developed to represent the variability in tool/air temperature and surface heat transfer coefficient. The heat transfer coefficient presents a variation across different experiments of 12.3%, whilst the tool/air temperatures present a standard deviation over 1℃. The boundary condition variability is combined with an existing model of cure kinetics uncertainty and the full stochastic problem is addressed by coupling a cure model with Monte Carlo and the Probabilistic Collocation Method and applied to the case of thin carbon epoxy laminates. The overall variability in cure time reaches a coefficient of variation of about 22%, which is dominated by uncertainty in surface heat transfer and tool temperature; with ambient temperature and kinetics contributing variability in the order of 1%.
The addition of Z-yarns to an S-2 glass composite can increase the material's resistance to interlayer delamination and damage. However, the presence of the Z-yarns also complicates the resulting constitutive behavior, as multiple individual failure mechanisms (e.g. yarn/matrix delamination, yarn breakage) may occur before the overall ultimate failure of the composite is reached. A mesoscale model of a 3D-weave composite was created to provide insights into these processes. Material models for the yarns, matrix, and interfaces were previously developed for a similar plain-weave composite. These were combined with a complicated unit cell geometry that could be used to construct specimen meshes that closely matched the physical characteristics of the 3D-weave composite. Numerical simulations conducted using this model can reproduce most aspects of the load–displacement curves and the observed failure mechanisms from tension, torsion, and delamination tests.
The flexural properties of plain-weave woven fabric-reinforced composites have been investigated to clarify the effects of equi-biaxially fabric prestressing on flexural characteristics. The prestressed composite samples were manufactured by applying the symmetrical tension load to both warp and weft yarns prior to matrix curing. The fabricated samples were tested under different fabric orientation angles, i.e. from warp to bias direction. The decline in the flexural properties of the prestressed composite due to matrix creep was checked. From three-point bending tests, the prestressed samples exhibited a maximum increase in the flexural performance, such as the strength and modulus, of ~16% at a prestressing level of 50 MPa when compared with unprestressed counterparts. The level of improvement in the flexural properties reduced with increasing fabric orientation angle. The creep was induced in the prestressed matrix and subsequent decline in the improved flexural properties was indicated in the prestressed samples. The decline in flexural properties occurred mostly during the short-term creep.
This paper presents the experimental and the analytical results of pultruded glass fiber reinforced polymer sheet pile panels subjected to flexural loading. Full scale four-point flexural tests were conducted up to failure on five sets of joined pairs of glass fiber reinforced polymer Z-pile panels by placing them flat-wise. The influences of transverse and longitudinal fiber volume fraction, span-to-depth ratio and steel plate on the carrying capacity and deformability were discussed. The test results displayed that increasing the fibre transverse and longitudinal volume fraction was helpful for tearing control at the flange-web junction on the compressive side, thus obviously enhancing the load capacity and the flexural rigidity of fiber reinforced polymer sheet pile panels. The additional steel plate improved the rigidity of fiber reinforced polymer sheet pile panels, but the load capacity increased insignificantly due to the debonding of steel and glass fiber reinforced polymer interface. The span-to-depth ratio has little influence on the ultimate load due to the local failure but large deformation occurred in the long-span specimen. Based on classical laminate theory for anisotropic materials, a new equation was proposed to calculate the flexural and shear rigidities of glass fiber reinforced polymer sheet pile panels with corrugated section, in addition to applying classical Timoshenko beam theory. The theoretical results agree well with those obtained from the experiments.
Ti-6%Al-4%V (Ti64) alloy powder coated with vapor grown carbon fibers was consolidated to fabricate titanium matrix composites by spark plasma sintering, followed by hot extrusion process. Four compositions of the additive vapor grown carbon fibers were 0.1, 0.2, 0.3 and 0.4 wt. %. The microstructure was changed from the full lamellar of monolithic Ti64 alloy to the bimodal structure by addition of vapor grown carbon fibers to Ti64 alloy. The changing of microstructure was attributed by an α-stabilize effect of carbon (vapor grown carbon fiber). Almost all the vapor grown carbon fibers reacted with Ti and result in formation of Ti6C3.75 carbide phases detected by x-ray diffractometer and energy dispersive spectrometer. Hardness was improved by effect of solid solution strengthening of carbon in the Ti matrix and reached the maximum value of 495.8 Hv. 0.2%YS and UTS were significantly increased by addition of 0.1 wt. % vapor grown carbon fiber. However, they were not much improved when more vapor grown carbon fiber content was applied. Ductility of the sample was affected by a change in microstructure when 0.1 wt. % of vapor grown carbon fibers was added because the bimodal structure shows higher ductility compared to the full lamellar structure. The carbide precipitation in TMCs was not much contributed in improving of strength and decreased a ductility of material.
A new kind of polymeric emulsifier was readily synthesized through one-pot three-stage processes. Fourier transform infrared spectroscope (FTIR) combined with conversion rate of epoxide groups tests was applied to characterize the chemical composition of the polymeric emulsifier. The emulsion sizing based on the self-made polymeric emulsifier was prepared through phase inversion emulsification method. The stability of the sizing agent was evaluated by centrifugal sedimentation and particle size investigations. The surface morphology of CF was observed by scanning electron microscope (SEM). The effects of the sizing agent on CF handleability were examined by abrasion resistance, hairiness amount and stiffness tests. The results indicated that after sizing, the CF obtained an appreciable enhancement in handleability. Finally, interlaminar shear strength (ILSS) of sized CF/epoxy resin composite increased by 55.39% compared with that of the desized one.
Multiwalled carbon nanotubes have been widely used as mechanical reinforcement fillers for polymers during the past few decades. However, high electrical conductivity of raw multiwalled carbon nanotubes hampers their application in some fields demanding not only good mechanical properties and/or high thermal conductivity but also electrical insulation. In this research, carboxyl functionalized multiwalled carbon nanotubes and organically modified montmorillonite were introduced to prepare epoxy nanocomposites with anhydride as curing agent. The obtained epoxy nanocomposites possessed improved impact toughness, and the electrical insulation was maintained. Compared to the volume resistivity of the raw multiwalled carbon nanotubes (0.6 wt%)/epoxy nanocomposites, the volume resistivity of the organically modified montmorillonite/carboxyl functionalized multiwalled carbon nanotubes (0.6 wt%)/epoxy nanocomposites increased more than four order of magnitude. These excellent properties were attributed to the synergistic effect of carboxyl functionalized multiwalled carbon nanotubes and organically modified montmorillonite on toughening epoxy, as well as the suppression of electron transport by multiwalled carbon nanotubes surface modification and the organically modified montmorillonite layer in the multiwalled carbon nanotubes conductive network. The effects of adding nanofillers on the dielectric constant and dielectric loss values of epoxy nanocomposites were also studied. This work has demonstrated the feasibility of using multiwalled carbon nanotubes as mechanical reinforcement fillers, while simultaneously giving electrical insulation in the polymer nanocomposites.
Automated image analysis of textile surfaces allowed determination and quantification of intrinsic yarn path variabilities in a 2/2 twill weave during the lay-up process. The yarn paths were described in terms of waves and it was found that the frequencies are similar in warp and weft directions and hardly affected by introduced yarn path deformations. The most significant source of fabric variability was introduced during handling before cutting. These resulting systematic deformations will need to be considered when designing or analysing a composite component. An automated method for three dimensional reconstruction of the analysed lay-up was implemented in TexGen which will allow virtual testing of components in the future.
This paper investigates the blast response of a glass fiber reinforced polymer pipeline using explicit finite element analysis. In this study, a fluid-structure interaction methodology was employed to obtain the deformation and damage of pipeline under explosive impact using LS-DYNA. The purpose of this research is to evaluate the influence of the stand-off distance, the tube diameter, the internal pressure of tube and the explosive quantity on dynamic response of composite pipe. Simulations were carried out not only in the case of empty pipe but also in the case of water-filled pipe with different internal pressures. The analysis was performed in two phases; initialization phase, where the pressurization and gravity loads are applied on the pipeline, and the blast phase. Comparing the analysis results, it was proved that the internal pressure influences significantly the deformation of the tube. The results of the present study can serve as a reference guide for the prediction of pipe response under blast loading, since no guidelines exist in pipeline standards for design under blast loading conditions.
Electrical conductivity of polyester filled with carbon nanotubes composites have been studied from 240 to 380 K in the frequency range from 100 Hz to 1 MHz as a function of the filler volume fraction above the percolation threshold. The frequency dependence of the electrical conductivity obeys the universal dynamic response. Positive temperature coefficient in resistivity and negative temperature coefficient in resistivity phenomena were observed at temperatures below and above the glass transition respectively. It was found that the mechanism responsible for the changes in resistivity is predominantly due to the tunneling effect. Positive temperature coefficient in resistivity intensity was also exploited which is strongly dependent on the carbon nanotube content.
Within the EUCLID project, ‘Survivability, Durability and Performance of Naval Composite Structures’, one task is to develop improved fibre composite joints for naval ship superstructures. In many practical situations, the structures are subjected to loading at very high strain rates like slamming, impact, underwater explosions or blast effect. Material and structural response vary significantly under such loading as compared to static loading. In this paper, the results from a series of Split Hopkinson Pressure Bar tests on the woven composites are presented. These tests were done in two configurations: in-plane and out-of-plan compression test. It is observed that the failure strength varies with the different loading directions. The results indicate that the stress–strain curves, maximum engineering stresses and strains evolve as strain rate changes. The woven composites have higher values of engineering stress and dynamic stiffness for in-plane than for out-of-plane compression at the same strain rate; however, the in-plane strain at maximum stress is higher than that of out-of-plane compression. During the experiments, a high speed camera was used to determine the damage mechanisms. The specimens are mainly damaged in a crushing and shear failure mode under out-of-plane loading, as for in-plane test, the failure was dominated by fibre buckling and delamination.
A novel technique to predict the manufacturing process-induced distortions and deformations in thick unidirectional carbon fiber reinforced plastics laminates is detailed in this article. An integrated numerical model is developed to account for non-isothermal resin flow, related compaction, transient resin cure, and resin shrinkage effects to predict the final shape of the autoclave cured thick prepreg laminate. The associated physics are mathematically coupled to solve for the process variables interactively. The results illustrate reduction in the thickness of the laminate prior to the start of curing when initial fiber volume fractions are pre-set. This confirms that the initial transverse deformation of B-stage prepreg is due to the applied vacuum and/or pressure. Once curing initiates, deformation of the laminate due to compaction increases, proportionally, with the increase in fiber volume fraction. Furthermore, the thermo-chemical residual strains contribute to additional compaction. The final distorted shape observed in simulation of the originally flat laminate matches with the shape of the fabricated laminate with 6.6 mm thickness. A solution to minimize the distortion is discussed in detail. This procedure is extended to simulate a curved laminate’s processing; where, the shear moduli were observed to influence the final shape of the laminate. The findings are presented and deliberated in this article.
Mechanical joining and adhesive bonding provide convenience for manufacturing of complex structures, which made of composite materials. Failure load is directly related with process parameters of mechanical joining or adhesive bonding. In this study, the effects of bonding angle, patching type (single side and double side) and patching structure on the failure load were investigated in the pultruded composite specimens. For this aim, the pultruded composite specimens, which bonded with five different bonding angles (45°, 51°, 59°, 68° and 90°) and five different bonding types as unpatched, single-side woven patch, single-side knitting patch, double-side woven patch and double-side knitting patch were exposed to tensile loads at room temperature. In the view of experimental results, the failure loads of bonded pultruded composite specimens were predicted by training six different artificial neural network algorithms. The only three best prediction results of Bayesian regularization, Levenberg–Marquardt and scaled conjugate gradient were given in the figures for better understanding.
The present study reports on the microstructural, physical, thermal and mechanical properties of two types (A and B) of carbon–carbon composites processed by an economical route. Skeleton composites were first made by pyrolysing laminated carbon fibre-reinforced phenolic composites and subsequently densified by liquid pitch impregnation–pyrolysis process. Both types of composite employed polyacrylonitrile-based 8-harness satin woven carbon fabrics, Type A being woven with tows of continuous fibres, whereas Type B used yarns of discontinuous fibres. Experimental results indicated that the Type A composite had better flexural and tensile modulus and strength values. However, the Type B composite had better interlaminar shear strength and through-thickness thermal conductivity, despite being less dense. These results are discussed and explained from microstructural and fractographic analysis using optical and scanning electron microscopes. Finally, the results were compared with those intended for similar applications.
In couple of years, epoxy UV curing has emerged as a necessity, yet it has some limitations, e.g. little curing conversion of thick films, especially with colored additives. Thick dimensions do not allow radiation to penetrate into the sample. Moreover, colored additives cover photoinitiators and reduce their efficiency. Therefore, UV-cured epoxy composites survive only in thin layers. Current research has resolved the issue, through a unique "smart approach," which resolves the problem by separating the initiation step of the reaction (which is true photo-induced reaction) from the propagation (which is not directly dependent on light). This approach completely photo-cures epoxy thick films with colored additives. Superb filler dispersion has been observed, along with strongly interconnected polymeric network. Augmentation in thermo-mechanical properties has been observed through different instrumental techniques.
The micromechanical prediction of mechanical properties of composites, using a finite element method, is highly affected by selecting a suitable representative volume element. In the present research, the effects of the representative volume element aspect ratio on the stiffness of aligned short fiber composites have been studied. First, an analytical method is suggested to predict the longitudinal and transverse moduli of composites considering the representative volume element aspect ratio. Then, the optimum aspect ratio of the representative volume element is estimated by comparing the results obtained by the present method with those of a micromechanical method. Comparing the stiffness of the optimized representative volume element with some experimental results available in the literature shows that the result of the present method is also in a good agreement with experiments.
Recycled polystyrene wood flour composites were developed with three different polystyrene maleic anhydride oligomers used as coupling agents. The thermal properties, morphology, kinetics, and thermodynamic parameters were investigated. The use of oligomers clearly improves the interfacial adhesion with the polymer matrix and increases the thermal stability of the composites. The oligomers with higher maleic anhydride content tend to reduce the composite thermal stability, while oligomers with intermediate quantities of maleic anhydride groups tend to promote a higher composite thermal stability. The thermodynamic results showed that the composite without coupling agent presented the lowest entropy value, which indicates that the thermal decomposition in this composite is slow. However, the entropy values and frequency factor increased when coupling agents were used. The addition of coupling agents likely changed the state near to the thermodynamic equilibrium by causing structural disorder, in the system increasing the treated composite reactivity.
In this paper we employ a nonlinear acoustic method, namely, the vibro-acoustic modulation method, for the detection of delamination and kissing bond in composites. Both a (large-amplitude and low-frequency) pumping wave and a (low-amplitude ultrasound) probing wave are used to vibrate the structure. Permanently bonded piezoceramic transducers are used for both excitation and measurement. A side-band ratio is used to estimate the extent of nonlinear interactions between the two input waves in the structure. Current results show that the proposed method successfully differentiates the intact specimens from both the delamination and the kissing bond specimens used in this study.
Damage mechanisms in uni-directional fiber-reinforced plastics have been studied at the micro-scale by developing three-dimensional-repeating unit cells with randomly distributed fibers. Three damage mechanisms have been considered, viz., matrix damage, fiber failure and fiber-matrix interface debonding. The development of these damage modes and their effect on the overall stress–strain response of the micro-structure due to varying fiber volume fractions, loading conditions (longitudinal, transverse and combined transverse tension and shear), spatial distribution of fibers and fiber strength distributions (deterministic and Weibull) are of interest. A numerical framework has been developed that allows for conducting such studies for the chosen parameters. Microscopic images of real micro-structure of uni-directional fiber-reinforced plastics have been used to develop the random micro-structure for the three-dimensional-repeating unit cell. In the longitudinal loading scenario, fibers with Weibull strength distribution are closer to reality. While the three-dimensional-repeating unit cell under longitudinal loading fails in predominantly fiber failure mode, debonding and matrix damage play a major role in determining the average response of the micro-structure under transverse and combined transverse and shear loading.
The main purpose of this work consists of the mechanical characterization of a copolymer polypropylene (PP) filled with natural Alfa fibres. The elaboration of the PP-based composite reinforced with these natural fibres is explained. The mechanical behaviour of these composite blends has been analysed. Contrary to classical studies of the mechanical characterization of these kinds of materials which focus only on monotonic tensile tests, cyclic loading/unloading tests and loading/unloading tests interrupted by relaxation steps have been investigated. These tests enable us to compare the mechanical response of the virgin PP and the PP filled with Alfa fibres (Alfa/PP). Reinforcement by these natural fibres shows an impact in the improvement of the mechanical properties of this composite. All these experimental results constitute a data base used to identify the material parameters of a phenomenological constitutive hyperelasto-visco-hysteresis model. These tests also provide indications about the hyperelastic, viscous and hysteretic stress contributions of the hyperelasto-visco-hysteresis model with these materials during cyclic loading.
Biopolymers as polylactic acid (PLA) have some drawbacks in processing and the final product is characterized by a high permeability to steam/gases and low thermal and mechanical resistance. The addition of reinforcement materials in the biopolymer matrix provides opportunities to minimize these disadvantages. Thus, this research focuses on the inclusion of non-wooden lignocellulosic fillers from bamboo Guadua angustifolia in a polymer matrix obtained from PLA. Three types of fillers were applied: sawdust, fiber from a Kraft process, and fiber with low lignin content. The blends of PLA reinforced with 5% organic fillers were performed on a torque rheometer at temperatures between 170 to 200℃ with the addition of glycerol as plasticizer and viscosity reducer of the system and maleic anhydride as compatibilizer. The evaluation on rheological, thermal, mechanical and morphological properties were investigated by rotational rheometry, differential scanning calorimetry (DSC), universal testing machine, wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). The results showed that the inclusion of the lignocellulosic material generated a positive response in obtaining polymeric biocomposite. The use of fibers subjected to a process of delignification with bleaching sequences OQPoPWa resulted in a greater influence on the mechanical strength, an adequate distribution of the fibers in the PLA matrix and the viscosity of the mixture.
The engineering applications of thermoplastics have been increased in the last few years by blending the thermoplastic polymer with fillers, which is a common methodology to develop thermoplastic polymer with boosted properties. In this investigation, the polypropylene thermoplastic polymer and Cloisite 30B (1, 2, 3, and 5 wt. %) as a nano filler with Elvaloy AC 3427 were mixed using Twin screw. The Elvaloy AC 3427 was used as a compatibilizer to enhance the dispersion of Cloisite 30B in polypropylene matrix. The polypropylene/Cloisite 30B/Elvaloy AC 3427 nanocomposite was investigated with X-ray diffraction shows an increase in d-spacing with an intercalated structure. Polypropylene/Cloisite 30B/Elvaloy AC 3427 nanocomposite was mechanically and metallurgically characterized. To validate the tensile properties of polypropylene/Cloisite 30B/Elvaloy AC 3427 nanocomposites, different mathematical modeling was used. The result shows an improvement in properties due to the fine dispersion of Cloisite 30B in polypropylene matrix in the presence of Elvaloy AC 3427 compatibilizer, and the same was justified with Field Emission Scanning Electron Microscopy.
Improving the numerical techniques to foretell the fatigue life of composite joints is a crucial work. The main goal of this study is to present a new algorithm for simulating the fatigue behavior of composite laminates. This algorithm was applied to the composite bolted joints and the effects of bolt arrangement and laminate configuration was considered. The fatigue life of defined layers was estimated by Hashin’s failure criteria. In contrast to the previous investigations, the influences of load ratio on fatigue curves were determined using Kawai’s rule. Also, negative mean stresses were considered using the constant life diagrams. The damage parameter was estimated by a modified form of Miner’s rule. To validate the proposed model, numbers of specimens were fabricated with [0/45/–45/90]s and [0/90/0/90]s lay-up. For each configuration, the samples were joined by two and four bolts. The fatigue life predictions of all configurations were compared with the experimental data and good agreement was observed.
The main aim of this paper is to introduce mechanical, thermal and surface properties of produced industrially HDPE-based nanocomposites. For this purpose, 1.0, 2.0, 3.0, 4.0 and 5.0 wt.% loading of nanoclay-reinforced HDPE nanocomposites made from the HDPE matrix were prepared by the melt mixing method using a compounder system, which consist of industrial banbury mixer, single screw extruder and granule cutting. The effect of nanoclay on mechanical, thermal and surface properties of nanoclay/HDPE nanocomposites was investigated. The tensile and flexural strength of nanoclay/HDPE nanocomposite increased by about 5% and 7%, respectively, with addition of 1.0 wt.% nanoclay. But then it decreased slightly as the nanoclay content increased to 5.0 wt.%. The tensile modulus and tensile elongation were decreased with the addition of 1.0 wt.% nanoclay, but did not increase further as more nanoclay was added. The flexural modulus of HDPE was significantly improved after (from 1.0 wt.% up to 5.0 wt.%) addition of nanoclay. It was found that the scratch resistance of nanoclay/HDPE nanocomposite improved with addition of the nanoclay by SEM examination. Density, melting flow index (MFI), differential scanning colorimetry (DSC), and vicat softening temperature (VICAT) were used to characterize the physical and thermal properties of the nanocomposites. The X-ray diffraction (XRD), the Fourier transform infrared spectrophotometry (FTIR), and the scanning electron microscopy (SEM) were used to analyze the structural characteristics of the nanocomposites. It is concluded that the addition of the nanoclay in HDPE has significantly influenced the mechanical, thermal, and surface properties of the nanocomposites.
A three-dimensional continuum damage model is developed for the prediction of the initiation and propagation of intralaminar damage mechanism. This continuum damage model is implemented using a user-subroutine in finite element software ABAQUS™/Standard and combined with the cohesive zone model for three-dimensional progressive damage analysis of angle-ply [0/α] (α = 90°, 75°, 60°, 45°, 30°, 15°, and 10°) laminated plates subjected to transverse loading. It is found that delamination is the major damage mode in large-angle (α ≥ 30°) laminates while fiber bridging is the dominant damage mode in small-angle (α ≤ 15°) laminates. The damage mode predictions are in good agreement with experimental observations reported in the literature. Matrix cracking within the bottom plies and its interactions with interlaminar damage are found to play an important role in the formation of the final damage modes, shapes, and sizes.
In this study, a mixture of vinyl ester (VER) and polyurethane (PU) prepolymer was used in the field of pipe manufacturing as a matrix material (interpenetrating polymer networks) with E-glass fibre as reinforcement. In order to evaluate the mechanical strength of the fibre-reinforced VER-PU interpenetrating polymer network composite pipe, tests like hydrostatic, hoop, stiffness, and axial compression were conducted systematically with five proportions of PU-loaded VER (0%, 10%, 20%, 30%, 40%, and 50%). It was observed that during the stiffness test, all the PU-loaded VER specimens have regained its original shape, size, and also retains its structural integrity. VER glass fibre composite pipes (0% PU) exhibited crazing, wall delamination, and rupture on compression. With increase in PU content, there were no such effects after the compression, but a slight drop in mechanical properties like hoop, stiffness, and axial compression was observed. The obtained experimental results were found to be in good agreement with finite element analysis data.
The potential of flax fabric in the production of ‘green’ composites based on polypropylene (PP) was evaluated. The laminate composites were prepared by compression molding of flax fabric (with and without treatment) and PP films (using different formulations of PP). The morphology and crystallization of the PP/flax composites were characterized using scanning electron microscope, X-ray diffraction, and differential scanning calorimetry. The mechanical performance of the composites was also evaluated. The results demonstrate that fiber treatment and PP formulation play important roles on the performance of composites. This study proved that the addition of the coupling agent maleic anhydride grafted PP to the composites improves the interfacial adhesion between hydrophilic flax fibers and the hydrophobic PP matrix and thus the mechanical properties. The combination of the coupling agent and reactive additive provides superior strength, modulus and brings further fire resistance to the PP/flax composites.
As the use of fibre reinforced polymer materials in bridge construction is becoming more popular, appropriate joining techniques, particularly for field joints, are necessary. Bolted joints are a common method for joining fibre reinforced polymer structures. The main advantage of bolted joints is their detachability, but they have a number of shortcomings. On the one hand, hole clearances, which are needed to facilitate on-site assembly, reduce the stiffness and joint efficiency. On the other hand, it is not possible to rely on the beneficial effects of bolt pre-defined tension loads, i.e. load transfer in friction, due to the considerable losses of bolt tension caused by the creep deformation in the composite material. To tackle these problems, a solution utilising metallic inserts in the hole is proposed in this paper. A series of experimental tests have been conducted to investigate the effect of inserts on the bolt-tension relaxation, the stiffness and the load-bearing behaviour of joints. Finite element analyses were also employed. The study demonstrates several benefits of the inserts: the bolt tension relaxation is minimised, the load transfer by friction may be feasible to be utilised in the bridge service state and the joint efficiency is increased in terms of stiffness and strength.
The objective of the present investigation is to study the alteration in flexural performance of glass/epoxy (GE) composite with and without multiwalled carbon nanotube (MWCNT) after liquid nitrogen conditioning. The epoxy resin was first modified by 0.1, 0.3, and 0.5 wt% MWCNT, which was then used along with E-glass fibers to fabricate laminates. Flexural strength and modulus were evaluated by 3-point bend test. Out of these four compositions, the maximum strength in the as-fabricated condition was obtained for GE composite with 0.1 wt% CNT, which is 32.74% higher as compared to GE composite. Decrease in strength and modulus was observed for short time span of liquid nitrogen conditioning i.e. 0.25 h. Long cryogenic conditioning resulted in increment in strength. To understand the failure mechanisms, post failure analysis was carried out using scanning electron microscope. The design parameters are calculated using Weibull distribution model.
The aim of the present work was to study the effect of fiber surface treatment on the structural, thermal and mechanical properties of luffa fiber and its composites. Fibers were treated with alkali (5% conc.), benzoyl chloride, and potassium permanganate (KMnO4) (0.05%) at room temperature. The untreated and treated fibers were characterized and morphologically analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM). The effect of fiber surface modification on the mechanical properties such as tensile strength, flexural strength, ILSS, and impact strength of the composites were investigated. It is observed that chemically treated Luffa cylindrica-reinforced epoxy composites significantly improved the mechanical properties of the composite. The maximum strength properties were found with benzoyl chloride-treated fiber-reinforced composite.
Acoustic Emission (AE) is a capable approach to characterize delamination initiation and propagation in laminated composite materials. One of the major issues of applying this methodology is to establish a reasonable correlation between delamination initiation and propagation and resultant AE features. To this aim, initiation and propagation of mode I delamination is investigated in glass/epoxy composite materials. Micro and macro observation and acoustical energy were used to develop novel AE-based approaches for investigation of delamination damage. The privilege of these novel approaches is to detect the exact time of initiation of delamination in different laminates which was the lack of previous literature. The Cohesive Zone Modeling (CZM) was used to simulate the delamination growth. Scanning Electron Microscope (SEM) images were also used to investigate damage mechanisms at the interface of crack growth. The results show that AE and CZM approaches present decent capability to characterize delamination. The output of this study could lead to enhancing automatic techniques for structural health monitoring of composite materials.
An in-situ ternary ceramic composite material is developed indigenously by carbo thermal reduction of fly ash in a plasma reactor. X-ray diffraction (XRD), Scanning electron microscopy (SEM)-Energy dispersive x-ray (EDX) analyses of the developed ternary mixture has revealed in-situ conversion of SiO2 to SiC in the vicinity of Al2O3, present in the fly ash material. Presence of carbon in the form of graphite is also confirmed by XRD and EDX analyses. The morphology of the fly ash powder particles is changed from irregular to rod shaped particles with the presence of whiskers (diameter ~ 0.2 µm, length 8–20 µm). Metal matrix based (Al6061) ceramic composites are prepared with the plasma synthesized fly ash and untreated fly ash composites by stir casting method with different volume fractions of ceramic powders of both the category. Progressive reductions of grain size by increasing volume fractions with segregation of ceramic particles at the grain boundaries are salient features of microstructures. Remarkable improvement in strength and wear resistance is the two significant features of the novel Al6061alloy based composite developed with plasma treated fly ash. Results are presented on the ageing behaviour of the alloy composite for both the categories and mechanical properties of age hardened alloy based composites are compared with monolithic alloy. Accelerated ageing behaviour is observed for alloy based composites, which is found to depend on the morphology of the incoherent dispersed phases. Accelerated precipitation during ageing has been confirmed by hardness versus time as well as by differential scanning calorimetry analysis. Sequences of precipitation for all materials are shown in differential scanning calorimetry thermogram.
In the present study, surface modification of novel environment-friendly Rice Bran Carbon was carried out by mixed acid treatment followed by reaction with multifunctional silane, (3-Mercaptopropyl) trimethoxysilane. Then a new class of styrene-butadiene rubber/Rice Bran Carbon composites were prefabricated by using latex compounding method on the basis of pretreatment of RBC. Specifically, extra 3phr 3-MPTMS was added into the composites to construct a systematic filler-silane-matrix net framework. The chemical interaction mechanism of silane agent with the oxidized RBC was confirmed by Fourier Transform Infrared Spectroscopy, Energy Dispersive Spectroscopy, Thermogravimetric Analysis and Field Emission Scanning Electron Microscopy. The effect of silane modified RBC on the mechanical, wear and friction property of SBR vulcanizates was investigated.
Non-traditional machining of carbon fiber-reinforced plastics, such as laser machining, has great advantages over mechanical machining in the aspect of machinability and flexibility. In this article, a laser milling method using diode pumped and frequency doubled Nd:YVO4 nanosecond pulsed laser system is presented. The effects of processing parameters including laser power, scanning speed, and hatch distance were analyzed. It was found that machining quality and efficiency are seriously influenced by laser power and pulses overlapping rate. The features of micro-pit and chopped fibers on the machined surface were observed with metallographic microscope, and ablation mechanism involved was studied to explain the phenomenon. Depth-controlled milling strategy was realized in the laser focusing environment. Finally, challenges and suggestions are presented for wide application of the method.
The quality of composite parts manufactured by resin transfer molding is sensitive to material and process variations during the preform impregnation. In order to improve the process robustness in two-dimensional injection cases, this work proposes a fast method for tracking and controlling the resin flow through the preform, using only a small number of pressure sensors embedded in the mold. The approach combines pressure signals and flow modeling in a quick algorithm that returns on-line estimations of the flow front profiles. Virtual and real injection tests demonstrated the accuracy of the methodology and provided information for designing the sensing system. Moreover, the investigation proved the feasibility of a simple and effective procedure of flow rate control. Based on a computer-controlled pressure regulator acting on the inlet gate, a feedback mechanism was implemented in the system and allowed keeping the flow front velocity within a target range of values. Such a control procedure may be used to limit the formation of voids and enhance the final part quality.
It is important to determine accurately the elastic properties of fibre-reinforced polymer composites material, considering that their member design is often governed by deflection rather than strength. In this study, the elastic properties of the pultruded glass fibre-reinforced polymer square sections were evaluated firstly using full-scale with different shear span to depth (a/d) ratios and tested under static four-point bending. Back calculation and simultaneous methods were then employed to evaluate the flexural modulus and shear stiffness and were compared with the results of the coupon tests. Secondly, the full-scale beams were tested up to failure to determine their capacity and failure mechanisms. Finally, prediction equations describing the behaviour of the pultruded glass fibre-reinforced polymer square beams were proposed and compared with the experimental results. The results indicate that the back calculation method gives more reliable values of elastic properties of glass fibre-reinforced polymer profiles. In addition, the behaviour of the beams is strongly affected by the a/d ratios. The shear was found to have a significant contribution on the behaviour of beams with lower a/d ratios while the flexural stress played a major part for higher a/d ratios. The proposed equation, which accounts for the combined effect of the shear and flexural stresses, reasonably predicted the failure load of pultruded glass fibre-reinforced polymer square beams.
In this work, the effects of extractives removal on the performance of used varnishes were studied. The experimental samples were prepared using defect-free alder (Subcordate alnus) and ironwood (Zelkova carpinifolia) with moisture content of 12% and were coated with polyester and two-part polyurethane (urethane alkyd) varnishes. Removal of extractive materials followed Tappi Test Methods using hot water and ethanol. Variable parameters were based on the solvent type (extraction method), wood species, and varnish type. Other parameters such as moisture content, sanding process, and dimensions were kept constant. Pull-off adhesion and dynamic absorption tests were used to assess adhesion and performance of the two clear varnish coatings. Sanding process was employed prior to coating, which helped prepare surfaces virtually free of damage. In general, the extractive-free samples had better adhesion strength and wettability compared with the untreated (control) ones. Based on the results of this study, it can be pointed out that all the variable parameters, including the type of wood, varnish, and solvent, had significant effect on the adhesion of varnishes, applied on the wood surface. The difference in wettability between extracted and unextracted samples is due to blocking of the free hydroxyl groups by extractive materials. In addition, it was found that the removal of extractives had the effect of increasing the surface wettability of both species, an important consideration for the varnish coating. The highest adhesion was obtained from polyurethane varnish, applied on ironwood specimens. It seems that the diffuse-porous anatomical structure of ironwood along with its high density of 0.79 g/cm3 could be responsible for its higher adhesion strength than alder species.
A new recycling process for fiberglass waste is proposed without any use of virgin materials or additives. This technology consists of a "direct molding" (i.e. compression molding) of powders from fiberglass pulverization. In order to show the feasibility of this recycling technology, several fiberglass laminates were molded with a polyester matrix and different glass fiber contents (from 0 to 40 wt%). The pulverization-molding cycle was repeated two times to evaluate further recyclability of recycled fiberglass. Particle size distribution of recycled powders was evaluated after each pulverizing step, as well as density and bending properties of all virgin and recycled samples. Even if final properties of recycled fiberglass are not comparable with virgin fiberglass, they are comparable with other technical materials such as plasterboard. In fact, in the best case, 40% glass-filled samples showed a bending strength over 4 MPa with a strain at break over 2%. Highest values were found after the second pulverization step because of particle size refinement. Applying a polyester coating on the molded sample surface always resulted in a strong increase on strength (up to + 200%).
One of the well-known advantages of the sandwich construction is its high specific stiffness with lightweight. The property of withstanding the shearing, peeling, and flatwise tensile loading is an important factor of designing the sandwich structure. In the present study, the looped fabric reinforced foam core sandwich composite (U-cor) is proposed, and its flatwise tensile, peeling and shearing responses are investigated. Experiments are performed to study the mechanical behavior of the rigid polyurethane foam (RPUF) of different densities under tension, compression, and shear loading. Specimens of U-cor and the traditional 2D woven fabric reinforced foam sandwich composite (2DRFS) with thick and thin fiber yarns are investigated. Results show that the main failure mode of the 2DRFS is skin-core debonding. For the U-cor, the damage patterns are far more complicated. Besides breakage of foam, breakage of loop yarn may also occur. The interface performance of the U-cor is much better than that of the 2DRFS. Finite element analysis of U-cor under shearing load in warp direction is performed. The predicted shear strengths and failure modes are in good agreement with the experimental results.
The present work was performed on two composites: Al-15 vol.% B4C and experimental 6063/15% B4C. Additions of 0.45% Ti + 0.25% Zr were made to both composites melts. The composites were cast from the respective melts at 730℃ using two metallic molds to produce tensile as well as impact test samples. All samples were solution heat treated for 24 h at 540℃ for both composites, followed by aging at 200℃ for 10 h. Samples for microstructure and fractography were examined using field emission scanning electron microscopy. The results show that the powder injection technique used in this study produces composites with B4C uniformly distributed throughout the matrix. The strength and impact toughness of the two composites are controlled by the simultaneous precipitation of Al3Zr and Mg2Si phase particles depending on the matrix type. Hardening caused by precipitation of Mg2Si is more pronounced than that caused by Al3Zr phase precipitation. No B4C particle debonding was observed due to the presence of Zr-/Ti-rich layers surrounding the B4C particles. The cracks mainly propagate through the B4C reinforcement particles.
Ply waviness is a commonly observed manufacturing defect of ultra-thick composite materials essentially affecting stiffness, strength, and fatigue behavior of the composite. A specimen's geometry is designed to represent the failure mechanisms in thick wavy laminates, typically observed in spar caps of today's wind turbine blades. A material model is developed to simulate the phenomenological processes in wavy laminates to obtain a strength knock-down. Particular attention is taken on the nonlinear shear behavior and kink band formation. The presented model correlates well with results achieved by experiments. This paper shows a significantly higher influence of compressive compared with tensile loading on the mechanical material behavior of wavy laminates.
There is an emerging interest in the aerospace industry to manufacture components with intricate geometries using discontinuous fibre carbon/polyether–ether–ketone moulding systems (obtained by cutting unidirectional tape into strands). This type of material system is termed randomly oriented strand composites and is appealing for structural applications as it bridges the gap between the lack of formability of continuous fibre composites and the lack of performance of short fibre composites. The objective of this study was to investigate mechanical properties (tensile, compressive, shear and fatigue) of randomly oriented strand composites and to quantify the effect of strand size on their properties. Overall, properties were found to be highly variable and dependent on the strand length. Interestingly, tensile, compressive and shear strength had similar magnitudes and exhibited the same failure mechanisms (strand fracture and debonding). This experimental work expands the knowledge base for randomly oriented strand composite materials.
In this work, the elasto-plastic material properties of particle reinforced composites are evaluated using three-dimensional finite element approach. Effective properties of the composite are evaluated using representative volume element and numerical homogenization for kinematic and static boundary conditions. A computer program is developed to generate non-overlapping spherical particles of arbitrary size in an representative volume element. The numerical modeling of representative volume element containing the randomly distributed spherical particles of different sizes is performed by finite element method. Convergence study is performed to determine the suitable size of representative volume element. Homogenization is carried out for two cases i.e. matrix with soft and hard particles for reinforcement volume fraction up to 25%. A numerical comparison of 2-D and 3-D results is presented in terms of material behavior for matrix with soft particles.
The present study was performed on two Al-10 vol. % B4C composite containing in common 0.48%Ti and 0.25%Zr. The matrix in the first composite was commercially pure aluminum and in the other was experimental 6063 alloy. The molten composite was cast in the form of slabs (25 x 20 x 400 mm). Prior to hot rolling, the slabs were annealed at 540℃ for 16 h and the last two rolling passes were done at room temperature. Final sheet thickness was about 2 mm. Samples for tensile testing were prepared from the rolled sheet using wire cutting technique. The tensile samples were solution heat treated at 540℃ for 8 h followed by furnace cooling. Samples for microstructure as well as fractography were also examined. Tensile samples were tested in the temperature range of 25–500℃ at a strain rate of 5 x 10–4 s–1. The results show that when composite samples were tested following solution heat treatment, the role of the matrix composition in controlling the preservation of the composite strength at high temperature is nil. Increasing the testing temperature caused rapid decrease in the composite strength in a non-linear pattern. The fracture surface of the deformed composites consisted mainly of well-defined dimple structure. The size of these dimples is a direct function of the testing temperature. The presence of Ti-Zr rich protecting layers surrounding the B4C particles resulted in good particle/matrix wettability throughout the entire testing temperatures (25–500℃). Cracks were observed to be initiated at the particle/matrix interface and propagate through the B4C particles. The length of crack propagation was mainly dependent on the particle size. Increasing the testing temperature appeared in the gradual fracturing of the reinforcement particles.
Discontinuities such as cutouts always cause stress concentration in the structure, which increases the local stress. Understanding the effective parameters on stress concentration and proper selection of these parameters enables the designer to achieve a reliable design. In this paper, the optimum values of the effective parameters on stress distribution around the cutout are determined by genetic algorithm. The fitness function for a genetic algorithm is defined by generalization of the analytical solution based on Lekhnitskii method for different cutouts. The finite element method is used to check the validity of the obtained fitness function. Also, the genetic algorithm was able to predict the optimal value of each effective parameter on stress distribution while keeping constant the values of the other parameters. The results showed that material properties, geometry of cutout and angle load have much effect on stress concentration.
This paper investigated the use of graphite with different configuration designs to improve the thermal energy storage of phase change material systems. Two types of graphite have been combined with paraffin in order to improve thermal conductivity of phase change material: synthetic graphite (Timrex SFG75) and graphite waste obtained from damaged tubular graphite heat exchangers. Paraffin/graphite phase change material composites have been prepared by the cold uniaxial compression technique. Their morphologies have been observed and analyzed by scanning electron microscope, and their thermophysical properties have been estimated using new experimental tools. Results show that the thermal conductivity and thermal diffusivity can be accurately measured by these new experimental tools. Moreover, results highlight the fact that the phase change material thermal properties are greatly influenced by the graphite addition.
Matrix damage, in the form of cracks parallel to the fiber, is a common failure mode for composite laminates, yet little is known concerning the associated shear modulus degradation. This study proposes a novel experimental technique to study shear modulus degradation under biaxial loading. An Iosipescu coupon was modified that induced transverse tension and compression loads while applying shear loading. The axial loads were induced using unbalanced layups with coupled normal–shear response. An analytical, closed-form solution was generalized to include unbalanced layups under a shear loading boundary condition. It was observed that the shear modulus degradation depends on the normal stress acting on the matrix transverse cracks. While compressive normal stress increased the effective shear modulus, the tension normal stress reduced the shear modulus, compared to pure shear loading.
Polymethyl methacrylate (PMMA)/modified fly ash (MFA) composite particles have been prepared via suspension polymerization. Scanning electron microscopic results show that PMMA/MFA composite particles have perfect sphericity and roundness. Fourier transform infrared and X-ray diffraction analysis demonstrate that MFA powder is integrated with PMMA in the composites. Acid solubility tests illustrate that the particles have a good resistance to acid. In addition, apparent density of the particles (20–40 mesh) is 1.049–1.135 g/cm3 which is much lower than sand and ceramic proppants. Thermodynamic tests reveal that thermostability of the composites is significantly improved with increasing MFA addition content, which proves the particles could be used in deep underground. Furthermore, the crushing rate decreases to 3% with MFA addition content of 50% under the pressure of 69 MPa. Therefore, the composite particles manifest a promising application in shale oil or gas hydraulic fracturing as ultra-lightweight proppants.
The evaluation of damage in multiphase materials plays a crucial role in their safety assessment under service mechanical actions. In this context, the quantification of the damage associated to fibre–matrix detachment is one of the most important aspects to be carried out for short fibre-reinforced materials. In the present article, the problem of progressive fibre–matrix debonding is examined and a mechanics interpretation of such a phenomenon is developed by relating the shear-lag and the fracture mechanics approach in order to determine the fibre–matrix interface characteristics. A multiscale approach is employed: at macroscopic level, composites with dilute dispersed fibres, arranged in a undirectional or in random orientation, are analysed through a homogenization approach, whereas the problem of axisymmetric debond growth in short fibres is examined at microscopic level. Moreover, a ‘structured’ linear elastic interface framework model for crack propagation analysis is applied by defining a microscopic truss structure, enabling to relate each other the classical shear strength approach and the fracture mechanics approach. Finally, a fibre pull-out test and some simple fibre-reinforced structural components are examined. This new proposed point of view on the debonding phenomenon allows a deep understanding of the mechanics of the fibre–matrix interface and enables to characterize such an interface layer that has a relevant role in mechanics design of composites materials.
We examine numerically the uniaxially compressed stability of triaxially woven fabric (TWF) composites employing a proposed geometrically nonlinear finite composite plate element model with volume segmentation ABD constitutive relation, taking advantage of greatly reduced degrees of freedom. From satisfactory agreement with results from literature, numerous boundary conditions are explored for various aspect ratios in the buckling analysis. High dependencies of post-buckling patterns on plate aspect ratios are observed, from which a computationally time-saving characteristic equations have been defined before the occurrence of post-buckling state for practical convenience, best described on the basis of logarithmic critical buckling load and stiffness factor. These buckling characteristics have a direct general correlation to TWF’s aspect ratios and boundary rigidities.
The use of filament-wound composites can be advantageous, especially for designing parts of aircraft structures, because of superior mechanical properties of the composites. On the other hand, various experiments are necessary for studying the environmental resistance of composites rather than that of conventional metals. A hoop ring burst test would be a convenient and reliable method for the evaluation of the environmental resistance of a composite pressure vessel. In this research, a method with 24 split disks was developed for estimating the impact and thermal shock resistance of a carbon-fiber-reinforced composite pressure vessel using hoop ring burst specimens subjected to high-speed impact. Impact tests were conducted with a 12.7-mm diameter ceramic ball at a speed of approximately 40–110 m/s. After the impact test, three cycles of thermal shock were conducted on the half of the specimens. C-scan analysis was conducted to determine whether there was any internal damage after the tests. Finally, the hoop ring burst test using 24 split pressure disks was conducted for modified ring specimens. Through this procedure, the proposed method was successfully verified with a number of tests.
In this investigation, wear and tensile properties of 6061-Al/Al2O3 and 6061-Al/SiC metal matrix composites were studied with respect to their microstructural features and residual stresses. Variation in the residual stress values in the composites differed from each other due to their particulate size and thermal expansion coefficient. Tribological behavior of the composites studied with the aid of nanoscratch tester showed higher amount of debris in 6061-Al/SiC composite than that in 6061-Al/Al2O3 composite due to the presence of higher residual stresses and shorter interparticle distance in the former. Variation of wear volume, coefficient of friction and wear rate at different load were analyzed and correlated with the variation of microstructure along the scratch tracks. It is seen that wear resistance of 6061-Al/Al2O3 is better than that of 6061-Al/SiC composite, where as tensile property revealed that ultimate tensile stress and yield stress is higher in case of 6061-Al/SiC composite compared to 6061-Al/Al2O3. Fractographic studies shows bimodal fracture failure where cleavage type brittle fracture failure is dominant in 6061-Al/Al2O3 whilst ductile failure is dominant in 6061-Al/SiC composite.
A computational constitutive model is presented to predict matrix cracking evolution in laminates under in-plane loading. Transverse cracks are treated as separate discontinuities in the micro-model that provides damage parameters for the macro-model. Both micro- and macro-models are implemented using finite element analysis, specifically, ANSYS, to avoid limitation of analytical micro-modeling. The computational cost of the micro-model is limited to constructing a database of micro-model predictions a priori. The macro-model is simply a finite element analysis discretization of the structure using plane stress or shell elements in ANSYS. The macro-model queries the database, which effectively becomes a constitutive model. The damage surfaces in the database are obtained from the results of large number of finite element micro-scale (unit-cell) analyses. The proposed procedure is implemented in ANSYS as a usermaterial subroutine for transverse crack initiation and propagation in symmetric cross-ply and [0r/( / –)s/0n]s laminates under in-plane loads. This method is also examined to study matrix crack evolution in tensile specimen with open hole, and the results found to be in good agreement with available experimental data.
This paper studies failure of birch pulp–polylactic acid composites. Stiffness and strength are calculated using the theory of short fibre composites and the results are compared to experimental data. The results differed from the experimental values by 0–6%. With less aligned fibres the short fibre theory is not feasible. The performance of the 40 wt% birch pulp – polylactic acid composite is predicted with X-ray microtomography based finite element modelling, and the results are compared with experiments. Stiffness results differed from experiments by 1–17% . By adding into the models a third material phase representing the interface between the fibres and the matrix, the stress–strain curve of the composite was obtained with good accuracy. The work presents finite element modelling methodology of wood plastic composites and the critical further steps needed in order to assess the stress–strain behaviour, strength and stiffness. Tools for comparing different wood plastic composite microstructures are also presented.
The interlaminar shear behavior of AS4/8552 laminated short beam shear (SBS) coupons were experimentally and numerically investigated under static and impact loading conditions. Experiments were conducted in a range from quasi-static (1.7 x 10–5 m/s) to 3.9 m/s impact velocity using a testing device (ILSS device) that has been developed and adapted in a universal testing machine and a drop tower apparatus. Regarding the interlaminar shear strength (ILSS) values, the experimental investigation showed a low to medium strain rate sensitivity with a 23% maximum ILSS decrease observed at the samples which were tested with the maximum impact speed. In the finite element framework, the novel "stacked shell" or "2.5D" approach is investigated in the simulation of the SBS impact tests; models comprising four stacked sublaminate arrangements were capable of predicting the respective experimental results, with the maximum deviations from the respective experimental data to appear in the cases of high impact velocity.
It is a challenge to predict the permeability due to the complex correlation between the permeability and structural parameters of fabric preform. In this paper, the correlation between the in-plane permeability and the structural parameters of the non-crimped fabrics is investigated by using the finite element method. The effect of the intra-ply structural parameters and ply orientations on the in-plane permeability of the fabric preform is investigated. In addition, the influence of four structural parameters on the in-plane permeability is analyzed by the Morris method. The results show that the in-plane permeability is most sensitive to the distance between fiber bundles, while the semi-major axis length of the ellipse section of the fiber bundle is relatively insensitive. The order of the in-plane permeability of non-crimped fabrics preform with different ply orientations is as follows: [0]2 > [0/90] > [–45/45]. The present research is helpful to predict permeability and to further simulate resin filling process.
In this present work, Ni/SiC metal matrix composite coatings were prepared from a modified Watt’s type electrolyte containing nano-SiC suspended particles by direct current plating method to increase the wear resistance of Ni. The influence of surfactant content on codeposition of SiC particles within the matrix and tribological properties were investigated. A wide particle size range (between 0.1 and 1.0 µm) was chosen to provide a high load bearing ability for the codeposited layers. The wear tests were carried out at different sliding speeds by using a constant load. The influence of sliding speed on the tribological performances of the coatings has been investigated by using a reciprocating ball-on disk apparatus. Wear resistances and friction coefficients of Ni/SiC composites were decreased by increasing sliding speed due to temperature-controlled surface oxidation. The change in wear mechanisms by changing surfactant content and sliding speeds were also comprehensively studied.
This study researched the effects of machining parameters on surface roughness and material removal rate in the wire electrical discharge cutting of high-density Al/B4C metal matrix composites produced via the hot pressing method. Wire tension, reinforcement percentage, wire speed, pulse-on time and pulse-off time were set as the control factors. The Taguchi L18 (21 x 34) orthogonal array was used in the experiment design and determination of the optimum control factors. Variance analysis was applied to determine the effects of the control factors on the surface roughness and material removal rate. The results showed the most effective parameters to be pulse-on time (30.22%) for surface roughness and wire speed (83.20%) for material removal rate, and the optimum levels of the control factors to be A2B1C2D1E1 and A2B2C3D2E2, respectively. Predictive equations were then developed by applying linear regression analysis, and the adjusted correlation coefficients were calculated as 0.61 for surface roughness and 0.785 for material removal rate.
This study investigates the effect of nanocarboxylic acrylonitrile butadiene rubber on the tensile fatigue behaviour of carbon fibre-reinforced polymer composites with dicyandiamide-cured epoxy matrix. The stress-controlled tension–tension fatigue behaviour at a stress ratio of R = 0.1 and maximum stresses between 400 MPa and 650 MPa was investigated for the case of carbon fibre-reinforced polymers with pristine and nanorubber-modified epoxy matrices with loadings of 5 phr, 10 phr, 15 phr and 20 phr. The results from the experimental tests show that the high-cycle fatigue life of the laminates with 15 phr of nanorubber-modified resin matrix was increased by a factor of two compared to the pristine matrix samples. Scanning electron microscopy images of the fracture surfaces also show an enhanced plastic deformation existing at the fibre–matrix interface and a lower extent of fibre pull-out; both contributing towards the enhancement of the fatigue performance of the carbon fibre-reinforced polymer composites.
The aim of the work is an assessment of the effect of the deformation (curvature) of reinforcing fibre preform caused by insertion into a mould on the laminate properties. The basis for the assessment is a comparison with an analogous laminate without deformation, i.e., made in a flat mould. The work includes the production of laminate panels by way of resin transfer moulding, on the basis of three types of preforms: plain woven fabric (classic), stitched plain woven fabric and three-dimensional woven fabric, as well as an evaluation of their mechanical properties in static bending tests (in the radial direction of the curved panels) and tensile tests (in the axial direction of the curved panels). The state of the laminate reinforcement structure was assessed by the means of computer tomography. The strength of the curved laminates, both in the radial and in the axial directions, is lower than in case of the equivalent flat laminates. The strength decrease proceeds together with the deformation degree. The stitched and three-dimensional laminates are much less prone to strength loss resulting from deformation than the classic laminate; they also exhibit lesser strength drops in the direction transverse to the stitch lines or the interweave strands than in the parallel direction. The curved classic laminate shows the biggest deviations of the fibre strands from rectilinearity as well as local structure anomalies, in comparison to the two other laminate types. In the curved three-dimensional laminate, the fibre strands are much more uniformly oriented than in the other two.
This paper reports for the first time the damping behavior of Mg-Al85Ti15 metastable composites synthesized using near dense blend-press-microwave sinter-hot extrusion methodology. Optical microscopy results show that the metastable particles were located along the grain boundaries and the formation of twins within the grains. In addition, scanning electron microscopy reveals reasonable distribution of particles, good matrix-particle interfacial bonding and minimal presence of microvoids. The damping test results show an increment in damping capacity and damping loss rate with the presence and increasing amount of particles. The effect of microstructure on damping characteristics of magnesium and damping mechanisms are discussed.
A phenomenological study was carried out on laminated carbon fibre-reinforced polymer composites subjected to constant amplitude fatigue loading. Visualization of damage progression was performed using a high-resolution Skyscan micro-computed tomography unit which provided detailed information on propagation of initially occurring cracks throughout fatigue life at specific intervals. Quantitative analysis of image sequences of virtual cross-sections throughout the three orthogonal planes of the sample resulted in defining fatigue crack growth rates, da/dn for each plane, which was interpreted in terms of the three damage modes: opening (mode I), in-plane shear (mode II) and out-of-plane shear (mode III). By applying linear elastic fracture mechanics laws, strain energy release rates were calculated and then used in a cohesive zone model formulation to define model parameters. Considering a bi-linear triangular cohesive zone model curve, maximum traction and maximum separation were calculated for each of the three damage modes, differentiating between modes II and III in a novel manner.
A monolithic silica glass doped with Pr3+ was prepared by immersing a mesoporous SiO2–polyvinly alcohol (PVA) nanocomposite in a Pr nitrate solution and then sintering it at 1100℃ in air. The sintered glass exhibited pink emission when excited by ultraviolet irradiation. The photoluminescence intensity of the glass depended on the total concentration of Pr ions, which was adjusted by changing the concentration of the Pr nitrate solution and the immersion time. The glass with the highest photoluminescence intensity was prepared by using a 1- or 10-mM Pr solution at an immersion time of 60 min. To reveal how the immersion conditions affected the photoluminescence intensity, we analyzed and discussed the adsorption kinetics of the Pr ions onto the surface of the mesoporous nanocomposite. At immersion times below 60 min, the Pr ions diffused into pores of the nanocomposite without aggregating. To prepare Pr3+-doped silica glass with high photoluminescence, both the concentration of the Pr solution and the immersion time must be controlled.
In this paper, a set of failure criteria for transverse failure in non-crimp fabric-reinforced composites is presented. The proposed failure criteria are physically based and take into account the orthotropic character of non-crimp fabric composites addressing the observed lack of transverse isotropy. Experimental data for transverse loading out-of-plane in combination with in-plane loads are scarce. Therefore, to validate the developed criteria, experimental data are complemented with numerical data from a representative volume element model using a meso-micromechanical approach. The representative volume element model also provides a deeper understanding of how failure occurs in non-crimp fabric composites. Strength predictions from the developed set of failure criteria show good agreement with the experimental and numerical data.
The creation of defects (internal porosity) inside epoxy-based carbon fiber-reinforced composites during the curing process is well known to have a harmful impact on most of the mechanical properties of those materials. One of the main sources of void creation is the moisture absorption of the composite plies during the manufacturing process steps before the composite part cure. In order to get a better control of manufacturing, the moisture absorption behavior of a carbon-epoxy prepreg, which is submitted to a wet environment, is investigated. Moreover, the impact of the resin polymerization state on the water absorption behaviors of the plies will be highlighted. The moisture content is therefore measured as a function of time, temperature and relative humidity for uncured and for different partially polymerized materials. An original model was established in order to predict the moisture content in function of time, temperature, relative humidity and polymerization degree. As a major conclusion, it is demonstrated that the water coefficient of diffusion decreases when the polymerization degree increases.
A specific technical expertise is required to machine metal–matrix composites (MMCs). It is well known, from the literature data, that MMCs cutting involves high costs and high machining times due to excessive tool wear and possible subsurface damage. In recent years, the emphasis of research has focused on understanding the mechanics and mechanisms that are responsible for the behaviour of MMCs during the material removal process. These kinds of studies would be the most effective way of understanding the machining characteristics and deformation behaviour of the material. This work is a contribution to understanding the effects of two antagonistic phenomena that determine MMCs behaviour during the cutting process, such as the thermal softening of the matrix and the matrix hardening due to strain. To this end, a simplified model that describes the behaviour of the reinforcements beneath the machined surface is proposed and experimentally validated.
Vacuum bag-only prepregs enable the out-of-autoclave manufacture of high-performance composite structures and increase the material, part, and process selection space. However, manufacturing choices involve economic as well as technical considerations. To understand these relationships, we developed a technical cost model that captures the distinctive characteristics of vacuum bag-only prepreg processing (including vacuum-induced air evacuation and resin cure) and estimates the costs associated with materials, equipment, and labor. We applied the model to realistic manufacturing cases and used a parametric study to evaluate the effects of part characteristics, material use efficiency, and cure efficiency. The results indicate that prepreg cost, part size, prepreg waste, and the air evacuation capacity of the material have the strongest influence on part costs and demonstrate that cost modeling can guide efforts to improve or optimize processing by identifying the most economically valuable modifications.
The wet pressing process represents a new production method for carbon fibre-reinforced plastics components. Due to the low cycle times, it is suitable for use in the automotive industry. Therefore, a sufficient degree of industrialisation needs to be achieved, which is characterised by a stable process. The knowledge about relevant process parameters, their interactions, and influence on the part quality builds the basis of an economic process. This is a major challenge, since in the early stage of process development the available amount of recorded process data is small and the data sets are not complete. As the implementation of time-, material-, and cost-intensive experiments represents no acceptable alternative, a theoretical approach is chosen. This article describes a theoretical procedure to define the critical factors of the wet pressing process with significantly less resource input.
For the analysis of the wet pressing process, which was presented in the first part of this paper, a theoretical approach was chosen. This enabled the pre-definition of three quality-related priorities which now will be considered in detail in the second part. For further analysis, real process data, recorded in an early phase of the process implementation, are used. The challenge is that in this process status, the availability of data is limited or the data sets are incomplete. Supported by the theoretical approach, an easier interpretation of the process data, and in case of ambiguous issues, an accelerated decision making is expected. The objective is to show that this combination is suitable for the process analysis in an early production phase.
The influence of carbon nanotube dispersion techniques (surfactant-assisted dispersion, ultrasonication and magnetic stirring) and surface functionalization of carbon nanotubes on mechanical properties of nanocomposites fabricated via powder metallurgy techniques is characterized. Functionalized multiwalled carbon nanotubes dispersed using a combination of techniques results in Al/functionalized multiwalled carbon nanotube composites with excellent microstructure and enhanced mechanical properties. Nonfunctionalized carbon nanotubes dispersed using zwitterionic surfactants show good dispersion patterns on Al powder surfaces and results in composites with good strength and relative densities are also obtained. Carbon nanotubes dispersed solely via shear mixing techniques resulted in highly embrittled composites with poor microstructure. Also discussed in this study is the usefulness of scanning electron microscopy in conjunction with image analysis techniques in characterizing porosity in Al/carbon nanotube composites.
The main aim of this study was the modification of jute fibres with stearic acid using new route called azide method. The surface changes after modification process were characterized by attenuated total reflectance – Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy analyses. It was shown that the stearic acid was bonded to JF using azide method and the surface modification through azide method did not deteriorate the surface properties of JF. Representative JF containing low-density polyethylene-based composites were also prepared and characterized by tensile testing, dynamical mechanical analysis and differential scanning calorimeter. According to mechanical test results, the tensile strength and elastic modulus were increased using stearic acid modified JF through azide method. According to differential scanning calorimeter results, surface modification with stearoyl azide reduced the degree of crystallinity with respect to alkaline treated one.
A method for computational description of morphology of dispersive components’ spatial structures in composites has been developed. The method is based on the calculation and comparison average values of the elongation coefficient (Ke), sinuosity coefficient (Ks), and fullness coefficient (Kf) for structures of the dispersed phase. Stationary systems such as composite oligodiene epoxide vulcanizate (PDI 3AK) as matrix with micro-dispersive aluminium powder and composite methyl methacrylate-styrene (PMMAS) as matrix with magnetite Fe3O4 nanoparticles were investigated by SEM and AFM, respectively. Time behavior of micro-dispersive carbon black in dynamic system comprised of polydivinylisoprene oligomer (PDI) as the matrix was studied by optical microscopy. The obtained results by method for computational description of morphology had showed universality of the method and enabled ascertaining quantitative correlations describing shapes of certain particles of the filler and morphology of spatial structure under formation. Dependence of form coefficients on the time was determined. This dependence enables studying kinetics of structure formation.
This paper revisits the problem of free vibration of delaminated composite plates with Lévy type boundary conditions. The governing equations are derived for laminated Kirchhoff plates including through-width delamination. The plate is divided into two subplates in the plane of the delamination. The kinematic continuity of the undelaminated part is established by using the system of exact kinematic conditions. The free vibration analysis of orthotropic simply supported Lévy plates reveals that the delaminated parts are subjected to periodic normal and in-plane shear forces. This effect induces parametric excitation leading to the susceptibility of the plates to dynamic delamination buckling during the vibration. An important aspect is that depending on the vibration mode the internal forces have a two-dimensional distribution in the plane of the delamination. To solve the dynamic stability problem the finite element matrices of the delaminated parts are developed. The distribution of the internal forces in the direction of the delamination front was considered. The mode shapes including a half-wave along the width of the plate accompanied by delamination buckling are shown based on the subsequent superimposition of the buckling eigenshapes. The analysis reveals that the vibration phenomenon is amplitude dependent. Also, the phase plane portraits are created for some chosen cases showing some special trajectories.
The effect of the incorporation of silica nanoparticles on structural relaxation and curing kinetics of unsaturated polyester resin was investigated. The evaluations were made monitoring the curing behavior of the neat unsaturated polyester resin and the resin containing 3 wt% SiO2 nanoparticles using dynamic differential scanning calorimetry at heating rates of 2, 2.5, 3, and 3.5℃ min–1. Results revealed a salient increase in the redox activation energy of the unsaturated polyester resin containing silica, while the activation energy of thermal decomposition of peroxide did not show any change. On the interface of nanoparticles and matrix resin, the dissociation reaction of cobalt naphthenate reduced the effect of the promoter. Differential scanning calorimetry was also used to estimate the average size of cooperative rearrangement region of the nanocomposite. Results revealed the possible grafting of unsaturated polyester onto the nanoparticle surface. The attractive interaction between the matrix and nanoparticle hindered segmental relaxation of the polymer chains.
In this study, the effect of the textile reinforcement type on the flexural response of textile-reinforced concrete beams under static and impact loads was investigated. In addition, to compare the flexural capacities with those of conventional steel-fiber-reinforced concrete, steel-fiber-reinforced concrete beams having similar compressive strength with that of textile-reinforced concrete were fabricated and tested according to the fiber content. Enhancements in the flexural capacities were obtained using polymer-coated textile reinforcement, and three-dimensional textile reinforcement resulted in slightly better flexural performance than two-dimensional textile reinforcement under both static and impact loads. Upon comparison with the results obtained from the steel-fiber-reinforced concrete beams, the textile-reinforced concrete specimen with polymer-coated textile reinforcement exhibited the best flexural performance in terms of the strength, toughness, and residual load carrying capacity (higher than or at least similar to those of the steel-fiber-reinforced concrete with a fiber volume content of 2%), whereas the textile-reinforced concrete specimens with uncoated textile reinforcement exhibited lower strength and toughness than those of the steel-fiber-reinforced concrete with a fiber volume content of 0.5%. Finally, the strain-rate sensitivity of the flexural strength for textile-reinforced concrete was found to be similar to that for steel-fiber-reinforced concrete.
In the present study, six different combinations of pultruded hybrid kenaf/glass composites were fabricated. The number of kenaf and glass rovings was specifically selected to ensure constant local fiber volume fractions in the composites. The volumetric composition of the composites was determined by using a gravimetrically based method. Optical microscopy was used to determine the location of voids. The short-beam test method was used to determine the interlaminar shear strength of the composites, and the failure mode was observed. It was found that the void volume fraction of the composites was increased as a function of the kenaf fiber volume fraction. A linear relationship with high correlation (R2 = 0.95) was established between the two volume fractions. Three types of voids were observed in the core region of the composites (lumen voids, interface voids and impregnation voids). The failure of the samples started with horizontal shear cracks that propagated into the core region, and ultimately the samples failed by a vertical crack. The interlaminar shear strength was found to decrease as a function of the hybrid fiber mixing ratio.
The dynamic fragility concept has been performed on kenaf and jute composites containing different reinforcement contents. Based on the average relaxation times, we have determined, as a function of the type of reinforcement and fiber content, the dynamic fragility concept of composites manufactured by resin transfer molding. It was constated that for these polymeric composite materials, the glass transition temperature is not a so significant feature to evaluate the characteristic parameters of the Williams–Landel–Ferry and Vogel–Fulcher–Tamman equations. The reason is that for up to 30% reinforcement volume, the glass transition values maintain similar values for these composites, whereas mechanical and dynamical mechanical properties shown an increase in the properties by fiber incorporation. Also, the activation energy as the structural relaxation energy would give a more realistic approach of the results found, following the McKenna supposition.
In the present study, an artificial neural network model has been used for predicting the corrosion behaviour, aging and hardness responses of aluminium-based metal matrix composites reinforced with silicon carbide particle. Hyperbolic tangent sigmoid and linear activation functions are employed as the most appropriate activation function for hidden and output layers, respectively. The developed artificial neural network model is used to predict the corrosion current density, peak aging time and peak hardness of the composites. Feed forward back propagation neural network has been trained by Levenberg Marquardt algorithm. The regression correlation coefficients (R2) between the predicted and the experimental values of the corrosion current densities are found as 0.99986, 0.99629 and 0.99671 for the training, testing and validation datasets, respectively. Also, some case studies have been predicted by artificial neural network model. Test results indicate that the proposed network can be used efficiently for the prediction of the polarization response, peak aging time and peak hardness of the composites for different SiC volume fractions and deformation ratio without using any experimental data.
To enhance the mechanical performance and ultraviolet shielding property, graphene oxide was incorporated as the functional nanofiller into a sodium alginate matrix to form a composite film via a solvent-casting method. The as-obtained films were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and thermal gravimetric; their mechanical and ultraviolet blocking properties were also systemically evaluated and compared. The results showed that the maximum tensile strength increased to 84 MPa at 6 wt.% graphene oxide loading, nearly 265% greater than that of pure sodium alginate film. The increased tensile strength may have resulted from the existence of hydrogen bonding and high interfacial adhesion between graphene oxide filler and sodium alginate matrix. Furthermore, the sodium alginate/graphene oxide films also demonstrated robust ultraviolet shielding capacity; the corresponding ultraviolet protection factor reached up to 133.61 at 8 wt.% graphene oxide loading.
The present study intends to examine ratcheting response of SiCP particle-reinforced Al 6061 matrix (Al 6061/SiCP) composite samples over asymmetric load cycles based on the kinematic hardening rule of Ahmadzadeh–Varvani. The Ahmadzadeh–Varvani hardening rule offered a simple framework by taking into account of both material and stress level dependent coefficients to predict the quasi-shakedown of ratcheting of composite samples with various volume fractions under single- and multi-step loading conditions. The coefficients in the Ahmadzadeh–Varvani rule were estimated by means of mathematical expressions involving material properties, mean stress, and stress amplitude for any given stress levels. Ratcheting strain progressively increased as composite materials experienced low–high loading sequences when stress level increased over steps of loading histories. Ratcheting strain curves of low-high and high-low loading histories were successfully predicted. The predicted ratcheting curves with high–low loading sequences have shown change in ratcheting direction consistent with the experimental data.
Low-velocity/low-energy edge impact and quasi-static experiments have been carried out on carbon fiber-reinforced plastic structures. A drop-weight testing machine has been used to impact four different uni-directional laminates at 10, 20 and 35-J impact energy levels. In parallel, a quasi-static study has been carried out to compare its results with the impact ones. The residual behavior will be provided by the compression after impact tests. The impact results show that the static and dynamic behaviors are different. The difference between static and dynamic edge impacts, to understand the impact damage scenario, is explained with the help of an analytical approach irrespective of the stacking or impact energy. This approach provides good results regarding the dynamic and static initial stiffness along with the crushing plateau. It has been observed that the fiber properties control the initial impact stiffness, while in the quasi-static indentation case, the properties of the matrix control the initial indentation stiffness; whereas the crushing plateau is also controlled by the matrix properties.
This paper presents a study and analysis on the thrust force in the drilling of dissimilar materials. The material used is armor steel-glass fiber reinforced plastic (GFRP) composites. The armor steel is sandwiched between the two GFRP layers. Experimental design technique is used for conducting the experiments. The three panels are drilled, and the influences of feed and spindle speed on the thrust force are measured and mathematical models are established between the drilling parameters. The results indicate that the thrust force required is more for the bottom panel than for the top, most probably due to part of the drill is engaged in the armor plate and the heat is also generated because of higher thrust force experienced in drilling the middle layer. The results are discussed and presented in detail.
Before being processed into composites, reinforcement fabrics may undergo repeated involuntary deformation, the complete sequence of which is here referred to as specimen history. To mimic its effect, fabric specimens were subjected to sequences of defined shear operations. For single fabric layers with unconstrained thickness, quantitative evaluation of photographic image data indicated that repeated shear deformation results in a residual increase in inter-yarn gap width. This translates into an increase in measured fabric permeabilities in multi-layer lay-ups at given compaction levels. The extent of both interrelated effects increases with increasing yarn density in the fabric and with increasing maximum angle in the shear history. Additional numerical permeability predictions indicated that the increase in permeability may be partially reversed by through-thickness fabric compression. The observations suggest that the effect of involuntary deformation of the fabric structure can result in variations in the principal permeability values by factors of up to 2.
Three-dimensional fabric composites have evolved as an attractive structural material for multi-directional load bearing and impact application. However, research on prediction of their behavior before being fabricated is inadequate. This article reports a two-step modeling approach for predicting the effective mechanical properties of three-dimensional fabric carbon fiber-reinforced composites. In step one, the micro-heterostructural composites were represented by microscale cylindrical, square, or hexagonal prismatic representative volume elements, containing a long carbon fiber surrounded by polymer matrix, taking into account the transversely isotropic properties of carbon fibers. The mechanical properties of each representative volume element were extracted from the modeling results of uniaxial tensile, lateral expansion, and transverse shear tests, using appropriately derived formulae, and averaged as the equivalent mechanical properties of the micro-heterostructures. In step two, a three-dimensional fabric composite unit cell was represented by a three-dimensional finite element model, taking into account the fiber orientations and fabric architecture. A three-dimensional orthogonal fabric carbon fiber-reinforced epoxy composite was selected as the case study material. The overall orthotropic mechanical properties of the composites were predicted by conducting tensile and shear tests on the unit cell. The modeling results show that the Young’s moduli in fiber oriented directions and shear moduli are significantly higher than those of the matrix. The shear moduli of shearing fiber cross-sections are even much higher than that of shearing along fiber longitudinal direction. Carbon fiber can improve structural stability by lowering the Poisson’s ratios of the composites. The modeling results were compared with and validated by the experimental tests.
A newly developed initiation technique, activators regenerated by electron transfer, was successfully employed to synthesize MCM-41/ polystyrene nanocomposites by in situ atom transfer radical polymerization. Hexagonal structure, surface area, and morphological study of the synthesized MCM-41 nanoparticles were performed by using powder X-ray diffraction, nitrogen adsorption/desorption isotherm, scanning electron microscopy, and transmission electron microscopy respectively. Conversion and molecular weight evaluation was carried out using gas chromatography and size exclusion chromatography respectively. Remarkable decrease of conversion from 69% to 43% was observed by the addition of only 3 wt% MCM-41 nanoparticles. However, polydispersity index increases from 1.14 to 1.41. Increasing of thermal stabilities and also reduction of Tg by increasing MCM-41 nanoparticles loading were comprehended according to the results of thermogravimetric analysis and differential scanning calorimetry, respectively.
Carbon nanofibers (CNFs) with silane coatings were used as the reinforcement to enhance the mechanical and tribological properties of high-density polyethylene (HDPE) at different loading levels (0.5 wt% and 3 wt%). To improve the interfacial bonding between the CNFs and HDPE matrix, two types of silane coating thicknesses, 2.8 nm and 46 nm, were applied onto oxidized CNFs. Mechanical properties of the HDPE/CNF nanocomposites including Young’s modulus, ultimate stress, strain at fracture, as well as work of fracture, were investigated through tensile testing. The wear tests were performed on a pin-on-disk tribometer under the bovine serum lubricated condition. The coefficient of friction of the materials in contact with steel balls was monitored over the duration of the wear test. The addition of CNFs not only decreased the coefficients of friction of the nanocomposites, but also reduced their wear rates. The thicker silane-treated CNFs were found to be more effective in reducing the coefficients of friction and elongating the strain of fracture compared with the pristine CNFs and thinner silane-treated ones. The biocompatibility of the nanocomposites against a mouse osteoblast precursor cell line was also evaluated. Among the several types of nanocomposites, the one reinforced with the thicker silane-treated CNFs (46 nm) at 0.5 wt% loading level yielded the highest strain at fracture, the best wear resistance with a wear rate reduction of nearly 57.9% compared to the neat HDPE, and good biocompatibility, making it a promising material for biomedical applications.
Titanium-coated silicon carbide whiskers were prepared by vacuum slow vapor deposition process. W-20Cu composite powder was fabricated by spray drying and calcining-continuous reduction technology. The paper adopted hot-press sintering to prepare titanium-coated silicon carbide whiskers/W-20Cu composites. Effects of addition amount of titanium-coated silicon carbide whiskers on the phases, microstructures, and properties of the composites were investigated. Titanium-coated silicon carbide whiskers/W-20Cu composites with improved mechanical properties and thermal conductivities were fabricated by hot-press sintering at 1350℃ under a pressure of 30 MPa for 2 h in pure Ar atmosphere. The transverse rupture strength and thermal conductivity of the 0.6 wt% titanium-coated silicon carbide whiskers/W-20Cu composite when compared with W-20Cu-based alloy were enhanced by 22% and 26%, respectively. With reasonable addition amount of titanium-coated silicon carbide whiskers, the composites without graphite, tungsten carbide, and silicon phases were obtained.
Aluminosilicate diphasic gel was synthesized by using inorganic salts and characterized by chemical analysis, Fourier transform infrared spectroscopy spectroscopic studies and surface area and bulk density measurements. The gel powder was compacted with nickel oxide additive in three different ratios (wt/wt). Differential thermal analysis of gel samples with additive and without additive was carried out at four different heating rates and for each sample, the activation energy of mullitization was calculated using Kissinger equation. The microstructure and phase analysis were carried out by scanning electron microscopic and X-ray diffraction studies. Observation showed that nickel oxide favored the mullitization process.
Toner composite is an important element in laser printing and copying processes which is included magnetite, carbon black and binder. The emulsion aggregation is a popular method for toner preparation that allows tight control on toner properties. This study was done to determine the effects of the basic ingredients (magnetite, carbon black, and styrene-acrylic copolymer) that affect the properties of toner composites. Particle size analysis (PSA), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FE-SEM) were used to study particle size, shape, and morphology of the toner composites. It was found that using carbon black, magnetite and styrene-acrylic copolymer with small particle size produced toner composite with small particle size. It was also shown that using a styrene-acrylic copolymer with high viscosity resulted in smaller and narrower particle distribution in the resultant toner. The results determined by SEM confirmed that carbon black, magnetite and styrene-acrylic copolymer particle size did not affect the toner’s shape, and that it remained spherical.
This paper presents the research on fracture failure analysis and failure criterion for PTFE-coated woven fabrics. First, groups of on-axial and off-axial tensile tests are carried out, and the corresponding failure mechanisms are analyzed. Then, the samples with initial defects are tested and the fracture toughness is discussed. Finally, several current strength criteria are compared to predict the failure strength of PTFE-coated woven fabrics and a new strength criterion is proposed. Results show the material failure strength depends on the stress ratio and off-axial angle of sample. The ratio of principal stress decides the material strength, and the angle between the principal stress and yarn orientation decides the failure mode. The predictions of current strength criteria agree well with most of the experiment data, except for the samples of small off-axial angles. The disagreements could be attributed to the unbalanced woven structure due primarily to the crimp interchange in the weaving and coating processes. The new strength criterion can make a good prediction of the failure strength of PTFE-coated woven fabrics.
In the present study, process-induced residual stresses of composite materials have been analysed on the mesolevel of a fabric unit cell. The given fabric was the satin-weave G0926, which is used for example in high-lift application of aerospace structures. In the study, a developed viscoelastic cure-dependent material model is applied to analyse the process-induced deformations of the fabric. In fact of the weave structure, this type of fabric tends to be out-of-plane displacement based on chemical and thermal shrinkage. This effect is studied experimentally and numerically, and the results are compared. The knowledge of the sensitivity of this fabric for out-of-plane displacement is an important factor to create real symmetric laminates. Disrespecting can be the source for large scattering warpage on the component level, and therefore, this awareness of the effect of textile structures on process-induced deformations is one key factor to develop robust processes.
Epoxy/glass fiber/organo-montmorillonite hybrid composites with triaryl phosphates and decabromodiphenyl oxide flame retardants were prepared by vacuum-assisted resin infusion technique. The effects of triaryl phosphates and decabromodiphenyl oxide on the flammability properties of epoxy/glass fiber/organo-montmorillonite composites were evaluated through UL-94 vertical flammability test and limiting oxygen index. Epoxy/glass fiber/organo-montmorillonite hybrid composite showed no rating in UL-94 test up to 40 phr of triaryl phosphates. Field emission scanning electron microscopy showed the amount of char formed increased, denser char structure and better coverage of glass fibers as triaryl phosphates loading increased. Interestingly, the hybridization of 30 phr decabromodiphenyl oxide with respect to 10 phr triaryl phosphates flame retardants showed V-0 rating with limiting oxygen index of 33%.
In natural-fiber-reinforced polymer, absorption of water or moisture is a significant issue in maintaining strength and stiffness. To enhance the understanding of water and moisture sorption behavior, the kinetics of moisture sorption in natural-fiber-reinforced polymers are investigated under immersion conditions. Samples of hemp-bast-fiber-reinforced polyethylene are prepared using an injection molding technique at different hemp fiber volume fractions (vf). The samples are then immersed in water for 274 days. Moisture content and uptake rate are analyzed at different fiber volume factions and matrix crystallinity percentages. A simplified two-dimensional contraction model is developed to investigate the contraction effect on the moisture uptake; it shows that a matrix with high crystallinity has more stiffness contraction on the reinforcing natural fibers, which limits the maximum amount of the absorbed moisture. The Fickian diffusion is found to be the dominant mechanism, shifting toward pseudo-Fickian or anomalous diffusion depending on the natural fiber volume fraction and the crystallinity percentages of the matrix. The natural-fiber-reinforced polymers diffusivity is evaluated and modeled to characterize the ability of liquid molecules to diffuse into these composites at different hemp fiber volume fractions. Both the crystallinity percentage of the matrix material and the volume fraction of the reinforcing fibers were found to interactively affect the sorption kinetics of the tested natural-fiber-reinforced polymers.
Long-term degradation and failure in high-temperature polymer matrix composites are driven by chemical changes due to oxidation reactions and damage evolution. In this paper, we present a methodology for simulating oxidation-induced damage in a unidirectional composite. This approach explicitly models the time-dependent growth of oxidation layers and the evolution of discrete cracking in a homogenized representation of the composite. Long-term isothermal aging is simulated with high-resolution tracking of morphological changes and damage evolution. An element-free Galerkin method is used to simulate the oxidation layer growth, and the extended finite element method is used for computing the stress fields and predicting damage. The developed model captures both oxidation and damage growth in the unidirectional lamina through long periods of oxidative aging. The model predictions correlate well with the experimental results for a carbon/polyimide composite system.
In this paper, a theoretical model to investigate the ballistic impact behavior of two-dimensional woven glass/epoxy/nanoclay nanocomposites is presented, developed on the basis of dividing of the impact duration into several time intervals and calculation of the energy absorbed during each time interval. The major components of energy lost by projectile during ballistic impact were identified, namely the primary yarns tensile failure energy, the secondary yarns deformation energy, the cone kinetic energy formed on the back face of the target, the delamination of layers of nanocomposite and the matrix cracking. Ballistic tests were performed by a flat-ended projectile by a gas gun. Finally, a good correlation has been observed, comparing the theoretical model presented in this paper to the experimental results.
In the present study, the feasibility of joining PA6-matrix thermoplastic composite panel and aluminum alloy is considered. Self-piercing riveting processes of PA6 panels, PA6 composite panels with reinforcing fibers (glass fiber and carbon fiber), and 5754 aluminum alloy sheets are systematically investigated. Macro characterizations of the appearance and transverse sections of the self-piercing riveting joints were conducted to provide more comprehensive understanding of the composite deformation during riveting process. In particular, the effect of reinforcing fibers on the composite deformation behaviors was characterized. Lap-shear testing of the self-piercing riveting joints was performed to evaluate joint strength, and characterization of the fractured joints as well as failure mechanics analysis was conducted to investigate the underlying failure mechanisms of the self-piercing riveting joints. The effects of reinforced fibers on the failure mechanisms and mechanical performance were also discussed. The results showed that self-piercing riveting is an effective technique for joining fiber reinforced thermoplastic composite panels and aluminum alloy sheets. In addition, self-piercing riveting joints with glass fiber reinforced composite failed in a bearing mode and exhibited improved joint strength compared with the PA6 matrix; while self-piercing riveting joints with carbon fiber reinforced composite exhibited slightly decreased joint strength but significantly reduced ductility, owing to the brittleness of the carbon fiber reinforced composite.
The aim of this study was to clarify the effects of acrylamide on mechanical and tribological properties of short carbon fiber–reinforced bisphenol-A-type epoxy composites. The short carbon fiber/epoxy-based composites with or without acrylamide were prepared by solution blending and moderate temperature-curing techniques. Obviously, the short carbon fiber resulted in the enhancement of flexural strength and modulus of the short carbon fiber/epoxy-based composites in contrast to the epoxy-based composites. When the loading-level of short carbon fiber was 8 parts (corresponding to 100 parts of the epoxy matrix), the short carbon fiber 8/epoxy composites containing the same loading-level of acrylamide showed the highest flexural strength and modulus. At the same time, it was found that the addition of acrylamide further improved strength, modulus of the short carbon fiber/epoxy-based composites. Furthermore, the friction coefficient values of the acrylamide-modified short carbon fiber 8/epoxy composites were lower than that without the addition of acrylamide. The worn morphologies observed by scanning electron microscope revealed that the wear mechanism of the acrylamide-modified short carbon fiber 8/epoxy composites was fatigue wear.
In this article, the dimensional stability of epoxy- and cyanate-based laminates is discussed, focusing on the thermal deformation, moisture-induced deformation, and deformation induced by relaxation of thermal residual stress. Each of the deformations was calculated independently based on the laminate theory. The material properties of the unidirectional laminates were obtained by conducting thermal mechanical analysis, moisture absorption tests, and tensile creep tests. These material properties were adopted to the laminate theory to predict the deformation of quasi-isotropic laminate, and it was calculated that each type of deformation induced micron-level dimensional instability. The moisture-induced deformation was an order of magnitude larger than that caused by the other factors. The validity of the calculations was confirmed by comparing the calculated results with the experimental ones. It is important to control moisture absorption even if cyanate resin, which has high moisture resistance, is used.
A small amount of commercial functional nanosilica was mechanically mixed with epoxy to enhance the composite fracture toughness. Nanosilicas with amino and epoxide functional groups show strong interfaces with epoxy, which suppress large aggregations and enhance resin-wettability, hence enhancing the fracture toughness of epoxy composites. Compared with other reports, less nanosilica content was needed to achieve the same fracture toughness values or similar enhancement ratio. Due to their commercial availability, the low-cost of the raw material and simple fabrication method, those nanocomposites have the potential for large-scale applications.
A highly stable and uniform dispersion of multiwall carbon nanotubes in aqueous solution was achieved to prepare multiwall carbon nanotubes/gelatin–polyvinyl alcohol nanocomposites with varying multiwall carbon nanotubes content using solution casting. Optical microscopic images confirmed the homogenous dispersion and distribution of multiwall carbon nanotubes in solution and polymer matrix. The tensile strength and tensile modulus of the composite containing 1% multiwall carbon nanotubes (wt/wt) were found to increase by 128.1 and 284.8% compared to that of the gelatin–polyvinyl alcohol blend. Dynamic mechanical analysis revealed an increase in the storage modulus and a decrease in the loss factor (tan delta) for the composite. Electrical properties exhibited a typical percolation behavior when a small amount of multiwall carbon nanotubes (0.1 wt%) was loaded. X-ray diffraction showed that incorporation of multiwall carbon nanotubes increased the crystallinity. Scanning electron microscopy also showed a homogeneous distribution of multiwall carbon nanotubes in the composite matrix. The nanocomposites were further characterized by Fourier transform infrared (FTIR), thermo-gravimetric analysis, and differential scanning calorimetry.
The present review focuses on the properties and developing applications of carboxylated styrene butadiene rubber (XSBR) as a modified grade of styrene butadiene rubber (SBR). Carboxylated polymers, and particularly carboxylated rubbers, are regarded as a new class of polymers with superior physical and mechanical properties over non-carboxylated counterparts. Upon introducing carboxyl groups, the properties such as elasticity range, strength, compatibility towards functional fillers and polymers, resistance to hydrocarbon solvents enhance and cross-linking by non-sulfur reagents becomes possible. The XSBR latex benefits from excellent mechanical and chemical stability, excellent liquidity, high bond and conjunction strength, high toughness and the strength after molding, low water absorption and air shrinkage, excellent durability, abrasion, oil and corrosion resistance of cement products. Due to the polarity offered by carboxyl groups, fiber, and particulate composites of XSBR exhibit improved physical and mechanical properties. The stable interface created between carboxyl group and the second polymer phase generally results in an enhancement for the host polymer.
In this work, manufacturing steps of composites were simultaneously analyzed to optimize four mechanical properties (flexural and tensile moduli, impact strength, and tensile stress at yield) of flax fiber/postconsumer recycled plastic composite. Eight parameters of the extrusion-injection process (extrusion: temperature profile and screw speed; injection: temperature profile in the barrel, mold temperature, injection speed, injection pressure, injection time, and back pressure) were selected. Process optimization, taking into account simultaneously all the mechanical properties (multi-responses optimization), required four steps: determination of influential factors by a screening design and an evaluation of the selected factors effects on the mechanical properties, modeling of the relationships between mechanical properties and significant factors by a Box–Behnken experimental design and a multiple linear regression analysis, identification of the potentially optimum conditions using the desirability function approach (Derringer–Suich model), and determination the optimum composite manufacturing conditions by a comparative analysis of the material relative qualities.
The development of variable angle tow technology has attracted growing attention in recent years due to its strong potential for structural tailoring. However, the full details of the failure mechanisms of variable angle tow laminates have been as yet unknown, and the design complexity also requires use of numerical analysis and novel techniques for variable angle tow composites. This paper addresses the two main problems for use of variable angle tow laminates in design. Firstly, a mathematical model is presented to build a three-dimensional variable angle tow model which exactly captures the features of as manufactured variable angle tow laminates. Secondly, impact and compression after impact models using three-dimensional detailed finite element analyses are presented to predict the failure behaviour of variable angle tow laminates including delamination evolution and crack propagation. Results obtained from the impact and compression after impact models are validated against experimental data.
Modification of lignocellulose materials, used as fillers in the composites with polyolefins, is applied to improve their adhesion to the matrix. One of the most often applied methods of such modification is the treatment with organic acid anhydrides. Rapeseed straw was modified with anhydrides of acetic, maleic and succinic acids. Such a modification changes the straw surface leading to the exposition of the wood tissue skeleton. The character changes depending on the type of anhydride applied. Esterification of repeseed straw by organic acid anhydrides resulted in changes in its chemical structure. According to infrared analysis of modified straw, new carbonyl groups were formed, as indicated by the absorption band in the range 1750–1730 cm–1. The degree of straw modification, measured by the weight percent gain index, informs about similar reactivities of the lignocelluloses material with all three anhydrides of organic acids used as modifiers. The starting temperatures of active thermolysis for the straw modified with maleic and succinic acid anhydrides were lower than that for native straw, while that for the straw modified with acetic acid was higher. Concentration of free radicals in rapeseed straw samples was measured by electron paramagnetic resonance spectroscopy. It was found that the maximum concentration of radicals for rapeseed straw was treated with maleic anhydride.
We report on the pioneering application of inkjet printing in depositing patterned thermoplastic microphases between composite plies and the beneficial effect of the printed thermoplastic on the interlaminar fracture toughness of carbon fibre-reinforced polymer laminates. Double-cantilever beam test and short-beam shear test were employed to investigate the mechanical performances of the engineered composites. The results from this work revealed that by printing thermoplastic poly(ethylene glycol) and poly(methyl methacrylate) between the carbon fibre-reinforced polymer plies, mode I interlaminar fracture toughness (GIc) is noticeably enhanced, whilst the shear strength has also been preserved. Scanning electron microscopy was used to investigate the fracture surfaces generated during the double-cantilever beam test. The microscopic addition of the thermoplastic polymers (approximately 0.015 wt%) did not increase the weight of the composites significantly, which compares favourably to other conventional toughening methods.
Compressive strength, micro-hardness indentation, friction, and wear tests of the poly(methyl methacrylate)/hydroxyapatite nanocomposite have been performed and evaluated in order to study the potential application of this nanocomposite in the field of bone tissue regeneration. PMMA/HA nanocomposite in the form of bulk has been successfully prepared by a simple mixture between PMMA and HA nanoparticles. The compressive strength value of PMMA/HA nanocomposite was found to be ~90 MPa; whereas the compressive Young’s modulus was 840 MPa, and the micro-hardness obtained was 55 MPa. According to the results, PMMA/HA nanocomposite exhibit an acceptable elastic modulus for its possible consideration in the tissue engineering. The wear rate value of the nanocomposite PMMA/HA was found to be at the same magnitude for its similar applied as coating reported previously, while friction coefficients showed higher values.
The objective of this study was to investigate the effects of core material and its thickness on impact behavior of sandwich composite plates subjected to low-velocity impact, experimentally. Poly(vinyl chloride) (PVC) and poly(ethylene terephthalate) (PET) foams were selected as the core material, having approximate density of 65 kg/m3 and 60 kg/m3, respectively, and thicknesses of 5, 10, and 15 mm. The stacking sequence of the sandwich composites is
Mechanical response of nano-based composites is generally influenced by interaction of filler and matrix at interface. Increasing filler-loading within the composite may cause spatial limitation toward best dispersion of filler, and since synthesizing a totally agglomerated-free nanocomposite is difficult, filler and matrix interaction needs to be perfectly modeled. A micromechanical model is developed in this study based on the common Halpin–Tsai theory to predict the elastic stiffness of vinyl ester/exfoliated graphite platelet nanocomposites. The model considers near-rational ideal (uniformly dispersed) mixed with clustered filler-network to simulate filler-distribution conditions. A filler-dispersion level based on the filler concentration has been proposed mathematically in this study. Predictions of the proposed model considering filler morphology were compared with the predictions of the Halpin–Tsai model and the experimentally obtained results as well. The proposed model shows better accuracy in terms of stiffness over predictions of the Halpin–Tsai model and appears in a very good agreement with the experimental results obtained for vinyl ester nanocomposites.
Biocomposites of recycled high-density polyethylene (rHDPE)/recycled polyethylene terephthalate (rPET) matrices with a high loading of rice husk flour (RHF) were fabricated through a two-step extrusion. The use of ethylene-glycidyl methacrylate (E-GMA) copolymer improved the compatibility of the immiscible rHDPE/rPET blend. Maleic anhydride polyethylene (MAPE) was used as a coupling agent to increase the adhesion of the fibre–matrix interface. In this study, the effect of natural fibre loadings on rHDPE/rPET blends was examined. The water absorption process in the RHF-filled composites followed the kinetics and mechanisms of Fickian diffusion. Compared with samples without RHF, the rHDPE/rPET/RHF system had 58–172% higher tensile modulus and 80–305% flexural modulus. The thermal stability of the composites slightly increased with the addition of the RHF filler. The storage modulus of biocomposites was greatly enhanced by RHF. From these results, we can conclude that RHF can work well with rHDPE/rPET for manufacturing high loading biocomposite products.
The compressive behaviour of epoxy based syntactic foams filled by ceramic microballoons is experimentally investigated in this study. Nine different types of syntactic foams are fabricated with three different microballoon sizes and three different microballoon fractions. All of the syntactic foam specimens are tested at various strain rates from quasi-static to high strain rates. Analysis of the results is carried out on the effect of the volume fraction, microballoon size and strain rate on the compressive behaviour of syntactic foams. Also, scanning electron microscopy is used to understand the fracture mechanisms of tested specimens. The results show that as the microballoon volume fraction increases the compressive strength, compressive modulus, failure strain and plateau stress decreases for all types of syntactic foams at all strain rates. Although, this decrease is slight for 20% and 40% volume fraction, it is considerable for 60% microballoon volume fraction syntactic foam. The results indicate that reducing the microballoon size or increasing the strain rate of testing would enhance the compressive strength.
Evaluating post-impact failure responses of single-lap adhesively bonded composite-to-composite joints in uniaxial static tensile loading was the main objective of the current experimental study. At first, axial tensile impacts having various energy levels (10, 15, 20, and 25 J) were applied to the joints at different temperatures (–20, 0, room temperature, 50, and 80℃). Afterward, the samples were secondarily subjected to static tensile loading at ambient temperature, so that reductions in joint strengths arising from the impacts performed under different loading conditions could be assessed. Consequently, it was definitely proved that each of the axial impacts performed in any loading case has a noticeable effect on ultimate joint strengths, proportionally to the acting condition. Besides, the combination of high energy and temperature sometimes appeared to be the reason of impact failure, which occurs instantly without being able to perform axial static tests. When applied energy and/or absolute difference from room temperature is increased, lower joint resistances could be measured during secondary tensile tests.
The hole wall defects created during drilling of carbon fiber reinforced polymer laminates are analyzed. First, an analysis of the location of the defects on the wall is performed. It is shown that, using results of orthogonal cutting, it is possible to predict the location of the main defects. Then refined scanning electron microscopic observation shows the different patterns of the defects. These observations raise the question of the quantification and measurement of the quality of holes drilled in composite laminates. Two roughness parameters, Ra and the bearing surface are compared and significant differences are found. This study is a contribution to a better definition of quality indicators for machined surfaces in composite structures, which should help to limit overquality and production costs.
Multi-layered glass and carbon-reinforced polymer composites may exhibit unique properties comparatively with the benchmark, proven they are being tailored bounded by several requirements. The paper herein approaches issues on the influence of the various contents and orientation of UD carbon fibre constitutive on the mechanical, dynamical and thermal expansion if embedded along with glass fibres in different stacking sequencing within an unsaturated polymer resin. The results show that the architectures with the highest content of carbon fibres (e.g. GF:CF(60:40) – 0° and 90°) provide the best tensile and flexural properties, and behave better under dynamical loading conditions and temperature variations, no matter the orientation directions. In addition, it was shown that a thorough understanding can be attained, with respect to the UD carbon fibre content, and different orientations influence on the overall composite material properties, taking into account the data retrieved from dynamical and thermal expansion runs.
Current energy absorbers in industrial applications are made of metals or fiber-reinforced polymers using glass and carbon fibers. These materials are extremely stiff and strong but exhibit low-energy absorption when subject to the impact load. Other issues in the use of these materials are their high cost (fiber-reinforced polymer) and weight (metal). Wood reinforcements on the other hand are light weight and economic but less stiff. This study investigated the impact resistance and fracture patterns of wood-reinforced polyester composites using a drop-weigh impact test and considers the potential of using wood as a natural reinforcement in the manufacturing of polymer composites. Densified and un-densified Douglas-fir veneers were used to create three different mat configurations: woven, cross, and unwoven (unidirectional) mats. A total of 350 specimens were tested following ASTM D5420, and their impact resistance was calculated using the staircase method. Scanning electron microscopy was used to examine the resin distribution and its penetration into the reinforcement. Additionally, light micrographs of the veneers before and after densification were examined to determine the effect of densification on the cell-wall structure. Glass fiber-reinforced polymer samples had significantly higher impact resistance than the wood composites. Densification of the veneer did not significantly improve the composite performance. The effect of reinforcement configuration on the final performance of the wood–polyester composites, however, was significant with the woven, and cross configurations having notably higher impact energy than unidirectional composites.
Polymer-layered silicate nanocomposites containing polypropylene, compatibilisers, muscovite, and organomuscovite were prepared through the hot compression technique. Muscovite particles were initially treated with lithium nitrate, followed by cetyltrimethylammonium bromide, to obtain the organomuscovite through an ion-exchange treatment. Maleic anhydride and polyhedral oligomeric silsesquioxane were used as compatibilisers to modify the polypropylene/organomuscovite nanocomposite systems. The effectiveness of the ion-exchange treatment and the implementations of both compatibilisers were successfully proven using the Fourier transforms infrared, X-ray diffraction, as well as the transmission electron microscopy. The overall thermal properties of the polypropylene/muscovite layered silicate composites were investigated by applying both differential scanning calorimetry and thermogravimetric analysis. It was found that the polypropylene reinforced with organomuscovite recorded higher thermal properties in terms of crystallisation temperature and thermal stability than its counterpart; whereas, the melting temperature and the degree of crystallinity displayed an opposite trend. For the effect of the compatibiliser, it was found that compatibilised polypropylene/organomuscovite nanocomposites recorded higher values of decomposition and crystallisation temperatures than that of non-compatibilised polypropylene/organomuscovite nanocomposites. However, the melting temperature and the degree of crystallinity of compatibilised polypropylene/organomuscovite nanocomposites were slightly lower, as compared to non-compatibilised polypropylene/organomuscovite nanocomposites.
This paper contains details of mechanical testing of double cantilever beam composite specimens comprising a vacuum-assisted resin transfer molding–infused parent plate and a hand laid-up upper plate simulating a typical blade repair. The testing was carried out as part of an effort to optimize a new repair resin for a wind turbine blade manufacturing company. A number of commercial suppliers were contacted and nine resins with different formulations were selected for screening. Out of these, six were shortlisted for initial testing which was carried out in accordance with ASTM D 5528 (Test Method for Mode I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites) using double cantilever beam specimens. The fracture toughness values obtained from testing these repair resin candidates were compared to values obtained from the repair resin currently being used and also to those of the main blade resin. The top three repair resin candidates were identified from the double cantilever beam testing.
The knowledge of the through thickness permeability is important for the design of modern liquid composite molding processes. In through thickness permeability measurements, the textile undergoes flow-induced compaction, changing the distribution of fiber volume content. As a consequence, results of permeability measurements show a dependency on the applied injection pressure, further denoted as apparent permeability. In the study presented in this paper, saturated permeability measurements were conducted for different fiber volume contents, varying the injection pressure. At the lowest fiber volume content used, the apparent permeability strongly varied with pressure. With increasing fiber volume content the dependency on injection pressure decreased, but it was not negligible. We used a simulation model for coupling of flow and fiber deformation in liquid composite molding in order to predict the apparent permeability in saturated through thickness permeability measurements. The input data for the simulation model were determined by measurements of saturated permeability, using a very low injection pressure, and textile compaction properties. With the proposed methodology, a good agreement of simulation and experimental results was achieved.
Tensile behaviors of aluminum/carbon fiber reinforced polymer hybrid composites with different carbon fiber reinforced polymer stacking sequences were measured at strain rates between 0.001/s and 100/s and properties of the hybrid composites were compared to the results of aluminum and carbon fiber reinforced polymer tested under the same conditions. In the aluminum specimen, negative strain rate sensitivity resulted in a significant decrease in the tensile strength and positive strain rate sensitivity resulted in an increase in failure strain at higher strain rates. However, in the hybrid composite specimen, both the tensile strength and the failure strain could be increased as the strain rate increased by reinforcing the aluminum with carbon fiber reinforced polymer. The differences in tensile properties and the degree of strain rate sensitivity were extensive depending on the stacking sequence of the carbon fiber reinforced polymer layer. A Weibull function with scale parameter 0 and shape parameter β was used in order to describe the relation between stress and strain of aluminum/carbon fiber reinforced polymer hybrid composites, and the constitutive equation established by the Weibull function described the tensile behavior of the hybrid composites very well.
This study deals with the determination of the response of woven glass-epoxy composites to the single and repeated impact loading at room temperature and at –50℃. Single impacts were performed at various energies up to perforation took place and the characteristics such as contact force, contact duration, deflection, and absorbed energy were obtained. In addition, some low energy values were selected for the repeated impact tests. The maximum contact force and absorbed energy versus repeat numbers were given. It was found that temperature is extremely important for determining the response of the composite subjected to single and repeated impact loadings.
This paper presents the simultaneous effect of alkali activator and water/slag cement ratios on composites properties by full replacement of Portland cement at different ages under steam curing condition. Twelve mortar composites divided into four different groups were prepared using municipal slag cement and alkali activator of sodium meta-silicate by varying the ratios of alkali activator/slag cement and water/slag cement. The results revealed that the addition of municipal slag cement by full replacement of Portland cement was largely worthwhile for slag mortar composites under steam curing condition. The preparation of the mortars by taking various ratios of alkali activator/slag cement and water/slag cement contributed significantly in improving the compressive strength with an acceptable drying shrinkage to slag composites. Scanning electron microscopic technique was used to analyze the microstructure of slag composites at different ages. The ACI 209 and EC 2 design equations were used to predict the compressive strength.
In this study, the effects of two variable parameters namely nanoclay and coupling agent on the durability against fungal attack and water absorption of wood plastic composites (WPCs) were investigated. Composites based on high density polyethylene (HDPE), sanding dusts (SDs), and nanoclay (montmorillonite type) were made by melt compounding and then hot-pressing. Maleic anhydride grafted polyethylene (MAPE) was used as coupling agent. Treated and untreated (control) composites were exposed to the fungal decay using white-rot fungus (Trametes versicolor) for 4, 8, and 12 weeks according to the modified ASTM D 1413 standard. The experimental results indicated that the exposure of the composites to a five-cycle of boiling and drying caused serious damage to the interfacial adhesion between SDs and HDPE matrix due to contraction and swelling stresses developed during the cyclic immersion. The composites filled with nanoclay were more resistant than the control sample to decay. However, based on statistical analysis significant differences in water absorption of the composites were not observed after addition of nanoclay. In general, fungal degradation and water absorption were significantly decreased by the incorporation of MAPE. However, addition of 6 wt% MAPE improved both properties more than 3 wt% MAPE. Morphological studies showed fungal colonization and some micro voids, which accelerate water absorption. In conclusion, this work has shown that addition of nanoclay and coupling agent can considerably decrease fungal decay and water absorption of WPCs.
Fabrication of nanocomposite polymer-blend electrolytes comprising polyethylene oxide, polyvinyl chloride, lithium perchlorate and titanium dioxide (TiO2) (27.03 nm) has been demonstrated using the famous solvent casting technique. AC impedance measurements were carried out in the frequency range of 1 Hz–10 MHz. The effect of filler on the ionic conductivity is explained on the basis of distribution of TiO2, Lewis acid–base interaction, polymer segment-ion coupling and aggloramation of TiO2. Fourier transform infrared spectroscopy spectra showed the occurrence of complexation and interaction among the components. The crystalline and amorphous phases of the complex were identified through X-ray diffraction. Atomic force microscope pictures of the samples confirmed the plasticizing action of nano TiO2.
Central composite designed experiments are conducted to study the independent effects of maleated polyethylene, dicumyl peroxide and nanoclay in the forms of natural (Cloisite® Na+) and masterbatch (Nanoblend™ concentrates MB2001) on the mechanical properties of fiber reinforced PE composites with different fiber levels. The optimum values are predicted on the results of designed experiments and there are linear regressions between fiber content and their mechanical properties. The deposition and formation of nanoclay particles in PE composites are ascertained by scanning electron microscope and transmission electron microscope observations. Both natural nanoclay and MB2001 can be delaminated and even exfoliated in polarized PE matrix. As wood fiber is introduced, natural nanoclay particles (nanoclay-natural) are deposited on fiber surface even loaded in fiber lumens, but the nanoclay particles of MB2001 (nanoclay-concentrate) are mostly dispersed in the matrix. In addition, the different reinforcements between nanoclay-natural and nanoclay-concentrates are also investigated to find out the influence of particle formation on the quality of composite materials.
This paper investigates the effects of hybrid interphase region on the calibration factors of the hole-drilling method for orthotropic composites. For this purpose, a three-phase composite has been considered which includes fiber, matrix as well as the interphase between them. By employing the available analytical predictions for elastic properties of the interphase and based on micromechanical equations, the mechanical properties of the three-phase orthotropic composite can be obtained. These properties are applied to the available exact solution in order to determine the calibration factors for the three-phase orthotropic plate. Four different composites have been considered in order to study the interphase influences on the calibration factors matrix. Analytical results show that for carbon/epoxy composite, bonding conditions affect all of the calibration factors importantly, while for boron/epoxy, glass/epoxy, and aramid/epoxy composites, some of these factors are not sensitive to interphase thickness. Sometimes, bonding conditions change some of the calibration factors considerably between 30 and 50%. Consequently, when a central hole-drilling experiment is performed in an orthotropic layer, the precision of residual stresses measurement is identically dependent on the elastic properties and the thickness of the hybrid interphase.
Steel fibers, with their high stiffness and high ductility, have a potential to provide a new range of properties in polymer composites, in comparison with carbon and glass fiber composites. However, the high stiffness contrast between the steel fiber and the polymer matrix plus the fiber’s non-circular cross-section are likely to generate high stress concentrations in a composite under transverse loading. In the present study, these stress concentrations are analyzed using finite element modeling and compared with the case of carbon and glass fiber composites. The study is performed for an isolated fiber and multiple fibers in hexagonal and random packings with 40% and 60% of fiber volume fractions. According to the results, in spite of a high contrast between the stiffness values of steel and glass fibers, no significant difference between the transverse stress concentrations was observed for steel and glass fibers in the hexagonal packing due to the difference in material properties. Differences in stress concentrations were noted for the case of randomly packed fibers. The polygonal cross-section of steel fibers was found to introduce extreme stress concentrations.
This study develops a fiber-optic-based technique for in situ characterization of direction-dependent cure-induced shrinkage in thermoset fiber-reinforced composites. A procedure is established to embed fiber Bragg grating (FBG) sensors in composite out-of-plane directions and to measure key through-thickness chemical cure shrinkage directly under practical curing conditions. First, sensitivity of the proposed method is evaluated through comparison with a standard technique (i.e. thermo-mechanical analysis (TMA)), and the effect of sensor tail length on measurement sensitivity is discussed considering shear-lag effect. Next, combined with double-sided vacuum bagging and demolding during curing, FBG sensors embedded in through-thickness and in-plane directions clarify direction-dependent cure-induced shrinkage in autoclaved unidirectional carbon/epoxy. Finally, the feasibility of characterizing through-thickness shear strain, which is important in complex-shaped parts but cannot be measured using conventional techniques, is confirmed. The developed technique will be a powerful tool for evaluating cure shrinkage in complex-shaped parts and for validating process-simulation tools based on internal strain.
In this paper, we look at the shear-out failure of carbon fiber reinforced plastics connections in the automotive industry. Contrary to the aircraft industry, the boundary conditions of automotive applications favor this failure mode strongly. Moreover, the use of other joining technologies than that used in the aircraft industry, such as joining by forming, leads to new challenges. The different influences, typical for joining by forming, on ultimate shear-out strength were first investigated separately and then transferred and validated on connections related to praxis by an analytical model. Special attention was given to effects that resulted from oversized pre-holes, acting clamping forces, and the reduced quality of the laminates in the immediate vicinity of the joint due to the joining process.
The effect of stress raisers in the form of a slit-like notch and an open circular hole on the tensile strength of a quasi-UD flax-fiber-reinforced composite is studied experimentally. A finite fracture mechanics approach is applied to determine the intralaminar fracture toughness of the composite and to predict the strength in the presence of stress concentration. Reasonably good agreement of the notch effect predicted using finite fracture mechanics with a coupled strength and toughness fracture criterion and test results is demonstrated.
In this experimental study, the performance of injection-molded short flax and hemp fibers in plasticized starch acetate were analyzed in terms of strength. Parameters involved in the analysis are a variable fiber and plasticizer content. The measured strength of the composites varies in the range of 12–51 MPa for flax fibers and 11–42 MPa for hemp fibers, which is significantly higher than the properties of the unreinforced starch acetate matrix. The micro-structural parameters used in modeling of composite strength were obtained from optical observations and indirect measurements. Some of these parameters were qualitatively verified by X-ray microtomography.
In this paper, the ultimate bearing strengths of pin-loaded double shear and T-bolt loaded connections were studied in thick composites, where the diameter of the pin equates to the thickness of the laminate. These bearing strengths were obtained for E-glass/epoxy laminates of [(±45, 03)n ,±45] and a Vf of 54%. It is found that the values for ultimate bearing failure and first non-linearity of pin-loaded connections should be reduced by 25% and 38%, respectively, when applied to T-Bolt connections. The failure modes prior to ultimate failure were primarily dominated by fibre matrix shear-out and delamination. As far as laminates with specific reinforcement architecture and a large percentage of reinforcement orientated to the load axis are concerned, the long-term service life of T-bolt connections may be impacted due to the visible onset of damage at a similar level to that accepted by Germanischer Lloyd for load introduction zones.
Multiwalled carbon nanotubes (MWCNTs) were modified to covalently attach the carboxylic moiety with their surfaces. Variant concentrations of functionalized multiwalled carbon nanotubes (F-MWCNTs) were introduced into polydimethylsiloxane (PDMS) adopting solution mixing technique. Fourier transform infrared spectroscopy (FTIR) confirms the carboxy functionalization presence on the surface of the nanotubes. X-ray diffraction (XRD) patterns for both MWCNTs and F-MWCNTs illustrate that the crystallinity does not alter with surface modification of the nanotubes. Experimental results simulated that electrical conductivity of the nanocomposites was augmented with increasing filler concentration in the host matrix. Thermal conductivity and thermal impedance of the nanocomposite specimens were evaluated according to developed methodologies and the accumulative data revealed the nanocomposites thermal transport dependence on the F-MWCNTs doping concentration in the host polymer matrix. Thermal stability enhancement with increasing filler incorporation into the polymer matrix was observed in thermogravimetric/differential thermal analyzer (TG/DTA) contours. Crystallization, glass transition, and melting temperatures were examined using differential scanning calorimeter (DSC) and it was observed that phase transition temperatures of the composite specimens can be tuned by varying the nanotubes to matrix ratio. Scanning electron microscopy and energy dispersive x-ray spectroscopy were carried out to analyze the surface morphology/composition of the fabricated nanocomposites and dispersion of functionalized and pristine MWCNTs in the polymer matrix.
This study contributes to the understanding of the mechanism behind thermo chemical aspects related to the resin transfer moulding manufacturing process of a composite part. The aim is to comprehend the phenomena, to identify related parameters and to get knowledge-based methods for the process development. Therefore, the first part of this study is an experimental study about the behaviour of material properties during the manufacturing process of the single component and the composite. It concludes with constitutive equations for single-process parameters and their associated homogenisation approach for the composite properties. During the manufacturing process, material values of the matrix are changing and influenced by a high number of effects. In the second part, a simulation strategy is been derived. This developed material model integrates a dependency of the time–temperature–polymerisation and fibre volume content. The model is validated in a test case of a manufacturing process of an aircraft component, a fuselage frame using online cure monitoring.
The use of polymer–matrix composites in structural applications necessitates certain degree of machining operations to meet the final product integrity. Drilling is an essential machining operation being used in composite industries for making of holes for bolted joints. The conventional drilling, which is frequently used for making holes in polymer–matrix composite parts, is not convenient anymore because of plethora of challenges encountered. The major drawback is the drilling-induced damage, which mainly occurs due to the direct interaction between the tool and composite laminate. Therefore, there exists a research opportunity to develop cost-effective high-quality machining methods for composite laminates. In the present research endeavor, rotary-mode ultrasonic drilling process has been conceptualized and developed for the drilling of fiber-reinforced polymer composites. The influence of various process parameters including power rating, slurry concentration, and abrasive size on material-removal rate, tool wear rate, and average surface roughness (Ra) has been experimentally investigated. It has been observed that the entry and exit delamination is prevented, and hole circumferential edge quality is improved when holes are produced through rotary-mode ultrasonic drilling as compared to the conventional drilling. It has also been found that with substantial modification in the conventional ultrasonic drilling process, the drilling performance in terms of material-removal rate of glass-epoxy laminates can be significantly improved. The major contribution of the present research endeavor is the development of a novel method of making clean-cut damage-free holes in fiber-reinforced composite laminates.
We report an efficient and novel method to functionalize graphene oxide nanosheets with hyperbranched polysiloxane and successfully compound them with dicyclopentadiene bisphenol dicyanate ester resin to prepare nanocomposites. Fourier transform infrared spectra were used to examine the surface functionalization of graphene oxide. The effects of functionalized graphene oxide on the mechanical, thermal, dielectric and water-resistant properties of dicyclopentadiene bisphenol dicyanate ester resin were investigated systematically. Results of differential scanning calorimetry (DSC) show that the addition of modified graphene oxide can facilitate the curing reaction of dicyclopentadiene bisphenol dicyanate ester. Compared with pure dicyclopentadiene bisphenol dicyanate ester resin, the impact and flexural strengths of the nanocomposite materials are improved markedly with up to 60% and 47% increasing magnitude, respectively. Meanwhile, the modified graphene oxide/dicyclopentadiene bisphenol dicyanate ester systems show lower and more stable dielectric constant and loss than pure dicyclopentadiene bisphenol dicyanate ester resin over the testing frequency from 10 to 60 MHz. In addition, the thermal stability and moisture resistance of modified graphene oxide/dicyclopentadiene bisphenol dicyanate ester nanocomposties are also superior to that of pure dicyclopentadiene bisphenol dicyanate ester resin.
Biodegradable poly(butylene succinate)/organo-montmorillonite nanocomposites were prepared at different organo-montmorillonite loadings, using maleic anhydride-grafted poly(butylene succinate) as compatibilizer. Poly(butylene succinate) nanocomposites were exposed to outdoor natural weathering for 180 days. Weight loss and decrease in mechanical properties after weathering revealed the degradation of poly(butylene succinate). Natural weathering caused photo-oxidation on poly(butylene succinate), leading to the formation of degraded products, as manifested in Fourier transform infrared spectroscopy. Gel permeation chromatography showed a significant reduction in molecular weight after weathering. It was noted that poly(butylene succinate) nanocomposite exhibited lower degradability as compared to neat poly(butylene succinate), due to the enhanced barrier properties after the addition of organo-montmorillonite. However, the incorporation of maleic anhydride-grafted poly(butylene succinate) increased the degradability. Degree of crystallinity of poly(butylene succinate) reduced after weathering, as shown in differential scanning calorimetry. Scanning electron microscopy analysis revealed fungal and bacterial colonization on the sample surface. In addition, the isolation and identification of bacterial strain were also performed.
An experimental and numerical study has been conducted to evaluate the underwater blast response of E-Glass/epoxy composite plates with polyurea coatings. The goal of the study is to determine the effects of these elastomeric coatings on the dynamic response of the plates, specifically the influence of coating thickness, location, and plate natural frequency. The composite material is a 0°/90° biaxial layup and the coatings are applied to either the loaded or non-loaded faces. A conical shock tube facility which produces shock loading conditions representative of the underwater detonation of an explosive charge is used to impart the shock loading to the plates during the experiments. The transient response of the plates is recorded using a three-dimensional (3D) digital image correlation system, consisting of high-speed photography and specialized post processing software. Computational models of the experiments are developed using the LS-DYNA finite element code. The simulations are shown to have a high level of correlation to the experimental data in terms of center point displacements and full field deformation profiles. Additional parametric studies using the correlated model show that the transient response of the composite plates is improved with increasing coating thickness, and that polyurea coatings located on the back face of the panels provide better performance than when located on the loaded surface.
Glass fiber polymer composites are used in wind turbine blades because of their high-specific strength and stiffness, good fatigue properties, and low cost. The wind industry is moving offshore to satisfy economies of scale with larger turbines. High humidity in this environment degrades mechanical performance of wind turbine blades over their lifetime. Here, environmental moisture conditions were simulated by immersing glass fiber-reinforced polymer specimens in salt water for a period of up to 8 years. The mechanical properties of specimens were analyzed before and after immersion to evaluate the degradation mechanisms. Single-fiber tensile testing was also performed at different moisture conditions. The water-diffusion mechanism was studied to quantify the diffusion coefficients as a function of salt concentration, sample geometry, and fiber direction. Three degradation mechanisms were observed: polymer plasticization, fiber stress corrosion, and interface degradation, where the latter was found to be the most detrimental for wind-industry applications.
Compressive high-strain rate behavior of thermoset epoxy resin dispersed with multi-walled carbon nanotubes is presented. The present study investigates the effect of strain rate as well as effect of multi-walled carbon nanotube dispersion on the compressive stress–compressive strain behavior of thermoset epoxy resin. The amount of multi-walled carbon nanotubes dispersion was varied up to 3% by weight. The studies were carried out on compressive split Hopkinson pressure bar apparatus in the strain rate range of 800–3700/s. Processing of multi-walled carbon nanotubes-dispersed thermoset epoxy resin, testing, damage, and energy absorbing mechanisms and key findings are reported. It is observed that the compressive strength of thermoset epoxy resin increases up to 147% at high-strain rate compared with that at quasi-static loading. The corresponding increase for multi-walled carbon nanotubes-dispersed thermoset epoxy resin is up to 140%. It is also observed that the compressive strength and energy-absorption capability of thermoset epoxy resin increase with the addition of multi-walled carbon nanotubes.
A simple method of compressive strength measurement of a continuously pultruded unidirectional glass-vinyl ester rod is developed. Measured compressive strength was quite high. Compared to a unidirectional composite laminate of the same composition, compressive strength values were doubled. The study shows that glass fiber composites have the potential for high compressive strength. This potential is reached with excellent fiber alignment and suitable matrix characteristics. Results of both laminate and pultruded composites are shown to be consistent with model predictions, with the primary delineating factor being fiber alignment.
This study describes an experimental investigation of the degradation of the tensile properties of basalt fibers and epoxy-based composites in various corrosive environments, including alkaline, acid, salt and water solutions, and clarifies the corresponding degradation mechanisms. Carbon and glass fibers and their composites are adopted as references. Accelerated experiments were conducted at temperatures of 25℃ and 55℃ and the variation in tensile properties was studied by means of tension testing, mass loss weighing, scanning electron microscope imaging and energy spectrum analysis. The experimental results show that basalt fibers posses relatively strong resistance to water and salt corrosion, moderate resistance to acid corrosion and severe degradation in an alkaline solution. The tensile properties of basalt FRP composites are much better than those of basalt fibers. The degradation mechanism of basalt fibers involves damage by etching in salt, water and alkaline solutions and by change in the chemical composites in an acid solution. The fracture properties of basalt FRP composites are controlled by the failure of corroded interfaces between the fibers and the resin, making the interface the critical factor, rather than the fiber itself.
This study addresses the effects of impact velocity (impact energy) and particle volume fraction and particle size on the impact behavior of particle-reinforced metal matrix composites (Al 6061/SiC). Their effects on the contact force and plastic dissipation histories, the residual stress, and plastic strain distributions were also analyzed. The impact velocity and particle volume fraction and particle size were found to have a significant effect. The contact forces and durations increased significantly with increasing impact velocity. The predicted peak contact forces were in minimal errors for lower impact velocities, whereas the predicted impact durations were slightly longer. The composite structures become stiffer when the particle volume fraction is increased. Consequently, the contact force was increased, whereas the impact durations were shortened. However, a larger particle size resulted in lower contact forces, but longer impact durations. Particle reinforced composites can dissipate more kinetic energy in case the particle volume fraction decreases and the particle size increases. Increasing impact velocity and particle volume fraction increased residual stress levels and the plastic strain. Increasing the particle size resulted in the residual stress distributions to become non-uniform but increases in the plastic deformations.
In the present investigations, LM13 aluminium alloy reinforced with 15 and 20wt.% rutile mineral of fine (50–75 µm) and coarse (106–125 µm) size range was prepared through stir casting technique. The microhardness on different phase of the composite was measured to check the interfacial bonding of particles with the base material. The wear properties of the samples were studied using pin-on-disc tribometer at high load (49 N) with variation in temperatures from 50℃ to 300℃. Wear results indicated that the composites containing fine size reinforced particles showed around two times higher wear resistance over a wide range of temperature than the composite-containing coarse particles. A transition in wear mode from mild to severe was observed above 200℃. Wear track and wear debris were analysed to understand the nature of wear.
Effect of amino-functionalized multi-walled carbon nanotubes (NH2-MWCNTs) on the tensile performance of epoxy and E-glass/epoxy composites was investigated. Low weight percentages (0.3 and 0.4 wt%) of NH2-MWCNTs were dispersed into a DGEBA epoxy resin using combination of sonication and three-roll milling methods. Composites with plain weave E-glass fabrics were fabricated by compression molding process. Tensile test results showed a significant enhancement in strength, modulus and toughness of epoxy and E-glass/epoxy composites at 0.3 wt% loading of NH2-MWCNTs. Micrographs of NH2-MWCNTs-incorporated epoxy and E-glass/epoxy composites revealed better dispersion of nanotubes in epoxy, better bonding between nanotubes and polymer and improved interfacial adhesion between fiber/matrix at 0.3 wt% loading. Micromechanical models were used to predict the tensile properties and compared with the experimental results. An improved dispersion and hence an enhanced crosslink interaction between NH2-MWCNTs and epoxy lead to the improvements in the tensile properties of the epoxy nanocomposites close to predicted values at 0.3 wt% loading. A similar rationale applies for the increase in properties of E-glass/epoxy nanocomposites.
Oil and gas pipelines are susceptible to various forms of damage where repair mechanisms have since been developed for rehabilitation. Once installed, structural health monitoring often incurs excessive costs and defects rectification of the repaired pipeline becomes a major challenge. Finite element analysis (FEA) provides a rapid route to predict the behaviour of the rehabilitated pipelines under prescribed in-service conditions. In this paper, burst strength of a commercially available composite sleeve repair system, Helicoid Epoxy Sleeve (HES)™, which combines the use of carbon fibre strip and epoxy grout as reinforcement to damaged pipe, was investigated through experimental and numerical approaches. Design standards for subsea pipelines are used in the calculation of design pressure and burst pressure. In the experimental setup, API 5L X52 steel pipe was machined with 50% metal loss defect in wall thickness to simulate external corrosion. Results from design calculations, experimental and FEA showed good correlation with margin less than 10%.
Due to the complexity of composite material, numerical methods are generally utilized in their analysis and design. Commercial finite element (FE) codes, such as ANSYS and ABAQUS, allow the implementation of user subroutines in the program, which provides the advantage of using high meshing and solving technologies besides the improvement of materials and/or elements models. Nonlinearities arise for many engineering problems, for example, the progressive damage of a composite element that contains sources of stress concentration or damage localization such as holes, bolts, and/or flaws causes nonlinear material behavior. In order to simulate this nonlinear behavior, especially in 3D, an accurate material constitutive model is required. Therefore, the objective of this paper was to simulate the 3D progressive damage model of composite materials by using simple numerical models. In this paper, ANSYS user subroutine (USERMAT) was used to simulate the progressive damage behavior of a composite plate containing holes using simple models. Three different material models were used: ply discount model (PDM), simple progressive damage model (SPDM) by adding an empirical progressive damage criteria to the PDM, and continuum damage mechanics model (CDMM). Good agreements were observed between SPDM, CDMM, and published experimental results. Furthermore, CDMM showed the least dependence on mesh size. Three different damage evolution laws, linear, quadratic, and degradation laws, were adjusted and tested. It was found that there was no significant difference in the predicted failure load between these selected laws.
We present a composite material consisting of a thermoplastic base material and embedded, networked sensing, actuation, and control to vary its stiffness locally based on computational logic. A polycaprolactone grid provides stiffness at room temperature. Each polycaprolactone element within the grid is equipped with a dedicated heating element, thermistor, and networked microcontroller that can drive the element to a desired temperature/stiffness. We present experimental results using a 4 x 1 grid that can assume different global conformations under the influence of gravity by simply changing the local stiffness of individual parts. We describe the composite structure and its manufacturing, the principles behind variable stiffness control using Joule heating, local sliding mode control of each polycaprolactone bar’s temperature and function, and limitations of the embedded multi-hop communication system. The function of the local temperature controller is evaluated experimentally.
Reinforcing polymers with the appropriate nanofillers is an effective way to obtain a variety of enhanced material properties. In this paper, high-density polyethylene nanocomposites reinforced with either pristine or silane-treated carbon nanofibers at various weight percentages (0.5 wt%, 1 wt%, and 3 wt%) were fabricated through melt-mixing and compressive processing. Silane coatings with two thicknesses, 2.8 nm and 46 nm, were applied on the oxidized carbon nanofibers to improve the interfacial bonding between the carbon nanofibers and the matrix. Scanning electron microscopy and transmission electron microscopy demonstrated the dispersion of carbon nanofibers and the strongly improved interfacial adhesion between the carbon nanofibers and high-density polyethylene matrix due to the silane coating. The thermal properties of high-density polyethylene / carbon nanofiber nanocomposites were characterized and compared with those of the neat high-density polyethylene. The measurement results showed that the thermal conductivity of the high-density polyethylene /carbon nanofiber nanocomposites increased with the carbon nanofiber loading. The enhancement of thermal conductivity was not only due to the high thermal conductivity of carbon nanofibers but also due to the interfacial quality between the carbon nanofibers and the high-density polyethylene matrix. The interfacial thermal contact resistance between the carbon nanofibers and the matrix was determined to be in the range of
Low-velocity impact testing was conducted on carbon/epoxy composite laminates exposed to hygrothermal environments. Besides normal laminates, staggered lay-up structures were made from unidirectional prepreg materials. The specimens were immersed in water at 80℃ for different durations before impact testing. Experimental data showed that moisture played a positive role by improving the impact resistance of composite laminates. Laminates with higher moisture level could behave elastically up to higher strain levels and retain their resistance to the striking impactor to larger deflections. After absorbing moisture, more of the impact energy was dissipated by the specimen through elastic deformation and less damage was induced to the laminate. Little degradation in the fiber/matrix interface was observed from SEM graphs. It is postulated that absorbed moisture improved the ductility of the epoxy resin by promoting chain segmental mobility of the polymer molecules, which eventually lead to the better impact response of laminates with higher moisture content.
This paper reports compression properties of three-dimensional carbon fiber/epoxy braided composites at temperatures from 23℃ to 210℃ under strain rate from 1200 s–1 to 2400 s–1. It was found that the elevated temperature has a negative effect on the compression properties, whereas the strain rate effect is positive. The compression modulus has a rapid degradation at the temperature of 120℃ which is close to the glass transition temperature of epoxy resin obtained from the dynamic mechanical analysis. The results also showed that the shear deformation is the main failure mode of three-dimensional braided composites at high strain rates of compression load and high temperatures.
Impact on glass fibre reinforced (GFRE) pipes, produced by filament winding, was experimentally and numerically tested. The influence of ring stiffness and impact energy on the residual structural strength was evaluated by testing resistance against implosion due to external hydrostatic pressure. An advanced 3-D finite element (FE) model, based on the combined use of interlaminar and intralaminar damage models, was used for simulating impact events. Puck and Hashin failure theories were used to evaluate the intralaminar damages (fibre failure and matrix cracks). Cohesive theory, by mean of cohesive elements, was used for modelling delamination onset and propagation. Material data for the models were based on commonly measured ply properties. The numerical simulations were able to accurately predict the impact forces and damage development of the experimental impact events. Pipes with high ring stiffness have high resistance to external pressure, but they were found to develop more impact damage and have subsequently less resistance to external pressure when damaged.
Among the mechanical properties of polymer-matrix composite materials, the interlaminar tensile strength is among the most difficult to characterize. ASTM Standard D 6415 uses a curved-beam configuration for measuring interlaminar tensile strength. Not only the manufacturing process to produce curved-beam coupons with uniform radius and thickness could be challenging but also the curved-beam strength data typically exhibits large scatter. One question is whether ASTM D 6415 curved-beam interlaminar tensile strength data are coupon-specific, that is the curved-beam strength is not really a coupon-independent material property, suggesting that ASTM D 6415 is not adequate to measure interlaminar tensile strength. The objective of this work is to develop efficient and accurate methods to capture interlaminar tensile strength of composites. The authors expand a recently developed short-beam method coupled with the digital image correlation full-field deformation measurement technique to measuring the interlaminar tensile strength. The interlaminar tensile strength data are presented for IM7/8552 tape composite system. However, average curved-beam strength value is significantly lower compared to the short-beam test results. Micro-focus CT measurements show that porosity in the radius area is the reason for the low average strength value and the large scatter in the curved-beam strength test data. Once the stress concentration effects of porosity are captured through transfer of CT measurements into three-dimensional finite element model, the short-beam and the curved-beam test results agree. The short-beam method, which measures the interlaminar tensile strength for a pristine material, and the refined curved-beam method which accounts for manufacturing defects, represent more complete interlaminar tensile strength assessment methodology for composite structural designs.
The aim of this study is to investigate the effect of low-velocity multiple impacts at ambient (35°C) and elevated temperature (65°C and 85°C) on unidirectional glass fiber reinforced plastic (GFRP) composites. Low-velocity repeated impact tests were conducted using a falling weight tower at a constant velocity of 1.5 m/s. The dominant parameters such as energy, contact force, and deflection were recorded during multiple impacts. The residual strength of laminates following repeated impact was evaluated by conducting three-point bending tests with acoustic emission (AE) real time monitoring. The temperature was revealed to play a key role in the impact response of composite materials, especially due to the progressive softening of the epoxy matrix. The nature and extent of damage during multiple impacts at ambient and elevated temperatures was investigated using real time AE monitoring: this analysis indicated delamination as a predominant failure mode, whose extent and criticality depended on temperature and number of impact events.
An experimental investigation aimed at identifying the presence and onset of an asymptotic stable fracture region in the mode II fatigue delamination growth behaviour of composites is presented. The study is motivated by the possibility that experimental data sets, particularly those obtained at high R-ratios, may unknowingly contain data points within the asymptotic stable fracture region that influence its perceived log-linear behaviour and the fit of various log-linear delamination growth models. Results from the experimental investigation indicate that the asymptotic stable fracture region can extend to GIImax values as low as 0.7GIIc . The implications of this result on characterizing and fitting of various delamination growth models (including models assuming log-linear behaviour) to delamination growth behaviour are discussed.
Shell waste has the potential to be used as a bio-filler. In this work, the commercial calcium carbonate and furfural modified clam shell were used as fillers in polypropylene. Both fillers were characterized and analyzed by X-ray diffraction, scanning electron microscopy equipped with an energy dispersive spectrometer, particle size analyzer, Fourier transformed infrared spectroscopy, and contact angle measurement. The mechanical and thermal properties of unfilled polypropylene and polypropylene composites were investigated as well. X-ray diffraction and energy dispersive spectrometer analysis indicated that the major phase of calcium carbonate was calcite; of modified clam shell, calcite and aragonite. The calcium carbonate displayed a cubic-like morphology with a smaller particle size as compared with that of modified clam shell. The contact angle measurements indicated that calcium carbonate was fully hydrophobic, while modified clam shell was amphiphilic. Thermal gravimetric analyses confirmed the reinforcement effects of both calcium carbonate and modified clam shell in polypropylene composites. Mechanical property studies showed that the inclusion of modified clam shell played the role mainly of toughening the polypropylene; of calcium carbonate, that of reinforcing, with a nonsignificant toughening effect. The optimal filler ratio of modified clam shell could reach 15 wt.%, as compared with 10 wt.% for calcium carbonate, making it possible for substituting calcium carbonate in polypropylene.
This paper devotes to predicting and analyzing the fluctuation of the thrust force during drilling of unidirectional carbon fiber reinforced plastics (UD-CFRPs). The fluctuation properties could be evaluated by the average value, maximum value, minimum value, change frequency and the amplitude of thrust force. Firstly, an orthogonal cutting model is introduced, and the model is modified in order to reveal how the drilling parameters affect the fluctuation. Secondly, the cutting lips are assumed to conduct orthogonal cutting, and the thrust force generated on cutting lips is calculated by using the orthogonal cutting model. Thirdly, the variable parameter, which leads to the fluctuation of thrust force, involved in the analytical model is analyzed separately. Lastly, experiments were conducted to determine the coefficients and testify the model. The comparison between the theoretical values and the experimental results prove that the analytical model can predict the fluctuation of thrust force well.
The coupling efficacies of four silanes, i.e. n-propyl-trimethoxysilane, -amino-propyltrimethoxysilane, -methacryloxypropyl trimethoxy silane, and allyltrimethoxysilane, were investigated in wood flour-filled polypropylene or polyethylene composites. Compared to untreated composites, treatment of wood flour with these silanes alone did not cause any improvement in the mechanical strength of the ensuing composites. In the presence of dicumyl peroxide, composites treated with -methacryloxypropyl trimethoxy silane and allyltrimethoxysilane exhibited an increase up to 90, 60, and 50% in flexural, tensile, and impact strength, respectively. Moreover, creep deformation was inhibited during the creep recovery test. The improvement was obviously greater in polyethylene-based composites than in polypropylene-based composites. This study demonstrates that the coupling efficacy of silane depends not only on the establishment of covalent bonds between silane and the matrix, but also on the chemical structure of the matrix.
Blends of agave fibres with wool waste, pineapple leaf fibres and polypropylene fibres were manufactured by needle-punching technique. Composites were prepared with polypropylene matrix by the process of compression moulding. The effects of blend nonwovens on the mechanical and dynamic mechanical properties of composites were investigated. Composites containing agave-polypropylene (A-PP) nonwovens exhibited superior mechanical properties compared to the other two. Storage modulus of the composites was found to be maximum for agave–pineapple (A-PALF) composites due to the increased stiffness of the composites. Damping was found to be decreased with incorporation of agave nonwovens. As part of product development, parcel trays for automotives were developed from A-PP nonwovens by compression moulding technique. This study looks at the effective utilization of plant and animal fibre waste in composites.
The aim of the present study is to investigate the effect of alumina particle size and its amount on relative density, mechanical properties, and fracture behavior of Al - Al2 O3 composite. To manufacture micro and nano-composites, alumina particles with various sizes of 50 µm, 10 µm, and 20 nm were separately milled with aluminum powders, and subsequently different volume fractions of milled powders were injected by argon gas into molten alloy and incorporated into A356 matrix by a mechanical stirrer (vortex method). Composites were fabricated at various casting temperatures, viz. 750, 800, and 900°C. Microstructural characterizations revealed that the dendrite size of the Aluminum matrix nano composite (AMNCs) is smaller than that of the non-reinforced alloy. The Scanning electron microscope (SEM) micrographs revealed that the Al2 O3 particles were surrounded by silicon eutectic and inclined to move toward inter-dendritic regions. Nano-particles were dispersed uniformly in the matrix when the volume fraction of nano-particles in the composite was less than 3.5 vol.%. The porosity content of the composites increased with increasing volume fraction and decreasing particle size. Also, hardness and tensile strength of the composites improved with decreasing particle size and increasing reinforcement content. The significant improvements in hardness and tensile strength were respectively attained in the nano-composites, reinforced with 1.5 and 2.5 vol.% Al2 O3 nano-particles. Alumina particle cracking was observed in the fracture surface of the micro-composites. Agglomerated nano-particles were observed on dendrites in the fracture surface of nano-composites.
This investigation describes the fracture mechanism which explains a significant part of massive failures due to the existence of a sand layer placed near the neutral axis in the core making the composite very sensitive to impacts in fibreglass-reinforced polyester pipes. These failures create interface delamination, and consequently fluid can leak into supporting sand backfill thereby initiating the fracture process that can, at times include resin hydrolysis. In order to assess the delamination magnitude, an analytic method is developed and a squared root law between delamination and energy applied proposed. Vertical blunt ram tests on samples extracted from complete pipes have been carried out to verify this theory, reaching a goodness of fit up to 92%. It is concluded that low-energy impacts, around 90–160 J in 800–1000 diameter PN 16–20 continuous filament winding pipes, can seriously compromise their structural integrity with no external trace.
Damage development due to impact needs to be understood to evaluate the consequences of impact on composite structures. This study concentrates on modelling and measuring damage development due to low velocity impact on thick industrial composites made from glass fibre epoxy by vacuum-assisted resin infusion. Cross-plied laminates were tested with different impact energy and different number of interfaces (clustering). Results were compared to a 3D finite element analysis. Interfaces and their damage development were modelled with cohesive elements. Intra ply properties were modelled by progressive failure analysis. Many elements and large memory use were needed to obtain sufficient modelling accuracy. However, all input parameters of the model were based on widely available and independently obtained material properties. Impact force and time to initiate damage and maximum force were measured and related to impact energy and clustering. Damage development was monitored optically in the translucent material for all test cases. The results show that the numerical model using only simple and independently measured material data was able to predict the impact behaviour for the different energies and different stacking sequences.
Due to the heterogeneous nature and electric anisotropy, it is challenging to establish a numerical model to analyze the electromagnetic properties of multilayer carbon fiber-reinforced polymer (CFRP) laminate. In this study, we focus on the exploration of an effective electromagnetic modeling approach for calculation of eddy currents in CFRP laminate composite, as well as eddy current testing signals due to surface cracks. In order to prove the feasibility of modeling CFRP laminate with homogeneous anisotropic layers, the electrical parameters in the three directions are measured, and eddy current path in CFRP is investigated according to the measurement results. A finite element solver based on reduced magnetic vector potential ( A r ) formulation and edge elements is developed to enable the eddy current simulation of anisotropic CFRP material, which avoids matching the discretization of source coils with the rest of conductor mesh, and can easily solve the field continuity problem in the interface between two adjacent fiber layers of CFRP laminate. The A r formulation and way to calculate the eddy current testing signals are described. To validate of the developed simulation code, a comparison is conducted between the calculated signals and experimental results of thin-opening cracks in a CFRP test piece, which indicates the simulation code can predict eddy current testing signals with good precision.
Due to their high specific stiffness, carbon-filled epoxy composites can be used in structural components in aircraft. Graphene nanoplatelets are short stacks of individual layers of graphite that are a newly developed, lower cost material that often increases the composite tensile modulus. In this work, researchers fabricated neat aerospace epoxy (EPON 862 with Curing Agent W) and 1 to 6 wt% of two different types of graphene nanoplatelets (XG Sciences xGnP®-M-5 and xGnP®-C-300) in epoxy composites. These materials were tested for tensile properties using typical macroscopic measurements. In addition, nanoindentation was used to determine modulus and creep compliance. The macroscopic measurements showed that the tensile modulus increased from 2.72 GPa for the neat epoxy to 3.35 GPa for 6 wt% (3.7 vol%) xGnP®-M-5/epoxy composite and 3.10 GPa for 6 wt% (3.7 vol%) xGnP®-C-300/epoxy composite. The modulus results from nanoindentation followed this same trend. xGnP®-C-300/epoxy composites had higher tensile strength and ductility compared to similar loading levels of xGnP®-M-5/epoxy composites. The creep compliance for the neat epoxy, 1 to 6 wt% xGnP®-M-5/epoxy composites, and 1 to 6 wt% xGnP®-C-300/epoxy composites were similar. The two dimensional randomly oriented filler Halpin-Tsai model adjusted for platelet filler shape predicts the tensile modulus well for the xGnP®-M-5/epoxy composites and the three-dimensional randomly oriented filler Halpin-Tsai model works well for the xGnP®-C-300/epoxy composites. Per the authors’ knowledge, mechanical properties and modeling for xGnP®-M-5 and xGnP®-C-300 in this epoxy system has never been reported in the open literature.
In this work, the thermomechanical viscoelastic response of a high temperature polymer matrix composite system made up of T650-35 graphite fibers embedded in PMR-15 resin is studied through a micromechanical model based on the assumptions of simplified unit cell method within a temperature range of 250–300°C corresponding to aerospace engine applications. The advantage of this particular micromechanical model lies in its ability to give closed-form expressions for the effective viscoelastic response of unidirectional composites as well as each of their constituents. Using the experimental data of the creep behavior of thermostable PMR-15 polyimide, the micromechanical model is first calibrated to account for the effect of temperature. The resulting elastic and viscoelastic responses are found to be in good agreement with the existing experimental data. The validated model is then used to predict the behavior of the composite material under different combinations of thermal and mechanical loadings. The results clearly demonstrate the importance of accounting for the viscoelastic effect of the matrix material as the temperature increases. Current works on modeling temperature-dependent viscoelastic behavior of polymer matrix composites are mainly based on the assumption of thermorheologically simple material. However, through the present approach where the matrix is modeled as a thermorheologically complex material, the effect of temperature on the elastic and viscoelastic response of the composite system can be individually investigated.
This study deals with the computational study of asymmetric glass reinforced plastic beams in off-axis four-point bending and the comparison of the induced results with experimental and analytical results. The measurement of the interlaminar shear strength of composite beams, an important design variable in many applications, may be successfully performed by the asymmetric bending test. A three-dimensional finite element analysis is adopted throughout the composite beams in order to, on the one hand, correlate with the experimental results and, on the other hand, to obtain the stress distributions at the supports and at the loading points where usually there is an abrupt variation due to the indentation existing because of the noses. From the finite element analysis and the experimental investigation possible crack initiation positions are determined.
Glass fiber reinforced polymeric composites are unique and unlike conventional construction materials (steel, timber, concrete), inadequate design standards are holding back the high volume use of fiber reinforced polymer composites in construction. To alleviate part of this difficulty, this paper investigates several column failure prediction models. Composite material properties were developed by testing coupons and 0.30 m long components under compression. Furthermore, a strain energy density failure model developed at West Virginia University in tension and bending was extended to axial compression, predicting the critical buckling load within 10% of experimental failure for different column lengths tested (1.83 m, 2.60 m, 2.75 m, 3.05 m). In conjunction with failure analyses, load–deflection effects enhanced by eccentricity due to loading and initial out-of-straightness were also investigated. The current research found that the preliminary design limit of column height (h/220) is acceptable under axial loads; however, manufacturing related imperfection limit state of h/700 is slightly low.
Although composite materials have numerous advantages, some disadvantages, including high manufacturing costs, are relevant. In particular, if the material is applied to large structural components, such as the wings, flaps or fuselage of an airplane, efficient manufacturing processes are required to generate products that are both high quality and cost effective. Therefore, monolithic designs often become integral due to the lower overall part count and simplified designs (e.g. reducing the number of joints and fasteners significantly). For highly integrated monolithic structures, developing a robust manufacturing process to produce high quality structures is a major challenge. An integral structure must conform to the tolerance requirements because those requirements may change. Process-induced deformations may be an important risk factor for these types of structures in the context of the required tolerances, manufacturing costs and process time. Manufacturing process simulations are essential when predicting distortion and residual stresses. This study presents a simulation method for analysing process-induced deformations on the structure of a composite multispar flap. The warpage depends on the thermal expansion and shrinkage of the resin. In this study, a sequentially coupled thermo-mechanical analysis of the process will be used to analyse temperature distribution, curing evolution, distortion and residual stresses of 7.5 m long composite part.
In this study, a comprehensive comparison was drawn between a single impact with certain amount of energy and two impacts with various amounts of energy, the sum of which is the same as the amount of energy in the single impact. A rational plan for repeated impact tests on fiber metal laminates (FMLs) has been designed so as to assess the effect of impact energy division and its sequence, by varying the energy amount in each of the two impacts, on FMLs under repeated low-velocity impact. The total impact energy is estimated by a quasi-static punch test to use a reasonable amount of energy in the repeated impact tests. The impacts are conducted by a drop hammer machine at the same point. Impact parameters and failure modes for all the repeated impact tests are discussed. The results show that the FML specimen responds stiffer in second impact; to understand the reason of this behavior, impact tests on composite specimens were conducted, and comparison of the results of FML and composite specimens showed that the stiffer behavior is due to the elastic–plastic behavior of aluminum sheets which causes strain hardening and creates a dent. Additionally, the effect of impact energy division and its sequence are so influential that failure modes and impact parameters of each arrangement are considerably different from one another.
This investigation explores the manipulation of carbon black particles for tailoring the electrical properties of unidirectional glass fiber reinforced epoxy composites. Carbon black particles were anisotropically networked along the through-thickness direction of glass/epoxy composite plates using an alternating current electric field applied during curing of the composite, with the objective of maximizing the electrical conductivity through the thickness. Anisotropic networking was observed microscopically and was quantified by measuring the DC electrical conductivity of the cured glass/epoxy composite material in the three principal directions. The effects of carbon black amount, electric field strength, and electric field frequency on the anisotropic conductivity are elucidated using a parametric investigation. It is shown that the through-thickness conductivity can be increased by a factor of roughly 104 relative to the case with no conductivity tailoring and can be of the same order of magnitude as the transverse and longitudinal in-plane conductivities, which are improvements well beyond the studies published until now. Moreover, for the first time, it is shown that the through-thickness conductivity of unidirectional glass/epoxy composites containing carbon black can exceed the in-plane transverse conductivity by selecting appropriate electric field parameters during processing.
This research aims to characterize the damping properties of fiber/epoxy composites containing different degrees of silica nanoparticles and rubber particles. Conventionally, adding rubber particles into fiber/epoxy composites would lead to dramatic reduction of stiffness although the vibration damping could be improved accordingly. In order to enhance the damping properties of the fiber composites without sacrificing the stiffness, silica nanoparticles together with rubber particles were introduced into the epoxy resin through the sonication process. The epoxy resin was then treated as matrix and impregnated into the fiber layer by means of the vacuum hand lay-up process to form the composite laminates. The vibration damping as well as the flexural stiffness of the fiber composite was measured using the forced vibration technique together with the half-power method. In addition, the vibration damping of the composite laminates, consisting of silica nanoparticles and rubber particles, was characterized using the micromechanical analysis. The repeated unit cell with the fibers displaying randomly in the matrix was employed to represent the microstructures of the unidirectional composites. The loss factor as well as the moduli obtained from the micromechanical analysis were regarded as the effective properties homogenizing within the fiber composites. In conjunction with the modal shapes, the vibration damping of the composite laminates with stacking sequence of [0]10, [90]10, [±45]2s, and [90/0]2s was calculated using the finite element analysis. Experimental results indicated that with the incorporation of the silica nanoparticles together with the rubber particles, the reduction of flexural stiffness of fiber composites, especially for the [90]10 laminates, was diminished while the damping properties of laminates were improved. Moreover, it was found that the effect of the particles in the [0]10 laminates is relatively minimal. The vibration damping responses of composites laminates obtained using the micromechanical analysis together with the modal analysis exhibit an agreement with the experimental data.
Conventional methods of analysis for drilling of composite materials usually study the amount of damaged area, thrust force, and effective parameters. However, these methods do not provide the investigator with sufficient information about drilling mechanisms. In the current investigation, a procedure for diagnosing different drilling mechanisms based on the analysis of the signals of acoustic emission is presented. According to the number of time domain acoustic emission parameters, using multi-variable methods of analysis is unavoidable. In this work, unsupervised pattern recognition analyses (fuzzy C-means clustering) associated with a principal component analysis are the tools that are used for the classification of the recorded acoustic emission data. After classification of acoustic emission events, the resulting classes are correlated with the different drilling stages and mechanisms. Acoustic emission signal analysis provides a better discrimination of drilling stages than mechanic-based analyses.
This paper presents an optimization of the mechanical properties of cement–Posidonia composite by means of experimental characterization. We are interested in producing composite of cement reinforced by Posidonia raw fibers. The ratio of fibers and water-to-cement mass is varied to determine its effect on the mechanical properties of the cement–Posidonia composite, particularly, its resistance to fracture. Three point bending and compression tests were carried out to study the mechanical properties of the composite. Scanning electron microscopy was used to examine the surface of the tested samples. The experimental investigation shows an improvement of flexural strength for a ratio W/C equal to 0.5 and fiber content equal to 10 vol%.
The paper presents a comparison of the test results for the fiber volume fraction, static bending and Charpy impact strength of glass fiber reinforced polymer laminates reinforced by 0/90 fabric and chopped strand mat, produced by the hand lay-up and the vacuum-assisted resin infusion (VARI) method. The laminates were produced under equivalent conditions, with polyester matrix and lay-up areal mass 2100 g/m2. In the comparison of the obtained measurement results, similar mechanical performance was stated in the case of the hand lay-up and the VARI production. However, significantly smaller scatter of results and better uniformity of the reinforcement in the matrix with smaller amount of local structure defects was observed for the samples obtained by VARI method. The laminates obtained by VARI method show a much more advantageous coefficient of variation than that of the hand lay-up method, especially in the case of the mat reinforcement.
This article is concerned with the effect of the strain rate on the strain hardening behavior of polymer-bonded explosives at a wide range of strain rates ranging from 0.0001 s–1 to 3870 s–1. Inert polymer-bonded explosive simulants are prepared as specialized particulate composites to acquire analogous mechanical characteristics to polymer-bonded explosives for safety reasons. Uniaxial compressive tests were conducted from quasi-static states to intermediate strain rates ranging from 0.0001 s–1 to 100 s–1 with cylindrical specimens using a dynamic material testing machine (INSTRON 8801) and a high-speed material testing machine. An experimental method was developed for uniaxial compressive tests at intermediate strain rates ranging from 10 s–1 to 100 s–1. Split Hopkinson pressure bar tests were performed at high strain rates ranging from 1250 s–1 to 3870 s–1. Deformation behavior was investigated using captured images from a high-speed camera. The strain hardening behavior of polymer-bonded explosive simulants was formulated as a function of the strain rate with the proposed rate-dependent hardening model based on the DSGZ model. The model is capable of representing the complicated strain rate effects on the strain hardening behavior for rate-sensitive materials with a second-order exponentially-increasing function of the strain rate sensitivity. The rate-dependent hardening model of polymer-bonded explosives can be readily applied to prediction of deformation modes of polymer-bonded explosives in a warhead that undergoes severe dynamic loads.
In this paper, the strength of braided textile composites is predicted using a multi-scale approach bridging the mesoscale and microscale regimes. Mesoscale finite element models of representative unit cells of biaxial and triaxial braided composites are developed for predicting strength. The constituent stresses of tows inside the braided unit cell are calculated using micromechanics. Correlations between mesoscale stresses and microscale constituent stresses are established by using stress amplification factors. After calculating microscale stresses, a micromechanics-based progressive damage model is employed to determine the damage statuses of braided composites. A volume-averaging homogenization method is utilized to eliminate damage localization in the matrix of tows, and a parametric study is performed to evaluate the effects of damage homogenization. Subsequently, the ultimate strength is predicted for braided composites in which the braiding angle ranges from 15° to 75°. The prediction results are compared with the experimental values, and good agreement is observed.
The damage evolution in composite material is a complex phenomenon, comprising several interacting failure modes like matrix cracking, fiber breakage, debonding and delamination. Damage initiation, its propagation and ultimate strength prediction of composite structure is of paramount importance for developing reliable and a safer design and utilizing them as primary load bearing one. During service life, these structures get damaged and are often repaired for extending their service life. In the present work, a 3D finite element-based progressive damage model is developed for predicting the failure and post-failure behaviour of notched and repaired panel under tensile load. Failure initiation load, ultimate strength and failure mechanisms are investigated through the developed progressive damage model. The accuracy of developed finite element model is assessed by comparing its prediction with the experimental results obtained from digital image correlation technique and they are found to be in good agreement. In this study, the panels made of carbon/epoxy composite laminates of pure unidirectional and quasi-isotropic stacking sequence are considered. The damaged panel is repaired with both single- and double-sided circular patch of same parent material. Stress-based 3D-Hashin’s failure criterion is used for predicting the damage mechanism. Maximum shear stress and strain criteria are considered to account for patch debonding. It is found that the damage in notched panel always initiates with matrix cracking around the hole. However, damage in repaired panel is influenced by localized patch debonding.
The use of knockdown factors (percent reduction of undamaged compression strength) to account for flaws such as impact damage or holes have been used to infer the notched strength of laminates. It has been observed that this criterion tends to over-predict the strength of laminates with a high percentage of 0° plies. This paper examines some limited data from the literature and presents new data that compares knockdown calculated notched compression strength values with those measured experimentally for laminates with various percentages of 0° plies. Results show that the trend of over-predicting the notched compression strength of laminates as the percentage of 0°plies increases, based on a knockdown factor, is observed, but the difference can be within scatter except at very high percentages of 0° plies.
Interfacial debonding frequently initiates composite failure in a fiber/matrix composite. A single-fiber fragmentation test and its modifications can be used to evaluate interfacial properties. However, they still have accuracy problems due to fiber impurities and friction work. This paper presents a new method of evaluating interfacial properties using a stress contour of composite matrix. A single-fiber fragmentation test model was developed to simulate the stress contour. The interface was modeled as a cohesive zone model. Four characteristic lengths on the stress contour were found after conducting simulations with many interfacial properties values. The stress contour was then captured from the single-fiber fragmentation test employing a photo-elasticity technique and the four characteristic lengths were measured. Iteration in simulation involved changing interfacial properties until corresponding characteristic lengths from experiment and simulation were obtained. The results were compared with those obtained with existing methods and found to be reasonable.
This paper investigates the fatigue failure and electrical resistance behaviors of carbon nanotube-based polymer composites at cryogenic temperatures. Tension–tension fatigue tests were performed on carbon nanotube /polycarbonate composites at room temperature and liquid nitrogen temperature (77 K), and the electrical resistance of the specimens was monitored during the tests. Based on the obtained results, the dependence of the mechanical and electrical responses of the nanocomposites on the temperature and the nanotube content was studied. Microscopic examinations were also carried out on the specimen fracture surfaces, and the failure mechanisms of the nanocomposites were discussed.
Hybrid fibre-reinforced polymer composites have extensive applications due to their high strength, cost effectiveness, improved product performance, low maintenance and design flexibility. However, moisture absorbed by composite components plays a detrimental role in both the integrity and durability of hybrid structure because it can degrade the mechanical properties and induce interfacial delamination failures. In this study, the moisture diffusion characteristics in two-phase hybrid composites using moisture concentration-dependent diffusion method have been investigated. The two phases are unidirectional S-glass fibre-reinforced epoxy matrix and unidirectional graphite fibre-reinforced epoxy matrix. In the moisture concentration-dependent diffusion method, the diffusion coefficients are not only dependent on the environmental temperature but also dependent on the nodal moisture concentration due to the internal swelling stress built during the diffusion process. A user-defined subroutine was developed to implement this method into commercial finite element code. Three-dimensional finite element models were developed to investigate the moisture diffusion in hybrid composites. A normalization approach was also integrated in the model to remove the moisture concentration discontinuity at the interface of different material components. The moisture diffusion in the three-layer hybrid composite exposed to 45°C/84% relative humidity for 70 days was simulated and validated by comparing the simulation results with experimental findings. The developed model was extended to simulate the moisture diffusion behaviour in an adhesive-bonded four-layer thick hybrid composite exposed to 45°C/84% relative humidity for 1.5 years. The results indicated that thin adhesive layers (0.12-mm thick) did not significantly affect the overall moisture uptake as compared with thick adhesive layers (0.76-mm thick).
Recycled polypropylene composites reinforced with quill from chicken feathers were prepared by extrusion process. Chicken feathers, a worldwide waste without any relevant application, may potentially replace nonrenewable reinforcements in composites. The effects of quill reinforcement on the density, as well as the thermal, thermo-mechanical and morphological properties of the composites, were evaluated. Quill showed an excellent compatibility with the polypropylene matrix, revealed by the good dispersion that was confirmed by the physical appearance observed with aid of scanning electron microscopy. This fact is due to the hydrophobic nature of keratin in quill. All of the composites showed higher storage modulus than simple polymer, particularly for the lowest quill content. In addition, the composite materials also had a lower density. The transition temperature remained almost unaltered compared with polypropylene. However, the thermal stability was observed to be strongly related to the quill content. Thus, this study reports a successful industrial process applied to a new natural reinforcement material: quill, used to synthesize composites with an amply used polymer: polypropylene; which can open an important gate towards the extended exploitation of keratin quill as a novel and renewable reinforcement material.
The objectives of this paper include identifying important architectural parameters that describe the SiC/SiC five-harness satin weave composite and characterizing the statistical distributions and correlations of those parameters from photomicrographs of various cross sections. In addition, realistic artificial cross sections of a 2D representative volume element (RVE) are generated reflecting the variability found in the photomicrographs and include explicitly modeled voids (something not routinely done for woven CMCs). These models are used to make preliminary observation of the effects of architectural variability on the thermo-mechanical properties (material constants). Lastly, information is obtained on the sensitivity of linear thermo-mechanical properties to architectural variations. Two-dimensional finite element analysis is used in combination with a response surface and it is shown that the present method is effective in determining the effects of architectural variability on thermo-mechanical properties and their variability.
The influences of basalt fibre on morphology, mechanical properties, structure, contact angle, melting and crystallization behaviours of polyamide 1012 were studied using scanning electron microscopy, material testing machine, X-ray diffractometer, differential scanning calorimeters and static contact angle measuring instrument. It was found that interface properties between basalt fibre and polyamide 1012 were improved effectively by silane coupling agent. Also, the tensile and flexural strength of treated basalt fibre-reinforced composites were found to be higher than untreated ones at the same fibre content. The best tensile and flexural strength of polyamide 1012/treated basalt fibre composites with 30 wt% basalt fibre were determined to be 84.4 and 120 MPa. Although basalt fibre did not change polyamide 1012 crystal structure, it altered the melting temperature and degree of crystallinity of polyamide 1012. In addition, by virtue of the changes in crystallization temperature, the nucleation ability of polyamide 1012 in composites was confirmed to be enhanced. The strong interaction between epoxy groups of silane coupling agent and amide groups of polyamide 1012 elevates the hydrophobic properties of composites. The results from nonisothermal crystallization kinetics testified that crystallization ability of polyamide 1012 in composite containing 30 wt% basalt fibre was strengthened, compared with neat polyamide 1012.
Since a few years ago, a topic of curiosity consists in evolving composites filled with the nanofillers to improve various properties of polymer matrix. This work aims to investigate the structure–property relationship in ternary nanocomposites containing of amorphous poly(vinylpyrrolidone) as the matrix, organo-modified-TiO2 nanoparticles and organo-modified-montmorillonite as the nanofillers. In this regard, poly(vinylpyrrolidone)/organo-modified-TiO2/organo-modified-montmorillonite nanocomposites were prepared by sonication process with 10 wt% of organo-modified-TiO2 and different loadings (3, 5 and 7 wt%) of organo-modified-montmorillonite. Nanostructure characterization and dispersion capacity of nanofillers in the polymer matrix were investigated by X-ray diffraction, transmission electron microscopy and field emission scanning electron microscopy. According to these data, the nanostructure of ternary nanocomposites was predominantly intercalated. Also, incorporation of the organoclay and TiO2 nanoparticles into the poly(vinylpyrrolidone) system was observed to have an obvious effect on the shape and size of polymer matrix. According to thermogravimetric analysis data, the presence of organo-modified-montmorillonite and organo-modified-TiO2 in the poly(vinylpyrrolidone) matrix, caused an improved in the thermal stability of the obtained poly(vinylpyrrolidone) nanocomposites.
The damage initiation and development in flax/epoxy laminates under quasi-static tension is studied. The laminates are made of quasi-unidirectional woven prepregs in different configurations [0°]8, [0°, 90°]2S, [–45°, 45°]2S and [0°, 90°, +45°, –45°]S, and processed using an autoclave. The damage was monitored during the tensile test using acoustic emission and observed by post-mortem microscopy of the samples. The stress–strain curves illustrate the ductile behaviour of the [+45°, –45°]2S composite, whereas in the other composites a more brittle behaviour was observed. Non-linearity of the stress–strain curves is explained by the intrinsic non-linearity of flax fibres in tension. The combination of the stress–strain data and the registered acoustic emission data is used to identify the damage initiation and propagation thresholds. The damage thresholds are the lowest in the [0°]8 laminate and the highest in the [+45°, –45°]2S laminate. The observed fracture zones and damage mode are cracks inside and on the boundary of technical fibres, cracks on the boundary of tows, matrix cracking, fibre pull-out and fibre breakage. A notable feature of the damage behaviour is almost full absence of transverse matrix cracks inside tows in 90° plies, which are the major damage modes in glass- and carbon-reinforced plastics. This is attributed to the low stress concentrations in transverse direction due to the low transverse modulus of flax fibres.
The use of micromechanical models to study composite material's behavior leads to save time and cost. In this paper, bridging micromechanical models have been used in order to observe the behavior of unidirectional laminate composite under fatigue loading. In order to study the fatigue behavior, stiffness degradation has been studied as well as the strength degradation and a driftnet model has been proposed for each of them. The strength degradation has only been studied for the unidirectional fiber, while the stiffness degradation has been studied for the fibers with different fiber angle. The results are compared with macro-mechanical models and other methods in literature.
Fused deposition modelling is one of the additive manufacturing processes in which a semi-solid polymer material is deposited line-by-line to construct 3D objects direct from computer aided design (CAD) models. Benefits inherent with additively manufacturing allowed effective competition with other traditional methods in specific applications and the process drew sufficient research attention. The nature of material deposition and the mechanics of solid state sintering lead to varying levels of inter-road and inter-layer bonding resulting in a composite structure of voids interspersed in the base material matrix. While there are other parameters also, the raster angle in particular has a direct bearing on the resulting meso-structure and together with the rate of inter-road coalescence plays a significant role in influencing the mechanical characteristics of parts produced. Experimental and analytical attempts were made in the past to evaluate the role of raster angle orientation, but the resulting material properties were taken to be those of the base material. The hypothesis for the current research is that the mechanical properties resulting from fused deposition modelling are structure-sensitive. Experimental and analytical models are developed to test this hypothesis and the results indicate the hypothesis to be true.
In recent years, graphene has attracted a great research interest in all fields of sciences due to its unique properties. Its excellent mechanical properties lead it to be used in nanocomposites for strength enhancement. In current work, a new magnesium-graphene nanoplatelets composite is fabricated for the first time using semi-powder metallurgy method. The effect of graphene nanoplatelets addition on the mechanical behaviour of pure magnesium under both tension and hardness is investigated. The results demonstrate that graphene nanoplatelets are distributed homogeneously in the magnesium matrix, therefore act as an effective reinforcing filler to prevent the deformation. Compared to monolithic magnesium, the magnesium/0.3 wt% graphene nanoplatelets composite exhibited improved elastic modulus, yield strength, ultimate tensile strength and Vickers hardness. The improvement in elastic modulus, yield strength (0.2%), ultimate tensile strength and Vickers hardness for magnesium/0.3 wt% graphene nanoplatelets composite relative to pure magnesium are up to +10.6%, +5%, +8% and +19.3%, respectively. In addition to tensile and hardness tests for the analysis of mechanical properties of as synthesized composite, scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction are used to investigate the surface morphology, elemental percentage composition and phase analysis, respectively.
The reinforcement of composite aluminium extrusions offers a high potential regarding weight reduction and improvement of mechanical properties, which is essential for components in lightweight constructions. The current work gives an overview of the quasi-static properties of spring steel wire reinforced EN AW-6082 with varying reinforcing ratio. The deformation and damage behaviour is investigated in detail for a reinforcing ratio of 11.1 vol.%. It is shown that the relatively ductile behaviour of the spring steel wire leads to multiple necking resulting in higher strains than expected. Current models are expanded and modified for a proper adaptation to the material system.
The aim of this study was the analysis and characterization of composites based on thermoplastics (ethylene vinyl acetate, polypropilene and high-density polyethylene) and chicken feathers. Several composite samples with a content of 20% v/v of chicken feathers have been studied to determine the optimal manufacturing conditions of temperature, mixing time, and mixing speed to achieve the best tensile properties. The results have shown that the addition of micronized chicken feather (20% v/v) to thermoplastic matrices increases stiffness and provides a more brittle behavior. Ethylene vinyl acetate matrix also shows an ability to participate in second-order intermolecular interactions with chicken feathers, providing better tensile properties (tensile strength and toughness) than polypropilene and high-density polyethylene. Optimal manufacturing conditions were found for a mixing time of around 5 min; a mixing speed of 50 r min–1; and temperature values of 160°C in case of high-density polyethylene, 120°C for ethylene vinyl acetate, and 170°C for polypropilene. Fourier transform infrared spectroscopy, differential scanning calorimetry and scanning electron microscopy analysis have been performed in order to provide further understanding of the compatibility and microstructural features that support the tensile properties of the materials.
The rapid increase in the use of polymer composite materials recently has led to the investigation of their behavior under thermal loads. During thermal loading, for example a fire incident, the polymer materials start to degrade. The degradation rate is heating-rate dependent. In this work, medium heating rates were applied to a neat resin and two (2) resin batches doped with fire-retardant nanomaterials. The thermal decomposition behavior of these systems was investigated using multistage kinetics modeling, the kinetic triplets for each system were extracted using the van Krevelen methodology and finally the thermogravimetry tests were reconstructed. This is the first part of the work dealing with the fire response of polymer composites. It is a material characterization process and the outcome is used in Part B, where a progressive degradation material model is developed and implemented in a Finite Element code.
Optically transparent glass-ribbon composite panels were made by reinforcing a clear epoxy resin with soda-lime silicate glass ribbons, as opposed to using cylindrical fibers, of matching refractive index. Cross-ply (0/90°) optically transparent glass-ribbon composite panels were made by stacking either 64 or 128 glass-ribbon layers and with two-ribbon dimensions. The haze and light transmission were measured between 10°C and 52°C. Additionally, 610 mm x 910 mm x 18.7 mm flat windshields were made by laminating 1.9-mm thick (0°/90°)40 optically transparent glass-ribbon composite panels to clear polycarbonate panels in an autoclave. The haze and light transmission for the optically transparent glass-ribbon composite panels and for a prototype windshield were measured as a function of temperature while optical distortion was measured at room temperature (22°C) only for the windshield. The haze changed with temperature, with a minimum at the temperature where the refractive index of the glass ribbons and the polymer were equal. The lowest haze value was found for the widest ribbon, while the light transmission was almost constant in the temperature range of study.
Taguchi method is used to find the optimal cutting parameters for turning Al2124/SiCp (45%wt) metal matrix composite using polycrystalline cubic boron nitride (PcBN) and polycrystalline diamond (PCD) tools. Experiments were performed with High Precision Lathe Machine Model No: CG6125C. The parameters selected were cutting speed, feed, depth of cut, and tools. The output responses considered for the investigation were surface roughness, tool wear, and specific power consumption. Results show that the optimal combination of parameters giving the best surface finish are at cutting speed of 60 m/min, feed rate of 0.2 mm/rev, depth of cut of 0.10 mm, and PCD tool. The analysis also show that the optimal combination of parameters is at cutting speed of 40 m/min, feed rate of 0.15 mm/rev, depth of cut of 0.20 mm, and PCD tool insert give the lowest specific power. Dominant wear mechanism observed is abrasion wear affecting flank face. Uncoated PcBN had greatest wear followed by coated PcBN and finally PCD tool. Tool insert has a dominant effect on surface roughness having a contribution of 82.04% and followed by cutting speed having a contribution of 4.51%. On specific power consumption tool insert has a dominant effect having a contribution of 65.14% and followed by feed rate with a contribution of 13.57% and interaction (ap x f) with a contribution of 9.92%. It is observed that optimization of the two characteristics for machinability of Al2124SiCp (45%wt) metal matrix composite simultaneously is difficult. On tool wear the results showed that PcBN tools suffered severe wear as compared to PCD tools.
Fibre-reinforced plastic (FRP) composites are used in weight-sensitive aerospace type of applications as well as in infrastructure where durability is of real concern. FRP structures are prone to fracture at interfaces or within the matrix which may not be visible from outside. Thus, a thorough knowledge of initiation and propagation of crack in FRP composites is necessary. The present investigation focuses on a combined experimental and numerical investigation to understand the fracture behavior of FRP properly. The finite element modeling of FRP plate type of specimen is done using ABAQUS to estimate the fracture parameters, such as fracture toughness, fracture energy, etc. The same parameters are also measured experimentally by three-point bending tests as per American Society for Testing and Materials (ASTM) standards. The mode-I fracture behavior seems to be predicted very closely, whereas the observed mode II behavior does not match closely and the reasons for discrepancies are explored.
In the present study, chitosan-ZnO composite was synthesized by stirring of chitosan purified from mud crab Scylla serrata shells with zinc chloride and sodium hydroxide. The physicochemical characteristics of chitosan-ZnO composite were studied using UV-Vis spec, Fourier transform infrared, X-ray diffraction and scanning electron microscopy. Chitosan-ZnO composite formation was confirmed by the functional groups stretching and bending vibrations in Fourier transform infrared. Scanning electron microscopy results showed the synthesized chitosan-ZnO composite was rod-like structure. X-ray diffraction results represent the hexagonal shape and crystalline size (30–60 nm) of zinc oxide in the chitosan-ZnO composite. Antibacterial activity of chitosan-ZnO composite demonstrated the effective growth control of a Gram-negative Vibrio parahaemolyticus and Gram-positive Bacillus lechiniformis bacteria isolated from aquatic environments. Light microscopy and confocal laser scanning microscopy also evidenced the antibiofilm activity of chitosan-ZnO composite against the V. parahaemolyticus and B. lechiniformis at the concentration of 40 and 60 µg/ml, respectively.
In this research, multi-walled carbon nanotubes were used to delay the propagation and growth of cracks in cement mortar on the nanoscale. To improve the dispersion of multi-walled carbon nanotubes in the cement mix a polycarboxylate superplasticizer was used. The mechanical strength of multi-walled carbon nanotubes-cement nanocomposites mix containing 0.1–2% nanotubes by weight (wt) and 0.5% superplasticizer by (wt) was measured and compared with that of cement prepared without multi-walled carbon nanotubes. It was found that the tensile strength of the specimens increased about 70% up to 0.3%, multi-walled carbon nanotubes. With further increase in multi-walled carbon nanotubes, a decrease in tensile strength was observed. Field-emission scanning electron microscopy used to observe the fracture surface of specimens containing 0.3 wt% nanotubes indicated that the multi-walled carbon nanotubes were well dispersed and there were no agglomerates visible in the matrix. Field-emission scanning electron microscopy observation also revealed good bonding between the multi-walled carbon nanotubes and the surrounding cement matrix. In addition, X-ray diffraction data showed the multi-walled carbon nanotubes accelerated the dissolution and growth of the calcium silicate hydrate hydration products compared with the control cement mortar. Mercury intrusion porosimetry test results showed that chemical species could not penetrate the specimens containing 0.1 wt% and 0.3 wt% multi-walled carbon nanotubes as easy as other specimens. Thermogravimetric analysis results indicated that the cement hydration was enhanced in the presence of the multi-walled carbon nanotubes.
The main focus of this paper is the experimental comparative analysis of the viscoelastic properties of acrylic- and silicon-based viscoelastic materials. These materials are widely used in the aeronautic industry for structural and/or damping applications. It is therefore required to determine their viscoelastic properties such as shear modulus and loss factor following their integration to aircraft composite structures. The influence of material thickness, bonding quality, curing and cocuring with composite material was evaluated using dynamic mechanical analysis machine in plan shear configuration. This comparative analysis in the 0–600 Hz frequency range provides useful information that should be taken into consideration when designing bonded joints for structural and/or damping applications. The small variation of the acrylic-based material loss factor over a wide range of frequencies suggests that this material is a good candidate for damping applications at room temperature in metallic structures where no curing is required. However, cocuring of acrylic-based material with composite laminae increases its shear modulus up to six times with respect to the uncured material whereas its loss factor is reduced by about 20%. On the other hand, the silicon-based material remains stable after thermal treatment (curing and cocuring) suggesting that it is well suited for in situ damping treatment involving laminate composite structure. However, at room temperature its shear modulus after cocuring is lowered by almost 25% due to poor bonding quality, which depends on the nature of the substrate.
This paper studies the flexural stiffness controllability of hybrid layered beams with woven carbon fiber reinforced polymer composite and shape memory polymer layers. The temperature of the shape memory polymer layer was controlled using electrical resistive heating of the woven carbon fiber reinforced polymer layers. Flexure tests were then conducted on the layered cantilever beams, and the temperature dependence of the beam flexural stiffness was examined. A three-dimensional finite element analysis was also performed to predict the flexural response of the layered beams, and a good correlation was observed between the predicted and measured results for the flexural stiffness. In addition, the deformation states in the layered beams were determined numerically and discussed in detail.
Tools used for fabricating polymers are often required to have low thermal conductivities, e.g. for pelletizing, because this lowers the risk of the polymer nozzle being obstructed by molten polymer solidifying as it exits. Latterly, advanced corrosion and wear resistant metal matrix composites (MMCs) are used for pelletizing tools. Therefore, with respect to polymer processing it is important to know how the thermal conductivity of MMC gets influenced by hard phase and metal matrix contribution. In this study, the thermal conductivity of a TiC reinforced corrosion and wear resistant MMC gets analyzed. Especially the influence of chemical interdiffusion between TiC and metal matrix on the resulting thermal conductivity gets analyzed. It is shown that changes in the chemical composition lead to distinct decrease in thermal conductivity of the TiC which has to be considered when MMC thermal conductivities have to be examined.
Vinyl ester matrix syntactic foams filled with hollow glass microspheres are characterized for unnotched Izod impact properties. The study is aimed to analyze the effect of wall thickness and volume fraction of the hollow glass microsphere on the impact properties of syntactic foams. The impact strength of syntactic foams was observed to be lower in comparison to the neat vinyl ester resin. The volume fraction of the hollow glass microspheres was found to have a more pronounced effect on the impact strength than the wall thickness. The energy absorbed until failure decreased with increase in the hollow glass microsphere volume fraction. The observed values decreased by 50–72.2% depending on the hollow glass microsphere volume fraction and wall thickness. The failure feature of syntactic foams under the current testing condition is explained using finite element analysis. The failure initiates from the tensile region, propagates through the specimen and is deflected near the compression region. The microstructural failure features are examined using a scanning electron microscope and matrix cracking, hollow glass microsphere-matrix debonding, and crack deflection by hollow glass microspheres are observed to be the failure features. Since the cracks were deflected around the compression zone, all types of syntactic foams showed tensile failure features, which include prominent matrix fracture and lack of hollow glass microsphere crushing. The understanding of the variation of impact properties with respect to the hollow glass microsphere volume fraction and wall thickness can help in tailoring the properties of syntactic foams.
Al-diamond-like carbon films were successfully deposited by the reactive magnetron sputtering of Al target (>99.9%) in the argon and methane gas mixture atmosphere under selected substrate negative bias with high pulse cycle duty (50%). The microstructure, mechanical and tribological properties of the as-prepared Al-diamond-like carbon films were investigated. Results showed that these films were dominated by the typical amorphous structure, and the internal stress of films dramatically decreased while the hardness remained a high level (~20 GPa) with increasing substrate pulse negative bias. Especially, the Al-diamond-like carbon film fabricated at substrate bias of –500 V displayed a longer wear life and lower friction coefficient due to both itself superior mechanical properties and the formation of continuous and compact graphitized transfer layer, making it a good candidate for solid lubricating film in engineering applications.
In this study, the dynamic response of the laminated composite beam with arbitrary lay-ups has been investigated within the framework of the third-order shear deformation theory using the finite element method. A new three-nodded finite element compliant with the theory is introduced next. To deal with the dynamic contact between the delaminated segments, unilateral contact constraints are employed in conjunction with Lagrange multiplier method. Furthermore, the Poisson’s effect is incorporated in the formulation of the beam constitutive equation. Also, the higher-order inertia effects and material couplings (flexure–tensile, flexure–twist and tensile–twist couplings) are considered in the formulation. Results are extracted based on two methods namely the Eigen-value techniques for frequencies and the Newmark method to calculate the transient response. Then, the obtained results have been verified with the other results available in the literature and very good agreements have been observed. Furthermore, the new results have been obtained for the case where the excitation was due to a moving/non-moving force.
Glass fabric reinforced thin sandwich panel and carbon fabric reinforced thin sandwich panel of thickness close to 2.5 mm were studied to explore an alternative skin material for the outer body of various machines and appliances. The polyester foam Coremat XM of 2 mm thickness was used as core material in the thin sandwich panels. The panels were fabricated by vacuum bagging process and characterized through two plate tests: (i) low-velocity normal impact loading under a drop weight impact test set up and (ii) transverse static loading of a plate. The damage area, indentation depth and permanent depression over damage area, energy absorption capability, load-deflection relation and failure modes were observed under the test. The impact drop test was simulated by LS-DYNA. The properties of glass fabric reinforced thin sandwich panel and carbon fabric reinforced thin sandwich panel were compared with those of 0.8-mm-thick MS sheet, a widely used skin material for the outer body of various machines and appliances.
By a domain decomposition method, free vibration characteristics of laminated orthotropic conical shells resting on Pasternak foundations are analyzed. The conical shell is divided into some conical shell segments in the meridional direction and separated from the geometric boundary and Pasternak foundation; the theoretical model is formulated based on a modified variational functional which includes energy of each conical shell segment, interface potentials (including the boundary potentials) and the energy due to the Pasternak foundation. Numerical comparisons with those published results are made to validate the high accuracy of the present method. The variation of the energy contribution of the shell with different thickness-to-radius ratio, cone angle and fibre orientation against various circumferential wave numbers are presented to help better understand the vibrational characteristics. Moreover, the effects of elastic foundation, boundary condition, stacking sequence and the variations in physical parameters of the shells on the natural frequencies are also investigated.
Composites of rubberwood flour (RWF) and recycled polypropylene (rPP) were produced into panel samples by using a twin-screw extruder. The effects on creep behavior of mixture fractions of rPP, RWF, maleic anhydride-grafted polypropylene (MAPP), and ultraviolet (UV) stabilizer were studied in a D-optimal mixture design. Creep was significantly affected by the composition. Increasing the fraction of RWF decreased creep, while MAPP and UV stabilizer increased it. The models fitted were used to optimize a desirability score that balanced multiple creep characteristics. The model-based optimal formulation 50.5 wt% rPP, 44.9 wt% RWF, 3.5 wt% MAPP, 0.1 wt% UV stabilizer, and 1.0 wt% lubricant was experimentally validated to have low creep closely matching the model predictions.
The AC-impedance and dielectric properties of hybrid polymer composites made up of epoxy (diglycidyl ether of bisphenol-A) matrix filled with various zinc oxide concentrations (0, 4.9, 9.9, 14.9 and 19.9 wt%) and reinforced with conductive carbon black nanoparticles (0.1 wt%) have been investigated as a function of fillers concentrations, applied frequency in the range from 20 kHz to1 MHz and temperature in the range from 30°C to 110°C. The observed data were analyzed using the dielectric permittivity and electric modulus formalisms (i.e. the inverse quantity of complex permittivity). The dielectric constant of the composites increases with increasing temperature and filler concentrations, and this case can be explained by hopping and tunneling processes. The AC-conductivity is apparently enhanced with increasing frequency, temperature, zinc oxide and carbon black fillers. The observed increase in the AC conductivity was explained based on the concept of conductive paths and connections between the zinc oxide–particles and the conductive carbon black–nanoparticles. The activation energy has been estimated from fitting the AC conductivity–temperature data and it was found that it is decreased by the addition of the zinc oxide content in the epoxy matrix reinforced with carbon black, which means that the composites have better electrical conduction. The scanning electron microscopy images revealed that the dispersed zinc oxide-particles and carbon black-nanoparticles were randomly distributed within the epoxy matrix with some paths and surface contacts between the fillings. It was found that the addition of carbon black nanoparticles to epoxy/zinc oxide composites enhances the electrical conduction due to the electronic and impurity contributions.
This paper reports the development of a generalized fracture law that can assess the mechanical behavior of ceramic matrix composites (CMCs). The established law differs from conventional bridging laws in that it accounts not only for the bridging effect but also for all major energy dissipation mechanisms including matrix cracking and fiber pull-out. As such, the formulation can be used to directly assess the original, experimentally recorded, fracture behavior of the material in the load-extension domain. The established expression successfully approximated the experimental load versus beam deflection (P-u) curves of a SiC-fiber reinforced glass-ceramic matrix composite tested under the single-edge-notched beam (SENB) configuration. The fracture law was found to be geometry-invariant by comparison with results from tensile specimens with radically different damage zone geometries. A parametric analysis is presented which demonstrates the potential of the model in the a priori prediction of the fracture behavior of hypothetical CMCs with similar fracture characteristics.
Shape-memory alloy composites are relatively new materials and their behaviour is not yet completely understood. The purpose of this work is to minimise the effect of impact damage on their structural performance. To do that, GFRP woven fabric reinforced epoxy composite panels, with and without additional superelastic shape-memory alloy wires, have been impacted at constant velocities using a servo-hydraulic testing machine, and a digital video camera has been used to monitor the impact event. Single, multiple and partial penetration impact tests have been carried out, and the energy absorption and damage development are similar in all cases for the same material. The benefit of using the superelastic shape-memory alloy wires was seen only at high displacements and when the volume fraction of the wires was high.
Poly (lactic acid) (PLA)/natural halloysite nanotubes (HNTs) films were prepared by solution casting method to investigate their properties for packaging applications. Tensile test results revealed that the maximum tensile elastic modulus (1.40 ± 0.05 GPa) and tensile strength (52.75 ± 1.80 MPa) were achieved at 5 w/w % of HNTs (in a range of 0–10 w/w % HNT concentrations). A nanoindentation test was performed to confirm the reinforcing effect of HNTs. Analysis of electron micrographs of the fracture surfaces suggested that the reinforcing mechanism was subjected to the interfacial interaction between HNTs and PLA. Infrared spectra revealed that the end hydroxyl groups of PLA chemically interacted with HNTs’ outer surface siloxane groups via hydrogen bonding. In addition, the contact angle test and thermogravimetric analysis were used to investigate the surface wettability and thermal stability of the PLA/HNT films, respectively.
Nano-phased polymers show strain rate dependent mechanical behaviors owing to the nature of polymers. Therefore, in this study a strain rate dependent constitutive equation is developed based on a micromechanics method, to predict the mechanical behavior of nanocomposites under various loading rates. The Goldberg et al. model, as a constitutive equation of polymers, has been used to predict the strain rate dependent mechanical behavior of pure polymers. Then, this model is combined with a micromechanics method (Halpin–Tsai model) to develop a constitutive equation for nano-phased polymers which predicts the stress–strain behavior of carbon nanotube (CNT) reinforced polymers at arbitrary strain rates. Also, contrary to the strain rate dependent behavior of the polymeric matrix, it is assumed that mechanical properties of carbon nanotube particles are not sensitive to loading rates. To verify the proposed model, predicted results are compared with the experimental data of multi-walled carbon nanotube/epoxy (tested in this study) and multi-walled carbon nanotube/polypropylene composites (available experimental data) under various tensile loading rates.
Thermal deformations that occur during formation of long-fiber-reinforced composites have been a continued challenge for manufacturers as the final shape of a given part can be different from the original mold shape. The ensuing dimensional distortions can be difficult to predict due to complex thermo-mechanical behaviour of composite laminates during different forming cycles. This study intends to model the fundamental mechanisms that lead to thermal deformations during forming of a thermoplastic matrix composite comprised of comingled polypropylene and E-glass fibers. While the discussion is framed around a custom-design multi-stage roll-forming process, it is also relevant to a wider range of thermoplastic composites manufacturing processes. A methodology is developed to characterize the thermal mechanical behavior of the material, optimize the manufacturing process, and predict the magnitude of resulting spring-in angle due to thermal deformations. It is found that the process control parameters can be optimized first such that the crystallization of the matrix occurs at an ideal position along the forming line. Once the process is optimized, the developed numerical model, with a thermoelastic material behaviour, can give an adequate prediction of spring-in at the end of the process. Finally, through a comparative study, it is discussed how for other manufacturing processes, such as compression molding, including a thermoviscoelastic liquid/solid material behaviour may be required to yield accurate spring-in predictions.
Coconut fiber reinforced chemically functionalized high-density polyethylene (CF-HDPE) composites (CNF/CF-HDPE) with in situ fiber/matrix interfacial adhesion have been processed by Palsule process. Chemically functionalized maleic anhydride grafted polyethylene without compatibilizers has been used as matrix (in place of polyethylene with compatibilizer), and no fiber treatment has been performed. In situ fiber/matrix interfacial adhesion has been established by field emission scanning electron microscope and Fourier transform infra red spectroscopy. Mechanical properties of the CNF/CF-HDPE composites have been found to be higher than those of the CF-HDPE matrix and increase with increasing amounts of fibers in composites. Measured tensile modulus of CNF/CF-HDPE composites compares well with values predicted by Rule of Mixtures, Hrisch Model, Halpin-Tsai equations, Nielsen equation and Palsule equation.
A mathematical correlation for the effective thermal conductivity of particulate-filled polymer composites is developed using the law of minimal thermal resistance and equal law of specific equivalent thermal conductivity. To validate this correlation, two sets of epoxy-based composites with micro-sized aluminum nitride and Al2O3 fillers (0–25 vol%) are prepared by simple hand-lay-up technique. Thermal conductivities of these composites are measured using the Unitherm™ Model 2022 tester. These values are then compared with the values obtained from the proposed model and are found to be in fairly good agreement. Effects of fillers on other thermal properties (dimensional stability, glass transition temperature) and dielectric behavior of epoxy resin are also studied. Perkin-Elmer thermal mechanical analyzer and HIOKI-3532-50 Hi Tester Elsier Analyzer are used for this purpose. It is found that the measured properties of the composites are suitable for certain applications like electronic packaging and printed circuit boards.
A combination of experimental, analytical and numerical techniques is used to characterise the shear response of lightweight corrugations based on glass fibre and carbon fibre reinforced epoxy resins. The corrugations were manufactured via a compression moulding procedure in which composite prepregs are cured between two serrated mould halves. The properties of the composite corrugations are compared with those offered by a similar aluminium system. Subsequent mechanical testing was undertaken using an Arcan rig capable of generating a range of loading conditions between pure shear and pure compression. As a result of difficulties in accurately measuring the displacement of the cores under mixed-loading conditions, an analytical model was used to predict the stiffness characteristics of the cores as a function of loading angle. The accuracy of the model was assessed using a finite element analysis. The final part of this investigation focused on fitting the measured values of maximum strength to an appropriate failure criterion. An examination of the corrugated structures during combined compression–shear loading indicated that the composite samples failed as a result of buckling in the strands and in certain cases, delamination between the composite plies. Both the analytical model and the finite element analysis indicated that the stiffness of composite and aluminium cores did not vary significantly with loading angle. An analysis of the strength characteristics of the corrugated cores showed that the aluminium corrugations could be accurately represented using a two-dimensional quadratic failure criterion. In contrast, due to the initiation of delamination within the composite struts, an additional component in the failure criterion was required to accurately capture the response of the composite corrugations.
In this study fire retarded HDPE/WF composites based on high density polyethylene (HDPE) and wood flour (WF) were investigated. Polymer and WF ratio was kept at 70/30 while concentration of fire retardant was 20 mass %. Ammonium polyphosphate and aluminum hydroxide were used as fire retardants. To diminish the influence of high loadings of fire retardants on mechanical properties two different types of organically modified nanofillers (CaCO3 and SiO2) were used. Surface modification of HDPE polymer and nanofiller was done to enhance the compatibility in composite and improve the mechanical properties and fire performance. Mechanical properties were characterized by dynamic mechanical analysis while compatibility of components in composites was followed through morphology by scanning electron microscopy. Thermal and fire properties were characterized by thermogravimetric analysis, pyrolysis combustion flow calorimetry, and limiting oxygen index. The obtained results show that addition of surface modified nanofiller considerably affects the morphology resulting in the enhancement of mechanical and fire properties. Ammonium polyphosphate fire retardant in combination with SiO2 nanofiller showed the highest limiting oxygen index value, the lowest heat release rate, and total heat released in pyrolysis-combustion flow calorimetry test indicating best overall fire performance.
Surface-modified silica nanoparticles, 20 nm in diameter and with a very narrow particle size distribution, are available as concentrates in epoxy resins in industrial quantities since 10 years. They improve many different properties like strength, modulus, toughness and fatigue performance. Some of these improvements can be found for fiber-reinforced composites as well. In this review, the research results obtained with commercial silica nanoparticles published in the last decade are studied, results are compared with a focus on mechanical properties and the mechanisms responsible for the property improvements are discussed. Silica nanoparticles present only in the interface between fibers and resin matrix might be a very promising approach for future cost-sensitive applications.
The growing demand for using high-performance materials for industrial sheet parts manufacturing actuated us to fabricate wave-formed sheets nanocomposite. A combination of impregnation method and plasma spray forming (PSF) was used for manufacturing of Al/multiwall carbon nanotube (MWCNT) sheets nanocomposite. By addition of 5 wt.% MWCNT to pure Al matrix, microhardness increased to 34.5 (VHN) compared to 30 (VHN) for monolithic pure Al. The effect of using coated MWCNT as a reinforcement material was also investigated. Our results showed that applying coated MWCNT not only lead to significant increase of microhardness (50 VHN), but also less defect was generated on the MWCNT sidewalls.
Micromechanical models usually applied to predict the mechanical properties of short glass fibre reinforced composites were used to evaluate the Young’s modulus and tensile strength of flax fibre reinforced polypropylene. Due to lack of accuracy between the experimental results and the existing models, a new adjustment to the Kelly-Tyson model was proposed. The changes were based on the understanding of the microstructure obtained in polypropylene/flax fibre composites produced by injection moulding with different flax fibre content. The mechanical properties were interpreted based on real fibre loading, fibre orientation, fibre dimension distribution and morphology of the composites. Lack of fibre/matrix adhesion, strong fibre damage and changes on the crystallization behaviour of polypropylene in the presence of flax fibres affect the mechanical strength, stiffness and elongation of the composites. The Kelly-Tyson’s model used for tensile strength prediction was modified to take into consideration the fibre property variability due to the large distribution of fibre shape ratio induced by the process. Finally, matrix modulus has been adjusted to take into account the change of crystallinity with fibre content. A better description of the mechanical properties is obtained using the proposed approach, resulting indeed in an excellent approximation to the modulus of the composite.
An advanced accelerated testing methodology (ATM-2) for long-term life prediction of CFRP laminates exposed to actual loading with a general stress and temperature history is proposed based on the conventional accelerated testing methodology (ATM-1) established for the long-term life prediction of CFRP laminates exposed to stress and temperature. The most important condition for ATM-1 is that the time–temperature superposition principle held for the viscoelastic behavior of matrix resin holds also for the static, creep, and fatigue strengths of CFRP laminates. Furthermore, three conditions that form the basis of ATM-2 are introduced along with their scientific bases. The long-term fatigue strength of CFRP laminates under an actual loading is formulated based on the three conditions. The viscoelastic coefficients of matrix resin, which perform an important role for the time and temperature dependence of long-term life of CFRP laminates, are also formulated based on the time–temperature superposition principle. The applicability of ATM-2 is demonstrated by predicting the long-term fatigue strengths of four typical directions of unidirectional CFRP laminates.
Application of natural fibers has attracted a great deal of attention among the composite research community in the past couple of decades. In this study, sisal fiber was utilized in fabrication of syntactic foam to improve the mechanical properties. Four sets of samples with different volume fractions of sisal fibers (0%, 1.5%, 2.5%, and 3.5%) were prepared. Viscoelastic properties of the samples were characterized with dynamic mechanical analysis. Storage and loss moduli, complex viscosity, and damping factor (tan ) of syntactic foam samples were recorded. Dynamic mechanical analysis results showed improvement in storage and loss moduli in glassy region (30°C) up to 12% and 300%, respectively. In rubbery region (150°C), the storage modulus of sisal fiber syntactic foam was three orders of magnitude higher than plain ones. Decrease in the tan peak also indicated improved interfacial bonding by addition and increase in the content of sisal fibers. All these improvements in viscoelastic properties were achieved without any significant change in the density of syntactic foam.
Fiber breakage occurs during the injection molding of fiber-reinforced thermoplastics, resulting in the deterioration of the mechanical properties of the molded parts. In this paper, we propose an effective screw geometry for improving both fiber dispersion and residual fiber length. The proposition is based on the results of our study in which we evaluated five types of screws by investigating the molded specimens. We found that the screw geometry in the compression zone dominantly affected the residual fiber length, and that the choice of an appropriate geometry for the melting and mixing processes was the most important factor for improving both fiber dispersion and residual fiber length. Our experimental observations were compared with the results of flow analysis and consistency was observed. We also discuss the effects of the screw size on the quality of the molded parts.
The composites were fabricated under a variety of different process conditions, ranging from a blend of a low-density polyethylene reinforced with agave bagasse fibers at concentrations of 5, 10 up to 20 wt%. The theoretical and experimental moduli were determined in conjunction with the fiber degradation. The results show that the moduli of the composites increased with fiber content but decreased linearly with increasing processing temperatures. The calculated theoretical module with the Cox model is significantly greater than that experimentally obtained due to the fiber degradation, poor interfacial fiber/polymer interaction, and structural defects in the interior of the composite generated by the thermal conditions. Thermogravimetric analysis shows an unexpected three-stage process of degradation of the fiber in the temperature range of 225–290°C.
To increase understanding of damage evolution in advanced composite material systems, a series of large deflection bending-compression experiments and model predictions have been performed for a woven glass-epoxy composite material system. Theoretical developments employing both small and large deformation models and computational studies are performed. Results (a) show that the Euler–Bernoulli beam theory for small deformations is adequate to describe the shape and deformations when the axial and transverse displacement are quite small, (b) show that a modified Drucker's equation effectively extends the theory prediction to the large deformation region, providing an accurate estimate for the buckling load, the post-buckling axial load-axial displacement response of the specimen and the axial strain along the beam centerline, even in the presence of observed anticlastic (double) specimen curvature near mid-length for all fiber angles (that is not modeled), and (c) for the first time the quantities eff – eff are shown to be appropriate parameters to correlate the material response on both the compression and tension surfaces of a beam-compression specimen in the range 0 ≤ eff < 0.005 as the specimen undergoes combined bending-compression loading. In addition, computational studies indicate that the experimental eff – eff results are in reasonable quantitative agreement with unwoven laminate finite element simulation predictions in the range 0 ≤ eff < 0.010, with the effect of the woven structure appearing to provide the key constraint for various fiber angles that leads to the observed consistency in the experimental eff – eff results on both surfaces.
A new four-node quadrilateral plate that accounts for shear deformation effect and all couplings from the material anisotropy is developed for laminated composite plates. Lagrangian linear interpolation functions are used to describe the primary variables corresponding to the in-plane displacements, while Hermitian cubic interpolation functions are considered for the transverse displacement. Since the present element is derived based on a refined plate theory that has strong similarity with the classical plate theory, it is capable of modeling both thin and very thick plates without shear locking. The accuracy of the present formulation is verified by comparing the results obtained with those available in the open literature. Numerical results are presented to investigate the effects of thickness ratio, lamination angle and lay-up on the shear deformation and response of laminates.
The Batdorf "unit-sphere" methodology has been extended to predict the multiaxial stochastic strength response of anisotropic (specifically transversely isotropic) brittle materials, including polymer matrix composites, by considering (1) nonrandom orientation of intrinsic flaws and (2) critical strength or fracture toughness changing with flaw orientation relative to the material microstructure. The equations developed to characterize these properties are general and can model tightly defined or more diffuse material anisotropy textures describing flaw populations. In this paper, results from finite element analysis of a fiber-reinforced matrix unit cell were used with the unit-sphere model to predict the biaxial strength response of a unidirectional polymer matrix composite previously reported from the World-Wide Failure Exercise. Findings regarding stress–state interactions, thermal residual stresses, and failure modes are also provided. The unit-sphere methodology is an attempt to provide an improved mechanistic basis to the problem of predicting strength response of an anisotropic and composite material under multiaxial loading as compared to polynomial interaction equation formulations. The methodology includes consideration of strength scatter to predict material probability of failure, shear sensitivity of flaws, and accounting for multiple failure modes regarding overall failure response.
Material property characterization tests were performed on satin weave carbon/epoxy composites, where the epoxy resin was modified with 2 wt% carbon nanofibers prior to infusion into a continuous carbon fiber preform. Uniaxial tension tests, tension-tension fatigue tests, and fracture tests were initially performed on 0–3 wt% carbon nanofibers-reinforced epoxy specimens in order to determine the carbon nanofiber weight fraction leading to the optimal mechanical properties of the modified epoxy matrix. In general, the elastic modulus of the modified epoxy increased with increasing carbon nanofibers weight fraction. The ultimate tensile strength, fatigue life, and fracture toughness also increased significantly with increasing amounts of carbon nanofibers and reached maximal values at a carbon nanofiber weight fraction of 2 wt%. The improvement in tensile properties of carbon nanofiber–modified epoxy became more pronounced for specimens loaded at higher strain rates. Further increases in nanofiber content, however, resulted in a relative decrease in nanocomposite strength and fatigue life; this is likely due to local stress concentrations associated with poorly dispersed carbon nanofibers. Vacuum-assisted resin infusion molding was used to fabricate hybrid composite panels consisting of woven carbon fabric and epoxy resin modified with 2 wt% carbon nanofibers. Uniaxial compression, open-hole compression, and short beam shear tests were performed to assess the effect of carbon nanofibers on matrix-dominated composite properties. Hybrid composites containing 2 wt% of carbon nanofibers in the epoxy matrix resulted in compressive strength, open-hole compressive strength, and interlaminar shear strength values that were 19.8%, 27.8%, and 15.8% greater, respectively, than those for woven fabric composites prepared with neat epoxy. Quasi-static uniaxial tension tests and tension-tension fatigue tests of hybrid composite specimens also led to similar enhancements in the composite ultimate strength and fatigue life relative to composites specimens infused with neat epoxy. Scanning electron microscopy images of composite micro-fracture surfaces indicated that randomly distributed carbon nanofibers provide some crack bridging, reduce crack opening, and lead to crack turning for small cracks. Such mechanisms are likely responsible for the improvement in mechanical properties.
A series of thin, low-cost and environment-friendly elastomeric composites consisting of reclaimed rubber, which is a waste product of roller processing of textile mill and seven-hole hollow polyester fibers, were fabricated. In this study, the damping property of the reclaimed rubber composites was tested in the dynamic mechanical thermal analyzer, the sound absorption property was investigated using the impedance tube method, the morphology was characterized by scanning electron micrographs and the mechanical property was measured by strength tester. The study concluded that reclaimed rubber/seven-hole hollow polyester fibers elastomeric composites exhibited an exceptional damping performance with a broad temperature range, and that acoustical absorption of materials increased significantly with increasing seven-hole hollow polyester fibers content. Meanwhile, reclaimed rubber/seven-hole hollow polyester fibers demonstrated an acoustic property of 1 mm thickness and a mass ratio of 100/25, giving a sound absorption coefficient peak, 0.407 at 2500 Hz. The analysis also revealed that the mechanical properties of the composites improved significantly with the increase of fibers. As an acoustical absorption material with high damping performance and a broad temperature range, reclaimed rubber/seven-hole hollow polyester fibers composites have potential applications in fields of engineering.
Nickel matrix composites reinforced with different weight percent of alumina short fibres have been fabricated by powder technology route. The alumina short fibres were encapsulated by nickel layers using the electroless deposition technique. The produced alumina/nickel composite powders underwent cold compaction and sintering at 850°C. The alumina/nickel powders as well as the consolidated composites were investigated by optical microscope, scanning electron microscope, XRD and VSM. It was observed that the surface of the alumina short fibres was completely coated with nickel layer and the microstructures of the consolidated compacts show homogeneous distribution of the alumina short fibres in the nickel matrix. The density, electrical conductivity, coercivity, retentivity, saturation magnetization and hardness of the produced alumina short fibres/nickel composites were measured. The relative sintered density and the saturation magnetization were decreased, but the retentivity, the coercivity and the hardness were increased by increasing the alumina short fibres weight percent in the nickel matrix.
The main objective of this work has been the characterization of wear behavior of the graphene nanoplates (GNPs) in Ni3Al matrix composites (NMC). The friction and wear behaviors of NMC with the addition of 1 wt.% GNPs against Si3N4 ball are tested under different loads using a constant speed of 0.2 m/s. Tribological test results have revealed that small amounts of GNPs are able to drastically improve the antifriction and antiwear properties of NMC. A possible explanation for these results is that, the GNPs not only provide an enhanced effect for NMC to produce better wear resistance, but also form a local protective layer on the contact surfaces to reduce the friction. The investigation shows that GNPs hold great potential applications as an effective solid lubricant for Ni3Al matrix composites and possibly other alloys.
This paper is concerned with the characteristics of the temperature-dependent tensile strength along the longitudinal direction of the unidirectional carbon fiber-reinforced plastic. The temperature-dependent tensile strength of the unidirectional carbon fiber-reinforced plastic was evaluated using resin-impregnated carbon fiber strand (carbon fiber-reinforced plastic strand) specimens with highly reliable co-cured tabs developed by the authors. The tensile strength of carbon fiber-reinforced plastic strand was measured at three constant temperatures below the glass transition temperature of the matrix resin for 50 specimens at each temperature. The results showed that the tensile strength of the unidirectional carbon fiber-reinforced plastic has a Weibull distribution at each temperature. The shape parameter of the tensile strength does not change with the temperatures and the scale parameter decreases clearly with increasing temperature. The degradation rate of the tensile strength of the unidirectional carbon fiber-reinforced plastic by the temperature raise against that of the viscoelastic modulus of the matrix resin agrees well with the predicted results based on the Rosen’s strength analysis.
Elastic properties were predicted for AS4, IM7, T300, and T650-35 graphite fibers. An inverse method was employed using lamina and epoxy properties taken from literature. Fiber properties predicted using finite element analysis based on a hexagonal microstructure, finite element analysis based on a random microstructure, and the Mori-Tanaka averaging scheme were compared. It was observed that the Mori-Tanaka averaging scheme and finite element analysis based on a hexagonal microstructure predicted nearly identical fiber properties. In contrast, randomness in the arrangement of fibers results in significantly different predictions for the longitudinal shear modulus of the fiber. The three microstructural models were also used to predict properties for isotropic E-glass 21xK43. It was observed that the three models predicted nearly the same properties for the glass fibers.
The total light transmittance of hand lay-up glass fiber-reinforced polymer laminates for building construction was investigated with a view to two architectural applications: translucent load-bearing structures and the encapsulation of photovoltaic cells into glass fiber-reinforced polymer building skins of sandwich structures. Spectrophotometric experiments on unidirectional and cross-ply glass fiber-reinforced polymer specimens in the range from 0.20 to 0.45 fiber volume fraction and artificial sunlight exposure experiments on encapsulated amorphous silicon photovoltaic cells were performed. Analytical models have been developed to predict light transmittance through glass fiber-reinforced polymer structures and the percentage of solar radiation reaching encapsulated photovoltaic cells. The total amount of fibers in the laminates was the major parameter influencing light transmittance, with fiber architecture having little effect and regardless of fiber volume fraction. Eight-three percent of solar irradiance in the band of 300–800 nm reached the surface of amorphous silicon photovoltaic cells encapsulated below structural glass fiber-reinforced polymer laminates with a fiber reinforcement weight of 820 g/m2, demonstrating the feasibility of conceiving multifunctional glass fiber-reinforced polymer structures.
Carbon-fibre epoxy panels have been subjected to rapid high temperature loads. The effects of temperature, exposure time and moisture content of the panels have been studied. It could be demonstrated that the combination of high moisture content and rapid heating can lead to excessive damage, such as sudden formation of delaminations up to the development of large bubbles on the panel. While it is well known that matrix and fibre–matrix interface strength generally decrease both with water uptake and temperature increase, a more severe damage mechanism has been observed here. The combination of high moisture content and rapid high temperature loads leads to an internal vapour pressure overload that can cause extensive cracks and delaminations. Subsequently, this permanent damage leads to serious changes in mechanical properties. Whereas heating dry panels to 350°C for 1 h and testing them at room temperature reduced interlaminar shear strength less than 25%, many of the moist panels were fully destroyed within the first minute of heating.
This paper reports the effect of sulfuric acid solution absorption on the dynamic mechanical properties of glass fiber reinforced polyester composites. An experimental device is presented that allows the effect of changes of environment and temperature on both the diffusion coefficient and the maximum moisture content to be measured. The results indicate that increased temperature raises both the diffusion coefficient and the maximum absorbed moisture of all the composite materials used in this study. Moreover, it has been noted that specimens exposed to higher temperature absorb much more water than those held at low temperature. The dynamic mechanical properties of glass fiber reinforced polyester were found to be highly affected by the presence of absorbed solvents in the specimen and increases in temperature. A fractographic study has been conducted using a scanning electron microscope of the surfaces of specimens broken by tensile testing. Observations indicate that increased immersion time increases the deterioration of the fibre/matrix interface.
The diffuse light transmittance of hand lay-up glass fiber-reinforced polymer (GFRP) laminates was investigated. Spectrophotometric experiments were performed on unidirectional and cross-ply glass fiber-reinforced polymer specimens with fiber volume fractions ranging from 0.20 to 0.45. Numerical ray-tracing analysis was used to investigate the experimentally observed wavelength dependency of the diffuse light transmittance. Refractive index mismatches between glass fibers and resin and the presence of air flaws in the laminates were the major parameters increasing light diffusion. Based on the experimental data, analytical models were developed to predict the translucency (haze) of glass fiber-reinforced polymer laminates as a function of the reinforcement weight and total light transmittance. The developed models demonstrate the feasibility of conceiving glass fiber-reinforced polymer skylights with a translucency of 0.90 and a total light transmittance of 0.50 for the daylighting of energy-efficient buildings. It is also shown that laminates with translucencies of lower than 0.30 satisfy minimum total transmittances of 0.83 as required for the encapsulation of photovoltaic cells.
Nanoindentation and nanoscratch tests made on the surface of Kevlar KM2 fibers impart contact geometries similar to a typical contact between a particle and a fiber in a particle-infused fabric. In this study, the forces required for indentation and scratching are used as a measure of a single particle gouge of the surface of a Kevlar fiber. The gouging forces and a geometric model are used to calculate the apparent friction and energy associated with particle gouging during impact. The friction associated with particle gouging can be increased up to ~240% compared to Kevlar yarn-yarn friction levels. The energy of gouging a distribution of particles is on the order of 10% of the energy required for axial tensile failure and 25% of the energy required for transverse compression of the fiber (for a limiting transverse strain of ~0.3). The relative contributions of the friction and energy associated with particle gouging to the overall energy dissipated by a textile composite are discussed.
Composites based on metallic fibers and thermosetting polymers are being increasingly used for molding blocks of hybrid injection molds, thereby improving the mechanical and thermal properties. However, an adequate study on the behavior of steel fibers in a reactive epoxy resin is necessary to understand how to maintain suitable mold properties. In this paper, the sedimentation velocity of short steel fiber suspensions in reactive epoxy resin was estimated using a model emerging from the Stokes equation and considering the resin rheology and correction factors for the fiber shape and concentration. DMP (2,4,6-tris (dimethylamino-methyl) phenol) was the accelerator more suitable for this type of composites because it increases the rate of cure and reduces the gel time more pronouncedly than any of the other common accelerators. Samples were manufactured with epoxy resin, short steel fiber and DMP as accelerator, and using anti-sedimentation equipment. The distribution of the fibers was observed in all composites. The viscosity data were used to predict the time in the anti-sedimentation equipment necessary to reach a minimum sedimentation velocity using the mathematical model. Results showed that this velocity is recommended to be below 3.28 x 10–8 m/s to avoid sedimentation of the steel fibers.
This work presents the application of an ultrasonic method to measure stresses in unidirectional carbon fiber composites with epoxy matrix (HexTow® AS4/Hexply® 8552). This kind of composite is largely employed as a structural material in the aeronautical industry. The ultrasonic method is based on the acoustoelastic principle, a principle that holds that wave speed is affected by variations in strain in the material. We employ critically refracted longitudinal waves (Lcr waves) and relate their time-of-flight with applied strains and stresses. We first performed tests on a polygonal specimen evaluating the influencing factors on the results, that is, temperature and transducer positions, as well as their effects on each fiber direction: 0°, 45°, and 90°. Tensile tests were next performed on rectangular specimens, as we sought, for each fiber direction, the relation between stress and wave speed variation, otherwise known as the acoustoelastic coefficient. The results showed that the wave speed was sensitive to the stress variation along the fiber direction (0°) and nearly insensitive to any other direction. Also, temperature (between 20°C and 27°C) was not a relevant factor for waves propagating along the fiber. These findings support the notion that the method holds promise as an alternative to measuring stresses in multidirectional composite materials.
The aim of the present study was to propose a unified anisotropic elasto-viscoplastic damage model and computational analysis method for predicting the damage growth and material behavior of initially anisotropic glass-fiber-reinforced polyurethane foam (RPUF). A plasticity damage mechanism-based mechanical model with no yield surface is introduced. To consider the microcrack closure effect according to different loading directions, two types of elasto-viscoplastic damage models were developed. The developed models were transformed into the implicit form and implemented in an ABAQUS user-defined material subroutine for the application of finite elements. Finally, the simulation results were compared with a series of tensile and compressive test results on RPUF to validate the proposed model and computation.
The influence of hot pressing conditions on mechanical properties of nickel aluminide/alumina composite has been investigated in the present paper. In particular, effect of the process parameters, viz. compacting pressure, sintering temperature and sintering time on the evolution of density, elastic constants and tensile strength properties of the intermetallic-ceramic composite has been studied. Elastic constants, the Young's modulus and Poisson's ratio, have been evaluated using an ultrasonic testing method, and the tensile strength has been determined by a Brazilian-type splitting test. Microscopic observations of microstructure evolution complemented the experimental procedure. Experimental results have been confronted with theoretical models showing a good agreement between the data compared.
This study investigates failure behaviors of woven glass fiber-reinforced epoxy resin composite plates with two parallel pins jointed and under the effect of seawater. The effects of joint geometry and immersion time in seawater were analyzed by experimental and numerical methods. In order to observe the effects of seawater, the samples were kept in seawater for periods of zero, three, and six months. For the observation of the joint geometry effect on the failure behaviour, the edge distance-to-upper hole diameter (E/D), the two hole-to-hole centre diameter (K/D), the distance from the upper or the lower edge of the specimen to the center of the hole-to-hole diameter (M/D), and the width of the specimen-to-hole diameter (W/D) ratios were selected as geometrical parameters. The numerical study where the progressive failure analysis was employed was carried out through a sub-program running in ANSYS 11.0 finite elements program. In order to predict the failure loads and failure types in the numerical analysis, the Tsai-Wu failure criterion was used along with material degradation rules. At the end of the study, it was determined that increase of the immersion time in seawater caused weaker mechanical properties and decrease in failure loads of samples. It was also found that the results of progressive failure analysis were consistent with the experimental results.
Functionalized multiwall carbon nanotubes (MWCNTs)-reinforced copper (Cu) nanocomposites have been fabricated by the combination of ball milling and hot-press sintering methods. The functionalization was carried out in two levels under reflux condition. Characterization of the fabricated nanocomposites revealed that the functionalization plays an important role in enhancing the hardness and electrical properties of nanocomposites. Enhancements of up to ~116% and ~58% in microhardness compared to pure un-milled and milled Cu were observed by adding 1 wt.% functionalized MWCNTs. On the other hand, the electrical resistivity of nanocomposites increased by increasing MWCNT contents. It was found that improved control of the Cu and MWCNT interface also leads to enhance electrical properties. Based on the experiments, the results indicated that the amount of chemically bonded oxygen atoms affects the electrical resistivity of nanocomposites. Accordingly, the nanocomposite containing 1 wt.% MWCNTs with lower amount of functional groups has lower electrical resistivity.
The monotonic and fatigue tensile strengths of acid-treated carbon nanotube/epoxy composites with various carbon nanotube contents were experimentally studied at –25°C, 0°C, 25°C, and 40°C. Experimental results reveal that the monotonic and fatigue strengths of the nanocomposites decreased as the temperature increased. The temperature-dependent S-N curves of the nanocomposites with various carbon nanotube contents were experimentally determined. The fatigue strength exponents in the power laws that described the S-N curves were independent of the carbon nanotube content and the testing temperature, and the reciprocals of the fatigue strength coefficients were related to the testing temperature according to the Arrhenius model. Nanocomposites with higher monotonic strength had a lower pre-exponential factor and activation energy. The epoxy-based nanocomposites presented obvious cyclic softening and dynamic creep characteristics in the fatigue tests conducted at 40°C. However, adding carbon nanotubes in the epoxy can diminish these phenomena significantly. The fracture surfaces demonstrated that the length of the pull-out carbon nanotubes increased with the testing temperature, indicating that a high temperature weakened the adhesion strength between the carbon nanotube surfaces and the polymer matrix.
The objective of this research was to study chip formation mechanisms in order to predict damage in the orthogonal machining of unidirectional polymer–matrix composites by experimentation and analysis. This study presents a new analytical formula, which is able to successfully predict the fiber orientations at which depth of machining damage is higher based on different cutting parameters and can be used to minimize machining damage by selecting proper fiber and matrix materials. Orthogonal cuttings were conducted by experiments and then compared to the developed theory. The variables that were presented for this research were tool geometry and fiber orientation.
In this study, buckling behavior of adhesively patch repaired composite plates was investigated experimentally and numerically. Unidirectional carbon/epoxy composite plates with circular cutout were repaired with an adhesively bonded patch. Critical buckling loads of composite plates were researched without cutout, circular cutout, single patch-repaired, and double patch-repaired conditions. In addition to circular hole dimensions, patch length and adhesive thickness were used as geometrical parameters. Numerical study was performed in ANSYS finite element software three dimensionally. As a result, the critical buckling loads of single and double patch-repaired composite plates were increased from 96 and 263 ratios higher than circular-cutout composite plates. The percentile errors between experimental and numerical studies were determined from 2 to 11.5.
Acrodur solution and dispersion have been developed as alternative wood adhesives to phenolic and urea formaldehyde resins. They are non-corrosive and do not emit carcinogenic gases. Contrary to most resins used in natural plant fibre composites, Acrodur has superior tolerance to moisture during composite fabrication and thus drying of the preforms may be minimised or eliminated. The aim of this study was to produce optimised flax fabric reinforced Acrodur biocomposites by varying the ratio between Acrodur solution and dispersion, relative humidity, curing time and temperature. The optimised biocomposites provided a combination of specific tensile strength 57.9 MPa-cm3/g and specific Young’s modulus 5.5 GPa-cm3/g at a low density 0.91 g/cm3. Thermogravimetric analysis and contact angle measurement showed that the biocomposites had higher thermal stability and hydrophobicity than the fabrics. The low loss of tensile properties upon water immersion was explained by tortuous wicking path in the biocomposites.
Compared to the laminated composite made of two-dimensional preform, the three-dimensional-woven composites have been evolved as an attractive structural material. Based on the classical textile technologies such as weaving, braiding, and knitting, complex preforms have been fabricated as reinforcement for technical applications. In the recent years, many studies have been published on the development of modeling techniques to design, analyze, and understand the effect of the preform architecture on its mechanical behavior. In this paper, we propose to use these numerical approaches to evaluate the mechanical performance of multiaxis three-dimensional-woven preform. Also, comparisons of the estimated mechanical properties of this structure with that of 3D orthogonal-woven preform and that of classical laminate are conduced.
In natural fibre based-composites, the interfaces within the fibre bundles are not as well described as those between fibres and classical polymer matrices. To compensate for this lack of information, which is however of great importance when trying to model the composite behaviour, morphological and mechanical characterization of the middle lamella present between flax fibres has been carried out. The interfacial strength of the pectic cement was found to be very low compared with that of the fibre/matrix interface. Assuming this fibre/fibre interface to be a cohesive zone material and using the experimental results obtained on pairs of fibres, a model has been developed at the scale of the fibre and applied to study numerically the tensile properties of bundles. The results of the simulations were in agreement with the experimental data obtained on bundles, both leading to a mean bundle strength of 500 MPa.
The tensile properties of two types of alumina (Al2O3 and Al2O3–SiO2) and short fiber-reinforced A366 alloy composite created by low-pressure infiltration have been studied. The applied pressure and temperature of molten alloy are 0.4 MPa and 1073 K, respectively. The composite containing 10 vol.% of fiber preform was sectioned and the microstructure was observed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Tensile properties of Al2O3 fiber-reinforced A366 alloy composites were investigated, in conjunction with investigation of effects of amounts of SiO2 sol added as binder to fabricated preform and effects of changed chemical composition of Al2O3 fiber. The composite with SiO2 sol of 8 mass% has higher relative density in comparison with composite with SiO2 sol of 2 mass%. SiO2 sol raised the relative density of composite by the reaction with aluminum. In addition, the ultimate tensile strength of composite which has Al2O3 fiber-reinforced A366 alloy composite containing SiO2 sol of 8 mass% was approximately 20% higher than that of Al2O3–SiO2 fiber-reinforced A366 alloy composite containing SiO2 sol of 8 mass%.
Hazelnut husk is widely available as a biowaste in the northeast of Turkey. In this study, biocomposites were manufactured using hazelnut husk, polypropylene, and coupling agent. Density, thickness swelling, water absorption, modulus of rupture, modulus of elasticity, tensile strength, tensile modulus, and elongation at break tests were carried out to determine its performance properties. The results obtained in this study indicated that the performance properties of the biocomposites manufactured from hazelnut husk could be comparable to neat PP composites. The biowaste-loading level positively affected the modulus properties of the biocomposites. It could be concluded that the hazelnut husk could be used as an alternative raw material to produce new biocomposites so that biowaste could be recycled. It can also help sustainable protection of nature.
Repeated impact of foreign bodies on laminated composites may give rise to delamination damage. In this paper, experimental information is employed to formulate a growth law for delamination damage in terms of the impact energy per impact and number of impacts. The growth of regions of delamination is considered a stochastic problem, and hence the growth law is placed in a probabilistic setting by considering the evolution of a probability density function of delamination damage as the number of impacts increases. For a specific number of impacts, this formulation is used to determine the probability of a delamination in a selected range of delamination sizes. The formulation has been extended to include the effect of probability of detection as well as the effect of variable impact energy according to a probability density function. Finally, random variation of impact location is taken into account by the equivalent effect of a discrete probability function for the number of impacts at a fixed location.
Coir fiber–reinforced polypropylene-based unidirectional composites were prepared by compression molding. Mechanical properties like tensile strength, tensile modulus and impact strength of the resulting composites were found to be increasing with increase in the loading of coir fibers, reached an optimum and thereafter decreased with further increase in fiber loading. Based on fiber loading, 30 wt% fiber-reinforced composites had the optimum set of mechanical properties. After alkali treatment of coir fiber, tetramethoxy orthosilicate treatment was conducted to promote adhesion between coir fiber and polypropylene matrix. Treatment of the coir fiber with tetramethoxy orthosilicate after the alkali pre-treatment enhanced the mechanical properties and water desorption of the resultant composites, resulting from the improved adhesion between the coir fiber and polypropylene matrix. These results were also confirmed by the scanning electron microscope observations of tensile fracture surfaces of coir fiber/polypropylene composites. The interfacial shear strength of the composites was also measured using a single-fiber fragmentation test and a microbond test.
Polysaccharide has become one of the most promising resources to substitute synthetic plastics because of its wide abundance, renewability, low cost and near-zero carbon footprint. In this work, a novel polysaccharide-derived biopolymer has been investigated as a potential candidate in developing hybrid biocomposite foam. Through the study of microcellular morphology and thermal degradation behaviour, an optimised processing methodology was identified. A synergistic technique of ball milling of modified biopolymer granules and two-step foam processing resulted in achieving reduced particle sizes, narrower particle size distribution and improved dispersion of biopolymer, which in turn resulted in enhanced microcellular morphology within the hybrid biocomposite foam. Considering the renewed interest for such bio-based materials and their potential use as reinforcements and fillers, a much needed study was undertaken to assess its health safety in terms of in vitro biocompatibility and cytotoxicity. The results from the studies indicated that modified biopolymer did not pose any health safety risks and were non-cytotoxic.
Composite laminates have low resistance under dynamic loading, particularly impact loading. A low-velocity impact on laminated composites causes various types of damage, such as delamination, fibre breakage, matrix cracking and fibre matrix interfacial debonding. Post-impact compressive strength is one of the greatest weaknesses in carbon fibre reinforced plastics laminates. After impact, due to the delaminations present in the laminates, local instability is triggered, which ultimately reduces considerably their residual strength. In this work, symmetric cross ply carbon fibre reinforced plastics laminates [(0°/90°)2]12 were subjected to falling weight impact at two different velocities, 2.5 and 3.5 m/s. Compression after impact studies showed substantial differences in failure mode between the two cases, passing from end crushing to crack propagation with higher impact energy. Acoustic emission technique was able to confirm this result and characterize the different types of failure modes during compression after impact test, in particular by frequency distribution.
Al-A206/5 vol.% aluminap cast composites were synthesized by the addition of reinforcing particles into molten Al alloy, semi-solid and liquid states, in two different forms: (1) as received alumina particles and (2) pre-synthesized composite reinforcement prepared via milling of alumina with Al and Mg powders (master metal matrix composite). The effects of powder addition technique, reinforcing particle size and interface bonding strength on tensile properties and fracture behavior of Al-A206/5 vol.% aluminap composites were then investigated. It was found that fabrication of Al-A206/aluminap composites by master metal matrix composite addition in the semi-solid state leads to considerable improvement in tensile properties.
Carbon nanotubes were decorated with silver nanoparticles (CNTs/Ag) using N,N-dimethylformamide as a reductant in this study. A new heteroaromatic azo-polymer, poly(azo-thiourea) (PAT) was also prepared using 4-(4-aminobenzyl)benzenamine and diazonium salt solution of 2,6-diaminopyridine. PAT was subjugated as a matrix material to synthesize new hybrids using CNTs/Ag as filler. To prepare the nanocomposites, melt compounding and solution mixing techniques were employed. The results showed that the CNTs/Ag effectively improved the electrical conductivity, mechanical and thermal properties of CNTs/Ag/PAT nanocomposites. The solution mixing method resulted in a better dispersion of filler leading to higher mechanical strength (52.47–54.36 MPa) relative to melt system (29.11–39.22 MPa). The solution method also gave better electrical properties due to lack of oxidization of silver nanoparticles. Consequently, increasing the filler content from 1 to 5 wt% increased the electrical conductivity from 4.89 to 6.12 S/cm. Scanning electron micrographs revealed fine dispersion of filler and adhesion of matrix to the nanotube surface in solution method. Thermal stability of the materials studied via 10% gravimetric loss was found to increase from 522°C to 543°C (solution) and 512°C to 521°C (melt). Similarly, glass transition of solution mixed system (242–249°C) was higher than the melted one (212–217°C).
Researches on the armor systems composed of composite materials with ceramic frontal face and polymer-based back-support are continuously developing further. This study, which mainly covers the impact behavior of ceramic composite armors, is a two-stage research. The first stage involves the investigation of component-level impact characteristics and failure mechanisms of the ceramic composite armors. At this stage, low-velocity impact behavior of ceramics and fiber-reinforced composites is investigated. Impact test results revealed that impact loading is of dynamic nature and strength of the composite materials under dynamic loading increases considerably as a result of strain rate sensitivity, which makes them the right choice to be used in conjunction with ceramics in armor systems. The second stage examines the ballistic impact behavior and ballistic performance of the armor systems. The extent and pattern of impact damage related to projectile velocity are determined for the armor components and the armor itself.
An experimental apparatus utilizing double cantilever beam specimens loaded with uneven bending moments was developed to study the mixed-mode fatigue crack growth in composites. The approach is suitable when large-scale bridging of cracks is present. To illustrate the testing method, cyclic growth of delaminations in a typical fibre-reinforced polymer composite was investigated under a constant cyclic loading amplitude. Pure mode I, mode II and mixed-mode crack growth conditions were examined. The results, analysed using a J-integral approach, show that the double cantilever beam loaded with uneven bending moments configuration provides a robust approach to investigate the fatigue crack growth of composites for pure mode and mixed-mode cracking. A steady-state crack growth regime was observed for mode I and mixed-mode loading. For mode II loading, steady-state was absent, and a progressively decreasing crack growth rate observed. In addition to details concerning the equipment, a general discussion of the development of cyclic bridging laws for delamination growth in the presence of large-scale bridging is provided.
The influence of geometry and structure of different commercial multi-walled carbon nanotubes (MWCNTs) have been analysed. MWCNTs have been characterized by transmission and high-resolution scanning electron microscopy, measurement of specific surface area by nitrogen isotherm and X-ray diffraction. The behaviour of carbon nanotube/epoxy composites has been studied by dynamic mechanical thermal analysis, differential scanning calorimetry, measurements of density and electrical conductivity. Composites manufactured at the same experimental conditions and with the same nanofiller content presented different thermal, mechanical and electrical properties. Despite using MWCNTs with similar aspect ratio, the presence of surface defects on the nanotube structure induces an important decrease in storage modulus and electrical conductivity of composites. The functionalization of MWCNTs leads to composites with lower electrical conductivity due to the breaking of sp2 carbon delocalization and also due to the insulating polymer film wrapping the nanotube. In contrast, resin reinforced with long MWCNTs present higher modulus and electrical conductivity than those filled with shorter nanotubes.
This study reports on the use of polyvinyl alcohol–based needle punched nonwoven fabrics in cement-based composites as a low-cost reinforcement. In this study, a nonwoven reinforcement using crimped polyvinyl alcohol fibers for cement-based composites was developed. The incorporation of nonwoven fabric in composites improved tensile and flexural properties compared to a discrete polyvinyl alcohol fiber-reinforced composite. Formation of multiple fine cracks, crimped fibers and mechanical interlocking of fibers in the nonwoven fabric structure resulted in superior mechanical performance. The failure mode was due to a single macro-crack for the discrete fiber reinforced composite as opposed to a number of fine cracks for the nonwoven reinforced composite.
New ceramic electrospun fibres, SiO2 nano-particles covered ZrO2 fibres (SiO2@ZrO2), are facilely fabricated, which have many hydroxyl groups on the surfaces, and their diameters and chemistries are controllable. Based on this, novel SiO2@ZrO2/cyanate ester composites with desirable interfacial adhesion were prepared, completely overcoming the drawback of ZrO2/cyanate ester composites; moreover, the SiO2@ZrO2/cyanate ester composites show much higher storage moduli and glass transition temperatures as well as lower dielectric constants and losses than ZrO2/cyanate ester composites. These interesting results originate from the double effects of the unique structure of SiO2@ZrO2 fibres. Specifically, the active groups on the surfaces of SiO2@ZrO2 fibres bring the chemical bonding with the matrix, while SiO2 particles on the ZrO2 fibres provide physical interlocking between the matrix and fibres. These attractive properties of SiO2@ZrO2/cyanate ester composites suggest that the method proposed herein is effective to prepare electrospun fibres with unique and controllable structure as well as high-performance ceramic/polymer composites.
New polyethylene matrix and alumina whiskers composites have been designed in order to combine the processability of common thermoplastics with improved physical properties. This work analyzes the influence of the composite formulation on the morphological, rheological and thermal properties of the new materials. Concerning rheological properties, a significant increase in viscosity and storage modulus is observed for high alumina whiskers content. Furthermore, the whiskers were functionalized with silane coupling agent in order to improve compatibility with the matrix. Two surface treatments were used for comparison purposes, and Fourier transform infrared spectroscopy was applied for evaluating the chemical changes on the surface of whiskers. Pre-treatment with the silane coupling agent brought about beneficial changes in morphology and rheology, related with improved dispersion of whiskers and increased filler–matrix interface. Finally, the inclusion of only 5 wt.% filler, functionalized with 100 wt.% silane, increased the thermal stability of matrix around 37%.
A meso-scale finite element model for static damage in textile composites was established. The impregnated yarn is taken as homogeneous and transverse isotropic material, whose mechanical properties are calculated using Chamis’ equations. The damage modes are determined by using the Tsai-Wu criterion and additional criteria. The Murakami damage tensor is used to calculate the post-damage stiffness matrix. The model has been validated using plain weave and twill weave carbon–epoxy composites. The initiation of inter-fiber matrix cracks and fiber rupture were analyzed using this meso-FE model.
The low-velocity impact response of sandwich structures based on fully-recyclable skin and core materials has been investigated. Particular attention has been focused on structures based on self-reinforced polypropylene skins combined with a polypropylene honeycomb core. Two types of skin designs were considered, the first being based on a single ‘as-supplied’ monolithic self-reinforced polypropylene laminate and the second being manufactured from thin self-reinforced polypropylene sheets bonded together using a hot-melt polypropylene film. For comparative purposes, a limited number of tests have also been carried out on a more conventional GFRP/aluminium honeycomb sandwich structure. Drop-weight impact test have shown that all-polypropylene honeycomb sandwich structures absorb significant energy through plastic deformation in the composite skins as well as plastic buckling in the honeycomb core. It has also been shown that the design of the self-reinforced polypropylene skin has a significant effect on the energy-absorbing characteristics of the sandwich structure, with the performance of systems based on multiple layer skins greatly exceeding that observed following tests on a monolithic design. Tests on plain laminates also yielded similar conclusions, with multilayer systems offering much higher perforation resistances than their plain counterparts. Finally, it has been demonstrated that when the impact data are normalised by their respective areal densities, the all-polypropylene composites significantly out-perform GFRP/aluminium honeycomb sandwich structures.
The effect of gamma radiation on the morphology, thermal behavior and mechanical properties of wood polypropylene composites has been investigated. Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been performed on wood polymer composite (WPC) samples of 9.5 ± 0.1 mg. These samples were exposed to different gamma dose in the range 10–100 kGy. The results indicated that gamma radiation improves the mechanical properties while the thermal stability is decreased. With gamma radiation, the scanning electron microscopy (SEM) of the micrographs became smoother and we can notice an improvement of interaction between polymer and wood fibers.
Glass fiber-reinforced epoxy resin (GFRE) composites filled with aluminum nitride (AlN) powder were fabricated, and their thermal and electrical properties were investigated. It is observed that with the increasing percentage of AlN particles, there is a significant enhancement in thermal conductivity and micro-hardness, but there is a decline in tensile strength. Experimental results demonstrate that the GFRE composites with 10 wt% of AlN loading show maximum dielectric breakdown strength of 30.26 kV/mm with minimum volume resistivity of 7.5 x 1014 cm and thermal conductivity value of 0.300 W/mK. Scanning electron microscopy studies were conducted to observe the voids and distribution of filler in composites.
This paper outlines the findings of a study on a range of stainless steel and titanium alloy lattice structures manufactured using the selective laser melting technique. The effect of varying key manufacturing parameters on the properties of lattice strands was studied through a series of single-filament tensile tests. The resulting failure mechanisms were investigated using a scanning electron microscope. The resulting observations have shown that the properties of these lattice strands are determined by the laser energy during the manufacturing process, which in turn is controlled by the laser power and laser exposure time. The quasi-static and low-velocity penetration behaviour of lattice core-based sandwich panels has been examined, and an aluminium foam and an aluminium honeycomb were chosen to benchmark their performance. The impact resistance of the lattice core-based sandwich structures were shown to be dependent on both the manufacturing parameters and lattice unit-cell geometry of the lattice structure. The impact resistances were improved by increasing manufacturing laser energy and lattice core density. A series of drop-weight tests at velocities up to 6 m/s have shown that the penetration behaviour of the titanium alloy lattice cores and the aluminium honeycomb cores is similar.
An experimental investigation was conducted to evaluate the structural behavior and chloride penetration characteristic of reinforced concrete columns to which patch repair was applied. A total of 24 reinforced concrete columns were fabricated. Four repair materials, with different types and contents of admixtures, were selected. The contents of the three admixtures (silica fume, zeolite, and polymer dispersion) were 0%, 10%, and 15%, based on the weights of the admixture and cement. For the experimental methods, a chloride penetration test and an axial compression test were used. As a result of the experiments, compression failure occurred in the control column. For the repaired columns, however, interfacial failure was observed, as the concrete core and repair material were debonded. The repair material that showed the greatest effect was mortar, for which polymer dispersion was used as the admixture. The ultimate strength and the strain at ultimate strength of the column that was repaired with this mortar increased by 40% and 80%, respectively, compared to the control column. Furthermore, the repair mortars that were mixed with the admixture showed much lower chloride concentrations than the control column. In particular, the chloride concentration of the mortar that was mixed with 15% polymer dispersion decreased by 95%, in the section with a depth of 20–30 mm from the column surface.
New innovative basalt fiber/epoxy composite materials are used in engineering applications such as aerospace, automotive, and civil structures due to the potential low cost of this material together with its mechanical characteristics and its failure mechanisms. Acoustic emission is a passive nondestructive testing technique for real-time monitoring of damage developed in materials and structures, which have been used successfully for the identification of damage mechanisms in composite joints under tensile loading. The present study is focussed on acoustic emission characterization of failure modes in three prominent joining methods namely, bonded, riveted, and hybrid joints during tensile test. Parametric analysis is performed on the acoustic emission data obtained during the tensile testing of these types of joints to discriminate the failure modes. Fast Fourier transform analysis using acoustic emission waveform analysis is carried out to analyze the different failure events and associate them with their dominant frequency ranges. The predominance of failure modes in each signal is used as a key in the study to discriminate failure modes on single-lap joints in basalt/epoxy composite laminate, and the results are validated with fast Fourier Transform analysis.
This paper investigates fabrication and mechanical properties of L-930 carbon fiber reinforced polymer woven prepreg composites cured using three different techniques: (1) thermal, (2) microwave, and (3) the combination of microwave and thermal. The third technique was established through parametric studies and proved to be an efficient method in curing the carbon fiber reinforced polymer prepregs without compromising mechanical properties, but with significant energy and time saving. With 20 min in microwave at 510 W followed by 60 min thermal curing in convection oven at 120°C provided optimum results in terms of time and energy saving. The process was 2.5 times faster than and consumed only 1/4th of energy required for the autoclave curing. The mechanical properties in tension and flexure of carbon fiber reinforced polymer [0/90] and [±45] laminates cured with this technique were 1–5% better than the autoclave cured laminates. The details of the composites fabrication, the curing techniques, the experiments conducted along with the rationale and underlying reasons for the success of the new "microwave–thermal" curing technique are presented in this paper.
Natural plant fibres are short fibres that must be spun into continuous length yarns for the production of structured composites. Fibres in a twisted singles yarn are poorly aligned. The fibre alignment can be improved without sacrificing the yarn strength by forming a two-ply yarn from two singles yarns. In this paper, we analysed the differential geometry of fibre trajectory using an idealised twisted yarn model and derived the optimum two-ply yarn structure that gives the maximum Krenchel fibre orientation factor. In the optimum two-ply yarn, the ply twist is in the opposite direction to the singles twist and the ply-to-singles twist ratio is 0.28. Such a two-ply yarn construction is beneficial for all twisted yarns aimed for structural composites applications, particularly for yarns made from low cost natural fibres which are usually of short length, low strength and poor uniformity and thus require high twist to achieve sufficient strength for yarn manufacture and further handlings in composite fabrication.
Polypropylene nanocomposites containing 1–5 wt% of nano α-alumina particles are prepared using a Hake internal mixer. Mixing of nano α-alumina particles is performed at 170°C and 50 rotational per minute is set for the rotor speed. To improve the dispersion of the nanoparticles, sodium dodecylbenzenesulfonate is used. X-ray analyses reveal that the basal spacing of polypropylene/nano α-Al2O3 composites compared to the pure polypropylene spreads out. The peak intensity for polypropylene / nano α-Al2O3 composites is stronger and narrower with larger amounts of the nano α-Al2O3 filler, in comparison with the virgin polypropylene. According to differential scanning calorimetry, the crystallinity of the nanocomposites is increased with increases in nano α-Al2O3 filler loading. The storage and loss modulus of the nanocomposites are found to be higher than that of pure polypropylene, because nanofiller increases the stiffness of the nanocomposites. The tensile strength and tensile modulus of the polypropylene nanocomposites are slightly improved up to 4% of nano α-Al2O3 particle filler adding. The addition of filler content higher than that amount leads to the reduction of these properties. The formation of filler agglomeration site within the matrix body affects the decreasing of properties. Transmission electron microscopy observations confirm these results.
The present paper deals with the mode I and pull-out performance of unreinforced and with pin-reinforced carbon-fibre-reinforced plastic specimen. Especially, the influence of the pin geometry on the aforementioned tests was investigated. The rectangular pins require a higher pull-out force and cause higher energy consumption during the frictional pull-out compared to circular pins. The influence on the mode I performance of the rectangular pins depends on their orientation relative to the carbon fibres of the laminate.
Based on the three-dimensional theory of elasticity, a comprehensive stress analysis is performed for the rotating bidirectional functionally graded thick axisymmetric circular/annular plates, for the first time. The plate may be subjected to arbitrary distributions of the transverse load and various mixed (Dirichlet-type and Neumann-type) edge conditions. Furthermore, the circular plate may be supported by a nonuniform elastic foundation or a rigid substrate. In contrast to the very limited works presented for the rotating functionally graded circular plates so far, the transverse flexibility and the transverse stress components are considered and studied in the present research. Since finite element and boundary element techniques, due to their integral natures, cannot adequately trace abrupt changes of the quantities, a second-order point collocation method with forward–backward schemes is adopted to solve the system of the governing and boundary conditions. Effects of the distributions of the various material properties (Poisson’s ratio, Young’s modulus, and mass density), angular velocity, foundation compliance, and edge conditions are evaluated. Results reveal that radially graded or transversely graded material properties significantly affect distribution and magnitude and location of the extrema of the stress components and the lateral deflections and orientation of the general neutral surface of the plate.
The effect of hot-wet environment on the structural properties of composite T-joints reinforced with z-pins is experimentally investigated. The properties of unpinned and z-pinned carbon fiber-epoxy T-shaped joints were determined following conditioning in a hot (75°C) and humid (85% relative humidity) environment for increasing times up to five months. Water absorbed by the z-pinned joint weakened the mode I bridging traction load generated by the z-pins. However, the structural properties of the z-pinned joint were not affected by the hot-wet environment because they are strongly influenced by the friction-induced traction load generated during z-pin pull-out, which was not affected by absorbed water. The effect of water removal by drying on the restoration of the z-pin bridging traction load and the structural properties of the z-pinned joint are also investigated.
The effect of introducing semi-circular shear keys in at the skin-core interface of the composite sandwich panels is illustrated numerically in the current study using ABAQUS software. Particularly, the effect of the shear keys orientation (shear grid), namely ±15°, ±30°, ±45°, ±60° and 90/0°, on the shear response of the sandwich panel is introduced. Composite sandwich panels consisting of polyurethane foam core sandwiched between stiff glass fibre reinforced polymer skins were used, while chopped strand glass fibre impregnated with epoxy resin was utilized for the keys. The nonlinear finite element model was built to capture the shear response and the damage mode of the sandwich panel with the shear grid. The finite element model nominated the model with ±60° grid orientation to be the most sustainable one among the other investigated models. In comparison to the model with shear keys in one direction (uni-axial model), the model with shear grid showed a significant reduction in the shear strength.
Macro and microstructural damage was studied in typical glass fibre-reinforced polymer laminates with two types of coating systems – (a) isophtalic gel coat: pure or mixed with SiO2 nanoparticles 5 wt% (5%N) or 10 wt% (10%N) and (b) a two-layer coating system: urethane-modified vinyl ester-based gel coat and additional polyester barrier coat layer. Accelerated tests were performed at 50°C for 50 days for specimens with nanoparticle-modified gel and for the specimens with or without a barrier coat. Long-term ageing behaviour at 23°C was assessed for the two-layer systems. Nanoparticle-reinforced specimens (10%) showed some advantage over ‘barrier coat systems' in terms of blister incubation time (16 and 13 days, respectively), while for 5% and 0% nanoparticles, blister incubation times were 3 and 7 days, respectively. Round and/or acicular blisters were observed on the examined surfaces. The varying size, shape and distribution of blisters was illustrated by macro and micrographs showing various failure modes associated with surface blisters.
Recent research works in the area of experimental and computational analyses of microscale mechanisms of strength, damage and degradation of glass fiber polymer composites for wind energy applications, which were carried out in the framework of a series of Sino–Danish collaborative research projects, are summarized in this article. In a series of scanning electron microscopy in situ experimental studies of composite degradation under off-axis tensile, compressive and cyclic loadings as well as three-dimensional computational experiments based on micromechanics of composites and damage mechanics, typical damage mechanisms of wind turbine blade composites were clarified. It was demonstrated that the damage mechanisms in the composites strongly depend on the orientation angle of the applied loading with the fiber direction. The matrix cracking was observed to be the main damage mechanism for tensile axial (or slightly off-axis axial) loading; for all other cases (off-axis tensile, compressive and cyclic tensile loadings), the interface debonding and shear control the damage mechanisms.
In this research, optically transparent composites were successfully fabricated by embedding polyacylonitrile hollow nanofibers into poly(methyl methacrylate) matrix. Hollow polyacylonitrile nanofibers were first prepared by coaxial electrospinning polyacylonitrile/mineral oil solution, followed by etching with octane to remove the mineral oil from the fibers. Polyacylonitrile hollow fibers were then homogeneously distributed in poly(methyl methacrylate) resins to fabricate the composite. The embedded polyacylonitrile hollow nanofibers significantly enhanced the tensile stress and the Young’s modulus of the composite (increased by 58.3% and 50.4%, respectively), while having little influence on the light transmittance of the composite. This novel transparent composite could be used for transparent armor protection, window panes in vehicles and buildings, airplane windshield, etc.
Surface treatment of sansevieria ehrenbergii fibers were carried out using various chemicals like alkali, benzoyl peroxide, benzoyl chloride, permanganate and stearic acid in order to improve the interfacial bonding between the fiber and matrix. Polyester composites were prepared using raw and surface-treated fibers. Morphology and physico-mechanical properties of the prepared composites are analyzed and compared with pure resin. Fourier transform infra-red spectroscopic analysis examined for the chemically treated fibers affirm the removal of moisture, hemicellulose, lignin and wax content. Scanning electron microscopic images prove the formation of rough surface on the fiber after chemical treatment due to the removal of lignocellulose content. The physico-mechanical properties of the treated fiber reinforced polyester composites are enhanced due to good physical interaction between the fiber and polymer matrix. The chemically treated fiber shows lower water absorption compared to untreated fiber composites.
In this research, the effect of adding graphene nanoplatelets on mechanical properties of epoxy resin under flexural bending stress in fatigue conditions was investigated. The fatigue tests of specimens were carried out under displacement-controlled bending loading at different displacement amplitudes at room temperature. Due to the addition of graphene nanoplatelets, a remarkable improvement in fatigue life of epoxy resin was observed. For instance, specimens comprising epoxy resin and 0.25 wt.% of graphene fatigued at a stress ratio of 0.43 revealed a 27.4-fold improvement in fatigue life in comparison with the neat epoxy resin.
Woven fabrics used in composite materials are designed to fulfill specific manufacturing or structural requirements. Knowledge of the influence of the weave structure on the mechanical properties of the composite is essential to properly optimize the design of structural components. The focus of this work is to investigate the influence of the type of weave used for fabric reinforcement in polymers particularly on the in-plane shear mechanical performance. The selected materials are carbon fibers and epoxy resin. The laminates are manufactured by vacuum infusion. Three woven structures are selected for manufacturing the composite laminates: (a) a plain weave with unidirectional orientation in the warp direction, (b) a plain weave with balanced properties in the warp and weft directions and (c) a 2/2 twill weave with balanced properties in the warp and weft directions. The laminates are tested according to the ASTM D 4255 standard by a two-rail shear test under quasi-static monotonic and cyclic loading conditions. The resulting stress–strain curves are used to study the initial in-plane shear modulus and its evolution (which directly correlates with material damage) and the hardening produced by plastic strain. The results show that for vacuum infusion manufacturing, the weave structure has an influence on the resulting fiber and void volume fractions and, consequently, on the mechanical performance. However, for similar fiber volumes, the weave structure is found to have little effect on the experimental results.
An experimental investigation has been carried out on skin delaminations buckling and growth phenomena in stiffened composite panels subjected to compression loading. Optical fibers have been used to monitor the delamination-related phenomena. The optical fibers have been embedded in the skin close to an artificial delamination following paths with minimum length, satisfying the grating sensor locations and direction requirements and fulfilling specific embedding/integrity constraints. The stiffened panel has been also instrumented with back-to-back strain gauges in skin and stringer locations to acquire additional information on delamination and panel buckling and on delamination growth. Finally, a lock-in thermography inspection activity has been performed at different levels of the applied compressive load to acquire information on the delamination buckling and growth shapes. The performed experimental activity was aimed to study the delamination-related phenomena by comparing experimental data obtained from different sources focusing on delamination growth initiation and delamination growth stability.
A numerical study has been carried out on stiffened composite panels under compression, focusing on the delamination-related phenomena. A robust numerical finite elements model has been introduced to simulate the compression behavior of the panel, including the delamination buckling and growth, and to provide reasonable predictions of the strain measurements in the delaminated area. The robustness of the novel approach, which adopts an improved (mesh and time step independent) virtual crack closure technique for the simulation of the delamination propagation, has been demonstrated by comparisons with standard commercial FEM (Finite Element Method) codes results and experimental data. Indeed, the numerical results, in terms of strains and delamination size as a function of the applied load, have been compared to experimental strain gauges readings, embedded optical fibers measurements, and thermography images of the delamination taken at different load steps. Actually, the performed numerical activity contributed to improve the knowledge on delamination-related phenomena in stiffened composite panels, focusing on delamination growth initiation and delamination growth stability, by providing reasonable justifications and interpretations of experimental strain measurements and thermography images.
Free-standing nanocomposite films of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/Fe3O4 were successfully prepared by mechanically blending magnetic Fe3O4 nanoparticles with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) solution and casting the mixed solution on polypropylene film substrates. The characterization of nanocomposite films was investigated by scanning electron microscopy and Fourier transform infrared spectra. The result indicated that the Fe3O4 nanoparticles were composited together well with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). It was found that the content of Fe3O4 has a significant effect on the electrical conductivity of the nanocomposite film. The maximum electrical conductivity could be up to 218 S/cm, at optical weight ratio (25 wt% Fe3O4). Simultaneously, the Seebeck coefficient fluctuated smoothly in a tiny range (13.1–15.8 µV/K), and the power factor was calculated to be 5.26 μW/mK2.
This article presents the results of an experimental study on the long-term durability of delaminated glass fibre-reinforced polyester composites that have been repaired using a rubber toughened cyanoacrylate adhesive. While several studies have investigated cyanoacrylate for glass fibre-reinforced polyester repair applications, none has addressed the impact of environmental conditions on bond durability. Glass fibre-reinforced polyester was fabricated in the laboratory and structural damage was induced before application of cyanoacrylate adhesive. Environmental chambers were used to expose the repaired specimens to the following three standard conditions for a duration of 1000 h: dry/wet cycles of salt mist (5% salt), dry/wet non-saline water mist condition (0% salt) and a cold chamber at –20°C. A fourth set were maintained at ambient temperature and pressure and served as the control. Double cantilever beam tests were used to determine the initial peak load for pre- and post-repair specimens. No degradation in the adhesive performance was observed for the specimens exposed to the salt mist and non-saline mist. A decrease in the peak load was observed for those specimens exposed to the cold environment, indicating some reduction in bond strength.
The boundary element method (BEM) is implemented in the simulation of the filling of anisotropic fiber reinforced preforms used in the resin transfer molding process (RTM). They are analyzed both as a homogeneous and a non-homogeneous domain. In the former case, two injection regimes are considered: constant pressure and constant flow. The BEM results of flow front positions, pressure history and pressure profiles in the two main directions of permeability are compared to analytical and experimental results. In the second case of non-homogeneous domain, the domain integral appearing in the boundary integral formulation is treated by the dual reciprocity (DR) technique; the DR-BEM results of the curve pressure versus radial distance Pressure vs. Radial distance in the main directions of permeability are compared to numerical results obtained from the solution of the equation of pressure using MacLaurin series. In this case, the influence of the linear radial change of the fiber volume fraction on the flow front positions and on the curves of pressure versus radial distance is analyzed Pressure vs. Radial distance . For the homogeneous case, an acceptable coincidence between BEM and analytical results is appreciated as the circular inlet effect, which is considered in the BEM formulation and is not considered in the analytical method, lessens with the time. In non-homogeneous case, the coincidence among the DR-BEM and the numerical MacLaurin series results indicates that the DR-BEM is a promising technique to deal with the infiltration phenomenon of anisotropic reinforced preforms in non-homogeneous domains.
The present work aims to analyze a study on the mechanical response of fiber-reinforced plastics in the presence of geometric discontinuity, as a result of the uniaxial tensile test. The geometric discontinuity is characterized by the presence of a circular hole in the longitudinal section of the composite. In this study, two different types of configurations are tested: one reinforced with glass/E fibers and the other is a hybrid reinforced with glass/E and jute fibers, with and without a hole, aiming to provide a better agreement on mechanical properties, regarding the residual strength. The residual strength determined using normative rules is used in the point stress criterion and average stress criterion failure theories for the semiempirical calculation of distances (d0 and a0) in the boundary of hole, area of failure stress. The results show the direct influence of the presence of geometric discontinuity and residual strength in all studied parameters since the direction of reinforcement in relation to application of the load is maintained regardless of hybridization.
The thermal properties of carbon/carbon composite are strongly affected by manufacturing porosity. This paper focuses on the characterisation of the manufacturing macro-porosity of a 2D carbon/carbon composite using X-ray computed tomography. The different types of manufacturing porosity were classified and quantified according to their size and location. Three types of macro-porosity were identified using computed tomography, namely trans-tow cracks, interfacial cracks and dry zones. A composite unit cell representing the three types of porosity was developed to model and investigate the effective transverse thermal transport properties (thermal conductivity and diffusivity) of the carbon/carbon composite. Finite element simulations and theoretical calculations were performed and compared with laser flash tests for validation. The influence of the stacking sequence of the laminates on porosity distribution and transverse thermal conductivity of the carbon/carbon composite was also investigated.
The knowledge of the mechanical behavior of woven fabrics is necessary in many applications, in particular, for the simulation of textile composite forming. However, in-plane shear behavior of textile performs is the most studied mechanical property, because this mode of deformation is necessary for forming on double curvature surfaces. In this paper, bias extension tests on non-crimp fabrics (NCF) have been conducted. Two different types of non-crimp fabrics were tested. Force and displacement were measured and then force displacement curves were plotted. Shear angles and normalized shear forces were determined. The shear stress was represented as function of the shear angle, and obtained curves were assigned polynomial regression equations. The derivative of those regression equations provides in-plane modulus of rigidity as a function of shear angle. The effect of small stitches on the rigidity of these fabrics was also examined.
A review on works that investigate the mechanical behaviour of variable stiffness composite laminated panels is carried out in this paper. The review mostly focuses on buckling, failure and vibrations in laminates reinforced by curvilinear fibres, although other issues related to variable stiffness laminates are also addressed. The peculiarities in the formulation of curvilinear fibre reinforced plates are briefly described. As an illustration, the natural frequencies of vibration of variable stiffness composite laminated plates with curvilinear fibres are computed by an h-version type finite element code and are compared with the ones calculated using another model, based on a Third-order Shear Deformation Theory. Areas of research to explore on variable stiffness composite laminates are suggested.
The effect of zinc phenylphosphonate on the crystallization of poly(lactic acid) using differential scanning calorimetry operating in dynamical mode at various cooling rates is reported. Experimental data were analyzed using the Avrami, Tobin, and Ozawa models. It is concluded that the addition of zinc phenylphosphonate modifies the crystallization process of poly(lactic acid) (changing the value of the Avrami exponent). Various parameters such as the crystallization half-time and crystallization rate constant reflect that zinc phenylphosphonate significantly accelerates the crystallization process. The activation energy value of the crystallization of poly(lactic acid), determined by the Kissinger method, increases with the addition of zinc phenylphosphonate.
In the present work, the synthesized ortho cresol novalac epoxy-based composites were produced by filling the microsize silicon carbide and aluminum oxide filler particulates. The friction and wear behaviour of these composites were carried out by a pin on disc apparatus at applied normal load of 10 N to 40 N and sliding velocity of 0.9 m/s, 1.8 m/s, 2.7 m/s and 3.6 m/s under dry sliding conditions. It was found that the microsize silicon carbide and aluminum oxide filler particulates contributed significantly to improve the mechanical properties, friction and wear resistance of the ortho cresol novalac epoxy. The coefficient of friction decreases and specific wear rate increases with an increase in the applied normal load. The mechanical properties such as hardness, tensile and flexural strength increases with an increase in the filler content from 0 wt% to 10 wt% and decreases with the filler content of 15 wt%. On the other hand, compression strength increases with an increase in the filler content from 0 wt% to 15 wt%, respectively. It was also seen that the composites filled with 10 wt% (silicon carbide and aluminum oxide) filler exhibit the better mechanical and tribological properties.
Multifunctional hybrid composites are proposed as novel solutions to meet the demands in various industrial applications ranging from aerospace to biomedicine. The combination of carbon fiber and/or fabric, metal foil, and carbon nanotubes is utilized to develop such composites. This study focuses on processing and fracture toughness characterization of the carbon fiber-reinforced polymer–matrix composites and the carbon nanotube modified interface between the polymer–matrix composite and titanium foil. Vacuum Assisted Resin Transfer Molding (VARTM) process is used to fabricate the laminate. Double cantilever beam tests at both room temperature and high temperature are conducted to assess the mode I interlaminar fracture toughness. The experimental and characterization efforts suggest that carbon nanotubes improve bonding at the hybrid interface. Simple computational models are developed to assist the interpretation of experimental results and further investigate the damage modes. The numerical results agree well with the limited experiments at crack initiation and furthermore support the absence of mode mixity.
The main objective of this experimental investigation was to evaluate the changes from accelerated ageing on selected properties of carbon fibre/polyamide 6 composites based on hybrid yarns. In this study, two types of mechanical tests were performed to measure the environmental influence on the material properties. They are three-point bending to measure the flexural strength and stiffness, and short beam three-point bending to measure the interlaminar shear strength. The 10-mm-thick quasi-isotropic carbon fibre/polyamide 6 composites with 52% volume fraction of carbon fibre to be tested were manufactured by autoclave consolidation. The test samples were dried, and subsequently exposed to 60°C and 100% relative humidity at different lengths of time up to 2500 h, followed by drying at 23°C and 50% relative humidity. Few samples were additionally completely dried at 70°C in vacuum for 21 months. Tests were also performed on as manufactured and dried material at low temperature (–45°C) and high temperature (115°C). The measured mechanical properties decreased with exposure time at 60°C and 100% relative humidity. Both the bending stiffness and the strength degrade to a level of about 65%, whereas interlaminar shear strength drops to about 87% of the property values of the unexposed (initially dried) material. The bending stiffness and strength at –45°C are about 87% of the properties at room temperature, whereas at 115°C the stiffness drops to 75% and the strength drops to 60% of the properties at room temperature. The interlaminar shear strength values also drop to about 75% at both –45°C and 115°C. Extreme temperatures and long-time exposure to humidity of quasi-isotropic carbon fibre/polyamide 6 laminates can thus reduce the bending stiffness and strength by up to 35% and the interlaminar shear strength by up to 25%.
Damage progression in unidirectional glass fibre reinforced composites manufactured of a non-crimp fabric subjected to tension-tension fatigue is investigated, and a quantitative explanation is given for the experimentally observed stiffness degradation. The underlying damage-mechanisms are examined using three distinct microscopic analyses, and the transverse crack density is measured. It is documented that the stiffness loss in fatigue is directly related to fibre fractures in the load-carrying axial fibre bundles, initialised by interface debonding and cracking in the transverse backing bundles. A simple stiffness spring model validates the stiffness loss observed. A fatigue damage scheme is presented, which suggests that damage initiates due to failure of the backing bundle causing a stress concentration in the axial load-carrying fibres. This stress concentration, along with fretting fatigue, gives rise to axial fibre fractures and a loss of stiffness, eventually leading to final failure. The uniqueness of the present work is identification of the mechanisms associated with tension fatigue failure of unidirectional non-crimp fabrics used for wind turbine blades. The observed damage mechanisms need further attention and understanding in order to improve the fatigue life-time of unidirectional glass fibre reinforced non-crimp fabrics.
Polylactide composites containing bamboo fiber treated with various coupling agents were prepared using melt compounding. 3aminopropyltriethoxysilane, vinyltrimethoxysilane, maleic anhydride, and acrylic acid were used as the coupling agents to improve the mechanical properties of polylactide and bamboo fiber composites. In addition, to investigate the effect of fiber diameter on mechanical properties, either split bamboo fiber or fibrillar bamboo fiber was used as reinforcement. The composites containing split bamboo fiber treated with 3aminopropyltriethoxysilane and acrylic acid exhibited the highest tensile strength of all composites prepared. In addition, the composites containing fibrillar bamboo fiber showed a marked improvement in tensile strength. To investigate the biodegradability of the composites, an enzymatic degradation test was performed using Proteinase K, a commonly used enzyme for polylactide degradation. All the polylactide composites containing bamboo fiber, regardless of which coupling agent was applied, showed a similar trend in enzymatic degradation to that of neat polylactide.
The thermoforming process is a manufacturing method to produce fibre-reinforced thermoplastic components within short cycle times (<2 min). During this process, the anisotropic material behaviour provokes residual stresses which furthermore induce unwanted deformations. Thereby, at the beginning, newly produced geometries have a quite high reject rate and the process parameters have to be adjusted iteratively. Thus, an analysis of the process-induced deformations has been carried out to investigate the connections between process parameters and final geometry. In this case, an L-angle bracket has been observed which shows a spring-in effect after the thermoforming process. For the experimental approach, the semi-crystalline polyphenylenesulphide was used as thermoplastic matrix material. In particular, the crystallisation kinetics of this polymer is described by adjusting Nakamura’s crystallisation model to different cooling rates. And furthermore, a simulation strategy has been developed to include the crystallisation behaviour in a thermal and mechanical analysis. The results of these analyses have been compared and evaluated with the outcomes of the experimental approach. Finally, some opportunities for future studies will be introduced to provide a way for improving the simulation analysis.
The diverse use of thermoset composite materials is increasing day by day in industrial applications. This has led to the development of several fabrication techniques, use of various reinforcement types, and different fabrication conditions to achieve a composite part with required properties. Despite all these technological advancements, there is a shear need to investigate and understand the effect of all these factors on the curing process. Volume chemical shrinkage of resin is one such property, which has been studied by several authors for a given value of applied pressure. A few studies have reported results on volume chemical shrinkage of composites for one type of reinforcement and for a single applied pressure. In the present work, experiments on vinylester resin and associated glass fibres composites were conducted under two different pressures. The tested composites were containing unidirectional fibres ([0] and [0/90]) and plain woven fabric with two different fibre volume fractions. The results of these experiments, carried out in a plunger type dilatometer, led us to show the effect of fibre fraction, type of reinforcement, and applied pressure on the volume chemical shrinkage of vinylester resin.
A composite-antenna-structure covering three bands of global positioning system (1.575 GHz), digital multimedia broadcasting (2.62 GHz) and direct broadcast satellite (11.7–13.5 GHz) was designed and fabricated as a part of the structure surfaces. A new concept of antennas integrated into a composite sandwich structure provides a design that is an electrically and structurally effective multi-functional antenna structure. We designed two types of antennas. One is an annular ring patch antenna for global positioning system and digital multimedia broadcasting. The other is a microstrip patch antenna for direct broadcast satellite. In the design process, the effects of the composites and adhesive films are considered for in the design processes, because structural materials affect the antenna performance. Additionally, the composite-antenna-structure was designed by considering the coupling effect in a strict substrate. The measured results of buckling tests have provided useful information regarding not only the compression behavioral characteristics but also its degradation in electrical performance before and after the test. The experimental results have shown good results for compressive tolerance, though the specimens after compression loading returns to their original shape before failure. Additionally, an acoustic emission system is implemented in the buckling test system to study the inner behavior of the composite-antenna-structure for the reliability of secure mechanical performance. The antenna performance, as measured by the return loss and radiation pattern, remained excellent after the compression tests. These electrical performance results suggest that antennas with composite laminates will function well despite damage.
The finite difference method is used to solve the time-dependent thermo mechanical response of a layered composite structure subjected to fire. State variables of the composite are chosen whereby the external and internal boundary conditions are derived for an irregular grid through the thickness of the structure. The homogenised mass flux and specific heat capacity of pyrolysis gases over a layered composite is also defined. The formulations are tested against documented results found in the literature.
Fiber Bragg gratings provide accurate and non-intrusive strain sensors. They can be embedded into fibrous preforms to deliver real-time information on ongoing processes. The possibility of using the strain-induced birefringence of the fiber Bragg gratings to extract information on the effective strain state of the composite at the end of the process has already been demonstrated. This effect can be used to estimate the residual strain field in composites manufactured by resin transfer molding. The strain fields calculated with the associated optoelastic three-dimensional model are shown to compare well with the strain fields calculated using classical laminated plate theory. The fiber Bragg gratings are then used here in order to study the influence of the nature of the mold on the residual strain field in composite plates and furthermore to infer the kinematic conditions applied by the mold on the part during cooling by using two different molds, an aluminum mold and a HexToolTM composite mold. It is demonstrated by both in situ measurements using fiber Bragg grating in a closed mold under thermomechanical loading and mesoscopical external observations that the thermal expansion of the mold is determinant in the residual strain field development from the onset of the resin curing stage.
As major historical periods such as Stone Age, Bronze Age, and Iron Age, the development of new materials was the fundamental to all the periods. In the present investigation, a new hybrid composite with epoxy as a resin and reinforcing both biowaste (jute) and traditional fiber (glass) as continues layered mat composites and also study experimentally the effect of the stacking sequence on tensile, flexural, and interlaminar shear properties. Composites were prepared by using hand lay-up technique. All the laminates were prepared with a total of four piles, by varying the position of glass and jute. One group of all jute and glass laminate was also fabricated for comparison purpose. Specimen preparation and testing were carried out as per ASTM standards. Tests were conducted on INSTRON H10KS Material Test System at room temperature using automatic data acquisition software. The results indicated that the jute fiber and hybrid composite give encouraging results when compared with the neat epoxy. The morphologies of the composites are also studied by scanning electron microscope.
The use of short palm tree lignocellulosic fibers as a reinforcing phase in polyester and epoxy matrices has been reported. Dielectric spectra were measured in the frequency range 0.1 Hz–100 kHz and at temperature intervals from ambient to 200°C. For the polyester/palm tree, different relaxations process were identified, namely the orientation polarization imputed to the presence of water molecules in palm tree fiber and the relaxation process associated with conductivity and interfacial relaxation. In the case of epoxy/palm tree, the α mode transition associated with the glass transition of the epoxy resin, the phenomenon of conduction, and the interfacial relaxation are present. The water dipoles relaxation was not observed in this case. Dielectric measurements show that interfacial adhesion is strong for epoxy-based materials.
An analytical method is developed for the calculation of the torsional stiffness of thick laminated rectangular plates of finite width. Through-thickness shear deformations are included in order to satisfy vanishing of in-plane shear stresses at the free edges, which constitutes a non-trivial effect in thick laminated plates. The present method is based upon the calculation of a characteristic value for the relevant laminate, which serves to quantify the relative rate at which in-plane shear stresses are attenuated near free edges. This characteristic value is subsequently utilized in order to calculate an "effective" width-to-thickness aspect ratio, which is then used to calculate the torsional stiffness of the laminated plate. The present analytical method is computationally validated against 22 unique cases that were synthesized from seven different laminate layups, two different width-to-thickness aspect ratios, and two types of torsional loadings.
Composites are significantly used in aerospace, automotive and civil structures due to their high specific strength, high stiffness, corrosion resistant and longer fatigue life. During service life, composite structures are susceptible to damage, which reduces their structural integrity. For extending its service life, the damage needs to be repaired. In case of low velocity impact damage adhesively bonded patch repair is found to be effective in extending the service life of damaged parts. The repair performance is mainly influenced by patch stacking sequence, patch shape, patch thickness, overlap length and adhesive thickness and its shear strength. In the present work, both numerical and experimental works are carried out to study the mechanics of composite patch repair on damaged carbon fiber reinforced polymer panel of configuration [45/–45/0/90]s subjected to tensile load. The influence of patch stacking sequence, patch thickness, adhesive thickness and overlap length on repair performance is investigated through a mechanics-based design approach involving finite element analysis. Stress concentration factor and shear stress in adhesive layer are considered to analyze the repair performance. Later, a genetic algorithm-based approach in conjunction with finite element analysis is implemented for arriving at an optimized patch dimension and adhesive thickness. Experimental study is then carried out with optimized geometry using non-contact optical-based technique namely digital image correlation. The strain field from digital image correlation is compared against the finite element results.
The friction and wear behavior of vinylester composites filled with uniformly sized micron and submicron cenosphere particles is discussed in this paper. Three distinct uniform size cenosphere particles (2 µm, 900 nm and 400 nm in diameter) were prepared in the laboratory. The experiments have been carried on a pin on disc arrangement at normal room temperature conditions. In this study, a plan of experiments, based on the Taguchi design technique, has been used to acquire data in a controlled way. An orthogonal array L27 (313) and analysis of variance have been applied to investigate the influence of process parameters on the coefficient of friction and sliding wear behavior of these composites under dry sliding condition. It was found that the submicron size particulates as fillers contributed significantly to improve the wear resistance of the vinylester composites. Scanning electron microscopy has been used to investigate the interaction between filler and matrix and various wear mechanisms involved.
This study examines the parametric effects of core density, core thickness, face-sheet stacking sequence, and indentor diameter on the compressive strength of aluminum honeycomb-core sandwich panels stiffened with eight-ply, quasi-isotropic, graphite/epoxy face sheets. The sandwich panels contained damage at the threshold of visual detectability created through quasi-static indentation with 25.4 mm or 76.2 mm-diameter spherical indentors. During compression-after-indentation testing, failure occurred due to: dent deepening followed by localized, compressive micro-buckling of fibers in the 0° plies; localized buckling of the near-free-surface sub-laminates; or unstable dent growth in the direction lateral to the applied compressive load. Regardless of failure mode or face-sheet type, the compression-after-indentation strength increased with increasing core thickness and with decreasing core density. Additionally, panels containing face sheets with the 0° plies near the mid-plane and 45° angle change between subsequent plies exhibited greater undamaged compressive strength and higher compression-after-indentation strength relative to panels containing 90° angle changes between subsequent plies and 0° plies near the free surface. The compression-after-indentation strength was found to be relatively unaffected by the indentor diameter size and the resulting variations in the face sheet and core damage. These results imply that precise representation of the damage state in models to predict the post-indentation response of sandwich panels may not be necessary in order to make accurate average residual strength predictions.
Finite element models are developed to predict potential failure initiation sites and associated failure modes in S2-Glass/SC15 three-dimensional (3D) woven composites under quasi-static indentation. As part of this modeling effort, experimental micrographs of the composite specimen obtained from a previous experimental study1,2 are analyzed. In conjunction with these micrographs, model outcomes demonstrate the ability of warp weavers or through-thickness Z-yarns to shield inter-laminar cracks. Quasi-static indentation is modeled as a contact interaction between a rigid cylindrical indenter and a deformable S2-Glass/SC15 3D woven composite laminate using ABAQUS®. Tow elements are modeled as transversely isotropic elastic-plastic material entities, whereas the inter-tow matrix is modeled as an isotropic elastic-plastic material. Through-thickness failure modes are predicted based on the Tsai-Hill criterion. Contour maps of these failure modes point to the location and corresponding damage initiation mode within the material. Experimentally obtained micrographs1,2 are then analyzed on the basis of these contour maps, thereby serving to validate the modeling methodology. The effect of Z-yarns is demonstrated with the aid of two-dimensional plane strain linear elastic fracture mechanics analysis. Crack shielding abilities of the Z-yarns manifest as the variation of strain energy release rate as a function of crack length and location. In the vicinity of a Z-yarn, the energy release rate decreases precipitously, indicating the inability of the crack to penetrate the Z-yarns.
Composite structures are usually subjected to fluctuating loads in service leading to fatigue failure. Because it is one of the main failure modes, fatigue behavior of composites has been extensively studied to be able to design fatigue-resistant composite structures. However, little attention has been paid to their design optimization under fatigue loading. In this study, a methodology is proposed to find the optimum fiber orientation angles of composite laminates under various in-plane loads to achieve maximum fatigue life. Fawaz–Ellyin’s model is used to predict the fatigue life of the laminates. A variant of simulated annealing algorithm is used as the search algorithm in the optimization procedure. A number of problems are solved to demonstrate the effectiveness of the proposed method.
Flaky hexagonal boron nitride particles-reinforced epoxy composites were prepared. The investigation on the thermal, electrical, and mechanical properties of hexagonal boron nitride/epoxy composites indicated that the incorporation of silane coupling agent-treated hexagonal boron nitride into epoxy slightly improved the glass transition temperature and the thermal stability of the composites; the epoxy containing silane treated hexagonal boron nitride exhibited higher thermal conductivity compared to the untreated ones. The dielectric permittivity increased slowly with an increase in hexagonal boron nitride content, as well as with a decrease in frequency. The obtained 50 wt% hexagonal boron nitride–filled epoxy composite had a low dielectric permittivity (less than 5.4) and dielectric loss (less than 0.02) in all frequencies ranging from 10–1 to 107 Hz, a high volume resistivity of 6.3 x 1014 ·cm, and a high dielectric strength of 16 kV/mm, together with moderate mechanical properties, which are of great significance for practical electrical materials applications.
Quasi-static, low-velocity impact (LVI) and ballistic impact loading conditions were used to find the material properties and dynamic responses of E-glass/phenolic composites. Standard American Society for Testing and Materials (ASTM) tests were used to find the density, Poisson’s ratio, tensile, compressive and shear strengths, and the elastic and shear moduli of the material. The quasi-static punch shear and crush strength tests were used to find the punch shear and crush strengths of the material. LVI tests were conducted to obtain force versus time curves for various loading conditions. Ballistic testing was conducted using a right circular cylinder (RCC) to find the V50 ballistic limit and the depth of penetration of the RCC at various impact velocities. The experimental results of this investigation can be used for structural design and to validate numerical solutions for both LVI and ballistic impact events.
The use of masterbatches for increasing the industry friendliness of polymer nanocomposite production has grown in popularity over the last several years but is lacking a proper method for quality assessment. As the quality of a masterbatch is a critical pre-requisite for obtaining resultant high quality nanocomposites, a quantitative dispersion analysis tool viable and practical for industry use was studied in this paper. Polycarbonate/carbon nanotube masterbatches (10 wt.%) were prepared with a controlled dispersion state and their dispersion quality was assessed with a new quantitative stereological macrodispersion analysis tool. At the same time, the dispersion quality of the resultant two-step nanocomposites (diluted to 1 wt.%) was also assessed with the same method. It was demonstrated that the dispersion quality of the masterbatch has a significant effect on the quality of the second step composites. To confirm this relation, the material properties closely related to the dispersion quality of the masterbatch and diluted composites were studied. Moreover, we show that the use of stereology for the estimation of the bulk agglomeration state of a masterbatch system may yield a significant increase in accuracy over conventional methods.
The aim of this paper is to replace the traditional fiber composites with a natural-fiber composite in perception of tribological and mechanical accepts. A systematic study has been carried out to investigate jute fiber properties when incorporated into epoxy matrix. Thermogravimetric analysis has also been carried out for jute and epoxy for thermal property analysis. For a comparison purpose epoxy and glass fiber composites are prepared. The investigation reveals that, due to incorporation of jute into polymer epoxy shows better properties than the resins alone; but the properties are inferior to those of glass reinforced in terms of mechanical. When considering the tribological application, the jute fiber shows superior properties than neat and glass-reinforced epoxy composites. The worn out samples were studied using scanning electron microscope.
Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam specimen is used to measure fracture toughness, GIc, and strain energy release rate, GImax, for delamination onset and growth in laminated composites under Mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy double-cantilevered beam specimens to evaluate the draft standard "Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional Fiber-Reinforced Polymer Matrix Composites." Static results were used to generate a delamination resistance curve, GIR, for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic GImax level equal to 90% of the fracture toughness, GIc, to determine the delamination growth rate. Two different data reduction methods, a two-point and a seven-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the non-normalized results. Paris Law exponents were found to decrease by 5.7 to 47.6% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at three lower cyclic GImax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.
This paper deals with the origin of permanent indentation in composite laminates subjected to low-velocity impact. The three-point bending test is used to exhibit a non-closure of matrix crack which is assumed as a cause of permanent indentation. According to the observation at microscopic level, this non-closure of crack is produced by the blocking of debris inside matrix cracking and the formation of cusps where mixed-mode delamination occurs. A simple physically-based law of permanent indentation, "pseudo-plasticity", is proposed. This law is qualitatively tested by three-point bending finite element model and is lastly applied in low-velocity impact finite element model in order to predict the permanent indentation. A comparison between low-velocity impact experiments and simulations is presented.
In this study, properties of natural rubber (NR) were enhanced by adding sisal fiber, which is a tropical plant in Thailand. Sisal fiber/natural rubber composites were prepared by the incorporation of sisal fiber at various loadings into natural rubber. Natural rubber grafted with maleic anhydride (NR-g-MA) prepared in house and epoxidized natural rubber (ENR50) were used to enhance the compatibility between sisal fiber and natural rubber. With increasing fiber loading, tear strength and hardness of natural rubber composites increased while their scorch time, cure time, tensile strength, and elongation at break decreased. Moreover, interfacial adhesion between sisal fiber and natural rubber was enhanced with the addition of NR-g-MA and ENR50. In comparison, NR-g-MA provided more effective improvement in mechanical properties of natural rubber composites than ENR50. For NR-g-MA compatibilized natural rubber composite at fiber loading of 10 phr, its tensile strength, modulus at 100% strain, modulus at 300% strain, tear strength, and hardness were increased 43, 44, 53, 42, and 13%, respectively, comparing with that of uncompatibilized natural rubber composite.
As a part of a world-wide study, a commercial code (General Optimization Analyzer), based on multi-scale (micro–macro) progressive failure analysis (PFA), is used to provide theoretical predictions for damage development for a set of challenging 13 test cases proposed in the Third World-Wide Failure Exercise (WWFE-III). Multiple failure criteria were utilized aimed at tackling issues related to a wide range of damage modes, being addressed by the WWFE-III. The critical damage events/indexes predictions tracked translaminar and interlaminar composite failures, namely matrix cracking/crack density, damage initiation/propagation, delamination initiation/growth, and their interaction with fiber failure. The composite laminates analysed were both with and without a central hole and the predictions were made using constituent fiber properties and matrix properties based on materials data or identification from ply stress–strain curve inputs. Loadings included uniaxial tension or compression, biaxial, bending, thermal, and loading–unloading.
This paper examines three sets of approximate formulae for the overall tetragonal effective elastic properties of two-phase fiber-reinforced unidirectional composites with isotropic phases. The fibers are of circular cross-sections and periodically distributed in a matrix in a square pattern. The formulae by Kantor and Bergman, Luciano and Barbero, and estimates based on non-interacting Maxwell’s type approximations are rewritten in unified notations. The latter approximations coincide with the most of well-known estimates of the effective medium theories (composite cylinder model, generalized self-consistent model and the Mori–Tanaka method), as well as with one of the Hashin–Shtrikman variational bounds. The approximate estimates are compared with the exact periodic solutions to determine the range of their applicability. The simplest and most accurate formulae are identified and suggested as a set of approximate expressions for accurate estimates of the effective elastic properties of composite materials with a square symmetry.
This two-part article examines the effects of thickness and stacking sequence of GLARE 5 (2024-T3 aluminum alloy-unidirectional S2-glass/epoxy) fiber–metal laminated (FML) plates subjected to ballistic impact. Part I presented experimental observations of damage development in the specimens, C-scan damage contours, projectile velocity profiles and ballistic limit velocities (V50). Part II concerns with finite element (FE) modeling of the FML plates. The 3D FE code, LS-DYNA, was used to model and validate the experimental results. Experimentally obtained incident projectile impact velocity versus the residual velocity (Vi~Vr), damage patterns and bullet residual length were used to validate the FE model. Good agreement was achieved between experimental and numerical results. It was found that for a given specimen thickness/stacking-sequence, by increasing the projectile incident velocity up to its V50 value, the maximum contact force increased. By further increasing the projectile velocity above its V50, the maximum contact force was relatively invariant with respect to an increase in the projectile incident velocity.
The viscoelastic properties such as damping behaviour, storage and loss modulus, etc. of polymer composites depend on matrix filler interaction, crystallinity and the extent of crosslinking. It was observed that the storage modulus of the composites increased with the addition of filler due to the enhancement in stiffness of the material. The damping behaviour was found to decrease as a function of filler loading and this was attributed to the restricted movement of the polymer segments. The higher surface area to volume ratio factor of the layered silicate resulted in the better interaction between the polymer matrix and the filler, which resulted in the change in glass transition temperature. The flow properties were studied with special reference to filler loading of different fillers at a specified temperature. It was observed that complex viscosity of filled systems is higher than that of unfilled system. Among the fillers, layered silicate filled systems showed higher viscosity in comparison with calcium phosphate and titanium dioxide fillers.
Sandwich structures with frequency selective surfaces have been widely used to fabricate low-observable radomes. Traditional frequency selective surfaces were made of metals, such as copper and aluminum. There were problems when used the frequency selective surfaces with the sandwich structures, such as a bonding layer and thermal mismatch. Because of the thermal mismatch between the sandwich structures and the metals, mechanical properties of the sandwich structures usually decreased. To avoid these disadvantages, a new type of composite frequency selective surface was put forward in this study. A four-legged slot-array frequency selective surface and a square-aperture array frequency selective surface were fabricated by cutting carbon fiber/epoxy composites. Free-space method and finite-element method were carried out to evaluate the electromagnetic transmission characteristics of the frequency selective surface specimens, respectively. Results show that the composite frequency selective surface with four-legged slot array can realize the function of frequency selection, and its minimum transmission loss can be decreased by increasing the electrical conductivities of the composite material, adjusting the thickness, and increasing the aperture-to-cell ratio of it. For the composite frequency selective surface with square-aperture array, grating lobes come closely after its resonant frequency, although its minimum transmission loss is very close to zero.
Thermal residual stresses arise in long fibre reinforced composites such as metal matrix composites due to the mismatch of the thermal and mechanical properties of the constituents and the change in temperature during processing. This paper presents an inverse axisymmetric model that uses the fibre deformation obtained when the matrix is selectively etched away in order to back calculate the inherent residual stresses. The model is tested using finite element method simulation and also on published experimental data. Although, an approximate inverse solution exists in the literature, there is an ambiguity in the value of Poisson’s ratio to be used which can lead to large errors relative to the full solution presented in this paper. A sensitivity analysis is also carried out to quantify the effect of variability of material properties on the stress values obtained using the solution.
A new micromechanical approach for predicting the compressive response of long fiber composites based on a periodic unit cell model with a centrally located imperfection is presented. A detailed comparative analysis between the proposed model and the existing models was performed at the macro and micro level. The results show that the periodic unit cell model with non-uniform fiber waviness is able to reproduce the mechanisms that govern the failure of fiber-reinforced polymers under compression and provide accurate characteristics of microbuckling.
A novel wheat straw-composite superabsorbent with high gel strength and high water absorbency was prepared by graft polymerization with acrylic acid, acrylamide and maleic anhydride-modified wheat straw, using N,N-methylene-bis-acrylamide as a crosslinker and ammonium persulfate and sodium bisulfite as redox initiators. Factors influencing water absorbency and gel strength of the superabsorbent composite, such as amount of maleic anhydride-modified wheat straw, crosslinker amount and initiator amount, were investigated. Morphologies and structure of the wheat straw-composite superabsorbent were characterized by Fourier transform infrared spectroscopy, scanning electron microscope and X-ray diffraction. Composite superabsorbent with modified wheat straw as high as 25 wt% still has water absorbency of 435 g/g. Fourier transform infrared spectroscopy spectra indicate the structure of wheat straw graft copolymer. Scanning electron microscope data show that the discontinuous sheet structures of wheat straw disappear and gel aggregates with many large microporous holes and small capillary pores are formed after wheat straw graft modification. The wheat straw-composite superabsorbent has better thermal stability than those of raw wheat straw and maleic anhydride-modified wheat straw.
A number of refined beam theories are discussed in this paper to trace the free vibration response of laminated beams, including thin-walled boxes. By expanding the unknown displacement variables over the beam section axes using Taylor type expansions, trigonometric series, exponential, hyperbolic and zig-zag functions, many new displacement fields were obtained and, for the first time, evaluated for the dynamic analyses of composite structures. The finite element method is used to derive governing equations in weak form. These equations are written using the unified formulation introduced by the first author, in terms of fundamental nuclei, whose forms do not depend on the expansions used. The natural frequencies are compared with results available in the literature or with those obtained by the finite element models related to commercial software. A number of analyses were conducted to compare various theories, including Euler–Bernoulli and Timoshenko models. The advantages/disadvantages of using the different theories are discussed for significant problems related to laminated beams as well as thin-walled boxes. It is shown that refined kinematic theories are able to yield a very accurate evaluation of fundamental as well as higher mode frequencies in a way comparable to three-dimensional analysis, but it is obtained with a strong reduction of computational costs.
In this study, boron doped and undoped Bi2O3-Er2O3 nanocomposite fibers were produced via electrospinning technique. Obtained fibers were turned into ceramics via calcination process. Obtained nanocomposite fibers and ceramics were characterized by Fourier transform infrared, x-ray diffraction, and scanning electron microscopy techniques. X-ray diffraction results show that boron undoped Bi2O3-Er2O3 ceramic consisted of face-centered cubic Bi2O3-Er2O3 phase. However, boron doped Bi2O3-Er2O3 ceramic consisted of orthorhombic phase. Crystallite sizes of the ceramics were evaluated using Scherrer’s equation. Crystallite sizes of boron doped and undoped ceramics were calculated as 50 and 17 nm, respectively. The average fiber diameters for boron doped and undoped poly vinyl alcohol/Bi-Er acetate nanofibers were calculated as 79 nm and 96 nm, respectively. The Brunauer Emmett teller results show that boron undoped and doped Bi2O3-Er2O3 nanocrystalline powder ceramic structures sintered at 800°C have surface area of 20.44 and 12.93 m2/g, respectively.
A new process is proposed to fabricate an intermetallic compound-reinforced aluminum alloy matrix composite using the reaction between porous nickel and molten aluminum alloy. The intermetallic compound-reinforced aluminum alloy composite was manufactured with the infiltration process method. Porous nickel reacted with the molten aluminum alloy at 973 K, and the intermetallic compound of Al3Ni was generated on the surface of the porous nickel. The generated intermetallic compound Al3Ni is delaminated due to different thermal expansion coefficient with nickel and moves in the direction of aluminum matrix. The effects of processing variables such as specific surface area of porous nickel, holding time after infiltrated molten aluminum alloy, and cooling rate on the formation and dispersion behavior of Al3Ni were investigated.
Microspheres of poly(vinyl acetate)/poly(vinyl alcohol)/disperse dye composite were prepared through suspension polymerization followed by the heterogeneous saponification. The effects of disperse dye on the rate of polymerization and saponification of poly(vinyl acetate) were studied. It was found that the rate of polymerization of poly(vinyl acetate) decreased when the concentration of the disperse dye increased. Also, the rates of poly(vinyl acetate) saponification decreased dramatically by the addition of disperse dye. The presence of disperse dye has almost no effect on the microsphere morphology.
This paper presents the findings of an experimental investigation on the effects of cutting speed, feed rate, depth of cut, and nose radius in the CNC turning operation performed on red mud-based aluminum metal matrix composites. The surface roughness, flank wear, and power consumption are considered as the output quality characteristics. The Taguchi-based Grey relational analysis with entropy method has been used to accomplish the objective of the experimental study. The entropy method is applied to evaluate the weighting values corresponding to various performance characteristics. The L9 orthogonal array design has been used for conducting the experiments. The Grey relational analysis with entropy reveals that the optimal combination of the machining parameters for the multi-performance characteristics of the red mud-based aluminum is the set of cutting speed of 275 m/min, feed 0.2 mm/rev, depth of cut 0.5 mm, and nose radius 0.4 mm. The optimal results were compared with the experimental results for verifying the approach, and it is observed that the surface roughness decreases from 2.56 to 2.27 µm, tool wear decreases from 0.3 to 0.28 mm, and power consumption decreases from 721 to 715 W.
The present paper reports on the study of the interfacial adhesion of an optical fiber embedded in a composite material aged in distilled water. Samples composed of optical fibers embedded in an epoxy vinylester resin or in resin/glass fibers systems were submitted to different aging duration in distilled water. Pull-out tests on optical fibers were carried out to measure the effect of water diffusion and glass concentration on fiber bonding. For resin/optical fiber samples, water diffusion leads to damage of the polymer/matrix interface from an aging duration of 15 days, and the interfacial stress values present a low decrease up to an aging duration of 60 days. For the case of composite/optical fiber samples, the greater the glass fiber content, the less is the water damage at the polymer interface. On the other hand, a linear development of interfacial debonding stress with increasing glass fiber concentration is reported.
Polyurethane green composites with varying amounts viz., 0, 2.5, 5, 7.5 and 10 wt% of finely powdered ginger spent have been fabricated. The fabricated polyurethane/ginger spent green composites have been performed for mechanical properties such as surface hardness and tensile behaviours. The swelling behavior of the composites has been studied in different organic solvents. Improvement in tensile behaviors of the polyurethane/ginger spent composites were noticed up to 7.5 wt% of ginger spent content. Thermogravimetric analysis of the green composites has been carried out in order to determine the thermal stability and their mode of thermal degradation behaviour. The thermogravimetric analysis curves of polyurethane/ginger spent composites indicated three-step thermal degradation processes, suffers no weight loss up to 193°C and completely degrades at around 540°C. Thermal degradation kinetic parameters such as energy of activation (Ea) have been calculated for the green composites using two mathematical models namely Coats–Redfern and Broido’s models. The microcrystalline parameters such as crystal size (<N>) and lattice strain have been computed using wide-angle X-ray scattering data. The structure-property relationship of the polyurethane/ginger spent composites has been established on the basis of these parameters.
An experimental investigation on the wear behavior of aluminium alloy LM 25 and its composites reinforced with 7.5% SiC + 2.5% TiO2 and 2.5% SiC + 7.5% TiO2 is carried out to ascertain which constituent is having more influence on the metal matrix composites. The hybrid composite material is prepared by stir casting process. The wear and frictional properties of hybrid metal matrix composite are studied by performing dry sliding wear test using a pin on disc wear tester. The experiments are conducted at a constant sliding velocity of 1.04 m/s and a sliding distance of 628 m over various loads of 3, 4 and 5 kg. The result shows that, the reinforcement SiC and TiO2 has improved the wear resisting property of LM 25 alloy composite. The results also indicate that the three-fourth volume fraction of the TiO2 in a 10% volume fraction of reinforcement has better wear and frictional property than the three-fourth volume fraction of SiC. The coefficient of friction decreases with the increasing load and particle reinforcement. The microstructure analysis reveals that SiC and TiO2 particulate are uniformly distributed in the matrix. The wear surfaces are examined by scanning electron microscope which indicates an abrasive wear mechanism due to hard ceramic particles exposed on the worn surface.
This paper presents experimental and analytical investigations about the creep behaviour of sandwich panels comprising glass-fibre reinforced polymer faces and rigid polyurethane foam core for civil engineering applications. A full-scale sandwich panel was tested in bending for a period of 3600 h, in a simply supported configuration, subjected to a uniformly distributed load corresponding to 20% of the panel’s flexural strength. Additionally, specimens of polyurethane foam core were tested in shear for a period of 1200 h, for three different load levels corresponding to 10%, 20% and 30% of the foam’s shear strength. Experimental results were fitted using Findley’s power law formulation. Creep coefficients, shear modulus reduction factors and time-dependent shear moduli were obtained for the polyurethane foam in shear. A composed creep model is proposed to simulate the sandwich panel’s long-term creep deformations by considering the individual viscoelastic contributions from (1) the core material in shear and (2) the glass-fibre reinforced polymer faces in tension/compression. The composed creep model predictions adequately reproduced the full-scale panel’s experimental results. In addition, a good agreement was found between the composed creep model predictions and the extrapolation of the power law fitting obtained from the full-scale panel test, for a 50-year period.
This study examines the use of polypyrrole and polypyrrole/multi-walled carbon nanotubes composite coatings for corrosion protection of 304 stainless steel in 3% NaCl solution. Polypyrrole and polypyrrole/multi-walled carbon nanotubes composite coatings were electropolymerised on 304 stainless steel surfaces by the potentiodynamic method in aqueous sulphuric acid solutions. The coatings were characterised via cyclic voltammetry, Fourier transform infrared spectroscopy and scanning electron microscopy. The corrosion behaviour of the polymer-coated stainless steel electrodes was investigated in a 3% NaCl solution by open-circuit potential measurements, Tafel polarisation and electrochemical impedance spectroscopy. Potentiodynamic polarisation, impedance measurements and open-circuit potential data revealed that the polypyrrole and polypyrrole/multi-walled carbon nanotubes composite coatings effectively protected the stainless steel substrates from corrosion in chloride solution. However, the corrosion resistance of the substrates was reduced after longer immersion times.
There has been an increase in the use of fiber-reinforced composite materials in many areas. In particular, glass-fiber-reinforced composites have gained in popularity owing to their low cost, high-strength properties, durability, ease of repair, and being simple to form. This paper analyzes the buckling load for the glass-fiber-reinforced plywood specifically used for the liquid natural gas cargo tank and carriers, especially the No 96 cargo containment system insulation box. The buckling load of the plywood reinforced with glass fiber composite on both outer surfaces was estimated with various composite thicknesses and compared with the buckling load of unreinforced plywood. The buckling load was also evaluated at various temperatures to verify the temperature dependence of the buckling load. A much higher buckling load for the glass fiber/epoxy-reinforced plywood was obtained as the number of glass fiber/epoxy prepreg composite was increased. However, the rate of increase in the buckling load decreased as the temperature decreased and as the number of glass fiber/epoxy prepreg composite increased.
Short graphite fiber/Al composites were fabricated by a modified two-step vacuum pressure infiltration technique. Copper-coated graphite fibers preform was infiltrated with liquid aluminum at 800°C under infiltration pressure of 1 MPa and solidification pressure of 30 MPa for 30 min. The effects of surface modification and the processing parameters of vacuum pressure infiltration on relative density and thermal conductivity of the composites were systematically studied. The results show that short graphite fiber/Al composite with relatively high density of 99.1% and thermal conductivity of 208 W·m–1·K–1 was successfully fabricated. Through the application of copper coating onto the graphite fibers, the in-plane thermal conductivity of the composite was effectively enhanced from 117 W·m–1·K–1 to 208 W·m–1·K–1 as a result of improved interfacial bonding. The obtained short graphite fiber/Al composites are promising materials for electronic packing applications.
Representative volume element-based three-dimensional models with various nanofiller geometries and process parameters are presented for the design and analysis of composite materials. Analytical, computer-aided design, and computer-aided engineering tools are integrated to develop user interface tools with automated three-dimensional models for mechanical and electrical analyses. Various process parameters in the manufacture of nanocomposites are quantified using image analysis techniques. A filler-to-filler distance algorithm is incorporated in developing a three-dimensional network of fillers within matrix representative volume element to account for filler–filler interactions and compatibility. The stress–strain behavior of metal matrix nanocomposites, the effective modulus, and the electrical conductivity of polymer nanocomposite fibers are presented as case studies to demonstrate the capabilities of the developed representative volume element-based design and analysis tools. The unique automated design and analysis framework presented in this study integrates various software tools, quantifies the effect of process parameters of experimental composites with nanofillers, and provides quick what-if analysis for manufacturing application-specific composites.
The accurate prediction of damage and failure in laminated composites is still a major issue in many structural applications. The paper provides a detailed description of a new damage mesomodel and examines its application to solve material and structural problems for the Test Cases proposed in the Third World-Wide Failure Exercise (WWFE-III). The cases cover various materials (glass/epoxy and carbon/epoxy), lay-ups (unidirectional, cross ply, quasi-isotropic, angle ply and general multi-directional laminates), loadings (uniaxial, biaxial, bending, thermal and loading-unloading) and features (ply sequence, size effects and open hole). The model deals with various damage scenarios and mechanisms of degradation, including diffuse intralaminar damage, diffuse interface damage, localized delamination, fibre breakage and plasticity, and can predict the evolution of a laminate’s response until final failure. Some issues concerning the limited information available in the exercise for the identification of the parameters of the mesomodel are also discussed.
Advanced structural analysis methods that account for manufacturing defects in composite parts are needed to enable accurate assessment of their capability and useful life and to enhance current design and maintenance practices. In particular, porosity/voids are typical defects in carbon/epoxy and glass/epoxy composite aircraft flight-critical components. High-fidelity nondestructive evaluation by X-ray computed tomography allows accurate defect measurement and automatic conversion to structural models to assess the effects of defects on structural properties. This study presents a comprehensive structural analysis methodology, which includes nondestructive detection and finite element modeling of the defects in composites. Effects of porosity/voids on interlaminar tensile and shear strength of unidirectional carbon/epoxy composite specimens are investigated. Failure predictions and subsequent test correlations are presented.
This investigation reports the phase, microstructure and hardness of ZrO2-reinforced iron-based metal matrix composites synthesized by powder metallurgy. The process involved the preparation of homogeneous mixture of electrolytic iron powder with different weight percentages (5%, 10%, 20% and 30%) of ZrO2 by milling, compaction in the form of rod and sintering in the temperature range of 900–1100°C for 1–3 h in argon atmosphere. The sintered samples were subjected to X-ray diffraction for phase analysis and to scanning electron microscopy for microstructure evaluation. The X-ray diffraction and scanning electron microscopy results show the presence of Fe, ZrO2 and Zr6Fe3O phases. This Zr6Fe3O phase is formed due to reactive sintering. From these studies it is concluded that two types of sintering are involved: (a) solid state sintering between Fe particles and (b) reactive sintering with the formation of iron zirconium oxide. Density and hardness of composite specimens were found to depend on fraction of ZrO2, the nature of sintering and the formation of Zr6Fe3O phases.
Composite-metal stack is an ideal combination of materials which unites the advantages of each dissimilar material in a substantial weight. However, drilling dissimilar materials has been a challenge since the composite-metal stacks are at demand in industries. It is important to choose the appropriate drill geometry regarding the stacking sequence and utilize proper machining parameters in order to achieve damage free and precession holes. This experimental study was conducted on dry drilling of CFRP/Al2024/CFRP (carbon fiber-reinforced plastic). Four types of twist drills with various geometries, both coated and uncoated, were utilized to study the effect of machining parameters on hole quality. It was observed that increasing feed rate entails an increase in entrance delamination, whereas exit delaminations and fiber fraying at 2nd CFRP exit diminished with increasing feed rate. It was also found that four facet tools performed better than two facet tools in terms of fiber delamination. Most accurate hole was attained on 2nd CFRP; however, it was found that increasing feed significantly affects the hole size on 1st CFRP.
In order to examine the effect of ply thickness on the crack initiation and propagation in the 90° layer in [0°/90°n/0°] laminates, we conducted numerical simulations using two-dimensional mesoscopic numerical models. We found that the stress increase in the thin layer with 40 µm thickness was restricted in the vicinity of the adjacent layers, leading to restriction of crack penetration through the 90° layer. In addition, we confirmed the effect of stiffness of adjacent layers. In the case where the 90° layer was sandwiched between 45° layers, which had lower stiffness than the 0° layers, crack propagation in the 90° layer was faster than that observed with the 0° adjacent layers. Thus, the crack propagation behavior in the 90° layer was significantly influenced by the change in the stiffness caused by the orientation angle of the adjacent layers.
The viscoelastic properties of nanocomposites based on epoxy and epoxy-phenolic blend matrix reinforced with multi-wall carbon nanotubes were studied. The content of carbon nanotube (0.2 and 0.5 wt.%) and its surface state (pristine vs. oxidized) varied. The characterization of the nanocomposites was done by means of infrared spectroscopy and chemical methods. From the viscoelastic properties of nanocomposites, characteristic properties such as storage modulus (E0), tan and damping peak (tan ) were obtained. The void content was also determined. Also, Weibull statistics are used to represent the failure of secondary bonds during the relaxation processes that lead to stiffness change over the full range of use temperatures. The incorporation of phenolic resin to an epoxy matrix leads to an increase of the relaxation modulus. The glass transition temperature of the epoxy-phenolic nanocomposites decrease when it is incorporated carbon nanotubes, independently of its state (pristine or oxidized).
This work reports the effect of nanotube aspect ratio on the free vibration characteristics of a functionally graded nanocomposite cylinders reinforced by wavy single-walled carbon nanotubes (CNTs) based on mesh-free method. In this simulation, an axisymmetric model is used and axisymmetric natural frequencies of CNT reinforced composite cylinders are presented. The material properties of functionally graded CNT reinforced composites are assumed to be graded in the thickness direction and are estimated by a micromechanical model. The effect of the waviness of the CNTs and its parameters are studied. In the mesh-free analysis, moving least squares shape functions are used for approximation of displacement field in the weak form of motion equation, and the transformation method is used for the imposition of essential boundary conditions. It is observed that the waviness significantly reduces the effective reinforcement of the nanocomposites. The effective moduli and frequency response are very sensitive to the waviness but this sensitivity decreases with the increase of the waviness. The validity of the Young's modulus and the frequency response were assessed by a comparison with available literature data, providing a good agreement.
This study about the detection of delamination initiation is integrated into a wider project on the definition of a criterion of delamination on composite materials. It must therefore provide experimental data on the behavior of material to allow the formulation of a criterion of edge delamination as accurate as possible. Reliable experimental methods were used to identify the intrinsic parameters of the studied material. One of the most important procedures of this study consists in determining the onset of delamination in real time in order to determine stress level of delamination. Acoustic emission was used for detection of real-time damage initiation, using a location algorithm and specific pattern recognition. After correlation between experimental results and data processing, an acoustic emission signature of a delamination initiation was defined. Almost all experiments can be stopped before failure, in order to characterize damage. Extensive experimental work enabled us to verify the validity, the accuracy and the reliability of real-time delamination acoustic emission criterion.
Composite materials possess high mechanical in-plane properties but suffer from very low out-of-plane properties due to limited inter-laminar strength. Common z-pinning techniques to improve the mechanical properties of composites materials are presented in this paper. Special techniques to reinforce liquid composite molding preforms are explained. A new, low-cost z-pinning method to create high-performance laminates of dry fabrics invented by the author will also be introduced. Mode I double cantilever beam tests are performed to compare rectangular and circular z-pins using the newly described manufacturing process.
The present study investigated the effects of three pressure-related process deviations (reduced ambient pressure, reduced vacuum and restricted air evacuation) on the consolidation and quality of flat laminates manufactured by out-of-autoclave prepreg processing. The evolution of laminate thickness, measured in-situ, was correlated with thickness and porosity data for woven and unidirectional fibre bed composites. The process deficiencies are shown to have had distinctive detrimental effects on the rate of thickness change and on the amount, distribution and morphology of voids, and to have been more pronounced for the woven fabric prepreg format due to its higher initial bulk factor.
Combination of cells and materials opens a new option for tissue repair and regeneration. The aim of this study is to investigate the effects of hydroxyapatite/collagen composite on the osteogenic differentiation of rat bone marrow derived mesenchymal stem cells. The hydroxyapatite/collagen composites are synthesized by bioinspired mineralization and characterized by Fourier transform infrared spectrometry, X-ray diffraction, scanning electron microscope, thermogravimetric analysis and laser particle size analyzer. Different concentrations of hydroxyapatite/collagen are co-cultured with the rat bone marrow derived mesenchymal stem cells in passages 2–4 on a culture plate. Osteogenic differentiation is evaluated using reverse transcription polymerase chain reaction, alkaline phosphatase spectrophotometry as well as western blotting. The results demonstrate that the developed hydroxyapatite/collagen composite has microstructure and composition that are similar to the natural bone matrix. Hydroxyapatite/collagen treatment can induce osteogenic differentiation of rat bone marrow derived mesenchymal stem cells, as confirmed by the expression of osteoblast-related markers at both messenger RNA and protein levels. The concentration of 75 µg/ml may be the optimal inducer. In addition, combining biochemical reagents and hydroxyapatite/collagen has a synergistic interaction on the osteogenic differentiation of rat bone marrow derived mesenchymal stem cells. This provides a new avenue for mechanistic studies of stem cell differentiation and a novel approach to obtain more committed differentiated cells.
In order to investigate the combined effect of expansive and shrinkage-reducing admixtures on shrinkage and mechanical properties of ultra high performance fiber-reinforced concrete, a series of experiments including three types of shrinkage tests, compressive and flexural tests, and penetration-resistance test were performed. Test results showed that about 51–53% of free shrinkage and 35–49% of restrained steel strains were reduced by adding 1% shrinkage-reducing admixture along with 7.5% expansive admixture with slight change of strength. Elastic stress, residual interface pressure and degree of restraint from ring specimens were also significantly reduced by using expansive and shrinkage-reducing admixtures. Although the elastic stress of the ring specimen without expansive and shrinkage-reducing admixtures was higher than the tensile strength, no shrinkage crack was observed for all test specimens due to the relaxation effect.
The effect of extrusion conditions on the performance of polycaprolactone /organo-modified clay nanocomposites was studied. It was demonstrated that the extrusion parameters have negligible effect on the molecular weight of polycaprolactone, on the morphology of the nanocomposites and on the final thermal/mechanical properties of the materials. This result was a consequence of the previous optimization of both polymer/clay compatibility and clay processing stability. Finally, the molten–polycaprolactone/clay mixtures were post-processed by different techniques submitting the mixtures to extensional flow. Clay platelets alignment was observed as a function of the extensional flow intensity which further improved the mechanical properties of the nanocomposites.
The addition of damping to structural components holds potential benefits across many applications. However, the dynamic properties, including damping, are often considered independently of the quasi-static mechanical properties. In this study, the effect of multi-walled carbon nanotube (MWCNT) content level was investigated for structural damping properties, including elastic modulus, toughness, loss modulus and glass transition temperature. The MWCNT reinforcement levels of 0.2, 0.6 and 1 wt% are compared with that of the neat epoxy. The largest property improvement found was an increase of 81% in the toughness of the 0.2 wt% composite over the neat epoxy. The dynamic mechanical analysis showed improvements in the loss modulus as the MWCNT content increased. The highest loss modulus, with a 15–20% improvement over the neat epoxy, was in for the 1 wt% composite. Overall, the mechanical properties were improved by the addition of MWCNTs, though damping is limited at low material strain.
The aim of this study was to understand the low-velocity impact energy absorption mechanism of the developed two-dimensional multistitched multilayer E-glass/polyester-woven composites. It was found that the specific front and back face damaged areas of the two-dimensional multistitched E-glass/polyester-woven composites were smaller than those of the two-dimensional unstitched structures. When the stitching density increases, the front and back face damaged areas generally decrease. In addition, when the number of stitching directions increased, the front and back face damaged areas decreased. Therefore, stitching density, stitching directions, stitching yarn, and stitching type on the composite structures were considered as important parameters. Impact load caused a small indentation in the center of front face and resulted in fiber splitting and fiber breakages in the center of the back face of the structure. On the surrounding area of the front and back face damaged zones of the structures, fiber-matrix debonding and matrix breakages were observed. These results indicated that multistitching suppressed the impact energy to a small area of the composite structure. Thus, the two-dimensional Kevlar®129 or E-glass-multistitched E-glass/polyester-woven composite structures showed better damage tolerance performance compared to the unstitched composite structures.
This two-part article examines the effects of thickness and stacking sequence on the ballistic impact behaviors of GLARE 5 (2024-T3 aluminum alloy-unidirectional S2-glass/epoxy) fiber-metal laminated plates. Part I deals with observations from tests of specimens with various thicknesses and stacking sequences. A high-speed camera was used to measure impact and residual/rebound velocities. Ballistic limit velocity (V50) was determined based on the incident projectile impact velocity versus the residual velocity (Vi~Vr) data and the classical Lambert-Jonas’ equation for each individual panel type. As expected, thicker specimens revealed more resistance to perforation. The results showed that the V50 varied in a parabolic trend with respect to the specimen thickness. Furthermore, the quasi-isotropic specimen offered relatively more resistance to impact as compared to other types of stacking sequences. The interfacial debonding/delamination as well as bending/stretching in aluminum layers were considered to be important factors in dissipating impact energy in the specimens. The damage characteristics were evaluated using both nondestructive ultrasonic and mechanical sectioning techniques. Only the contour of the entire damage area could be obtained using ultrasonic C-scan, whereas more details of the damage were provided through the mechanical cross-sectioning technique. It was observed that the damage contour increased as the impact velocity reached its V50 value for a given specimen thickness and then it slightly decreased for impact velocity above the V50 value. In addition, the shape and orientation of damage contours were affected by lay-up orientations.
Poly-dicyclopentadiene matrix composites with different concentrations of mineral wollastonite particles (CaSiO3) were fabricated with possible applications as high volume structural materials. This represents a significant reduction in costs. A planetary Thinky mixer was used to initially mix the resin with the curing agent, followed by incorporating grubbs catalyst. Finally, the product was mixed together with different loading of the particles. The microstructure and compositions were identified by scanning electron microscopy and X-ray diffraction. Particles were found to be homogeneously distributed over the polymer matrix. Quartz was found as a byproduct of the calcium dissolution in the resin. The thermo-mechanical behavior was evaluated by compression, curing, dynamic mechanical analyzer and thermo-gravimetric analysis. For all wollastonite loadings it was found that compression strength was over 100 MPa. Wollastonite was found to decelerate the curing of the resin by the release of calcium ions that enhanced the exothermic reaction.
The traditional approach to mitigate problems resulting from stress concentrations around cutouts for windows and doors in aeronautical structures, either made of metallic or of composite materials, is to locally thicken the structure increasing its weight as well. A more effective solution, without weight penalties, is to reduce the peak stress by redistributing loads to supported regions of the panels such as frames and stiffeners. This can be achieved by means of fibre-steered laminates with variable in-plane stiffness. In this work, the potential of these novel designs for the purpose of stress alleviation around cutouts in composite panels is explored. The optimal configurations in terms of shear buckling and postbuckling failure responses are identified by means of parametric numerical studies. It is predicted that there are steered-fibre configurations that outperform the optimal straight-fibre ones for hole sizes up to two-thirds of the panel width.
Fibre-reinforced composites consist of three key components: the reinforcing fibres, the matrix and the interface between the fibre and the matrix. The efficient impregnation of the reinforcing fibre bundle by the matrix is a primary prerequisite for the production of advanced fibre-reinforced composites. This process can be significantly enhanced by spreading the filaments in the reinforcing fibre bundle. The authors previously reported on a manual technique for spreading the filaments in a bundle. This involved subjecting a fibre bundle to a series of reciprocating motions over a rod. The effect of releasing the tension on the bundle was also considered. On the basis of the observations made in the previous study, a mechanised rig was designed, manufactured and optimised to enable the lateral spreading of the filaments in a bundle of E-glass fibres. A Taguchi-based approach was used to optimise the variables on the rig such as the number and configuration of rollers, haul-off speed of the fibre bundle, pre-tension in the bundle and the rotational speed of the roller carrier hub. The maximum degree of fibre spreading achieved for a commercially available 2400 tex E-glass fibre bundle was 250%.
Using the principles of classical micromechanics, analytical equations are developed in this paper to estimate the effective orthotropic properties of a unit cell of strand-based composites according to their constituent phase properties and their microstructural features such as resin thickness, void content and strand geometrical characteristics. Although a special type of strand-based wood composite product, Parallel Strand Lumber, is considered here as an illustrative example, the methodology can be used for other wood composites consisting of high volume fraction of wood strands. The predictive accuracy of the derived analytical equations is investigated through comparisons with numerical results. Finally, applications of these equations in a linear viscoelastic analysis are discussed. The analytical micromechanics models developed here provide an efficient means of computing effective properties of a unit cell of strand-based composites. These models can then be used within a multi-scale modelling framework that has been developed previously to simulate the macroscopic behaviour of structures made of such materials.
Montmorillonite nanoclay was modified with octadecylamine and compounded with natural rubber by the melt mixing method. Influence of the modification on the properties of natural rubber/organoclay nanocomposites was investigated. Furthermore, structures of the natural rubber/clay nanocomposites were characterized by X-ray diffraction and scanning electron microscopy. It was found that organoclay was well dispersed in the natural rubber matrix. Curing characteristics of the rubber compound indicated that octadecylamine modification accelerated the vulcanization reaction of natural rubber and caused a higher degree of crosslinking. The storage modulus (E') of the natural rubber vulcanizates was higher with the modified clay than with unmodified montmorillonite (Na-MMT). The effects on the storage modulus are more pronounced at temperatures above the melting temperature of octadecylamine. The low thermal stability of this modifying agent does not affect the thermal stability of the natural rubber/organoclay nanocomposite.
Poly(3-octyl thiophene) (P3OT)–polycarbonate (PC) polymer blends were prepared at different P3OT weight ratios to investigate the effect of P3OT content on the optical, electrical, mechanical, rheological, and thermal properties of P3OT–PC polymer blends. Optical results showed that addition of P3OT to neat PC enhanced the ultraviolet/visible absorption of the neat PC. The optical energy gap of blend system was decreased with increasing P3OT up to 10% P3OT and the blend system appeared to be immiscible when P3OT content was 20% P3OT. Electrical results showed that incorporation of P3OT into neat PC up to 20% P3OT elevated the electrical up to six orders of magnitude and enhanced the polar character of the neat PC. Also, electrical results showed that P3OT–PC blend system became percolated above 0.8% P3OT. Mechanical and rheological results indicated that incorporation of P3OT into neat PC up to 10% P3OT will enhance elastic modulus, complex viscosity, storage modulus (G'), and loss modulus (G') of the blend system while these parameters were found to be decreased when P3OT content in the blend system was 20% P3OT. Generally, all obtained results indicated that P3OT–PC blends are partially miscible blends up to 10% P3OT while this blend system appeared to be immiscible blend at 20% P3OT.
This article focuses on the development of a comprehensive method for optimization of casting parameters including stirring time and speed of stirrer. Particulate AA6061 aluminum alloy matrix composites were produced by compocasting. First, the effects of extrusion and particles coating on the microstructures and mechanical properties of the composites were investigated. Then, a hybrid algorithm based on PSO and GA was implemented in order to solve the global problems and optimize the coefficients of equations with much better performance, resulting in higher possibility for industry application. PSO evolved the population over a certain number of generations, retained the best M particles and excluded the remaining pop size-M particles. Selection, crossover and mutation GA operators generated pop size M new individuals and combined them with the best M particles to form a new population for the next generation. The model based on combination of GA and PSO is capable to predict with greater reliability than single optimization methods including ACO and multiple linear regression.
Drilling is a key factor in the manufacturing of holes required for the assemblies of composite laminates in aerospace industry. The quality of holes can be controlled by the choice of tool geometry and process parameters. Simulation of drilling process is an effective method in optimizing the drill geometry and process parameters in order to improve hole quality and reduce the drill wear. In this research, we have developed three-dimensional finite element model for drilling carbon fiber-reinforced. A three-dimensional progressive intra-laminar failure model based on the Hashin’s theory is considered. Also, an inter-laminar delamination model which includes the onset and growth of delamination by using cohesive contact zone was developed. It is shown that the induced thrust force, torque, damaged area and delamination are predicted very well with the given drill geometry and process parameters. The delamination area resulted from drilling of carbon fiber-reinforced polymer was predicted successfully using the developed model. The current finite element model using three-dimensional elements and improved damage models showed much better capability in simulation of the drilling process of CFC compared to the previous model using shell elements.
Recently, as a part of the third World-Wide Failure Exercise (WWFE-III), the author provided a modelling capability, entitled ‘Energy methods for modelling damage in laminates’, to be published in this special issue. This paper describes full details and the mathematical basis of the author’s methods used to predict the properties of undamaged laminates and the development of damage in laminates, based on an energy balance methodology.
A novel approach for evaluating the torsional performance of single carbon fiber filament was established based on a self-designed apparatus. The number of torsion turns and twist angle at fracture from the direct torsion failure test were defined to characterize the torsion-bearing capacity of single carbon fiber. The fiber fracture strain under torsion stress was calculated based on a single fiber torsional deformation model and was compared with the fiber elongation in tension, which showed that the toughness of carbon fiber under torsion shear stress was inferior to its resistance to tensile stress. In addition, the resistance of carbon fiber to torsion shear stress was changed after changing the fiber surface structure by removing the sizing agent or coating the fiber surface with epoxy resin. The established method will enrich the existing research work on mechanical properties of carbon fiber as well as those for different types of fibers.
The modified tilted sandwich debond (TSD) test method is used to examine face/core debond fracture toughness of sandwich specimens with glass/polyester face sheets and PVC H45 and H100 foam cores over a large range of mode-mixities. The modification was achieved by reinforcing the loaded face sheet with a steel bar, and fracture testing of the test specimens was conducted over a range of tilt angles. The fracture toughness exhibited mode-mixity phase angle dependence, especially for mode II dominated loadings; although, the fracture toughness remained quite constant for mode I dominated crack loadings. The fracture process was inspected visually during and after testing. For specimens with H45 core the crack propagated in the core. For specimens with an H100 core, the crack propagated between the resin-rich layer and the face sheet.
In this article, the carbon fiber/epoxy resin matrix composite laminates were fabricated using a co-curing process combining resin film infusion (RFI) process with prepreg-autoclave process, called co-resin film infusion. A kind of unidirectional prepreg and its corresponding resin film were adopted. The compaction and defects of laminates cured by different processing were studied. Mode I and mode II interlaminar fracture toughness were adopted to evaluate the co-cured interlaminar properties of the co-curing laminates and were compared with those of laminates processed by the prepreg-autoclave process and the resin film infusion process. Moreover, the effects of lay-up type of prepreg part and resin film infusion part, isothermal dwell and epoxy tackifier of fiber preform were also studied. The results show that these factors have important effects on the processing qualities of the laminates cured by co-curing process, including resin-rich regions and voids, resulting in different interlaminar fracture toughness at the co-cured interface. Affected by the prepreg part and the resin film infusion part in the co-resin film infusion laminates, the mode I and mode II initial interlaminar fracture toughness of co-cured laminate lie between those of the prepreg laminate and resin film infusion laminate, and GIC at crack propagation stage for co-resin film infusion laminate are higher due to fiber bridging and deflection of crack. These results have close relationships with the compacting structure of fibers in prepreg and resin film infusion parts and the interfacial bonding between the two kinds of fiber and the matrix.
Ceramic particle-reinforced aluminum matrix composites are gaining attention especially from the automotive and aerospace industry requiring light-weight and high-strength materials for various applications. Since the AlB2 reinforced composites are relatively new materials, there is no investigation into the machinability of such materials in the literature. Therefore, the aim of this paper is to study machinability of AlB2/Al-Mg3 composite materials to optimize cutting force and surface roughness during turning operation. Taguchi’s statistical approach has also been utilized for the optimization of the process parameters. The optimum conditions providing the lowest cutting force and surface roughness were estimated. For the experimental planning, L8 (27) orthogonal array and the smaller-the-better response criterion were selected to obtain optimum conditions. Analysis of variance was used to determine the most significant parameters affecting the cutting force and surface roughness generated. The optimum condition was predicted from the combination of following factors and their respective levels; the second level of material type (AlB2/Al-Mg3), first level of cutting tool (TiN coating), first level of feed rate (0.08 mm/rev) and first level of cutting speed (350 m/min). Confirmation tests were performed to determine the effectiveness of Taguchi’s optimization method using the optimal levels of test parameters. A good agreement with a confidence level of 99.5% has been observed between the predicted and tests results for both cutting force and surface roughness.
Composites of untreated and treated kenaf fibres with recycled polypropylene were fabricated by melt-cast technique with and without using maleic anhydride-grafted polypropylene. To improve interfacial bonding between kenaf fibres and recycled polypropylene, surface modifications of fibres were performed through ultrasound, enzyme and alkali pre-treatments. Klason lignin test, Fourier-transform infrared spectroscopy and scanning electron microscopy were used for the characterization of fibres. For characterization of composites, the samples were examined by density measurements, mechanical tests, field-emission scanning electron microscopy, X-ray diffraction study, differential scanning calorimetry and thermogravimetric analysis. Results revealed that ultrasound was able to remove the highest amount of lignin (32%). Tensile strength of the composites was increased by 57%, 58% and 40% due to the treatment with alkali, ultrasound and enzyme, respectively. Optimization of treatment parameters was carried out by means of the design expert software. The optimum treatment parameters, such as alkali concentration, soaking time in alkali, sonication power, temperature, enzyme concentration and soaking time in enzyme were found to be 4.6 wt%, 4.95 h, 99.96%, 94.46°C, 1.26 wt% and 3.89 h, respectively, which are reasonably close to the experimental ones. The preferential b-axis orientation in recycled polypropylene crystal was found to be more apparent due to treated kenaf fibres with maleic anhydride-grafted polypropylene than untreated kenaf fibres with maleic anhydride-grafted polypropylene. A correlation among crystallinity, surface morphology, and tensile and thermal properties of composites with the fibre-matrix interactions has been established.
Composite pyramidal lattice structures with hollow trusses afford a convenient means to enable functionality by inserting elements into free volumes within or between trusses. In this study, vibration and low-velocity impact tests were carried out to investigate the dynamic behavior of hollow composite pyramidal lattice structures filled with silicone rubber. Frequencies and the corresponding damping ratios were obtained, which revealed that the damping ratios of space-filled composite pyramidal lattices increased by two times but those of hybrid composite pyramidal lattices decreased by 2% for the first three orders compared with hollow composite pyramidal lattices. Energy absorption capability for rubber-filled structures increased and the rubber filled between trusses can prevent tup penetration. Desired functional potentials can be realized for composite pyramidal lattice structures by serious selection of filling materials and the corresponding geometry.
A suitable anchoring system is required to anchor a CFRP tendon due to its sensitivity in lateral pressure. Recent developed anchors are still relying on lateral pressure in anchoring CFRP tendons. A new CFRP unit equipped with U-anchor at both end of the rod body without any jointing (namely of Super CFRP, S-CFRP) has been developed. This paper presents the mechanical behavior as well as failure mechanism of U-anchor under direct loading and loaded under embedded within concrete, respectively. The rupture occurred on the circular part of U-anchor under direct loading. The stress concentration on circular loop was the cause of U-anchor rupture. Loading of U-anchor embedded within concrete indicated an optimum capacity. The failure was out of the anchor system. Finite element model of U-anchor embedded within concrete showed better stress distribution on anchor at rupture load such that the stress on U-anchor was lower than CFRP strength.
The potential of accurate modelling of the shear modulus reduction of laminates with cracked 90-layers using models based on the minimization of the complementary energy with improved stress description in the constraint layers is evaluated.
This group of models refine Hashin’s model by introducing shape functions with unknown parameters to represent the out-of-plane shear stress distribution across the constraint layer thickness. The Hashin’s model becomes a particular case of the presented when the shape parameter approaches to zero. The most accurate shape parameters are found in the result of minimization. Three models are compared: the present variational model, Hashin’s model and the shear lag model introduced by Soutis which assumes linear out-of-plane shear stress distribution over an unknown part of the layer. It is shown in this paper that the size of this part may be determined by minimization of the complementary energy. The present model is the most accurate amongst the three, whereas Soutis’ model is more accurate than the Hashin’s model for laminates with constraint layer, thicker than the cracked layer. The comparison with finite element method results shows reasonable agreement. Agreement can be improved developing models with better description of the stress state in the cracked layer.
In the present work, a mechanistic modeling approach is pursued for material characterization and for modeling inelastic deformation of polymer matrix composite components. The model attempts to capture the dominant micromechanical deformation mechanisms in laminated composites caused by matrix inelasticity at elevated temperatures. Given material characteristics of the constituent materials, the model can be used in predicting stress, time and temperature-dependent response of a composite under a broad range of thermal and mechanical load conditions. This article describes the modeling approach and examples of its use in a finite element analysis framework. Examples include analyses of simple test specimen coupons, stress concentration at holes and a structural element configuration of a polymer matrix composite. In each case, the model predictions are compared with the experimental measurements.
A water-based emulsion method was used in this study to produce hollow epoxy particles. The hollow epoxy particles were then chemically treated with hydrochloric acid solution to eliminate the excessive calcium carbonate on the surface of the hollow epoxy particles. The hydrochloric acid–treated hollow epoxy particles were added to the polyester matrix at different loading values (0–9 wt%). The addition of 5 wt% hollow epoxy particles to the polyester matrix optimized the mechanical properties (i.e., tensile strength, tensile modulus and flexural strength) of hollow epoxy particle–filled polyester composites. The interlocking of the polyester matrix into the pore regions of the hollow epoxy particles strengthened the polyester composites. The addition of hollow epoxy particles to the polyester matrix increased the thermal stability, storage modulus and the glass transition temperature of the polyester composite. The water absorption and diffusion coefficient of the polyester composite were also increased with the increase of the hollow epoxy particle loading.
The current study investigates the impact characteristics of polymeric-based nanocomposites strengthened with carbon nanotubes, nanoclay, aluminum oxide and silicon carbide particles nanofillers. Different weight percentages of each nanofiller were prepared. A dead weight drop mechanism was utilized to compare the impact characteristics of different nanocomposite materials. An X-ray technique was utilized to characterize the formation of microstructural defects of the laminated composites post impact. The results showed that the nanoclay fillers were the best to enhance the impact and mechanical properties of the composite materials with 4.3 wt% of nanoclay being the optimum percentage.
In this article, we suggest a theoretical weight-reduction method to partially substitute an alternative light-weight material for a material in a box-type bodyshell with cut-outs using a material selection method. The box-type bodyshell with cut-outs was simplified as an equivalent box-type bodyshell model without cut-outs, which is denoted as an equivalent bodyshell model and has the same vertical stiffness as the case with cut-outs. The thicknesses of the roof and the walls of the equivalent bodyshell model were then determined such that the deflection of the equivalent bodyshell model was equal to the sum of the theoretical deflections of each box-type bodyshell section with cut-outs under a distributed vertical load condition. Next, the material selection method for weight-reduction design was applied to the equivalent bodyshell model to derive the hybrid-type equivalent bodyshell model. Finally, a weight-reduction design of the box-type hybrid bodyshell with cut-outs was derived from the hybrid-type equivalent bodyshell model. To demonstrate this method, we derived a weight-reduction design of the under-frame in the box-type hybrid bodyshell with cut-outs and then compared the finite element simulation results of the derived weight-reduction design with the suggested theoretical results. The comparisons yielded good correlations. The proposed method was useful to reduce the weight of a hybrid bodyshell with cut-outs by changing materials.
Multi-wall carbon nanotubes (MWNTs) with different loadings (0, 1, 3, 5, 7 and 9 wt%) were incorporated into polyamide 11 using a powder–powder mixing technique to form nanocomposites. The density and specific heat of each nanocomposite were measured. Thermal conductivities of these polymer nanocomposite powders were measured in a nitrogen atmosphere using a commercial transient plane-source device. Results indicate the thermal conductivity of preheated powder was larger than the fresh powder and similar to that at the melting temperature. The powder formulations were also pressed into films using a heat press and room temperature electrical conductivity measurements were performed. Substantial improvements in electrical conductivity were observed with increasing loading of MWNT. Microstructures of the nanocomposite specimens were examined using scanning electron microscopy.
An exploratory work has been carried out in the field of three-phase composites in which fly ash nanoparticles were incorporated in addition with 2D and 3D glass woven fabrics while preparing composites. These fly ash nanoparticles were prepared by high-energy ball milling technique and its effect on functional properties of composites were observed. In thermo-mechanical analysis, the composites with the 3D woven fabrics as reinforcement showed higher storage modulus. Further improvement in the storage modulus due to addition of fly ash can be explained as the nanoscale dimension of fly ash nanoparticles have got very large surface area which leads to interaction sites and hence efficient load transfer between reinforcing agent and matrix. Though the addition of fly ash nanoparticles improved the mechanical properties of composites marginally, a significant improvement was observed in functional properties of composites.
As an anode material for lithium-ion batteries, silicon/disordered carbon/carbon nanotubes composites were prepared by a sucrose-aided combustion method using two different mixing methods, namely mechanical stirring and ball milling. In this study, sucrose was used as a carbon source. The as-prepared composites were characterized by means of X-ray diffraction, field emission scanning electron microscopy, and electrochemical impedance spectroscopy. Both composites contain silicon and a small amount of carbon as predominating phases. The SDC-M composite prepared using mechanical stirring shows higher cycle performance (834.8 mAh/g) and first charge/discharge efficiency (72.4%) than the SDC-B composite prepared using ball milling (cycle performance of 815.2 mAh/g and first charge/discharge efficiency of 68.8%). The difference in the electrochemical performances of the SDC-M and SDC-B composites can be attributed to the distribution of carbon nanotubes and silicon particles in the disordered carbon matrices, which was greatly affected by mixing method.
In this study, we developed a new way to increase the efficiency of dye-sensitized solar cells by using TiO2/silver/carbon nanotube composites as the working electrode. Silver nanoparticles and multi-walled carbon nanotubes were mixed with TiO2 nanoparticles and used as working electrodes in a dye-sensitized solar cell. The effect of the silver nanoparticles and multi-walled carbon nanotubes on the efficiency of the dye-sensitized solar cell was studied as function of their volume fractions using several microscopic and spectroscopic characterization techniques such as scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultra-violet-vis and electrochemical impedance spectroscopy. It was found that the silver nanoparticles could induce surface plasmonic phenomena, where the light absorption was enhanced in the ultra-violet wavelength range. Additionally, the carbon nanotubes could increase the electron mobility in the working electrode due to their high surface-to-volume ratio and superior electrical conductivity. The efficiency of the silver/carbon nanotube/TiO2 nanocomposite working electrode was compared with that of a conventional TiO2 working electrode under one-sun illumination (100 mWcm–2, AM 1.5 G). The TiO2/Ag/carbon nanotube nanocomposite working electrode had a two-fold higher efficiency (3.76%) than the conventional pure TiO2 working electrode (1.88%).
Since flax is the most promising plant for the reinforcement of polymer-based composites in structural applications, we have chosen to investigate its hygrothermal characteristics which can be useful for the understanding of the behaviour of other plant fibres. The flax fibres were exposed to different hygrothermal conditions: in an oven at various controlled temperatures (–40 to 140°C) and measured relative humidity, in a climate chamber at 50% relative humidity for define temperatures between 25°C and 85°C, or different determined aging conditions. The correlation of these hygrothermal conditions to the evolution of the mechanical properties gives evidence of the prominent influence of water over temperature on the microstructural changes of flax fibres. The mechanical parameters drastically decrease in usually prescribed hygrothermal aging conditions for organic matrix composite materials, the strength being particularly sensitive to the presence of water. These evolutions were correlated to the fibre microstructure modifications induced by water absorption as revealed by electron microscopy analyses. These findings could be useful for understanding the behaviour of polymer matrix biocomposites in severe hygrothermal conditions.
The effect of alkaline treatment in combination with air bubbling in banana fibre mat on the water absorption property of the fibre composite has been experimentally studied. The effect of sonication on the water absorption property of the composites is also studied. Hot-water immersion test is done on the samples. Water uptake was quantified for the composite disc under different processing conditions. The studies have revealed that the fibre surface modification rate is enhanced by the combination of air bubbling with the general process of alkaline treatment. Water absorption is highly reduced by the sonication process. This shows the importance of this method of fibre treatment and this shall encourage other researchers to develop an adequate system to expedite fabrication for better quality of woven banana fibre composites for various sectors such as household utility or automobile parts.
Coated composite corrugated panels have wide applications in engineering, especially in morphing skins where extreme anisotropic stiffness properties are required. The optimal design of these structures requires high-fidelity models of the panels that would be incorporated into multi-disciplinary system models. Therefore, numerical and experimental investigations are required that retain the dependence on the nonlinear static and dynamic behavior of these structures. Considering the nonlinear effects due to the material properties and mechanism of deformation, the mechanical behavior of composite corrugated laminates with elastomeric coatings is studied in this paper by means of numerical and experimental investigations. The importance of this work is that it provides detailed experimental and numerical models of the panel that can be used for further static and dynamic homogenization and optimization studies. In this regard, an investigation of the manufacturing method and an evaluation of the mechanical characteristics of the materials are presented. Then the tensile, hysteresis and three-point bending tests of the coated corrugated panels are analyzed and the mechanical behavior of the panel is simulated. The comparison studies demonstrate the accuracy of the finite element model to predict the mechanical behavior of the coated corrugated panels. Finally, two concepts to deal with the non-smooth surface of the panel during bending for the morphing skin application are proposed.
In order to design parts or mechanical structures made up of heterogeneous materials, composed from a polymer matrix and a natural biodegradable fiber like the ALFA, material properties of such a composite have to be known. The purpose of the present study is to propose a method for identifying the visco-elastic properties of an ALFA/PMMA composite. Unidirectional and woven composites made of the ecological ALFA fiber are studied. The properties of the ALFA fiber were determined via multiple physical tests the results of which were processed with the Weibull distribution law. Given the relaxation law of the matrix and the properties of the natural ALFA fiber, the method determines the relaxation law of the homogenized composite. To achieve that, a representative cell is chosen and then subjected to unitary displacement numerical relaxation tests. The numerical tests results are used in a non-regression identification algorithm in order to find, as a mathematical reusable form, the composite visco-elastic Prony series of the stiffness matrix components. From the stiffness matrix, using a direct and an inverse Fourier transformation, the compliance matrix is computed and the orthotropic engineering property expressions are identified. The obtained results are compared to an ultrasonic measurement test of an ALFA/PMMA 45% specimen.
This article presents the application of waste sanding dusts in order to evaluate its suitability as reinforcement for thermoplastics as an alternative to wood fibers. The effects of sanding dust loading and nanoclay content on the physical and mechanical properties were also studied. Overall trend showed that with addition of sanding dusts, tensile and flexural properties of the composites were significantly decreased, due to the reduction of interface bond between the fiber and matrix. It was found that flexural, tensile and withdrawal strength of fasteners were moderately enhanced by the addition of 2 wt% nanoclay in the matrix. However, with increase in the nanoclay content (4 and 6 wt%), the flexural and tensile properties decreased significantly. The results also showed that the withdrawal strengths of screws are much higher than those of nails. At certain amount of sanding dust, with increasing nanoclay loading the withdrawal strengths of fasteners (screws and nails) were considerably decreased. The thickness swelling and water absorption of the composites dramatically decreased with the increase in nanoclay loading. Except thickness swelling, both variable parameters (sanding dust and nanoclay contents) showed significant influence on physico-mechanical properties.
In this study, the notched strengths of woven hybrid composite laminates each with a center circular hole were investigated. The effective engineering moduli of the composite laminate were first predicted by the spring model method and the stiffness averaging method. Then, a finite element-based point stress criterion proposed previously by the authors was employed to predict the notched strengths of those woven hybrid composite laminates each with a center circular hole. It is shown that the predicted notched strengths by this approach agree well with the experimental results. It is also noted that the notched strength decreases with the radius of the center circular holes for both the drilling and water jet cases. However, the decreasing rate of the notched strength for water jet is obviously higher than those for drilling.
This is a contribution to the exercise that aims to benchmark and validate the current continuum damage and fracture mechanics methodologies used for predicting the mechanical behaviour of fibre-reinforced plastic composites under complex loadings. The paper describes an analytical approach to predict the effect of intra- (matrix cracking and splitting) and inter-laminar (delamination) damage on the residual stiffness properties of the laminate, which can be used in the post-initial failure analysis, taking full account of damage mode interaction. The approach is based on a two-dimensional shear lag stress analysis and the equivalent constraint model of the damaged laminate with multiple damaged plies. The application of the approach to predicting degraded stiffness properties of a multidirectional laminate with multilayer intra- and inter-laminar damage is demonstrated for [0/90/0] and [0/908 /0] cross-ply laminates made from a specific glass/epoxy system under in-plane uniaxial and biaxial loading damaged by transverse and longitudinal matrix cracks and crack-induced transverse and longitudinal delamination.
Although silicones possess low dielectric constant, they are between the most used polymers in actuation due to their appropriate mechanical properties (low modulus and high elongation). These can be easily tuned by the preparation strategy: proper choice of the molecular mass and microstructure of the polymer matrix; adding or not of more or less active fillers; whether these are incorporated in the polymeric matrix (ex situ) or generated in situ; crosslinking mode (through the side or ending functional groups) or mechanism (condensation, radicalic or by hydrosilylation). A relatively low cost and easy scalable procedure was used in this article to prepare silicone composites based on high molecular weight polydiorganosiloxane copolymer and hydrophobized silica and titania nanoparticles. The matrix polymer was synthesized by bulk ring opening copolymerization of different substituted cyclosiloxanes and characterized by FTIR, 1H NMR and gel permeation chromatographic analysis. The composites prepared by the mechanical incorporation of the fillers were crosslinked by radicalic mechanism and investigated by dielectrical spectroscopy, mechanical tests, dynamo-mechanical analysis and dynamic vapor sorption. The actuation measurements revealed displacement values in the range 0.04–5.09 nm/V/mm, while energy harvesting measurements revealed impulse electrical voltage in the range 6–20 V for a dynamic force of 0.1–1 Kgf. The robustness of these composites supported by their thermal, mechanical and surface properties recommends them for use inclusively in harsh environmental conditions, when their behavior is not significantly affected.
663-tin bronze, Ni, W, nano-Al2O3 and Ni-coated graphite mixed as the fundamental alloy powder was added in concentrations of 5 vol.%, 7 vol.%, 9 vol.%, 11 vol.% and 13 vol.% NH4HCO3 respectively to prepare composite powders. Powder metallurgy method was applied to prepare porous materials with these composite powders. Polytetrafluoroethylene was hot-dipped into the pores of the materials to prepare novel polytetrafluoroethylene/copper-matrix self-lubricating composite materials. It was observed that the mechanical properties of the porous materials first increase and then decline with the increase of the added volume fraction of the pore-forming material (NH4HCO3) from 5% to 13%. When the added volume fraction of NH4HCO3 was 9%, the mechanical properties of the sample reached the maximum: its density was – 5.52 g/cm3, hardness – 35.5 HV and crushing strength – 148 MPa. The samples containing 9% (volume fraction) NH4HCO3 were hot-dipped with polytetrafluoroethylene in the water bath at 50°C, 60°C, 70°C, 80°C and 90°C respectively for 1.5 h, as a result of which the content of polytetrafluoroethylene in the samples first increased and then declined with the rise of temperature. When the temperature was 60°C, the content of polytetrafluoroethylene in the sample reached the maximum, indicating that the frictional performance obviously improved by the polytetrafluoroethylene: the wear loss was 6 mg and friction coefficient was 0.08, which proved that the novel polytetrafluoroethylene/copper-matrix composite material had excellent self-lubricating properties.
The vacuum-assisted resin transfer molding process uses a compliant vacuum bagging material which allows the thickness of the saturated preform to change as a function of pressure. The final thickness of the part is dependent on the post-filling phase where excess resin is bled from the preform to achieve a desired final part thickness. The flow of resin during the bleeding process is dependent on the compaction and permeability characteristics of the preform material. This paper examines the post-filling stage of the vacuum-assisted resin transfer molding process through finite element-based flow simulation and experimental studies. The studies focus on thick laminates where both in-plane and through thickness flows are considered. Comparisons of experimental and simulation results for two different post-filling scenarios provide insight into the post-filling phase of the vacuum-assisted resin transfer molding process including model validation as well as understanding of limitations of compaction/relaxation constitutive models in predicting final preform thickness.
The current rationale for development of composite combat helmets is to either maintain performance at reduced weight or maintain weight with a significantly higher level of ballistic performance. Typically, weight reduction with maintained performance is the design approach used. In order to reduce weight with the same materials requires a reduction of material thickness. Thinner structural materials then introduce the complicating and often limiting factor of greater back face deflection. To further understand the tradeoffs of ballistic performance and efficiency, weight and back face deflection, a research project was undertaken. In this research project, a set of 17 composite materials were investigated. The digital image correlation method was used to directly measure the characteristics of the dynamic back face deflection of targets engaged by a set of threats. The analysis of this data, which includes dynamic deflection time histories, back face velocity time histories, strain time histories and spatial distributions of these quantities, allowed for assessment of candidate material performance and characterization of back face deflection. The details of this experimental program and key data results are presented in this paper.
An attempt has been made to study the influence of wear parameters like applied load, sliding speed, sliding distance and weight percentage of breadfruit seed husk ash particles on the dry sliding wear of the Al-Si-Fe alloy composites. A plan of experiments, based on factorial design of four factors–two levels (42) techniques was performed to acquire data in controlled way. Analysis of variance and linear regression were employed to investigate the influence of process parameters on the wear of the composites. The results show that applied load has the highest effect followed by sliding distance. The comparison was made between actual values and predicted values, showing an error associated with dry sliding wear of the composites to be about 2%. The results showed that the addition of breadfruit seed hull ash as reinforcing materials in Al-Si-Fe alloy composites can be used for increasing wear resistance of the composite greatly.
The Kirchhoff and Mindlin plate theories are applied in this study to calculate the stresses and the energy release rates in delaminated orthotropic composite plates. A novel double-plate system is developed with the imposition of the kinematic continuity constraints in the interface plane. The governing equations of the system were derived in both cases. As a demonstrative example a simply-supported plate subjected to a point force was analyzed using Lévy plate formulation and the problem was solved by a state-space model. The distribution of the stress resultants and the interlaminar stresses in the uncracked part were also determined. Moreover, the distributions of the mode-II and mode-III energy release rates along the crack front were calculated by the J-integral. The 3D finite element model of the plate was created providing reference data for the analytical model. The results show that the displacement and stress fields obtained from the Kirchhoff and Mindlin theories are quite similar, but in the case of the energy release rates, transverse shear effect is necessary to consider to obtain reasonably good agreement between the analytical and numerical results.
Flax and coir fibres have the potential to be used as reinforcement in fibre reinforced polymer and concrete, respectively. This study investigates the effect of coir fibre inclusion and flax fibre reinforced polymer thickness on the dynamic and static properties of flax fibre reinforced polymer tube confined coir fibre reinforced concrete. Plain concrete and coir fibre reinforced concrete beams are considered as references. The properties investigated include natural frequencies, dynamic and static modulus of elasticity, Poisson’s ratio, damping ratio, compressive strength, stress–strain behaviour, ductility and confinement effectiveness. Axial compression test revealed that flax fibre reinforced polymer tube confinement significantly increases axial compressive strength and ductility of the confined concrete composite. The increase in compressive strength and structural ductility is directly proportional to an increase in tube thickness. Dynamic test revealed that both coir fibre and flax fibre reinforced polymer tube improve the damping of the structure considerably, thus reducing the effect of dynamic loading on the structural response.
The protection of steel against corrosion by nano-glass flake containing coatings has been evaluated in this article. Nano-glass flake was incorporated into epoxy vinyl ester resin by mechanical agitation, homogenizer and sonication process. The dispersion morphology and agglomeration degree of nanoadditives were analyzed by optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The effect of micro and nano-glass flake on anticorrosion performance of epoxy vinyl ester coatings were also compared in a salt spray chamber. The influence of nano-glass flake on the thermal behavior of resins was studied using differential scanning calorimetry and thermogravimetric analysis. The results showed that corrosion resistance of coatings improves as the amounts of nano-glass flake increases to 1.5 phr. Nano-glass-flake-filled specimens display a better corrosion protection than the micro-glass-flake-filled ones.
Many researchers are now working on composites systems where the fibers and polymers are either totally or predominantly based on renewable materials. The aim of this study is to develop renewable resource-based composites based on spinifex fibers and a polyurethane matrix prepared primarily from spinifex resin. Chemically cleaned and cryo-ground spinifex grass fibres have been used as a reinforcing phase in thermoplastic polyurethane prepared from spinifex resin biopolymer. The morphology, thermal and mechanical properties of the prepared composites were characterized using attenuated total reflection Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis and tensile tests. The results obtained by various characterization techniques indicate that interfacial interaction between the spinifex grass fillers and the polyurethane matrix occurs and that the ground spinifex cellulose fillers act as promising reinforcement for the relatively weak polyurethane matrix synthesized from renewable spinifex plant resin products.
In this study, novel acrylated epoxidized hemp oil bioresin was used in the manufacturing of jute fibre reinforced biocomposites. The 100% biocomposite laminates were characterised in terms of mechanical properties (tensile, flexural, Charpy impact and interlaminar shear), thermo-mechanical properties (glass transition temperature, storage modulus and crosslink density) and water absorption properties (saturation moisture level and diffusion coefficient). Comparisons with the equivalent synthetic vinylester resin based jute fibre reinforced biocomposite panels were performed. Scanning electron microscopic analysis confirmed panel samples containing acrylated epoxidized hemp oil to display improved fibre–matrix interfacial adhesion compared with the vinylester resin based samples. Furthermore in terms of mechanical properties acrylated epoxidized hemp oil based biocomposites compared favourably with those manufactured from vinylester resin synthetic resin. Dynamic mechanical analysis found acrylated epoxidized hemp oil based biocomposites to have lower glass transition temperature, storage modulus and crosslink density than vinylester resin based samples. Increasing acrylated epoxidized hemp oil content resulted in a marginal increase in saturation moisture content and diffusion coefficient.
The effect of matrix cracking on hysteresis behavior of cross-ply ceramic matrix composites is investigated in the present analysis. The cracking of cross-ply ceramic composites was classified into five modes, where cracking mode 3 and mode 5 involve matrix cracking and fiber/matrix interface debonding in 0° ply. The matrix crack space and interface debonded length are obtained by matrix statistical cracking model and fracture mechanics interface debonding criterion. Based on the damage mechanisms of fiber sliding relative to matrix in the interface debonded region, the unloading interface reverse slip length and reloading interface new slip length of cracking mode 3 and mode 5 are determined by the fracture mechanics approach. The hysteresis loops of four different cases for cracking mode 3 and mode 5 are derived respectively. The hysteresis loss energy as a function of interface shear stress of mode 3 and mode 5 are analyzed. The theoretical results have been compared with experimental data of two different cross-ply ceramic composites.
This study, part of the European JTI ‘Clean sky’, presents a simulation model designed to predict the evolution of the degree of cure of the paste adhesives used in carbon fiber reinforced polymer bonded systems cured by induction heating. The simulation combines induced Eddy currents in electrical conductive materials, which cause the heating of the carbon fiber reinforced polymer adherents by Joule effect, and the consequent chemical reaction which gives the relation between temperature, time and degree of cure of the adhesive. The model is validated and used to analyze the impact of different parameters on the degree of cure of the paste adhesive.
Poly(lactic acid)/synthetic mica nanocomposites were obtained by melt blending in an internal mixer in an investigation designed by experimental factorial planning. The results indicated that low speeds and temperatures favoured intercalation. In the range of concentrations tested (3–7 wt%), the added mica did not act as a nucleating agent, and it produced no significant change in the thermal properties of the poly(lactic acid). The thermogravimetric analyses suggested that temperatures below 190°C and short mixing times (<10 min) promoted better dispersion of the synthetic mica in the matrix nanocomposite.
Dynamic response of thermo-viscoelasticity of 3D braided composites is studied by a numerical method proposed in this article. The strain response of 3D braided composites to alternating stress load at room temperature is obtained by the theoretical derivation and finite element analysis. The complex compliance of 3D braided composites is derived from the creep compliance, and the relationship between the complex compliance and angular frequency is investigated. With the increase of angular frequency, 3D braided composites behave elastically gradually. Based on the equivalent thermal expansion coefficient assumption, the thermal effects on the dynamic viscoelasticity are characterized by the time–temperature equivalent relationship. The dynamic response derived in this article for 3D braided composites with thermal effects agrees well with the finite element analysis. The influence of braiding angles and fiber volume fractions on the steady-state strain response of 3D braided composites is investigated.
In this study, an elastic–plastic stress analysis is carried out in fiber glass-epoxy adherends with DP460 ductile adhesive. Multi-linear plastic solution is applied to the joints. Analytical results are checked using a multi-linear finite element solution, ANSYS 12. Analytical and numerical solutions are found to be in good agreement. The thickness of the adherend is chosen as 1.8 mm, 2.5 mm and 3 mm. It is observed that when the thickness of the adherend is increased, the intensity of the peel stress raises. Shear and peel stresses are found to be the highest at or around the free ends of the adhesive. The multi-linear solution yields a closer solution with respect to the bilinear solution since it follows the stress–strain diagram.
A novel concept of structural supercapacitors based on carbon fibre-reinforced composites has been introduced that can simultaneously act as a structural component and an electrical energy storing device. Supercapacitors consisting of woven carbon fibre mat electrodes; filter paper insulator and crosslinked poly(ethylene glycol) diglycidylether/diglycidylether of bisphenol-A polymer electrolytes were fabricated. Brunauer–Emmett–Teller surface area analysis and tensile tests were conducted on the as-received and activated carbon fibre reinforcements. Compression tests and ionic conductivity measurements were conducted on the polymer electrolytes while charge/discharge electrochemical tests and shear testing were done on the structural supercapacitors. This was to investigate the implications of increased diglycidylether of bisphenol-A loading in crosslinked poly(ethylene glycol) diglycidylether polymer electrolytes and carbon fibre activation on the multifunctionality of structural supercapacitors. The addition of diglycidylether of bisphenol-A increased the compressive stiffness, although the ionic conductivity was compromised. Specific capacitance of the structural supercapacitors was increased with the chemical activation of carbon fibre electrodes. Carbon fibre activation led to improved specific capacitance of the structural supercapacitors and the addition of diglycidylether of bisphenol-A increased the shear modulus, although the specific capacitance was compromised.
Flax and jute fibres are inexpensive and easily available bast fibres and they are extensively used as reinforcement in polymer matrix composites. However, due to their susceptibility to moisture absorption, their application is restricted to non-structural interior products. In this study, flax- and jute fibre-reinforced bioresin-based epoxy biocomposites were fabricated using hand lay-up method and their nanoindentation and flexural properties were investigated. In order to study the effects of water absorption on the nanoindentation and flexural properties, the biocomposites were subjected to water immersion tests by immersing specimens in a de-ionised water bath at 25°C for a period of 961 h. The nanoindentation behaviour and flexural properties of water-immersed specimens were evaluated and compared alongside with dry specimens. The percentage of moisture uptake and diffusion coefficient (D) was recorded higher for jute-reinforced specimens compared with flax. The flexural properties for both types of specimens were found to decrease with increase in percentage moisture uptake. Comparison of flexural strength and flexural modulus between flax dry and flax wet biocomposites showed that wet samples lost almost 40% of strength and 69% of modulus compared with dry flax samples. The jute wet samples lost 60% of strength and 80% of modulus compared with dry samples. The nanohardness value decreased from 0.207 to 0.135 GPa for dry flax sample after immersion in water.
The joining of composite materials has traditionally been achieved by adhesive bonding or mechanical fastening. Mechanical fastenings require little or no surface preparation, and are easy to inspect for joint quality. However, they require machining of holes that interrupt the fiber continuity and may reduce the strength of the adherend. We proposed the new composite key joint to minimize the fiber discontinuity and strength degradation of adherend. Mechanical key joints with three different d/t (slot depth to thickness) ratios and two different e/w (edge length to width) ratios were manufactured and tested. Carbon-epoxy unidirectional prepreg (USN125) and plain weave prepreg (HPW193) were used for the composite key joints. Tensile tests were performed on the composite key joints and their failure modes were evaluated. Finite element analyses for the mechanical key joints were performed and the stresses around the key slot were calculated. From the tests, it could be concluded that the failure load of the key joints was about two times larger than those of a mechanical joint for the investigated composite joints.
Previous studies show that fabric construction is one of the key factors affecting the performance of ballistic fabrics. In order to investigate the effect of yarn gripping in fabrics, plain woven narrow fabrics with different widths have been designed and studied. In terms of narrow fabrics, it was found that narrower fabric demonstrates better performance than wider ones under low impact energy due to the better weft yarn gripping effect of the selvages. In the case of higher impact energy, wider narrow fabric shows better ballistic performance in having sufficient material to dissipate the impact energy. Different ballistic panels were made for ballistic testing, with each panel only formed from the same fabric. Performance of narrow fabric panels is disappointing compared to the broad fabric panel due to the discontinuity of the fabric material. A novel broad fabric has been engineered providing both enhanced yarn gripping and material continuity. Tests showed that panels made from such fabrics have improved performance against both back face deformation and projectile penetration.
In-Cu composite solders have been proposed as an effective thermal interface material. Here, finite element analysis and theoretical treatment of their mechanical and thermal behavior is presented. It was determined that the stresses and the strains were concentrated in the narrow and wider In channels, respectively. Furthermore, it is suggested that an In-Cu composite with disk-shaped Cu inclusions may not only further improve the thermal conductivity but may also reduce the stiffness of In-Cu composites in shear.
In this paper, we utilized a bottom-up method to predict the transverse thermal conductivity of pitched-based carbon fibers. We used molecular dynamics simulations with Green-Kubo formalism to calculate the in-plane thermal conductivity and out-of-plane thermal conductivity of the graphite sheets. The effects of waviness on the thermal conductivity of the graphite sheets were studied by MD simulations. The calculated in-plane thermal conductivity and out-of-plane thermal conductivity of graphite sheets from MD simulations were then used for the prediction of transverse thermal conductivity of the pitch fibers by finite element method. In the finite element simulations, the waviness in the graphite sheets was found to decrease the transverse thermal conductivity of pitch fibers, though not significantly. The defects observed in the pitch fibers were simulated by the damage elements in the finite element analysis. The simulation results showed that the proposed model, in which 12.5% of damage was included, predicted the effective transverse thermal conductivity well compared to the value measured from experiments.
A new innovative infusion technology, pulsed infusion, has been developed for the manufacturing of fiber-reinforced thermoset-based composites. Pulsed infusion is a double-bag vacuum infusion process that is based on the use of a proper designed reusable pressure distributor and able to better control the vacuum pressure in pulsed way. Thus, the transverse resin flow through the dry fiber reinforcement is promoted and a better adhesion between the resin and the fibers is achieved. The new process allows to obtain laminates with the same fiber volume fraction and tensile properties of those produced by conventional infusion technologies. An average increase up to 9% for the flexural modulus and up to 24% for flexural strength has been assessed for pulse-manufactured composites compared to traditional vacuum infusion ones. Furthermore, due to a minor consumption of resin and the absence of the distribution net, pulse infusion provides a material cost-saving advantage around 19% and a significant waste reduction.
Sodium montmorillonite was organized by a series of alkyl quaternary ammonium to make an organic modified montmorillonite (OMMT). The organic montmorillonite-resorcinol-formaldehyde-latex (OMMT/RFL) film was prepared by introducing OMMT into RFL, and aging and curing later. The OMMT was valued by X-ray diffraction and Fourier Transform Infrared Spectroscopy (FTIR). The adhesion of the OMMT/RFL-coated fiberglass was evaluated using an H-adhesion technique. The mass-gain of OMMT/RFL and the tensile properties of the coated fiberglass were studied under different humidities. The results show that the best moisture resistance could be gained while 16MMT at 1.5 wt%. Compared with the tensile properties of RFL, the tensile strength of OMMT/RFL increases by 21.2%, the tensile modulus by 23.1% under humidity of 98%; the OMMT/RFL displays a better dynamic storage modulus at elevated temperature.
Novel biocomposites made of an acrylated epoxidized hemp oil based bioresin reinforced with random hemp fiber mat were manufactured by the vacuum infusion technique. Mechanical properties (tensile, flexural, Charpy impact and interlaminar shear), dynamic mechanical properties (glass transition temperature, storage modulus and crosslink density) and moisture absorption properties (saturation moisture level and diffusion coefficient) were investigated and compared with samples manufactured under the same conditions but using a commercial synthetic vinylester resin as the polymeric matrix. Results showed that the 100% biocomposites mechanical performance is comparable to that of the hybrid composites made with the synthetic resin. Moisture absorption tests showed that acrylated epoxidized hemp oil based samples displayed both higher diffusion coefficient and saturation moisture content; however, fiber reinforcement was the dominant transfer mechanism. Vinyl ester based samples were found to have higher storage modulus, glass transition temperature and crosslink density than acrylated epoxidized hemp oil samples.
Epoxy polymers, although often used in fiber-reinforced polymeric composites, have an inherent low toughness that further decreases with decreasing temperatures. Second-phase additives have been effective in increasing the toughness of epoxies at room temperature; however, the mechanisms at low temperatures are still not understood. In this study, the deformation mechanisms of a diglycidyl ether of bisphenol-A epoxy modified with MX960 core-shell rubber particles were investigated under quasi-static tensile and impact loads at room temperature and liquid nitrogen temperature. Overall, the core-shell rubber had little effect on the tensile properties at room temperature and liquid nitrogen temperature. The impact strength decreased from neat to 1 wt% and 3 wt% but increased from neat to 5 wt% at room temperature and liquid nitrogen temperature, with a higher impact strength at room temperature at all core-shell rubber loadings. While a large toughening effect was not seen in this study, the mechanisms analyzed herein will likely be of use for further material investigations at cryogenic temperatures.
This work deals with the delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/III fatigue loading at cryogenic temperatures. Fatigue delamination tests were carried out with eight-point bending plate (8PBP) specimens at room temperature, liquid nitrogen temperature (77 K), and liquid helium temperature (4 K), and the delamination growth rate data were expressed in terms of the energy release rate range during fatigue loading. The energy release rate was calculated using the three-dimensional finite element method. Moreover, the fracture surface characteristics of the woven GFRP laminates under mixed-mode I/III fatigue loading were examined by scanning electron microscopy (SEM).
This paper represents the authors' contribution to Part A of the third World Wide Failure Exercise where a constitutive model is proposed which considers stiffness degradation and plastic strain accumulation for the prediction of stress–strain curves and failure envelopes of 13 test cases, involving various continuous fiber-reinforced laminates with polymeric matrix materials. The model calibration by means of the provided material data is described and the limits of applicability of the proposed constitutive model are discussed. Finally, the predictions are presented as being obtained without any experimental results available. The test cases consider unidirectional and multidirectional laminates under biaxial loads, laminates under various loading conditions (uniaxial, bending, thermal, loading and unloading) and laminates with open hole under tension or compression. Most of the predictions are documented in terms of stress–strain curves and curves presenting the evolution of brittle damage and of plastic strains.
This study aims at investigating the effects of electron beam irradiation on the montmorillonite and alumina trihydrate–added low-density polyethylene and ethylene vinyl acetate blends. The mechanical, flammability and electrical resistivity of the montmorillonite and alumina trihydrate–added low-density polyethylene and ethylene vinyl acetate blends were investigated. The addition of montmorillonite provided the reinforcing effect to the alumina trihydrate–added low-density polyethylene and ethylene vinyl acetate, whereby the tensile strength of 150 kGy and 250 kGy irradiated samples gradually increased with increasing of montmorillonite composition (i.e. 5 phr to 20 phr). Furthermore, the addition of montmorillonite into low-density polyethylene and ethylene vinyl acetate blends increased the limiting oxygen index and thermal decomposition temperatures of samples, thus improving the flame retardancy and thermal stability. The increasing of montmorillonite and irradiation dosage promoted the char formation during combustion. Besides, the increasing of montmorillonite loading levels gradually decreased the surface and volume resistivities of the polymer blends. The high irradiation dosages (i.e. 150 kGy and 250 kGy) were found to slightly decrease the electrical resistivity of the alumina trihydrate–added low-density polyethylene and ethylene vinyl acetate blends especially at high loading of montmorillonite. The irradiation effect improved the mobility of montmorillonite ions in polymer matrix, subsequently causing the reduction of the electrical resistivity of the polymer blends. The storage capacity of electrical charges of samples was slightly declined with the increasing of montmorillonite loading levels and irradiation dosages as shown by the dielectric constant results. The dielectric loss tangent of samples slightly increased at the increasing of montmorillonite loading level. However, the dielectric loss tangent was declined with increasing of irradiation dosages.
Composite lattice structures can be regarded as hierarchical when trusses have their own structure (e.g. different stacking sequences are incorporated). In this study, hollow composite pyramidal lattice sandwich structures in end compression were analyzed, measured and evaluated with respect to the designable properties of sandwich cores, such as relative density and truss stacking sequence. Collapse mechanism charts were constructed for both component elements and sandwich columns to illustrate the influence of structural geometries and properties of composite pyramidal lattice cores on failure modes. Operative failure modes were identified and the analytical models were shown to be accurate when compared to the measured response. The minimum weight design for the hollow composite pyramidal lattice sandwich column in end compression was carried out and the structural efficiency was also discussed.
Since fiber-matrix interface strength is critical to properties of carbon fiber-reinforced composites, measurement and analysis of interface strength are crucial steps in tailored design of composites. In the present work, the single fiber push-out test and the short-beam shear test were applied to measure the fiber-matrix interface strength in uni-directionally and two-directionally carbon fiber-reinforced phenolic resin matrix composites. The technical difficulties in processing the specimen and in realizing the fiber push-out were also discussed and clarified. For obtaining the successful test, typically, the thickness of the specimen should be smaller than 100 mm. During the fiber push-out, the de-bonding and fiber sliding at the interface were analyzed from the load-displacement curve features. The results indicated that both methods could be applied to determine the interface strength. The single fiber push-out and the short-beam shear tests resulted in a similar phenomenon in regard to the interface strength of uni-directionally and two-directionally carbon fiber-reinforced phenolic resin matrix composites, but expressed different values. The low interface strength measured from the short-beam shear test could be associated with multiple interlaminar shear failures. Furthermore, it was found that the interface strength of uni-directionally carbon fiber-reinforced phenolic resin matrix composites is somewhat higher than that of two-directionally carbon fiber-reinforced phenolic resin matrix composites. The difference in the interface strength could be attributed to the thermally induced residual stresses caused by the coefficient of thermal expansion mismatch of fiber and matrix. The approaches applied in the current work can be used for the evaluation of the interface strength of carbon fiber-reinforced phenolic resin matrix composites with different fiber-matrix bonding properties.
Flow properties of aluminum and aluminum-boron carbide (Al-B4C) composites, containing 5, 10 and 15 wt% B4C, were investigated by compression tests at strain rates of 10–4, 10–3 and 10–2 s–1 over the temperature range 25 to 500°C. The nature of stress–strain curves as a function of reinforcement, temperature and strain rate revealed that (1) flow stress initially increases as the reinforcement increases, but it decreases for Al-15% B4C composite, (2) flow stress increases with the increase in strain rate, with the strain rate sensitivity index varying from 0.01 for aluminum at 200°C to 0.30 for Al-5% B4C composite. The activation energy for deformation is found to vary from 124 to 187 kJ/mol for Al-15% B4C and Al-5% B4C composites, respectively.
Carbon microspheres were prepared using cellulose as a precursor by hydrothermal carbonization and the composites were prepared using carbon microspheres as the filler and poly(lactic acid) as the polymeric matrix. The morphologies and structures of carbon microspheres were characterized by scanning electron microscopy and Fourier-transform infrared spectrometry. The crystallinity, tensile properties and thermal properties of the composites were tested. The results show that the tensile properties and thermal properties of the carbon microsphere/poly(lactic acid) composites were improved by the addition of CMSs.
A novel elastomer foamed nanocomposite has been developed with high electromagnetic dissipation and shielding properties. This light-weight foamed fluorocarbon incorporates multi-walled carbon nanotubes at low loading concentrations to achieve levels of conductivity and energy shielding that surpass the requirements for electromagnetic static discharge and electromagnetic interference shielding. These nanocomposites are significantly lighter than their metallic filled counterparts and foaming the material can further reduce the density by another 20% with minimal impact on electromagnetic static discharge or electromagnetic interference characteristics. The measurement of mechanical properties was performed and compared with commercially available electromagnetic interference shielding gasket materials. Currently available metal-filled electromagnetic interference shielding materials suffer from poor mechanical properties due to the high loading of metal fillers. Not only are these material systems heavy, but they are not durable. Replacing the metal fillers with a lower concentration of multi-walled carbon nanotubes provides excellent mechanical properties and lighter weight. Stress/strain curves show a high Young’s modulus and display the ability to be able to tailor the physical properties to meet a variety of functional requirements.
Modified nanokaolinite was uniformly dispersed in acrylonitrile butadiene rubber using dispersion kneader and two roller mixing mill. In order to evaluate ablation characteristics of the fabricated composites, high-temperature ablation testing was carried out. The peak incorporation of nanoclay in the rubber matrix has enhanced the ablation resistance up to 97% and reduced the temperature elevation at the back face of the nanocomposite up to 51% during the 200s oxy–acetylene flame exposure on the surface of the ablator. Thermogravimetric and differential thermal analyses elucidate the thermal endurance improvement with increasing filler contents in the polymer matrix. Thermal conductivity and thermal impedance of the composite specimens were evaluated on the domestically manufactured thermal conductivity measuring apparatus. It is scrutinized that the former property reduced up to 49% at and the subsequent characteristic of the rubber composite is elevated up to 43% at 373 K with the 30 wt% incorporation of the nanoclay into the base rubber composition. Mechanical properties of the acrylonitrile butadiene rubber composite specimens were significantly enhanced with increasing filler introduction into the host matrix. Porous char morphology, polymer pyrolysis and composition of the ablated samples were analyzed using scanning electron microscopy and energy dispersive spectroscopy analyses.
In this paper, the influence of nanoclay on the hail impact damage resistance of glass fibre/epoxy composites under single and repeated high-velocity hail impact has been investigated. The damage extension was investigated to gain insight into the damage behaviors of the composite materials in the presence of nanoclay. Nanocomposite specimens containing nanoclay in 0, 0.5, 1.5 and 3 (wt%) were prepared by liquid-state mixing method using a high shear homogenizer. X-ray diffraction and transmission electron microscopy analysis confirmed intercalation and exfoliation of the nanoclay in the epoxy resin matrix used. Woven roving fabric with ±45° stacking sequence showed highest resistance to damage for composite laminates under high-velocity hail impact. Glass fiber/epoxy containing nanoclay resulted in smaller damage area and higher strength. The existence of nanoclay in the epoxy matrix induced the transition of failure mechanisms of glass fiber/epoxy laminates during the high-velocity hail impact test. Addition of 1.5wt% clay proved to be an optimized value with the highest damage resistance.
It is well-known that asymmetric composite laminates can have bi-stable response to different kind of loadings. In this research, the non-linear temperature-curvature relationship for the asymmetric composite laminates is studied using Rayleigh–Ritz technique. Attention is focused on studying the effect of material temperature dependency and resin layers; especially in the bifurcation point by use of analytical method. To this end, the well-known analytical theories are extended and used to consider the temperature dependency of material. The results obtained from the theory are then compared with the finite element simulations results and a good correlation is obtained. Finally, an experimental investigation is carried out and several specimens with [90/0]T, [60/–30]T and [30/–30]T compositions were manufactured. In order to study the effect of resin layers, optical microscopy is utilized and the exact thickness of different layers in the manufactured specimens is determined. The thermal responses of the manufactured plates were measured and used to validate the results obtained from the analytical theory and finite element simulations.
This work describes for the first time the flexural and compression mechanical behavior of Manicaria saccifera fibers–polymer matrix composites. These lignocellulosic fibers are obtained from a palm of natural occurrence at the Amazon region. Specifically here the fibrous natural mat that protects the fruits’ palm is used. The results obtained are comparable to the data from other lignocellulosic fiber–polymer matrix composites, demonstrating the feasibility of using Manicaria saccifera fibers as reinforcement. The wear behavior of the composites was also analyzed and their use as parquet floor is pointed out as a possibility.
In liquid resin infusion processes, the compression phase and the resin flow are important stages that influence the quality of the obtained parts. In this study, an experimental device is presented to measure thickness and pressure variations in order to obtain an experimental compressibility curve that can be implemented in a software dedicated to numerical simulations. To perform these experiments no typical compression testing machine is required and there is no test fluid. The aim of this work is to present a new methodology in order to obtain an empirical relation to determine the compressibility as a power law function. In order to evaluate the reliability of the proposed methodology, the obtained experimental curve is compared with another curve achieved performing standard compaction test.
Polyamide-6-based composites with pristine or functionalized multi-walled carbon nanotubes were produced using melt extrusion technique. After chemical functionalization, defect formation and attachment of carboxylic (–COOH) or amine (–NH2) groups on carbon nanotubes was confirmed from high-resolution transmission electron microscope and Fourier transform infra-red spectroscope studies. Carbon nanotubes incorporation promoted growth of α-form crystals with enhanced thermal stability through increase in crystallization temperature from 162 to 192°C. Dynamic mechanical thermal analysis (DMTA) indirectly pointed out to a homogeneous, uniform dispersion of nanotubes with reduction in free volume of the polymer, exhibiting a slight increase in glass transition temperature and a significant drop in coefficient of thermal expansion value. Composites containing 0.5 wt% NH2-carbon nanotubes show increases in elastic modulus and tensile strength by ~60 and 76%, respectively. Uniform dispersion and high interfacial strength was manifested by drop in strain to failure and lack of evidence of carbon nanotubes debonding from the matrix.
Triangle-shape carbon fiber reinforced polymeric composites is a plausible material for fabricating the radar absorbing structure. In order to design the effective electromagnetic wave absorber, the dielectric properties of its constituents are required in the target frequency band. In this paper, the unidirectional triangle-shape carbon fiber/epoxy composites were manufactured and the carbon fiber volume fraction of composite materials was measured to be about 46%. And then the permittivities of specimens were tested with the rectangle waveguide method in the vertical direction and in the parallel direction. The theoretical models and prediction equations for estimating permittivity of the unidirectional triangle-shape carbon fiber/epoxy composites were proposed with respect to the fiber volume fractions, the cross-section shape of fiber, the fiber orientations and the permittivity of components. It was found that the theoretical values agreed well with the experimental results.
The current studies give a brief account of analysis of static and low-velocity impact on foam sandwiched composites comprising composite faces from carbon/epoxy and cores from Rohacell (polymethacrylimide). The face sheets consist of four different stacking sequences as unidirectional, cross-ply, angle-ply and quasi-isotropic, which were fabricated by using hand lay-up process. Later, the composite panels were subjected to quasi-static and low-velocity impact loading using MTS and an instrumented Drop-Weight Machine (Instron 9250HV), respectively. The load-displacement curves have been obtained to characterize the failure mechanisms in the face sheets and the core. Impact parameters were evaluated and compared for different types of sandwich structures. Failure modes were studied by sectioning the samples at the impact location and observing under optical microscope. The results evaluated from static test have shown that the unidirectional have the highest peak load. However, the dynamic test data indicated that the foam sandwich with unidirectional face sheet have lowest peak load, lowest displacement at peak load and minimum energy absorption. It has also been observed that largest damage size, highest penetration depth and shear cracking have been experienced by unidirectional as compared to cross-ply, angle-ply and quasi-isotropic face sheets.
In this article, the nonlinear equations of motion for anti-symmetric angle-ply composite rectangular plates have been derived using the first-order shear deformation theory, including shear deformation and rotary inertia. By using the Galerkin method, five coupled nonlinear partial differential equations of motion are reduced to a nonlinear ordinary differential equation. Then, the multiple time scales method is used to solve the obtained equation and to derive an analytical relation for the nonlinear frequency. Results are compared with the literature and good agreement is achieved. After proving the validity of this study, nonlinear free and forced vibration of a fiber metal laminated rectangular plate have been studied and the effects of some system parameters on the nonlinear behavior of the FML rectangular plate have been investigated.
Since train’s frontal nose is the first part of the train which is damaged at the frontal impact, specific attention should be paid to the design of this part. In this study, an effort has been conducted to the design of a nose with light weight which can absorb maximum amount of energy that is possible during a frontal collision. To this aim and with attention to aerodynamic considerations, application of aluminium honeycomb sandwich panel has been studied. This paper includes two main parts. The first part is dedicated to the simulation of aluminium honeycomb sandwich panel, while the frontal collision of nose with different internal layer thicknesses of honeycomb and various nose lengths have been simulated in the second part. Finite element method using LS-DYNA commercial package has been used for the numerical simulation. The results have been validated with available experimental results and an acceptable agreement has been observed.
A group of nickel-coated carbon fiber reinforced Stellite 25 composites are produced using the hot isostatic pressing technique. The focus of this research is on obviating the problems related to the presence of carbides in Stellite alloys by substituting carbides as the main strengthening agent in Stellite alloys with nickel-coated carbon fibers. The principal reason for selecting Stellite 25 is because of its low carbon content and thereby relatively carbide free microstructure. The nickel coating is intended to eliminate any chance of carbide formation due to the possible reaction between carbon fibers and the matrix alloying additions. The tribomechanical and corrosion properties of the composites are characterized. The results show that the composites exhibit better corrosion resistance than medium-carbon Stellite alloys. The addition of carbon fibers into Stellite 25 improves its hardness and tribological properties. The wear rates of the composites are lower than that of medium-carbon Stellite alloys and comparable with that of high-carbon Stellites.
Composites based on polyester matrices reinforced with fiber glass mats are widely used in the automotive industry, incorporating in many cases a gel coat surface layer. This paper presents the results of a current study in polyester/short fiber glass mat composites processed by hand lay-up moulding. Different lay-up architectures were tested under impact and fatigue bending loadings. Furthermore, the influence of the gel coat surface layer on fatigue and impact loading performances was also analysed. The composites tested with gel coat layer under compression loading exhibit significant higher bending stiffness, static and fatigue strength than when the gel coat layer was under tensile loading. The impact response showed a slight variation of the peak load and on the elastic recuperation and a significant increase on maximum displacement and absorbed energy with the increase of the incident impact energy. Both energy recovery, in percentage of the incident impact energy, and residual bending strength showed a marked and almost linear decreasing with the increase of the impact energy for all loading conditions, while the absorbed energy increased with the incident impact energy, reaching about 80% of impact energy for the highest impact energies
Effects of different volume fractions of reinforcement and various loads applied in wear testing on dry tribological behavior of Al-Al3Ti in-situ composite were investigated. Mixture of aluminum and different amount of titanium powders has been compacted and combustion synthesized to obtain Al-Al3Ti composite with different percent of reinforcements. The results of pin-on-disk wear tests indicated that addition of Al3Ti to aluminum matrix increase wear resistance of the composite samples by 4 times compared to that of pure aluminum. In addition, presence of Al3Ti reinforcement reduced friction coefficient of the composites. Oxidation/abrasive mechanism was revealed to be dominant on wear behavior of Al-Al3Ti composites; adhesive wear decreased as volume fraction of the intermetallic increased in the composites.
Basalt and E-glass fibers fabrics were combined with carbon fiber fabrics in order to prepare epoxy-based interlaminar hybrid composites and to investigate the hybridization effect on the flexural and impact properties of the resulting laminates. The flexural modulus of the composites depended on their composition according to a rule of mixture, while an important synergistic effect was detected for the ultimate flexural properties. Charpy impact tests evidenced a strength increase as basalt and glass fibers content increased. Interestingly, hybridization with basalt fibers promoted an increase of the adsorbed impact energy due to an enhancement of the fracture propagation component.
In this study, an ultrasound visualization system has been set up for in-situ monitoring of the resin flow impregnating through opaque carbon fiber reinforcements during the vacuum-assisted resin transfer molding. The flow front advancement through the carbon fabrics covered by a bagging film can be identified and visualized by the high frequency B (brightness) mode ultrasound imaging technique. The resin advancement in the opaque carbon fabrics has turned out to form a non-uniform plug flow along the pressure gradient direction and the potential void formation can be observed from the mesoscopic resin flow behavior: the inter-tow regions have been preferentially filled by the resin fluid and the fiber tow region behaves as a sink that probably remain as a void defect. The local unsaturated transient velocity of the resin flow has been also evaluated, which is particularly important in understanding saturation behaviors in dual-scale fabrics and is hardly measurable by other means. In spite of the hardware limitations on the resolution, the proposed ultrasound visualization system can provide a less expensive and portable visualization tool to understand the microstructure of opaque reinforcements and monitor the resin flow behavior during the vacuum-assisted resin transfer molding process in the industrial composite manufacturing environment.
In this article, the kinetic behavior of a glass/epoxy prepreg, HexPly® 1454, has been investigated using differential scanning calorimetry data. The kinetic parameters of three different kinetic models have been calculated using non-linear curve fitting of isothermal differential scanning calorimetry data. Curve fitting was done based on genetic algorithm to allow to obtain a unique value for each kinetic parameter. The non-isothermal differential scanning calorimetry experiments were predicted using the obtained models. Prediction results demonstrated that only model with the minimum number of kinetic parameters and unique value for each of them can correctly predict non-isothermal experiments at all heating rates.
To increase understanding of damage evolution in advanced composite material systems, stereo digital image correlation has been integrated with a compression–bending mechanical loading system to obtain full-field deformations on both compression and tension surfaces throughout the loading process. The integrated system is employed to simultaneously quantify full-field deformations along the length of the specimen. Specifically, the integrated system is employed to experimentally study the progressive failure behavior of thin, woven glass–epoxy composite specimens undergoing both cyclic and monotonic compression–bending loading resulting in large out-of-plane bending deformations with end conditions that allow free out-of-plane rotation. Experimental results obtained using the measurement system for specimens undergoing both linear and highly non-linear deformations during monotonic loading are presented. Results clearly show (a) the presence and magnitude of anticlastic (double) specimen curvature near mid-length for all fiber angles, (b) the distinct differences in the strain fields between the tension and compression surfaces at the critical location, (c) the corresponding disparity in local material failure mechanisms between the tension (e.g. matrix cracking) and compression (e.g. fiber buckling) surfaces in the critical regions and (d) the highly localized character of the strain fields, focused in regions of increased damage.
One class of multifunctional composite structures is one that is capable of load-bearing while transferring electrical current across a given span. An example is large antenna systems integrated into composite skins of aircraft and spacecraft. Current aircraft missions that require large amounts of sensing equipment can potentially involve heavy and bulky wiring systems. The bulk and mass inefficiency of these systems can be avoided by using multifunctional structures that mimic printed circuit boards to carry both load and electrical current for power or data. The fatigue life and failure modes of these types of systems were experimentally investigated to understand the capability of embedded metals inside composite host materials. ‘Dog bone’ specimens with copper traces were used to measure the fatigue life as a function of loading level. The specimens were fabricated by mass-production printed circuit board methods using woven glass/epoxy composite and chemically etched copper traces that were embedded or surface mounted. Functional failure, described as failure of the copper trace while still maintaining load-bearing capability, was of primary interest. This occurred when the specimen was loaded at levels below 55% of static ultimate tensile strength. Fracture of the copper was caused by cracks forming in the composite surface and propagated through the copper trace. Completely embedded copper traces exhibited longer life than traces that were surface mounted on the composite. An elasto-plastic analysis and summary of experimental results in the form of copper strain amplitude allow results to be applicable to other configurations, i.e., host structures of different stiffness than woven glass/epoxy.
This research work deals with aspects concerned with delamination detection in composite structures as revealed by an approach based on vibration measurements. Variations in vibration characteristics generated in composite laminates indicate the existence of delaminations because degradation due to delamination causes reduction in flexural stiffness and strength of the material and as a result vibration parameters like natural frequency responses are changed. Hence, it is possible to monitor the variation in natural frequencies to identify the presence of delamination, and assess its size and location for online structural health monitoring (SHM). The approach to this paper, therefore, typically depends on undertaking the analysis of structural models implemented by finite element analysis (FEA). The numerical solutions using FE models known as the simulator computes the natural frequencies for the delaminated and undelaminated specimens of composite laminates. However, these FE models are computationally expensive, and surrogate (approximation) models are introduced to curtail the computational expense. The simulator is employed to solve the inverse problem using algorithms based on computational intelligence concepts. An artificial neural network (ANN) model is developed to also solve the inverse problem for delamination detection directly and to provide surrogate models integrated with optimization algorithms (the gradient-based local search and non-dominated sorting genetic algorithm-II) to contain the computationally expensive simulations by FEA. This approach is termed as surrogate assisted optimization and it is seen that the engagement of surrogate models in lieu of the FE models in the optimization loop greatly enhances the accuracy of delamination detection results within an affordable computational cost and provides control over handling different variables. Meanwhile, to aid with the building of effective surrogate models using substantial number of training datasets, K-means clustering algorithm is harnessed and this effectively reduces the large training datasets usually required for ANN training. This paper demonstrated that ANN and optimization algorithms with surrogates show immense potentialities for use in delamination damage detection scenarios. Prediction errors of the algorithms were quantified and they were shown to be satisfactory when applied to previously experimental data. The algorithms in their inverse formulations are capable of predicting accurately delamination parameters. Hence, these algorithms should be employed for application in the domain of SHM where their small computational requirements could be exploited for online damage detection.
Adhesive bonding of structural aircraft materials depends upon a thorough understanding of factors that affect bond strength and durability. It is well known that surface preparation is critical to adhesive bond performance. However, little information is available regarding the effect of surface preparation procedures on adhesive bonding of high-temperature polyimide composites. Mechanical treatments such as hand sanding are commonly used for composite materials, and in general are known to improve the wettability of the surface as well as effect contaminant removal. Less is known about the influence of mechanical treatments on the surface chemistry. This article discusses the effects of mechanical surface preparation on the adhesive bond strength, durability, and failure mode of several high-temperature polyimide-based composites and adhesives.
In order to provide another way of reducing the stock of used tyres and polyvinyl chloride waste, a new material is developed and studied. Formulation includes a matrix constituted by a compound of recycled polyvinyl chloride with plasticized polyvinyl chloride and a reinforcement of ground tyre rubber. Acoustic and mechanical properties of different compositions of polyvinyl chloride/ground tyre rubber were tested in order to determine their suitability for applications fulfilling industry requirements. Sound absorption has been analyzed, showing interesting results at frequencies higher than 2500 Hz. The obtained values are found to be depending on the thickness of the sample, the content of the ground tyre rubber and the existence of gaps, pores and voids either between layers or in the interphase between the matrix and reinforcement. From the study of the mechanical properties, we may observe that the ground tyre rubber act as filler, improving stiffness of polyvinyl chloride/ground tyre rubber composites with an increase of Young Modulus. The tensile strength, elongation at break and toughness decrease slowly. The decrease of these mechanical properties is observed to be lower than in the case of composites made by using high-density polyethylene as a matrix.
In order to meet the clinical application requirements of biodegradable materials in orthopedic surgery field, novel Mg–3Zn–0.5Zr/xHA (x = 0, 0.5, 1 and 1.5 wt%) composites have been developed by melting under the mechanical stirring and the heat extrusion route. The thorough examination on the effect of nano-hydroxyapatite (nano-HA) particles on the microstructure, mechanical properties and corrosion resistance of Mg–3Zn–0.5Zr alloy was carried out. The results indicated that the gelatin-coated nano-HA particles could more uniformly distribute in the matrix and the grain size was refined to be approximately 10 µm. More non-basal dislocations presented in the composite with the increment of nano-HA particles. Compared with the Mg–3Zn–0.5Zr alloy, the ultimate tensile strength, yield strength and elongation of the Mg–3Zn–0.5Zr/1.5HA composite had been improved and reached 302 MPa, 275 MPa and 20.9%, respectively. Furthermore, Mg–3Zn–0.5Zr/1HA composite showed the optimal degradation rate of 6.45 mm/yr in the in vitro corrosion tests. From the clinical application requirements of biodegradable materials point of view, the composite containing 1 wt% nano-HA particles could be evidently believed to be a promising bone fixation material for the application.
In this article, multi-walled carbon nanotubes were noncovalently functionalized with different metal phthalocyanines by – stacking method. Multi-walled carbon nanotubes were dispersed by sonication into the phthalocyanines solution in chloroform or N, N-dimethyl formamide before refined purification by centrifugation. It could be observed that the metal phthalocyanine molecules adhered to the surface of multi-walled carbon nanotubes in the transmission electron microscopy images. These composites were characterized by thermo gravimetric analysis and UV/Vis absorption spectrum. The optical limiting responses of these composites in N, N-dimethyl formamide solution were studied by Q-switched Nd:YAG laser instrument with nanosecond pulse of 1064 nm and repetition rate of 10 Hz. It was noted that the optical limiting ability of these composites solution was stronger than that of multi-walled carbon nanotubes or phthalocyanine alone. In addition, carbon nanotube was commonly purified in concentrated nitric acid, which usually introduced carboxylic groups on the carbon nanotubes. Herein, the effect of introduced carboxylic groups in the nitric acid treatment on the – stacking was also studied based on thermogravimetric analysis of composite multi-walled carbon nanotube/copper phthlaocyanine.
Fiber-reinforced composites are of great interest to NASA for deep-space habitation missions due to the specific strength, modulus and potential radiation shielding properties. However, the durability of these materials on long-duration missions has not been evaluated. Few studies have been conducted on the radiation effects of fiber-reinforced composites in space and even fewer have been conducted with high-energy protons, which replicate portions of the deep-space radiation environment. Furthermore, previous studies of carbon fiber-reinforced composites focused on pure epoxy composites, and aerospace composites in use today include toughening agents to increase the toughness of the material. These toughening agents are typically either rubber particles or thermoplastics, known to be susceptible to ionizing radiation, and could affect the overall composite durability when exposed to high-energy protons. Thus, NASA has undertaken a study to understand the long-term radiation effects on one such potential composite for use in deep-space habitats (boron fiber, carbon fiber and semi-toughened epoxy). Samples were irradiated with 200 MeV protons in air to different doses and evaluated via tensile tests, differential scanning calorimetry, Fourier transform infrared spectroscopy and scanning electron microscopy. The results showed evidence of a weakened matrix due to scission effects and interfacial failure as a result of resin debonding from the boron fibers.
Determination of gel point is important for a modelling assessment of residual stresses developed during curing of composite materials. Residual stresses in a composite structure may have a detrimental effect on its mechanical performance and compromise its integrity. In this article, the evolution in bending stiffness of a glass/epoxy composite material during an isothermal curing process is examined to identify different material stages and behaviour. Differential scanning calorimetry and dynamic mechanical analysis are used to analyse the material behaviour. Gelation is identified as a clear onset in bending stiffness, and vitrification is seen as a decrease in the bending stiffness rate. Often gel point predictions for composite materials are based on neat matrix measurements. However, the results presented in this article demonstrate that the gel point is affected by the presence of the fibre reinforcement.
A significant improvement in fiber-reinforced polymeric composite materials can be obtained by incorporating a very small amount of nanofillers in the matrix material. In this study, an ultrasonic liquid processor was used to infuse carbon nanofibers into the polyester matrix which was then mixed with a catalyst using a mechanical agitator. Both conventional and carbon nanofibers-filled glass fiber-reinforced polyester composites were fabricated using the vacuum-assisted resin transfer molding process. Low-velocity impact tests was performed at 10 J, 20 J, and 30 J energy levels on conventional as well as 0.1–0.3 wt% carbon nanofibers-filled glass fiber-reinforced polyester composites using Dynatup8210. The morphology of fractured specimens was examined using digital photographs and optical microscopy. There was an increase in the peak load for the nanophased glass fiber-reinforced polyester composites compared with the conventional one. The absorbed energy of nanophased glass fiber-reinforced polyester composites was less than that of conventional one at different energy levels. The extent of damage was more pronounced in the conventional glass fiber-reinforced polyester composites compared to nanophased ones. Failure mechanisms comprised of indentation, debonding, delamination, matrix cracking, and fiber fracture. The extent of damage was pronounced in conventional composite compared to nanophased ones.
This paper showcases the authors’ predictions for the 13 challenging test cases of the third World Wide Failure Exercise. The cases involve the prediction of lamina biaxial stress–strain curves, matrix cracking and delamination in various cross-ply and quasi-isotropic laminates under uniaxial loading, variation of thermal expansion coefficient of a laminate with matrix cracking, bending of a general laminate, loading-unloading behaviour and the strength of various thin and thick laminates containing an open hole. The laminates were made of various glass and carbon fibre/epoxy materials. The constitutive model is based on plasticity theory, includes hydrostatic pressure effects and accounts for multiaxial load combination effects. The failure criteria distinguish between matrix failure, fibre kinking and fibre tensile failure. In-situ strengths are used for matrix failure. Propagation of failure takes into consideration the fracture energy associated with each failure mode and, for matrix failure, the accumulation of cracks in the plies. The model is used to make blind predictions of all test cases from the third World-Wide Failure Exercise.
This study investigated the surface roughness and wettability of polypropylene composites filled with Paulownia elongata wood flour with and without maleic anhydride-grafted polypropylene at different wood flour contents (30, 40, 50, and 60 wt%). The surface roughness values of the filled polypropylene composites decreased with increasing content of the polypropylene. The polypropylene composites without the maleic anhydride-grafted polypropylene were found to have higher surface roughness but better wettability as compared with the ones with the maleic anhydride-grafted polypropylene. The wettability of polypropylene composites increased with increasing content of the wood flour. The incorporation of the coupling agent in the polypropylene composites decreased the wettability of the specimens compared with untreated ones. The test result showed that P. elongata wood flour could be utilized in the production of the filled polypropylene composites because of their satisfying surface properties of the composites.
A biaxial non-crimped fabric, 400 ± 10 g/m2, +45°/–45° lay-up protocol, was made from a unidirectional tape comprised of a 60/40 wt% carded blend of virgin waste carbon fibres, 60 mm chopped length, and polyester resin fibres, 60 mm staple length. The non-crimped fabric was used as a thermoplastic prepreg to produce laminated composite panels. The prepreg exhibited a high degree of drapeability. The physical and mechanical properties of composite samples were determined; the density, void contents, tensile and flexural strengths and moduli were found to be 1.5 g/cm3, 10%, 180.7 MPa, 260.5 MPa, 34.2 GPa and 30.4 GPa, respectively. Modification of the consolidation process and the use of finer polyester fibres should decrease the void content. It was concluded that waste carbon fibres can be converted into flexible/drapeable dry prepreg materials, potentially useful for the manufacturing of thermoplastic composite products by hot press compaction.
This article deals with the computation of effective elastic properties of braided textile composites assisted by finite element analysis. In this approach, dynamic representative unit cells are first constructed to model typical geometry of braided textile preform. After establishing the elastic properties of braiding yarns, the effective Young’s moduli, shear moduli and Poisson’s ratios corresponding to varying braiding angles are obtained by analysing these geometric models of preform with the help of the commercial finite element analysis code Abaqus. Effects of fibre volume fraction on the elastic properties of both biaxial and triaxial composite unit cells are also examined. Finally, the bending behaviour of a simply supported beam with braided composite skin is evaluated via the finite element analysis assisted multi-scale modelling, which is further verified experimentally. The predicted results were compared favourably with the experiment, backing the accuracy of the proposed modelling approach.
The limits of processability of solid-state thin film lithium-ion batteries embedded into composite laminates are identified through testing under pressure, temperature and a liquid resin environment representative of composite processing cycles. Battery failures are characterized based on optical microscopy and capacity retention, and three distinct types of failures are recognized and analyzed. Failures are associated either with the battery polymeric sealant failure or with the physiochemical degradation of the electrolyte or the anode. Results give evidence that the cure temperature is the most influential parameter for battery survivability. Based on these experimental results, the optimum curing cycle is identified and embedding tests that retain full battery capacity are successfully performed. The minimal three-layer battery packaging proves to be an efficient air and moisture barrier within the above conditions.
The study investigated the use of waste tea (Camellia sinensis) leaves mixed in various proportions with Paraserianthes falcataria (moluccan sau) wood particles for the manufacture of particleboard. Boards containing waste tea leaves alone showed low thickness swelling and water absorption after 24 h soaking in water. Addition of P. falcataria wood particles from 20% to 50% to waste tea leaves resulted in boards with satisfactory thickness swelling, water absorption, internal bond, stiffness, and strength well above the minimum requirements for general use particleboards set by EN 312-2 (1996). Results of the study showed that waste tea leaves can be used as an alternative material either alone or in combination with wood particles for the manufacture of particleboard.
SiC monofilament-reinforced copper is a potential heat sink material for the divertor of future fusion reactors, where heat loads up to 15 MW/m2 must be removed. The composite combines high mechanical strength (300 MPa at 300°C) with sufficient thermal conductivity (at least 200 W/mK) at temperatures up to 550°C. The bonding between fibres and matrix is essential for the mechanical behaviour. We investigated two different fibre types (SCS0 and SCS6, Specialty Materials Inc.) with different interlayers in a galvanic copper matrix. Tensile tests in combination with acoustic emission analysis showed the dependence of fracture behaviour of the composite material on the fibre type. The average ultimate tensile strength of composite samples with 20% fibre reinforcement was in the range of 600 MPa, with a Young's modulus of 160 GPa. The tensile tested composite with SCS6 fibres generated roughly twenty times the number of acoustic emission events in comparison with the SCS0 fibres composite. Acoustic emission events in tensile test specimens with SCS6 fibres are correlated with the failure of the carbon-enriched surface layer of the fibre. In the case of SCS0 fibre-reinforced specimens, acoustic emission events were only caused by fibre failure. Fracture area analysis after tensile tests showed the failure of the SCS6 fibres at the interface between the two outer carbon layers and no interface failure for the composite with SCS0 fibres.
In this study, to estimate the impact resistance of steel fiber-reinforced concrete slabs strengthened with fiber-reinforced polymer sheets, a series of 50 x 100 x 350 mm3 sized slabs with 0.5%–1.5% (by volume) of steel fibers and two types of fiber-reinforced polymer sheets were impact-tested using drop-weight impact test machine. From the test results, the maximum impact load, dissipated energy and the number of drops before failure were all increased, whereas the maximum deflection and support rotation were reduced by strengthening the steel fiber-reinforced concrete slabs with externally bonded fiber-reinforced polymer sheets in the tensile zone. It was noticed that the impact resistance of the steel fiber-reinforced concrete slabs was substantially improved by externally strengthening the fiber-reinforced polymer sheets. In addition, the dynamic response of the steel fiber-reinforced concrete slabs strengthened with fiber-reinforced polymer sheets under a low-velocity impact load was analyzed using explicit code LS-DYNA with strain rate-dependent material models and debonding failure analysis. These numerical analyses were verified by comparing with the experimental results.
A three-step finite element model has been implemented to predict the spring-in of L-shaped parts. The material property development during the cure has been modelled as step changes during transitions between viscous, rubbery and glassy states of the resin. The tool-part interaction is modelled as a sliding interface with a constant sliding shear stress. The effect of various material and geometric variables on the deformation of L-Section parts are investigated by a parameter sensitivity analysis. The spring-in predictions obtained by the finite element method are compared to experimental measurements for unidirectional and cross-ply parts of various thicknesses and radii. Results indicate that although a 2D plane strain model can predict the spring-in measured at the symmetry plane fairly well, it is not sufficient to capture the complex deformation patterns observed.
The conventional strain gauge slitting method is usually used to determine the residual stress component normal to the slit plane. Therefore, it is assumed that residual shear stresses released in the slit plane have no effect on the released strains measured by strain gauge. This assumption is valid when measuring released strains by back surface strain gauges. However, in the top surface measurements, it is not true and neglecting residual shear stresses could create a biased estimation of residual normal stresses. In this research, a method based on the use of two symmetrically-placed strain gauges is proposed to separate the measured strains due to residual shear stresses from those due to residual normal stresses in the slitting method. In order to validate the proposed method, two series of slitting experiments were carried out on angle-ply carbon/epoxy laminated composites. The near-surface residual normal stresses were measured using the proposed method. These stresses were also determined using strain data obtained from the back surface strain gauge. Results of both experiments showed good agreement and thus verified the validity of the proposed method for the elimination of residual shear stresses effects in the slitting method.
In the present article, new polylactide/alginate fibers composites were investigated. Composite pre-pregs were made by solution casting method. The aim of the study was to define physico-mechanical properties of developed materials. The scope of the studies included: examining the static mechanical properties, properties of the surface and their changes during degradation. Moreover, intensity of the release of degradation products to the environment and a change of the mass of examined samples were analyzed. Obtained results were evaluated taking into account possibility to use prepared composited as materials for vascular implants.
Steel-reinforced concrete structural components are often associated with significant maintenance costs as a result of reinforcement corrosion. To mitigate this problem, fiber-reinforced polymer bars have been used in place of traditional steel-reinforcement for some applications. The non-ductile response of typical fiber-reinforced polymer bars is a concern, however. To overcome this problem, hybrid ductile fiber-reinforced polymer bars have been developed for use in concrete flexural members with resulting ductility indices similar to sections reinforced with steel. In this study, five different hybrid ductile fiber-reinforced polymer bar concepts are analyzed and compared in terms of ductility, stiffness, and relative cost. Of primary interest is the effect that the number of materials used in bar construction has on performance. Reinforced concrete beam and bridge deck applications are considered for analysis. It was found that all hybrid ductile fiber-reinforced polymer-reinforced flexural members czonsidered could meet code-specified strength and ductility requirements for steel-reinforced sections, although service load deflections were approximately twice that of steel-reinforced sections of the same depth. In general, ductility increased, and overall material cost decreased, as the bar material layers increased from 2 to 4. The four-material continuous fiber bar approach was found to be most promising, with high ductility as well as relatively low cost.
The aim of present study is to develop a toughened polylactic acid/talc composite. Talc and epoxidized natural rubber (ENR-50) were compounded with polylactic acid using counter-rotating twin-screw extruder followed by preparation of samples through injection molding. The effect of silane-treated talc and epoxidized natural rubber on mechanical, thermal, and morphological properties of polylactic acid was investigated. The Young's and flexural modulus of polylactic acid improved while the impact strength values dropped with increasing talc content (20–30 wt%) indicating that talc increased the stiffness of polylactic acid with a sacrifice in toughness. Subsequently, the blending of epoxidized natural rubber (20 wt%) to polylactic acid/talc (30 wt%) revealed that the impact strength of polylactic acid/talc composites improved 448% with considerable drop in Young’s and flexural modulus. Polylactic acid/talc/epoxidized natural rubber composite contains 60% polylactic acid, 30 wt% talc, and 10 wt% ENR display optimum stiffness and impact strength. Scanning electron micrographs demonstrates that talc agglomerates at higher loadings. Thermogravimetric anlaysis indicated that thermal stability of polylactic acid/talc composite was reduced by the addition of epoxidized natural rubber due to increasing talc agglomeration.
This article focuses on understanding the progressive failure behavior and predicting the failure strength of the E-glass/epoxy-woven composites. Two types of the plain weave composite material are investigated. A homogenized woven composite model is developed to obtain the properties of these materials based on micromechanics. Failure criteria are embedded into this model simultaneously and the material properties are degraded and updated when damage occurs. The model is incorporated into a finite element code, ABAQUS, via a user-defined material subroutine. Using this model, the progressive failure behavior can be easily traced and the predicted stress–strain curves and failure strength show good accuracy compared with the experimental results.
In this article, we present a study on the properties of ethylene vinyl acetate copolymer/silica nanocomposites prepared in absence and presence of EVA-g-maleic anhydride as a compatibilizer between silica nanoparticles and ethylene vinyl acetate matrix. A series of ethylene vinyl acetate/silica nanocomposites with different contents of silica nanoparticles were prepared by solution method. EVA-g-maleic anhydride with 0.5 wt% maleic anhydride groups was added to all ethylene vinyl acetate/silica nanocomposites. Fourier transform infrared, field emission scanning electron microscopy, rheology behavior, and thermogravimetry analysis were used to characterize the structure, morphology, rheological, and thermal properties of the nanocomposites, respectively. The Fourier transform infrared spectra and field emission scanning electron microscopy micrographs showed that the hydroxyl groups on the surface of silica nanoparticles interact with maleic anhydride groups in EVA-g-maleic anhydride and lead to a finer dispersion of individual silica nanoparticles in the ethylene vinyl acetate matrix. The rheological properties and thermal stability of ethylene vinyl acetate/silica nanocomposites were significantly increased after adding EVA-g-maleic anhydride into the nanocomposites. Mechanical properties including tensile strength and elongation at break of the nanocomposites were mainly affected by the content of silica nanoparticles. For the tensile strength as well as elongation at break of the nanocomposites, a maximum value was observed at the content of 0.5 wt% of silica nanoparticles. The addition of EVA-g-maleic anhydride into ethylene vinyl acetate/silica nanocomposites resulted in a further improvement of mechanical properties of the nanocomposites.
Three-point bending tests were conducted to reveal the failure mechanisms of the three-dimensional WTSC. Sixteen groups of experiments were designed and performed based on the orthotropic core structure and different thicknesses of the two skins. Bended along the weft direction, skin yielding and crimpling renders WTSC ductile failure mode. Bended along the warp direction, skin cracking renders WTSC brittle–ductile failure mode, while core shearing renders a quasi-brittle mode. After the initial failure, the progression of plastic hinges renders WTSC residual load capacity in a long deformation plateau, while fractures of lower skins lead to complete and abrupt failures. According to the experiments, failure mechanisms of WTSC include skin fracture, skin yielding, skin crimpling, tensile failure, indentation, and core shearing. Strength of the skins and the core in the warp and weft directions were predicted. The woven skin and the woven core are stronger in the weft direction. These failure mechanisms lead to six typical styles of flexural deflection curves having different brittle and ductile characteristics.
This article describes a novel technique for manufacturing functionally graded materials with tailored properties for thermal management. These materials are ceramic/metal composites with a gradient microstructure, elaborated by producing a porosity gradient preform made of alumina phase subsequently infiltrated by the molten aluminum alloy (Al) phase. In order to model these particulate composites and to evaluate their effective thermal conductivities, a numerical approach based on both discrete element method and finite element method has been developed. The study presented here deals with alumina/Al composites without gradient microstructure and is conducted to numerically investigate the effects of the particle size distributions and the interconnection sizes between particles on the effective thermal conductivity. The situation in which an interfacial thermal resistance is present between both phases of composites to simulate a non ideal contact has also been considered.
Wood polymer composites were manufactured from several types of tropical wood species by impregnating the woods with acrylonitrile monomer solution. The physical and mechanical properties of wood polymer composites were then investigated in this study. The vacuum-pressure method was employed to impregnate wood samples with monomer and in situ polymerization. Acrylonitrile reacted and incorporated with wood, which was confirmed through Fourier transform infrared spectroscopy and scanning electron microscopy test analysis. The mechanical properties of wood samples in terms of modulus of elasticity and compressive modulus were found to be improved on acrylonitrile treatment. Besides, the fabricated wood polymer composite samples had lower water absorption and higher surface hardness (Shore D) value as compared to their corresponding raw one. For wood polymer composites, a significant improvement was found in physical and mechanical properties compared to the raw woods.
A finite element micromechanical model of a high strength composite material is subjected to a range of loading conditions to demonstrate its ability to predict failure. An investigation into the relative magnitude and distribution of the normal and shear stresses within the interface region of a single fibre embedded in a matrix region is compared to that of a multi-fibre representative volume element, where the fibre placement is statistically equivalent to that of a real material. A study is subsequently undertaken in which the relative magnitudes of the shear and normal strengths of the fibre–matrix interface are varied under transverse tension and shear. The results are interpreted with relation to the yield strength of the matrix. The predicted performance of the composite is shown to compare well with published experimental data, under transverse tension and in-plane shear. It is concluded that a single set of interface strength parameters can be used to represent the behaviour of the composite material. The results also show that interfacial shear strengths are expected to be equal, and higher than the interfacial normal strength.
This article presents a model, known as generalized Daniels’ model, describing the process of micro-damage accumulation, deformation and failure of multilayered fibre reinforced composite structures under complex internal states of stress responding to external plane-stress loading. The model considers three independent kinds of ply micro-damage: longitudinal, transverse and shear. Non-linear analysis, taking into account scissoring effects, is used to make theoretical predictions for all the 13 test cases involved in the third world-wide failure exercise. The cases cover the biaxial failure behaviour of a unidirectional and multi-directional laminates, failure of thick and thin cross ply and quasi-isotropic laminates under tension, damage due to thermal loading, bending of a general laminate, loading and re-loading and the predictions of the tensile and compressive strength values of test specimens with an open hole. For the prediction of notched strength of laminates with hole, the model is used with a non-local approach based on the specific size of ply microstructure and Neuber’s hyperbola of specific deformation energy. The results are presented for all the 13 test cases.
This article focuses on the contact between layers in forming processes of composite laminate. The link between the degree of intimate contact and the consequent thermal contact resistance between layers is investigated. A hot plate forming process experiment allows to propose a relation and determine the missing parameter for APC2 thermoplastic prepreg composite. Besides the new proposed relation, this work showed that the internal thermal contact resistances in the laminate is significant. Therefore, thermal modeling of forming processes of composite laminate (such as automatic tape placement) should account for this phenomenon.
This article reports the preparation and characterization of newly developed hybrid composites consisting of epoxy resin (EP) matrix, acetylated kenaf bast fiber (AKF) and conducting polyaniline (PANI) nanowires. Initially, the EP/AKF composites were prepared by varying the AKF loading (5–30 wt%). The EP/AKF displayed an optimum tensile strength at 20 wt% AKF loading which was higher than that of untreated kenaf fiber EP composites (EP/UKF). The hybrid composites of EP/AKF/PANI were then prepared by using 20 wt% AKF loading with PANI inclusions from 2 to 14 wt%. The addition of PANI into EP/AKF induced positive electrical properties without considerably sacrificing its mechanical integrity. It was found that the electrical percolation threshold of these hybrid composites was at 11 wt% of PANI loading. PANI inclusions at above the percolation loading resulted in reduction of tensile and flexural strength. Meanwhile, no significant mechanical loss was observed below the threshold. The fracture morphological analysis revealed the occurrences of PANI nanowires pull out from the matrix. The Fourier transform infrared spectroscopy showed that the PANI component still maintained its doped condition inside the EP/20AKF. Water absorption and thermal analysis indicate that the PANI incorporation induced lower water uptakes and greater thermal stability to the EP/20AKF, respectively.
Integrated composite structures can significantly reduce the assembling cost and improve the performance of the aircraft. All-composite joint, which is a connector of integrated composite structures, has good ability to reduce the weight and assembly cost while retaining good load-carrying capability. To disclose the mechanics behavior of the out-of-plane, complex all-composite joint, the progressive damage methods are investigated for its good ability to trace the damage onset, damage propagation, up to collapse of composite structures. A progressive damage model is established using a new modified maximum stress failure criterion and a material degradation model developed from Chang’s model. The material degradation model takes account of the matrix crack direction, which is predicted by the modified maximum stress-failure criterion. Meanwhile, five other progressive damage models are established and applied to static tensile analysis to study the mechanics behavior of the joint. To evaluate the prediction of the six progressive damage models in joint initial failure, damage propagation, joint strength and stiffness, the numerical results of the six progressive damage models are compared with the experiment results. The effects of failure criteria and material degradation model on the prediction results are discussed in detail. By the comparison, it can be concluded that the progressive damage model consisting of the modified maximum failure criteria and its corresponding material degradation model is considered as the best one for the analysis of the all-composite joints, because its initial failure prediction, failure process and ultimate failure prediction agree well with the experiment result.
This paper is a part of research carried out for the manufacturing and evaluation of riot helmet shells having continuous textile reinforcement. Low energy impact tests were carried out on the developed riot helmet shells at different locations on the helmet shell with different impact energy levels. Energy absorption and force blocking effectiveness at different impact locations at the helmet shells were evaluated. The result shows the helmet top location is the least vulnerable location against the impact as compared to the helmet back and helmet side location.
Polymer/perovskite manganese oxide (epoxy resin/La0.8Sr0.2MnO3) composites are prepared using bonded method. There is no reaction between La0.8Sr0.2MnO3 and the polymer. The nonlinear current–voltage property is significantly affected by the content of polymer. The resistivity and nonlinear coefficient increase with the increase of polymer content. The resistivity of the composites is 5–9 orders of magnitude higher than that of the sintering ceramics. The nonlinear electrical behavior is an intrinsic property. Compared with conventional sintered ceramic varistors, the polymer/manganese oxide composite varistors have greater nonlinear coefficient above 45.
This work developed a shear thickening fluid enhanced fabrics and the influence of the shear thickening fluid types on the knife stab and puncture resistance performance were investigated. The rheological properties of the shear thickening fluids were tunable by varying both the dispersing particles (silica, polymethylmethacrylate and polystyrene-ethylacrylate) and the mediums (ethylene glycol, polyethylene glycol 200 and polyethylene glycol 600). The mechanical properties of the shear thickening fluid reinforced fabrics were evaluated by the knife and spike drop tower testing, respectively. The hardness of the particles was the dominant factor for the knife stab resistance, while the inter-yarn friction played as the critical role for improving the puncture resistance. In comparison to neat fabric, the knife stab and puncture resistance of the shear thickening fluid-fabrics exhibited significant enhancement, which can be proven by the results of yarn pull-out testing and optical microscope images investigation. The enhancing effect was systematically discussed and the improving mechanism was analyzed. Because the influencing factors for the knife stab resistance and puncture resistance were different, the enhancing effect of the dispersing particles and the mediums for the shear thickening fluid-fabrics should be also different.
The study described here combines materials science and engineering and mathematical analysis to fundamentally obtain insights underlying nanoscale deformation in polymer nanocomposites. The objective was achieved by examining the surface deformation response of two polymer nanocomposite systems with significant differences in ductility during scratching with a nanoindenter. The model host systems are a ductile polyethylene and a less ductile polypropylene. The model reinforcement material is nanoclay. The two systems are unique nanostructured materials because the distance between the individual clay layers is comparable to their thickness and also to the polymer segments. Thus, it is a morphology that is truly dominated by nanoscale features. Furthermore, we have used pressure-induced crystallization approach to alter the structure of the investigated two systems and study nanoscale surface deformation response. Our hypothesis is that the susceptibility to scratch deformation of nanostructured materials compared to their respective unreinforced counterpart depends on the shift of von Mises stress from the surface to the sub-surface region, leading to reduction in the maximum tensile stress induced by the scratch. To test the hypothesis, we have addressed two important aspects influencing the mechanics of surface deformation response of nanostructured materials. They include: (a) the determining role of nanoparticles on physical and mechanical properties and (b) the primary effect of nanoparticles on the structure of the host matrix. In the test of hypothesis, we have elucidated the relationship between structure, physical, and mechanical properties on the mechanism of nanoscale deformation via careful electron microscopy of the deformed surface. The nanoscale deformation response of polypropylene with large spherulites and α-phase was characterized by ripple-type deformation tracks that formed by a stick–slip mechanism. However, with decrease in spherulite size and evolution of toughness enhancing -phase, induced by high crystallization pressure and nanoclay, ironing represented the primary mode of deformation. In contrast, the highly ductile polyethylene system experienced multiple or single ironing process. In summary, the surface deformation topography implied that the nanoscale deformation response was material specific. Furthermore, electron microscopy topography of scratch tracks confirmed that reinforcement of polymers with nanoclay is a viable route to decrease the susceptibility of polymeric materials to nanoscale deformation and can be discussed in terms of physical and mechanical properties of materials, notably refinement of structure, percentage crystallinity, and elastic recovery. It was also intriguing to note that during nanoscratching, elastic recovery was high, in the range 77–85%, implying that a large fraction of the scratch-deformed volume was recovered. Finally, the electron microscopy findings and mechanical properties are related to mathematical analysis.
This article compares the test results of the conventional beam approach to those of the Brazilian test related to the durability of fibre-reinforced concrete. The flexural test was found to have some limitations for durability evaluation. On the other hand, the Brazilian test was found to have unique advantages for ingress of saltwater solutions in a short period of time and obtained relatively uniform stress distributions at the failure surface. The evaluation of the rate of stiffness reduction showed that polyvinyl alcohol fibres in saltwater solutions have the weakest resistance to crack propagation and polypropylene fibres have the greatest. The observed fracture surfaces after testing were found to have a uniformly damaged surface and first cracks were exactly initiated at the centre of the crack tip.
The article presents a user interface for modelling of textile composites, which combines open XML input and output formats with scripting for definitions of the reinforcement models under (local) shear, tension and compression deformations. The open input format (parameters of the textile reinforcement) allows easy integration of a meso-level (unit cell) textile processor upstream – with, for example, textile process modelling software or forming models, which define local parameters of the textile reinforcement. The scripting command format makes possible automated processing of the information on local reinforcement deformation conditions, and offers itself to be integrated in user software without accessing or knowing internal computational procedures or native data formats of meso-level textile processor. The open output format allows transferring the results of the meso-level textile processor downstream to meso-level models of the micromechanics or the permeability of a textile composite unit cell. These results can be further streamed to allow macro-level structural, forming or impregnation analysis. The integration approach is illustrated for meso-level textile processor – WiseTex software, integrated with processing of digital images and calculations of the fabric permeability tensor, micromechanics models and meso-level finite element ABAQUS model.
Fiber-reinforced polymer composites are engineered materials commonly used for many structural applications because of the high strength-to-weight and stiffness-to-weight ratios. Although the service life of these materials in various applications is usually between 15 and 20 years, these often keep the physical properties beyond this time. Recycling composites using chemical, mechanical, and thermal processing is reviewed in this article. In this review of carbon, aramide, and glass fiber composites, we provide, as of 2011, a complete view of each composite recycling technology, highlight the possible energy requirements, explain the product outputs of recycling, and discuss the quality (fiber strength) of recyclates and how each recyclate fiber could be used in the market for sustainable composite manufacturing. This article also includes the new concept of ‘direct structural composite recycling’ and the use of these products in the same or different applications as low-cost composite materials after small modifications.
Different montmorillonites were added to poly(vinyl alcohol) in order to improve their properties. The used montmorillonite’s were: Cloisite Na+ (Na), Nanofil (NF), and Cloisite 30B (30B). Poly(vinyl alcohol) + montmorillonite films, obtained by casting, were characterized by means of Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction pattern, transmission electron microscopy, water absorption, contact angle, and mechanical properties. Sodium and organically modified montmorillonites were used. The montmorillonite basal peak shifted to a lower angle for composites, with sodium montmorillonites, and the films were exfoliated–intercalated nanocomposites. Composites with organically modified montmorillonites presented a micro-morphology and the lowest water absorption. The best mechanical properties were obtained for the composite with sodium montmorillonites.
Central composite design method was used to design and optimize the best content of initiator, cross-linker, neutralizer and plant natural fibers of cotton and oil empty fruit bunch. The effect of buffer and saline solution on plant natural fiber based polymer hydrogel composites was investigated. The influences of plant natural fiber content and swelling behavior of polymer hydrogels were studied. Natural fibers had positive effect on polymer hydrogels in terms of absorption capacity and swelling rate. Polymer hydrogel composite of natural fiber with higher cellulose content had better swelling properties than that of less cellulose content and plain hydrogels. Polymer hydrogel composite base of high cellulose fiber content is less affected by the pH and ion concentration of the external solutions.
The off-axis constant fatigue life diagrams for a unidirectional carbon/epoxy laminate in different fiber orientations are identified over the whole range of stress ratio. The experimental results show that the off-axis constant fatigue life diagram plotted in the plane of alternating and mean stresses tends to shrink and incline to the left of the alternating stress axis more significantly as the off-axis angle of a specimen increases. The overall shapes of the off-axis constant fatigue life envelopes for different constant values of life are highly non-linear and asymmetric about the alternating stress axis, regardless of fiber orientation. These observations suggest that the sensitivity to mean stress in off-axis fatigue differs depending on the mode of fatigue loading, i.e. tension–tension, tension–compression, and compression–compression loading, and the difference is associated with the different modes of failure under different modes of fatigue loading. To deal with the off-axis fatigue strength of a unidirectional composite for any fiber orientation over the whole range of stress ratio, a most general form of the anisomorphic constant fatigue life diagram is developed that can take into account different sensitivities to mean stress in fatigue and distinguish between the tensile- and compressive-dominated failure modes. It is demonstrated that the generalized anisomorphic constant fatigue life diagram allows accommodating itself to a significant change in shape of a constant fatigue life envelope that depends on the range of stress ratio, and thus it can successfully be applied to accurate description of the off-axis constant fatigue life diagram for the unidirectional carbon fiber reinforced plastic laminate, regardless of fiber orientation.
The interply friction of prepreg is commonly found from the pre-forming to the final curing process during the fabrication of composite materials. In this study, a testing device was established to evaluate the slipping behavior of two kinds of carbon fiber/epoxy prepreg systems and the dominated friction mechanisms under different processing parameters, including temperature, pressure, and pulling rate, are determined. The interply frictional resistances of prepreg stacks were studied with the surface morphology observation and the surface roughness measurements. The results indicate that the friction mechanism of the two prepreg systems is the mixed friction, which could change from the Coulomb-dominated friction to the hydrodynamic-dominated friction under different processing conditions. The surface morphology and surface roughness of prepreg have significant effects on the slipping behavior controlled by the mixed friction. The Coulomb-dominated friction is not conducive to the slippage of prepreg and may result in fiber wrinkles in the laminates. The hydrodynamic-dominated friction should be achieved by adjusting the processing conditions to avoid manufacturing defects.
The reduced graphene/poly(vinylidene fluoride) nanocomposite films were prepared by the solution casting-thermal reduction process using graphene oxide and poly(vinylidene fluoride) resin. The results show that with the presence of reduced graphene nano sheets in the nanocomposite, the structure of poly(vinylidene fluoride) tends to transform from α- to β-phase and the β-phase fraction and its crystallinity are largely affected by the reduced graphene content. The thermal conductivity of poly(vinylidene fluoride) can be effectively improved by the reduced graphene nano sheets introduction and when the reduced graphene content increased to about 10.0 wt%, it is about two times higher than the pure poly(vinylidene fluoride), due to the excellent thermal conductivity of reduced graphene and formation of thermal conductive networks. With comparison of the pure poly(vinylidene fluoride), the reduced graphene/poly(vinylidene fluoride) nanocomposite films show a better tribological property at a low reduced graphene content (below 4.0 wt%), mainly owing to the lubricant and heat spreading effects of reduced graphene nano sheets. The friction coefficient and wear rate for 0.75 wt% reduced graphene/poly(vinylidene fluoride) nanocomposite film compared to the pure poly(vinylidene fluoride) are improved by 41.2% and 49.4%, respectively.
Rubber-thermoplastic-based composites represent widely used materials due to their good mechanical properties that can be controlled according to the composition and interfaces. Considering the mechanical properties as output, recycled polyethylene terephthalate-rubber-based composites with optimized curing temperature were obtained, where rubber acts as a matrix, polyethylene terephthalate as filler and high-density polyethylene as compatibility agent. By adding another recycled material wood (sawdust), a novel composite with tailored properties was obtained via compression molding. The composites were mechanically tested in terms of compression and tensile strength. The optimized compositions were investigated by X-ray diffraction and further characterization was done to outline the interfaces contribution: surface/interface topology (atomic force microscopy) and bonding (Fourier transform infrared); contact angle measurements were developed, to evaluate the surface energy as a tool for estimating possible (thermal) degradation. Water stability was investigated and its plasticizer role was outlined. Wood sawdust addition at recycled rubber–polyethylene terephthalate–high-density polyethylene composites had a remarkable effect on the mechanical properties, recommending this composite type as structural material.
Carbon nanotubes with unique physical and mechanical properties have shown great potential for biological applications, including tissue engineering and mimicking the structure and properties of human bones. In the present work, sol-gel synthesized nanocomposite powder of multi-wall carbon nanotube/hydroxyapatite characterized using field-emission scanning electron microscopy, transmission electron microscope, X-ray diffraction, Fourier transform infra-red spectroscopy and thermal analyses. The results show homogenous dispersion of nanotube in well-crystallized hydroxyapatite ceramic matrix. Scanning electron microscopy and transmission electron microscope observations show the sodium dodecyl sulfate–adsorbed multi-wall nanotube almost wrapped completely by crystals of hydroxyapatite that will help better integration of bone substitute materials with the surrounding bone tissue. Eventually, in vitro study confirms the biocompatibility of composite powder comparable to monolithic hydroxyapatite.
The effects of aluminum triacrylate on vulcanization, mechanical and morphological properties of filled peroxide-cured ethylene-propylene-diene rubber reinforced with carbon black are investigated. Curing kinetics is studied by rheometer and the results indicate that the curing characteristics are influenced by combination of conventional filler (carbon black) and reactive filler (aluminum triacrylate), because the former one enhances the physical cross-linking and the latter one introduces the ionic cross-linking. The results obtained show that the tensile strength, modulus, hardness and cross-link density were tremendously increased with increase of aluminum triacrylate loading in carbon black–filled ethylene propylene diene monomer. Moreover, the observation on the compounds showed that introduction of aluminum triacrylate reduces the elongation at break, resilience and abrasion of carbon black–filled ethylene propylene diene monomer. Morphology of the fractured surfaces of ethylene propylene diene monomer/carbon black/aluminum triacrylate composites was studied by scanning electron microscope. The morphological study indicates that interaction between the fillers and ethylene propylene diene monomer matrix has been improved by substitution of aluminum triacrylate filler with carbon black. Thus, the formation of ionic cross-links structures from both homo-polymerization aluminum triacrylate and graft co-polymerization of it onto the ethylene propylene diene monomer were the main reasons for the significant improvement in mechanical properties of ethylene propylene diene monomer/carbon black composites.
A series polyacrylamide/Na-montmorillonite nanocomposites was prepared by in situ free radical polymerization in aqueous medium using benzoyl peroxide (Bz2O2) as a radical initiator. To characterize the resultant nanocomposites, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis techniques were used. The moisture retain, water uptake, antibacterial properties, BET specific surface area and specific nanopore volume of nanocomposites were determined. The X-ray diffraction pattern showed that the polyacrylamide was intercalated up to 60.0 mass% acrylamide concentration, and then montmorillonite layers exfoliated into the polyacrylamide matrix. The interlayer spacing (d001) of the montmorillonite increased from 1.19 to 1.90 nm by intercalation. Transmission electron microscopy views showed that the Na-montmorillonite partially exfoliated in polyacrylamide. The nanocomposites exhibited better thermal stability than the pure polyacrylamide.
This article presents the latest developments of a constitutive modelling framework, CODAM (COmposite DAmage Model), for predicting the non-linear in-plane response of composite laminates using continuum damage mechanics. The methodology is best suited for non-linear structural analysis of large-scale laminated composites whose boundaries do not interfere/interact with the damage zone that develops and grows within the structure. The new development presented here, CODAM2, addresses the deficiencies in both the numerical and material objectivity of the original version of CODAM. While the previous CODAM formulation was essentially a local smeared crack model that was augmented with crack band scaling to overcome one aspect of the numerical objectivity, namely the mesh-sensitivity, CODAM2 introduces a non-local regularisation scheme to alleviate both the spurious mesh dependency and mesh orientation problems that plague all local strain-softening models. Two of the 13 test cases, provided in the third-world wide failure exercise, which were related to the in-plane tensile and compressive loading of open hole specimens, were used in order to demonstrate the effectiveness of CODAM2 in predicting the damage development and the corresponding overall response in such structural loading configurations.
The aim of this article is to study the variability in tensile, flexural, in-plane shear, and interlaminar shear properties of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composite laminates. Six kinds of laminated composites of average thickness 5.5 ± 0.2 mm and total fiber volume fraction (VfT) = 37% were manufactured using hand lay-up technique and tested under tensile, bending, in-plane shear, and interlaminar shear loading conditions. The failure modes of all test specimens have been recorded and discussed. Two-parameter Weibull distribution function was used to statistically analyze the composite samples experimental results. Weibull graphics were plotted for each sample using experimental monotonic mechanical properties results. The Weibull distribution has also been employed to show both the failure probabilities as well as the scatter in the experimental results for the above mentioned composites. The obtained failure probability graphs for the manufactured composites under different static loading conditions are important tools for helping the designers to understand and choice the suitable material for the required design and development.
The main objective of this article was to study the capillary effects during the impregnation of polypropylene resin in the jute fiber mats. The static equilibrium contact angles between polypropylene matrix and fibers were measured and the dynamic impregnation of matrix in the fiber mats was researched in detail. The results show that the static contact angles are clam-shell shape. The capillary effects present little values in the dynamic impregnation. The preparation for low void composites should be performed below the critical resin flow velocity of 0.13 µm/s for polypropylene resin and 0.21 µm/s for polypropylene/maleated polypropylene blend resin, respectively.
In this article, the integral hole drilling method, developed in the first part of this research, has been employed to calculate the non-uniform residual stresses trapped within various composite laminates. Test specimens were fabricated using hand layup assisted with vacuum bagging technique. In order to study the capability of the model in predicting the residual stresses in different layups, three different ply configurations are used in this article as: symmetric cross-ply ([0°2/90°2]S), unsymmetric cross-ply ([0°4/90°4]), and symmetric quasi-isotropic ([0°/±45°/90°]S) specimens. Using the developed method in the present research, calculated residual stresses in each layer of the laminates showed good agreements in comparison with the theoretical values.
Low-velocity dynamic compression tests were performed to reveal the failure mechanism and the energy absorption capacity of the integrated woven sandwich composite. Shear deformations were induced by the tilting of fiber piles in the core of the integrated woven sandwich composite. Ductile load–displacement curves are featured by a long deformation plateau originated from rotations of the core piles. Densification is apparent in the later stage of compression. Stout piles in the core also lead to plastic compression failure mode accompanying with much smaller rotations of core piles. Controlled by the latter failure mode, the dynamic strength and the energy absorption of the panel are stronger. In dynamic compression experiments, the integrated woven sandwich composite panels exhibit similar failure modes with those observed in quasi-static compression tests. The dynamic strength is much greater and the corresponding deformation plateau is much more stable, which leads to greater energy absorption. The dynamic effects of the strength and the energy absorption were explained by the dynamic buckling of the woven struts in the core. The tests suggest that the integrated woven sandwich composite is ideal to serve as a lightweight anti-impact material in engineering structures.
High particle content B4C/Al composites reinforced with different particle sizes were fabricated. Based on the tensile and compressive stress–strain behaviors of these composites, the effective medium approach was employed to investigate the load partition behaviors between the matrix and reinforcement. The results showed that the load borne by the particles increased in compression, which lead to larger compressive strength than the tensile strength. Meanwhile, the load borne by particles increased with decreasing particle size during compression. These observations were rationalized based on the combined theories of geometrically necessary dislocations and metal-based cemented granular material behaviors.
A holistic approach to strain monitoring in fibre-reinforced polymer composites is presented using embedded fibre Bragg grating sensors. Internal strains are monitored in unidirectional E-glass/epoxy laminate beams during vacuum infusion, curing, post-curing and subsequent loading in flexure until failure. The internal process-induced strain development is investigated through use of different cure schedules and tool/part interactions. The fibre Bragg grating sensors successfully monitor resin flow front progression during infusion, and strain development during curing, representative of the different cure temperatures and tool/part interfaces used. Substantial internal process-induced strains develop in the transverse fibre direction, which should be taken into consideration when designing fibre-reinforced polymer laminates. Flexure tests indicate no significant difference in the mechanical properties of the differently cured specimens, despite the large differences in measured residual strains. This indicates that conventional flexure testing may not reveal residual strain or stress effects at small specimen scale levels. The internal stresses are seen to influence the accuracy of the fibre Bragg gratings within the loading regime. This study confirms the effectiveness of composite life cycle strain monitoring for developing consistent manufacturing processes.
Investigations were carried out on glass-epoxy composite specimens with varied fibre orientations (±2° to ±88°), prepared by both braided and filament winding techniques. The experimental strength and stiffness of these composites while exhibiting similar trends, correlated well with those predicted by classical laminate theory. The filament wound composites exhibited (20–40%) initial higher properties vis-a-vis the braided composites, while a correlation index between the experimental and theoretical values has been evolved.
Carbon fiber reinforced cyclic butylene terephthalate composites have been processed by vacuum infusion under two different non-isothermal processing routes starting from a one-component cyclic butylene terephthalate resin system. One of them was processed under a short cycle with fast cooling, and another one was processed under a long cycle with slow cooling. Both the micro-structure and low-energy impact properties of the composites have been investigated. On one hand, the fast cooling generates randomly dispersed voids and porosities in the resin-rich regions during the crystallization-induced shrinkage. On the other hand, the slow cooling generates a highly crystalline and brittle matrix without porosity. However, many micro-cracks appear in the resin-rich regions due to the combination of the brittleness and longitudinal shrinkage of the matrix. The critical delamination energy of the slow cooled composite is slightly higher than that of the fast cooled one, whereas this latter absorbs over 25% more energy before being penetrated, as well as performing in a less brittle way. The lower interlaminar shear strength of the fast cooled composite is suggested to be the origin of its higher energy absorbing capability and less brittle behavior.
Conventional wisdom dictates that adding more 0° plies in the load-bearing direction of a laminate will increase its stiffness and strength. While this is true for undamaged laminates, the compression strength of laminates with impact damage may not be as straightforward. In this study, compression after impact strengths of relatively thin laminates with 25%, 33% or 50% of plies aligned in the 0° load-bearing direction were measured for three different damage severity levels. Results show that the increase in compression strength of the laminates with a higher percentage of plies in the 0° direction is lessened as impact damage severity increases indicating that a laminate that is stronger in compression when undamaged may not be stronger in compression when impact damage is accounted for.
As the application of fibre-reinforced polymer composite material continue to increase day by day, so the knowledge about the impact behaviour of fibre-reinforced polymer composite structures in the areas such as automotive and aerospace is very much needed. This article attempts a comprehensive review of recent literature in the broader area of impact damage. Testing methods and standard parameters as well as discussion of important aspects such as impactor shape, weight of impactor, velocity of impact, environment in which impact takes place are presented. Furthermore, the damage area, energy absorbed, contact time and many other considerations are discussed. Finally, an effort is made to review the research work by considering all aspects related to impact on such type of composite materials.
Random poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) and poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate)/organoclay nanocomposites were synthesized via in situ polymerization of terephthalic acid, ethylene glycol, 1,4-cyclohexane dimethanol, and aminosilane-modified Cloisite30B. An amorphous copolyester was produced with incorporating 30 mol% of 1,4-cyclohexane dimethanol in glycol part. Samples were characterized using Fourier transform spectroscopy, nuclear magnetic resonance, 13C-NMR and 1H-NMR, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, thermal gravimetric analysis, and dynamic mechanical thermal analysis. Intrinsic viscosities of samples were in the 0.51–0.55 dL/g range. A commercial organoclay was modified prior to add into the reactor. 1H-NMR and 13C-NMR spectra were applied to determine cis:trans isomer ratio of 1,4-cyclohexane dimethanol in polymer chains, Molar ratio of 1,4-cyclohexane dimethanol: ethylene glycol in polymer chains, degree of randomness, mean length of sequences, Mn, and Mark–Houwinks equation parameters. X-ray diffraction results of nanocomposites, including 0.5 and 1 wt% of organoclay, showed no peak in small angles, hence, modified organoclay presented exfoliated structure in polymer matrix. Differential scanning calorimetry analyses revealed that all samples were amorphous. Oxygen, nitrogen, and carbon dioxide diffusion rates in samples were investigated. Nanocomposite, including 3 wt% of organoclay, has about 60% less permeability in comparison to neat copolyester.
A nonlinear finite element model is developed to simulate the mechanical behavior of fiber networks exposed to moisture. The full-scale three-dimensional Reissner beam model is geometrically nonlinear and handle large deformations and rotations and can be applied to simulate any moisture-sensitive fiber-based network material. Moisture in the surrounding air is transported into the fibers, which leads to chiral curl of individual fibers, which in turn introduce stresses in the network since the fibers are connected to other fibers in a complex manner. Deformations of networks subjected to various degrees of moisture are analyzed. The effects of fiber orientation and network geometry are examined. Numerical results obtained with the model show qualitative agreement to experimental results reported in literature on cellulose materials.
High-strength carbon fibers were treated with nitric acid and periodically analyzed by several different methods to develop an understanding of overall property changes and how they relate to composite design. Fiber diameter, tensile strength, surface morphology, surface chemistry and surface energy were all evaluated as a function of treatment time and two distinct stages of change were identified; the first characterized by surface modification and the second by carbon material loss. Initially, the tensile strength, degree of surface oxidation and surface energy all increased. The surface oxidation consisted primarily of carbonyl and carboxylic acid types. Then in the second stage, both the tensile strength and surface oxidation reached stable levels and the fiber diameter began to rapidly decrease. The surface morphology and energy were the only properties that showed no obvious changes from one stage to the next. The surfaces were found to be smooth through all treatment times and the energy increased steadily throughout. It is believed that the variation of all of these properties is related to the fiber microstructure and how it varies through the cross-section of high-strength fibers. Specifically, high-strength carbon fibers are known to have better microstructural organization and alignment in the near-surface layer than within the interior.
This paper presents a multiscale hybrid approach for predicting damage and failure of laminated composite structures based on the thermo-mechanical properties (stress/strain behaviour and strength) of the unidirectional plies. This kind of approach is thus predictive for different stacking sequences. The approach introduces viscosity of the matrix in order to obtain an accurate description of the mesoscopic behaviour, especially the non-linearity under shear loading. The failure criterion used is based on physical principles and introduces micromechanical aspects (such as the effect of the local debonding on the non-linear failure behaviour) at the mesoscopic scale. The main improvements, over those proposed in the second world-wide failure exercise, are related to (1) the evolution and effects of the mesoscopic cracks and (2) the coupling between those cracks and delamination (inter-ply damage). This approach has been implemented in an implicit finite element code in order to predict the strength of composite structures, exhibiting different levels of complexity (unnotched plates, open-hole plates) and subjected to complex loadings (membrane or bending loadings). All the 13 Test Cases of the third world-wide failure exercise have been solved.
This study examines the effect of particle size and wood flour content on the properties of polystyrene filled with white oak flour. Wood-plastic ratios 10:90, 30:70 and 50:50% (wt/wt) and particle size 40, 50, 65 and 100 mesh were used. Tensile, bending and impact bending strength as well as the melt flow index were evaluated. Additionally, composite density and water absorption capacity were also tested. Scanning electronic microscopy revealed good adhesion between wood particles and polystyrene. Results show that mechanical properties are strongly influenced by wood flour content and particle size. A reduction in tensile module, elongation and deflection were observed, however, the bending module was increased. Impact strength increased with particle size and content. Melt flow index values are reduced with the increasing amount of filler content while water absorption increases with the amount of wood particles.
Modeling the infiltration of reinforcements during the processing of composite materials by liquid composite molding techniques is an important instrument for the prediction of flow front patterns, filling times and pressure gradients. Darcy’s law is widely used to model most of these processes. However, when polar fluids are used together with natural fibers, fiber swelling may occur and introduce further complexity to the simulation. In this work, a model that includes the aforementioned phenomena is proposed, leading to a more accurate prediction of the flow front position than the classic models that use a constant permeability value.
A composite material with anisotropic microstructure was fabricated by DC electric field. Alumina was used as filler for thermal conductivity and polysiloxane resin was used as matrix. According to the alumina morphology and loading amount, various anisotropic microstructures were assembled. The thermal properties of the composite material were investigated parallel to the direction of the applied electric field accordingly. For plate-like alumina with 20 vol% loading, a thermal conductivity of 0.44 W/mK was achieved. With spherically shaped alumina, with an equal loading of 20 vol%, a higher thermal conductivity of 0.46 W/mK was achieved. The increase in the thermal conductivity of the fabricated alumina composites with anisotropic microstructure was attributed to increased filler-to-filler connectivity.
We treat selected test cases in the third world wide failure exercise by the approach described as synergistic damage mechanics. This approach utilizes micromechanics and continuum damage mechanics to predict the overall mechanical response of composite laminates with ply cracking in multiple orientations. The material constants needed in the continuum damage mechanic formulation are calculated from stiffness property changes incurred in a reference laminate. For other laminate configurations, the stiffness changes are derived using a relative constraint parameter which is calculated from the constraint on the opening displacement of ply cracks within the given cracked laminate evaluated numerically by a finite element analysis of appropriately constructed representative unit cell. The number density of ply cracks (cracks per unit length normal to the crack planes) under quasi-static loading is calculated by an energy-based approach. Finally, the stress–strain response of a laminate is determined by combining stiffness property changes and evolution of crack number density.
High-density polyethylene/Mg2Al-layered double hydroxide nanocomposites with different ethylene-acrylic acid random copolymer/Mg2Al-layered double hydroxide master batches were prepared by melt-mixing. The effect of the ethylene-acrylic acid random copolymer melt index on the exfoliated/intercalated nanocomposite structures was investigated by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. The crystallization behavior and thermal property of the nanocomposites were determined via differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis and cone calorimetry. The Mg2Al-layered double hydroxide layers were well-dispersed at the nanometer level, providing direct evidence of the formation of intercalated/exfoliated nanocomposites, as well as the important effect of ethylene-acrylic acid random copolymer melt index on these structures and their properties. Lower ethylene-acrylic acid random copolymer melt index resulted in higher exfoliation and better dispersion of layered double hydroxide in the high-density polyethylene matrix. Reductions in the heat release rate, total heat release and the carbonic oxide and carbon dioxide production of the nanocomposites were also observed. These phenomena led to higher thermal stability and changes in the storage modulus, tan and loss modulus of the nanocomposites, as well as improvement in the heterogeneous nucleation effect of layered double hydroxide on high-density polyethylene.
New Fe-Cu-graphite-Ni-TiO2 (2 to 8 wt% TiO2) composite materials are studied for friction applications. The tribological characteristics of these materials were monitored by a pin-on-disc method against a cast iron plate. An increase of the friction coefficient when the titanium dioxide content was increased up to 6 wt% was noticed. Instead, adding 8 wt% of titanium dioxide determined the decrease of friction coefficient. A gradual decrease of the wear rate was marked out when titanium dioxide content was increased. The best behavior determined by the optimal ratio friction coefficient/wear rate was the Fe-Cu-graphite-Ni-TiO2 composite containing 6 wt% titanium dioxide.
An atmospheric pressure helium and oxygen plasma has been used for the surface preparation of 410 stainless steel and carbon-fiber epoxy laminates prior to bonding to themselves or to each other. Lap shear results for stainless steel coupons and carbon-fiber epoxy laminates demonstrated an 80% and a 150% increase in bond strength, respectively, after plasma activation. Following 7 days of aging, wedge crack extension tests revealed a crack extension length of 7.0 mm and 2.5 mm for the untreated and plasma-activated steel. The untreated stainless steel had 30% cohesive failure compared to 97% for steel activated with the plasma. Surface analysis by X-ray photoelectron spectroscopy showed that carbonaceous contamination was removed by plasma treatment, and specific functional groups, e.g. carboxylic acids, were formed on the surface. These functional groups promoted strong chemical bonding to the epoxy film adhesive. Atmospheric pressure plasmas are an attractive alternative to abrasion techniques for surface preparation prior to bonding.
The use of natural fibres in the development of composite materials is a sector in full expansion. These fibres were used for their low cost, availability and renewable character. The fibres of the palm (palm tree) were used as reinforcement in polypropylene. The date palm fibres have some potential because of their ecological and economic interest. Both unmodified and compatibilised fibres are used. Compatibilisation was carried out with the use of maleic anhydride copolymers. The morphology and mechanical properties were characterised by scanning electron microscopy and tensile tests. The influence of fibre content on mechanical properties of composite polypropylene/date palm has been evaluated and demonstrated that the maximum stress and elongation decreases with increasing fibre content, on the contrary, they notice an increase of the tensile modulus, but after the improvement of fibres, the maximum stress increase significantly up to 25% weight.
The thermomechanical properties, thermal stability and flame retardancy of the organoclay-vinyl ester nanocomposites and the glass fiber reinforced plastic composites made from the vinyl ester nanocomposites matrix were studied. The results show that nanoclay addition increases both the storage modulus and glass transition temperature of the vinyl ester and glass fiber reinforced plastic composites due to the reinforcing effects and the molecular relaxing constraining effects of clay platelets. Both the vinyl ester and glass fiber reinforced plastic composites show different thermal degradation behaviors in nitrogen and in air due to the oxidizing effect of oxygen. Nanoclay has little effect on the thermal stability of vinyl ester in nitrogen, while increases the 2nd peak decomposition temperature of vinyl ester in air, resulting from the shielding effect of silicate platelets. However, the thermal stability of the glass fiber reinforced plastic composites in both atmospheres is reduced by nanoclay with unknown reasons. The flame retardancy of vinyl ester and glass fiber reinforced plastic composites is significantly improved due to clay that promotes the formation of carbonaceous char platelets acting as mass and heat barrier. Glass fiber reinforcement alters the thermal dynamic, thermal degradation and combustion behaviors of the vinyl ester nanocomposites.
PLA nanocomposites containing 2 wt% of OMMT clay were prepared using twin screw extruder followed by injection moulding. EPR-g-MAH (5–20 phr) was used to improve the impact properties of PLA/OMMT nanocomposites. The mechanical properties of PLA nanocomposites were studied through tensile, flexural and impact tests. The morphology and dispersion of OMMT were examined using TEM and XRD. The thermal properties were characterised using differential scanning calorimetry and thermogravimetric analysis. The impact strength and thermal stability of the PLA/OMMT nanocomposites were improved significantly in the presence of EPR-g-MAH. The degree of crystallinity of PLA/OMMT was influenced by the loading of EPR-g-MAH. TEM and XRD results revealed the formation of PLA nanocomposites as the OMMT was exfoliated in the presence of EPR-g-MAH.
Stitch-bonded, non-crimped fabric composites are among the most common forms of structural fiber-reinforced polymer composites. A complex three-dimensional finite element model is usually required for the accurate prediction of the mechanical properties of these composites. The objective of this work was to develop a model to predict the in-plane elastic properties of non-crimped fabric composites (without structural stitching) with no inputs from the experimental characterization of the composites themselves. The motivation for this work was to develop a swift, accurate methodology that would be very beneficial with ever reducing design cycle times (for screening different fabrics) and as these fabrics find new application areas. The modeling approach used only the properties of the dry non-crimped fabric and the resin as inputs. Models were constructed to account for the geometrical aspects of the non-crimped fabric such as yarn width and yarn spacing, which depend on the stitching pattern employed. They included regions of pure matrix between fiber tows as well as between fabrics parallel to the stacking plane. The stitching threads, voids, crimping of the fiber tows and damage or disturbances to the fiber due to the stitching were not modeled. The effective elastic properties of uni-directional, bi-axial and tri-axial non-crimped fabric composites are computed using classical composite lamination theory and finite element analysis. The predicted results are found to be in good agreement with experimental results obtained using composites fabricated by vacuum-assisted resin transfer molding.
This article relies on the effect of two types of nanoparticle on morphology and dynamic-mechanical properties of polyvinyl chloride (PVC)/acrylonitrile butadiene rubber (NBR) blends. The results of mechanical investigation revealed that tensile strength and modulus of PVC/NBR nanocomposites reinforced with 1 phr of single-walled nanotube (SWNT) are very close to the case of reinforced with 5 phr of nanoclay. The outcomes of dynamic-mechanical properties revealed that the storage modulus increases with the addition of nanoparticles and the intensity of tan get reduced, in two cases. Morphological investigation of nanocomposites was determined by scanning and transmission electron microscopy. In the case of PVC/NBR/nanoclay, fracture surface of specimens were much rough while the fracture surface of virgin PVC/NBR was very smooth. In PVC/NBR/SWNT nanocomposites, with introducing of carbon nanotubes to polymer matrix, dispersion and distribution of NBR as minor phase in PVC as matrix got improved.
The stress concentration factor in the matrix of a fibrous composite reinforced with transversely isotropic fibers is determined upon constituent elastic properties as well as the fiber volume fraction available before fabrication of the composite. The matrix in situ transverse strengths are then defined from its original counterparts divided by the stress concentration factor, which are further used together with the other original constituent property parameters for calculating strengths of the composite in terms of the bridging model. The thus obtained strength envelopes of several unidirectional composites subjected to combined bi-axial loads are compared with independently measured results. Favorable correlation indicates that a composite ultimate strength can be reasonably well estimated without using any experimental data of the composite itself. The article clearly shows that an accurate prediction for a composite strength must take all of the three influencing factors, i.e. stress concentration, matrix inelasticity, and thermal residual stresses, into account. It is further demonstrated in the article that the stress concentration factor with transversely isotropic fibers involved can be sufficiently accurately approximated by a much simpler stress concentration factor formula derived upon isotropic fiber reinforcement.
This article is the author’s contribution to the third World-Wide Failure Exercise which aims at benchmarking current damage models for composites. Reduction of thermo-elastic constants of laminates and their nonlinear behaviour due to intralaminar cracking and nonlinear shear response of the composite are analysed using global–local approach. The macroscopic properties of damaged laminates are expressed in simple forms containing density of intralaminar cracks and their surface displacement features obtained from local solutions. The initiation and evolution of the intralaminar damage is analysed using strength-based approach for laminates with thick layers and fracture mechanics approach for thin layers. Due to a lack of information, certain characteristics, such as statistical failure properties distribution parameters and transition point (thickness) from strength to fracture mechanics applicability, were assumed. All calculations are based on analytical expressions, some of which were developed previously through numerical analysis. The present method was applied to solve 9 out of the 13 test cases of the third World-Wide Failure Exercise and that was sufficient to illustrate the capability of the damage model.
The paper represents the author’s contribution to the Third World-Wide Failure Exercise, which is aimed at benchmarking current models of damage, matrix cracking, initiation of delamination and their interaction with fibre failure. The approach used for the development of damage in laminates is based on an energy methodology that requires knowledge of the dependence of thermo-elastic constants on damage. The various models, developed by the author, are applied to the majority of the Third World-Wide Failure Exercise Test Cases, which included thin and thick cross ply and quasi-isotropic laminates, loading and unloading of an angle ply laminate, bending of a general laminate, and cracking under thermal loadings. Methods used to predict ply properties from those of the fibres and matrix are also described. Crack density in the 90 degree plies was modelled using a ply refinement technique. Detailed discussion is made on a number of relevant issues (initiating defect size and shape, fibre strength, ply saturation, off-axis ply cracking, delamination, mixed mode ply cracking) and their likely effects on design.
Two composites consisting of non-woven poly-96L/4D-lactide copolymer meshes and a bioactive glass containing silver and silver free were obtained in polyvinyl-alcohol solutions by slurry dipping method under slow mechanical stirring. The development of hydroxyapatite type nanocrystals on samples surface in simulated body fluid proved for both composites the tendency for high level of bioactivity. Their antibacterial effect was evaluated against Gram-negative Escherichia coli and Gram-positive Staphylococcus epidermidis. Both composites showed inhibitory effect on bacterial growth, but only the composite with silver containing bioactive glass proved to have bactericidal effect. The AgCl nanocrystals and Ag3PO4 particles self-assembled on the surface of this composite proved the release of silver ions into simulated body fluid.
Failure initiation locations were determined in the tows and matrix pockets of a plain weave textile composite for a wide variety of loading conditions in an attempt to identify characteristic failure initiation sites. Failure was found to initiate in a limited number of locations in the composite for a wide variety of loading conditions. It was also found that the different degrees of weave undulation studied shared similar characteristic failure initiation sites. Additionally, it was observed that some idealized features of the textile geometry led to fictitious stress concentrations that could artificially bias the predicted failure initiation locations. Steps were taken to reduce the influence of such stress concentrations, including modifying the textile geometry and excluding certain problematic regions of the textile from consideration when searching for failure initiation locations.
Forming complex shape composite parts with a good production rate/cost ratio is of particular importance for the automotive industry. The sheet forming of woven reinforcements is a promising technique, especially if complex shapes with singularities such as case corners can be obtained. Due to the more and more important recycling needs, the use of flax fibre based reinforcements may be considered for structural or semi-structural parts. During the sheet forming of a tetrahedron shape, the tows constituting the architecture of the reinforcement material are submitted to tensile strains. When using glass or carbon fibre tows, strains to failure are generally not reached. When flax-based fabrics are considered, the failure/degradation strength of the tows constituting the fabric may be reached. Even if no apparent failure is visible when observing the tows during forming, the strains measured by a mark tracking method indicate that the degradation limit of particular tows of the preform has been reached. This could lead to local lack of fibre density and to possible zones of weakness for the composite part. As a consequence, it is essential to improve the tensile performances of the tows constituting the fabric without losing their good impregnation characteristics and good ability to reach high mechanical properties for the composite part.
The aim of this work is to underline the influence of the wetting behavior on bubble formation and transport during the impregnation of fibrous preforms for Liquid Composite Molding processes. The void prediction within the elaboration of composite material sparks off interest because it represents a significant issue to keep the expected mechanical properties of the final product. Voids or bubbles are mainly formed due to the resin flow competition inside and outside the tows. However, the experimental characterization of void formation and transport during the flow inside fibrous media remains delicate. Since the direct visualization of the liquid flow and the entrapped bubbles in a practical material is uneasy, we used model networks to study the formation of bubbles and their transport. Thus, our experimental study deals with a simple configuration of connected pores: two capillaries converging at a node perpendicularly (T-junction). We emphasized on microfluidic and millifluidic approaches where the wetting is significant during bubble formation mechanisms. For the same flow conditions, the experimental results reveal that the bubble lengths are higher for partially wetting liquids than for completely wetting ones. In a wetting case, it was demonstrated that the lubrication hypothesis can be a good approximation to describe the bubble transport in fibrous media.
In this study, experimental studies on hole quality and machinability in drilling of unreinforced polyamide (PA6) and reinforced polyamide with 30% of glass fibers (PA66-GF30) using cemented carbide (K20) tool have been carried out. The experiments have been planned as per full factorial design of experiments. The effects of spindle speed, feed rate, and point angle on hole quality such as hole diameter and circularity error; the machinability characteristics such as thrust force and specific cutting coefficient have been analyzed by developing response surface methodology based second-order mathematical models. The parametric analysis shows that the quality of holes can be improved by proper selection of cutting parameters. The analysis also indicates the influence of reinforced fiber on proposed machinability characteristics during drilling of polyamides.
In recent times, composites made out of polymers and paraffin waxes were thought to be good thermal energy storage materials, in which the heat is stored as latent heat of fusion in the paraffin wax. In this study, phase change composite material with spherical shape calibrated based paraffin wax (RT27) was produced. The properties of the prepared composite phase change material have been characterized. The objective of this article was to study the energy storage and the energy recovery by using a phase change composite material. An experimental set-up consisting of fluxmetric measurement has been constructed to provide the thermal performance of the composite. In addition, a differential scanning calorimetry analysis was carried out. The experimental apparatus allows providing heat storage capacities and "apparent" thermal conductivities of the composite at the solid and liquid states, and also a measurement of the latent heat of fusion. The proposed test provides temperature and heat flux measurements at the material borders. The amount of energy exchanged during the variation of the thermodynamic state samples could be calculated when the boundary temperatures vary. In this article, one shows how it can allow the study of complex composite material with PCM. In particular, heat flux measurements make it possible to highlight very specific behaviors of these products and are thus a very interesting experimental source of data which comes to complete the traditional measurement methods like calorimetric device (differential scanning calorimetry).
Fatigue tests were conducted on non-prestressed and prestressed composite box sections. The approach to prestressing of composite structures seeks to eliminate compressive stress excursions under fatigue loading. This approach increased the fatigue life of composite structures; the experimental gains in fatigue life were in agreement with theoretical prediction. The tensile prestress developed in the composite structure mitigate compressive stress excursions under fatigue load, which significantly enhance the fatigue life of composites. Flexural fatigue tests were conducted through repeated application of a constant deformation, with the maximum load level monitored throughout the tests. The repeated (constant) deformation was selected to produce 70% of the ultimate (quasi-static) flexural strength of each of the prestressed and non-prestressed composite box sections. Two prestressed and two non-prestressed specimens were tested under (displacement controlled) fatigue loading–unloading conditions. The fatigue life of non-prestressed composite sections was reached upon local buckling of top flange (which experienced the maximum compressive stress). For both non-prestressed and prestressed composite sections, failure was defined as the number of cycles after which the peak load (at constant repeated deformation) dropped to 70% of the value in the first cycle. The fatigue life of prestressed composite sections was about 100% greater than that of non-prestressed sections. The residual strength of prestressed sections after fatigue loads was, on the average, 56% greater than that of non-prestressed sections. The experimental results indicated that contribution of prestressing to the fatigue life of composite sections was significant.
Aluminum nitride reinforced glass fiber epoxy resin composite was prepared by simple hand lay-up technique and its mechanical as well as erosion wear behavior were investigated. The interactive influence of various operational variables on specific wear behavior of composite materials has been studied thoroughly. It was observed that with increasing percentage of filler particles, there is a decline in tensile strength, but there is a significant improvement in hardness and erosion wear performance. Among all the factors, impact velocity is the most significant factor followed by filler percentage and impingement angle, while temperature has the least significance on erosion of the hybrid composite. Taguchi’s orthogonal arrays were used to identify the controlling factors influencing the erosion wear rate. Scanning electron microscopy studies were conducted to understand the erosion mechanism involved during the material removal process.
The interface reaction and its effect on the mechanical properties have been experimentally studied for aluminum-based composites reinforced with titanium dioxide nanoparticles (TiO2). Aluminum-based composites containing TiO2 nanoparticles (20 nm in size) are developed by hot-rolling the ball-milled powder. High chemical potential energy of the nanoparticles induces the fast formation of the interface layer during the heat treatment process, and high yield stress of 514 MPa in a composite containing 5 vol.% TiO2 nanoparticles can be achieved. Furthermore, dislocations are emitted at the nanoparticle/matrix interface during deformation due to the high stress concentration. This study can provide useful insights for the design of metal-matrix composites.
This article investigates the interlaminar shear behavior and damage detection of woven carbon fiber reinforced polymer composite laminates at cryogenic temperatures. Short beam shear tests were performed at room temperature and liquid hydrogen temperature (20 K), and the temperature dependence of the apparent interlaminar shear strength was examined. The electrical resistance of the composite specimens was also monitored during the tests. A detailed observation of the tested specimens was made to assess the damage, and the relationship between the damage and the electrical resistance was discussed. In addition, the stress, strain and current density distributions in the short beam shear specimens were determined by the finite element method. The numerical results were used to better understand and explain the experimental findings.
This article presents the efficient mixed model analysis, often called global–local approach, to obtain interlaminar stresses at free edges in the laminated composite system under extension and flexure. For the proposed analysis, first, the discrete-layer elements are adopted as three-dimensional elements in the local region. Second, the equivalent single-layer elements are considered as two-dimensional elements in the global region. Finally, the mixed-dimensional transition elements are formulated to connect two different element types used in the global as well as the local regions, respectively. All the elements have higher order shape functions derived from the Lobatto shape function. Modes of the elements are classified into nodal and nodeless modes. The nodal modes have physical meaning, while nodeless modes with respect to the increase of order of the Lobatto shape function do not have physical meaning but improve accuracy of analysis. Therefore, fixing mesh arrangement of present analysis, the quality of the analysis can be enhanced without remeshing work. Some results obtained by the proposed approach are verified with comparison with published references. Also, efficiency of the mixed model analysis using the mixed-dimensional transition element is shown through comparison with numerical results obtained by the single model analysis using only discrete-layer elements.
A broad class of structural problems has been recognized as being non-conducive to the use of conventional state-of-the-art structural analysis techniques involving the finite element method. This class of problems includes long fibre reinforced polymer structural members having non-uniform, continuously variable material properties and cross-sectional geometry. A novel composite leaf spring is being developed (Thunder Composite Technologies, Ltd.), which will likely prove to be superior to the conventional leaf springs that it aims to replace. However, this new spring belongs to the aforementioned class of problems and will therefore necessitate unconventional and highly complex design and analysis techniques due to the anisotropy and non-homogeneity of its constituents. As such, a design and analysis software tool was developed to be capable of collecting user stipulated parameters and performance specifications, generating a design that is capable of meeting these requirements and performing a high-fidelity structural analysis on the resulting design in order to verify its performance. The following paper summarizes the engineering science and programming methodologies employed by this design tool and discusses how similar methodologies could be employed in the design and analysis of other structural members that reside within this class of problems.
In this study, microwave-dried oil palm trunk core lumber was impregnated with phenol formaldehyde resin using high pressure vacuum chamber. The impregnation of oil palm trunk core lumber was performed under 3 bar pressure and cured in an oven at 150°C for 2 h. The impregnation of oil palm trunk core lumber was carried out at different time intervals (15, 30, 60, 90, 120 min) to obtain different density lumber and compared with microwave-dried oil palm trunk core lumber and rubberwood. The physical and thermal properties of microwave-dried oil palm trunk core lumber, impregnated oil palm trunk core lumber and rubberwood were studied. In general, the impregnated oil palm trunk core lumber obtained better physical properties than microwave dried oil palm trunk core lumber but slightly lower than rubberwood. The thermal stability of oil palm trunk core lumber was analyzed by using thermogravimetric analysis and it shows that rubberwood exhibited better thermal stability than impregnated oil palm trunk core lumber.
Wood fiber/poly(lactic acid) composites with 70 wt% wood fiber were prepared using a super mixing-hot press method. The biodegradability of composite panels made with one unmodified and three types of modified poly(lactic acid) was evaluated by comparing the mechanical strength, molecular weight, thermal properties and microstructures of composite wood fiber/poly(lactic acid) and 100% poly(lactic acid) panels before and after burial in soil. We found that wood fiber/poly(lactic acid) composites degraded more readily than 100% poly(lactic acid) panels during burial. The type of poly(lactic acid) used in wood fiber/poly(lactic acid) composites affected biodegradability. The flexural strength and modulus of wood fiber/poly(lactic acid) panels decreased sharply after burial in soil for 6 months. The molecular weight of poly(lactic acid) in wood fiber/poly(lactic acid) composites decreased more rapidly than 100% poly(lactic acid) panels that had been buried in soil for 6 months. Changes were observed in both the amorphous and crystalline structure of specimens buried in soil. The wood fiber/poly(lactic acid) composites prepared with unmodified poly(lactic acid) have better mechanical properties and degraded more easily during burial than did composites made with modified poly(lactic acid).
Drilling composite materials is one of the secondary processes of manufacturing industrial structures. However, drilling composite materials presents a number of problems such as degradation of mechanical behavior. In this study, effects of spindle speed, feed rate, and drill point geometry on residual tensile strength are studied. Acoustic emission technique with a wavelet-based signal processing method is developed to monitor the residual tensile strength of drilled laminates. Cumulative count, amplitude, and energy are used as time-domain parameters to characterize the process. According to wavelet analysis, frequency distribution and energy percentage of each damage mechanism (matrix cracking, fiber breakage, and fiber slipping) during tensile test are determined.
The objective of this research is to investigate the effect of long-term exposure of plain woven 240-D S2 glass epoxy composite laminate to dry, moist and various temperature cycles. Interlaminar shear stress was evaluated to assess the delamination tolerance on both virgin (preexposed) and harsh environment-exposed composites specimens. Delamination tests were performed with the pattern of four-point bending and tensile and compression shear tests under different combinations of humidity and temperature exposure, ranging from zero to 32 weeks. During the tests, the stress at the onset of delamination was taken as the first deviation of the load-displacement curve. Experimental study revealed that the delamination load-carrying capability reduced to 40% with the exposure time. Throughout the aging process dimensional stability was almost unchanged; however, 1.29% moisture absorption was noticeable in laminated composites. Microstructures of the delaminated surface revealed that failure occurs suddenly in a macroscopically fragile mode by crack initiation and proliferation. The delamination mechanism involved interlaminar processing flaws that favored the initiation and propagation of the interlaminar cracks, which are responsible for the delamination of the composite. As load carrying capacity dwindles substantially, therefore the effect of temperature and moisture must be taken into account in the experimental characterization of the laminates when establishing a design limit for composite structure.
This paper proposes a novel pseudo-elastic model for polymer-bonded explosive considering the Mullins effect for isotropic, incompressible, hyperelastic, particle-filled materials. Polymer-bonded explosive, an energetic material in which small explosive crystals are bonded in a polymer matrix, is known to exhibit highly nonlinear behaviors of deformation such as the Mullins effect of stress softening, hysteresis, residual strain, and frequency-dependent responses. The Ogden-Roxburgh model is modified for the unloading state to describe the Mullins effect accurately, which is the most important unloading behavior of polymer-bonded explosive. Uniaxial compressive loading and unloading tests at quasi-static states were undertaken to obtain the mechanical properties of the polymer-bonded explosive simulants. The pseudo-elastic model by Ogden and Roxburgh is subsequently modified for consistency with the test results of the polymer-bonded explosive simulants for the case in which the Mullins effect is dominant. The predictions from the new model exhibit good agreement with the experimental data, demonstrating that the model properly describes the Mullins effect and the loading-unloading behavior of deformation.
The objective of this paper is to study the influence of thickness of individual elastomer layers (first shape factor) on the vertical and horizontal responses of fiber reinforced elastomeric bearings that are not bonded or mechanically fastened to their top and bottom supports. By definition, the first shape factor, S1, is the ratio of the plan area to the perimeter stress-free area of a single elastomer layer within the bearing. Results of a comprehensive 2D-finite element study on a large group of bearings with different shape factors of 10, 15, 20, 30, and 40 suggest that the shape factor S1 is a critical parameter in controlling both the vertical compression modulus of the bearing and the level of stress in the fiber-reinforcing layers. Furthermore, the secant horizontal stiffness and the stress demand in the bearing elastomer material are to a limited extent influenced by the parameter S1.
The tensile test of a multidirectional laminate of strip geometry is analysed assuming that the clamping system imposes elastic restrictions to the rotations at both ends. The problem has five redundant unknowns that are determined by applying Engesser’s second theorem. Thus, the stress field can be obtained including clamping effects. The displacement field is determined by applying Engesser’s first theorem in conjunction with the unit load method. Hygrothermal effects are also included in the analysis. A method for determining the end compliances based on longitudinal strain and out-of-plane displacement measurements is proposed. Experiments are simulated by numerical results corresponding to AS4/3501-6 carbon/epoxy composite. Finally, two examples concerning multidirectional laminates with general coupling are analysed.
This article presents relationships for the through-thickness elastic constants of composite laminates, needed for modelling three-dimensional stress problems. Using the individual ply elastic constants together with the in-plane laminate elastic constants (from laminate theory) results in explicit relationships for, 13, 23, E3, G13 and G23. The Poisson’s ratio and Young’s modulus expressions apply to all laminate types. The shear modulus relationships apply to balanced laminates, but are extendable to other laminate types. Relationships are developed here for angle-ply, cross-ply and quasi-isotropic laminates. Results are presented for glass/epoxy and carbon/epoxy laminates, above and below the resin Tg. The importance of coupling stresses and their influence on 13, 23 and E3 is underlined. It is shown, for instance, that equating the laminate through-thickness modulus to the individual ply value can result in a significant under-estimate.
Composites made of polypropylene, a mixture of polypropylene and poly(lactic acid) and spruce wood fibres both non-modified and modified with toluene 2,4-diisocyanate – were prepared by melt blending. The chemical modifications of wood fibres with toluene 2,4-diisocyanate were evidenced by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Wood fibre reinforced polypropylene/poly(lactic acid) composites prepared with addition of maleic anhydride polypropylene as coupling agent were further investigated for structural and morphology properties before and under controlled accelerated weathering conditions by attenuated total reflectance Fourier transform infrared spectroscopy, mechanical testing and scanning electron microscopy analysis. Some specific indexes (carbonyl and vinyl) were also calculated.
The AC electrical conduction and thermal conductivity of casted thin films of poly (ethylene oxide)/carbon black composites were investigated as a function of applied frequency in the range from 100 Hz to 5 MHz, temperature and carbon black concentrations 0, 1, 2, 4, 6, 8 and 10% by weight. The average films thickness was about 90 µm. The AC conductivity and dielectric properties were determined from impedance measurements. The study showed that the thermoelectrical properties are affected by adding carbon black filler into the poly (ethylene oxide) polymer matrix. The study observed that the dielectric constant and dielectric loss decrease with frequency, and increase with temperature, and carbon content. The AC-conductivity increases with increasing frequency, temperature and carbon black content. The observed percolation threshold of the AC-conductivity occurs at about 2 wt% carbon black concentration. The thermal conductivity of the prepared films was studied as a function of temperature and CB concentration. It was found that the thermal conductivity is enhanced by addition of the carbon black content and increasing temperature. In case of raising the temperature, the phonons are activated and electrons hopping to higher energy states, thus enhancement in the thermal conductivity is produced.
In this study, epoxy/clay nanocomposites with different content of nanoclay were prepared by shear mixing followed by ultrasonication. In order to study electron beam irradiation effects, nanocomposites were exposed to electron beam irradiation in 100, 500 and 1000 KGy doses. X-ray diffraction and transmission electron microscopy were used to study the morphology of nanocomposites. The mechanical and thermal behaviors of nanocomposites were studied by tensile test and thermo-gravimetric analysis. The results showed that the irradiation brought about improvement in mechanical properties in most specimens and the most improvement was achieved in 100 KGy irradiation doses.
This paper investigated the effect of porosities, environmental factors and impact energies on the impact resistance properties of Carbon Fiber Reinforced Polymer/Plastic (CFRP) laminates. Impact tests on the CFRP laminates with three porosity levels were subjected to five energy levels from 3 J to 15 J at the room temperature. The damage area was assessed with combined effects of different impact energies and porosity levels. The damage area was evaluated by visual inspection and ultrasonic C-scan measurement. The thermal deply technique was used to evaluate the damage evolution behavior. Three-point bending tests were conducted on the non-impacted and impacted specimens that were exposed to the room temperature, the hygrothermal and the drying environments. The curves of the impact resistance of the tested laminates mutate at the impact energy of 9 J. The thermal deply technique reveals the failure mechanism of the impact damage mutation of the tested laminates when the impact energy exceeds the threshold 9 J.
This paper examines the application of a cohesive zone model to predict the open hole compressive strength of an IM7/8552 carbon fibre/epoxy quasi-isotropic multidirectional laminate and investigates the level-ply scaling or ply blocking effect on notch sensitivity. Cohesive zone models have been successfully applied to predict the damage from notches in engineering materials loaded in tension. They have also been used to determine the growth of fibre microbuckling from a hole in composite laminates under compression. The usual strategy is to replace the inelastic deformation associated with plasticity or microbuckling with a line-crack and to assume some form of stress-displacement ( - ) bridging law across the crack faces. Here a plastic fibre kinking analysis and a linear reduced (softening) - relationship are used for the prediction of the unnotched and open hole compressive strength; the theoretical results will be compared with experimental data in Part B of the 3rd World-Wide Failure Exercise.
Multiwall carbon nanotube composites with epoxy matrix were prepared by sonication followed by long curing. The electrical conductivity of the composite samples was measured and found to follow percolation behavior with low threshold mass fraction of multiwall carbon nanotubes and smaller value of critical exponent. The lower values of threshold and exponent are associated with higher purity and better interfacial contact between the multiwall carbon nanotubes and with the epoxy. The temperature dependence of electrical conductivity showed that both variable range hopping and fluctuation-induced tunneling mechanisms are followed. The dependence of conductivity on the width of the epoxy barrier film between multiwall carbon nanotubes is used to establish that fluctuation-induced tunneling through interfacial regions is the appropriate mechanism of conduction in the composites. The activation energy for viscous deformation of the composites was evaluated using dynamic mechanical analysis and found to increase for higher mass fraction of multiwall carbon nanotubes in the composite. These results indicate that interfacial interaction is significant in the molecular movement in the epoxy.
A multiscale numerical approach is established to model damage in random glass fiber composites. A representative volume element of a random glass fiber composite is employed to analyze microscale damage mechanisms, such as matrix cracking and fiber-matrix interfacial debonding, while the associated damage variables are defined and applied in a mesoscale stiffness reduction law. The macroscopic response of the homogenized mesoscale damage model is investigated using finite element analysis and validated through experiments. A case study of a random glass fiber composite plate containing a central hole subjected to tensile loading is performed to illustrate the applicability of the multiscale damage model.
This study, for the first time investigates the applicability of basalt fiber as a reinforcing material for metal matrix composites through various experimental works for thermal stability and mechanical properties. The residual tensile strength values of basalt fibers after being exposed at different temperatures in a furnace for pre-determined times and/or after being immersed into molten aluminum for different lengths of time were evaluated. Throughout these experimental studies, a processing method for fabrication of these composites was optimized. In this method, basalt preforms were coated with a thin layer of aluminum by immersion into aluminum melt for a short period of time. These laminates were stacked in a mold and consolidated by hot pressing (300°C, 7 min and 630 MPa). The microstructural studies confirmed a good bonding between aluminum and basalt together with a reasonably uniform distribution of fibers within the matrix alloy. Scanning electron microscopy studies revealed the fractured basalt fibers in the matrix alloy as well as occasional improper infiltration of the matrix alloy into the fiber bundles. Consequently, the mechanical properties of aluminum/basalt composites were far inferior to those expected by law of mixtures predictions. However, the strength values of these newly developed metal matrix composites are still adequate for some engineering applications.
Poly(vinyl alcohol)/montmorillonite/silver hybrid nanoparticles are successfully fabricated by one-step electrospraying process in aqueous solution. The aim of this project is to design an optimum solution parameter for electrospraying system and evaluate inorganic material effects to antibacterial performance and thermal properties. Hybrid nanoparticles could be obtained at low molecular weight and concentration of the polymer. Transmission electron microscopy analysis showed montmorillonite and silver were well dispersed in poly(vinyl alcohol) nanoparticles and enhanced thermal properties of the composite material. In anti-bacterial test, the poly(vinyl alcohol)/montmorillonite/silver hybrid nanoparticles showed an excellent anti-bacterial performance.
In this paper, a traction-separation-based cohesive modeling approach is proposed to predict the effect of z-pinning on laminated composites. A detailed experimental characterization of the z-pin pullout process using the flatwise tension test is presented. Utilizing these flatwise tension results, numerical simulation of the progressive damage due to delaminations in a double cantilever beam with z-pinning has been performed. Experimental details of the z-pinned double cantilever beams are presented for IM7/977-3 graphite/epoxy. The approach taken in this study utilizing the cohesive elements within the Abaqus® finite element software has proven that the models can predict the behavior of z-pinned composites close to experimental observations. It was found that the discretization of the fracture resistance curve along the z-pin field is essential to capture the dynamics of the delamination accurately.
Kenaf fibre reinforced polyester biocomposites fabricated by hand lay-up technique by using propionic and succinic anhydride-modified kenaf fibres. Chemical structure, mechanical, thermal and morphological properties of kenaf fibres reinforced polyester biocomposites evaluated. The Fourier transform infrared study of modified kenaf fibres carried out to look at changes in functional groups after modification. It confirmed from Fourier transform infrared spectroscopy the variation in positions of functional groups. The mechanical (tensile, flexural and impact) property results revealed that biocomposites with modified kenaf fibres exhibited better properties as compared to untreated kenaf fibres-reinforced polyester biocomposites. Morphological studies show that treated kenaf biocomposites show better fibre/matrix interaction. Thermal analysis results of modified biocomposites exhibited higher initial and final decomposition temperatures. Modified biocomposites display less char residue as compared with unmodified kenaf fibres reinforced polyester composites.
This study presents an evaluation of the physical and mechanical properties of medium density fibreboards produced from mixture of rubberwood from RRIM 2020 clone and empty fruit bunch fibres. The evaluations are conducted to determine variance in density, moisture content, modulus of rupture, modulus of elasticity, internal bonding, thickness swelling and water adsorption for these mixtures. The significance level of p < 0.05 was used to determine the variations between different ratios of rubberwood and empty fruit bunch fibre blends effect on all properties studied. Most properties of medium density fibreboard panel made with 100% rubberwood fibres are significantly better than medium density fibreboard made with 100% empty fruit bunch fibres. Additional of 30% of empty fruit bunch in mixture had decreased modulus of elasticity by 148 MPa, modulus of rupture by 1.21 MPa and water adsorption by 0.05% compared to 100% rubberwood. Generally, among mixtures, panels containing less than 50% oil palm empty fruit bunch in the mixture exhibits better strength properties but lower physical properties. The dimensional stability (thickness swelling) of the panel made from the mixture of 70% rubberwood and 30% empty fruit bunch board displays the best performance among the mixture.
Temperature-dependent modulus of glass fiber/epoxy composite laminates was studied at temperatures ranging from room temperature up to 120°C. The storage modulus, loss modulus, loss factor, and glass transition temperature of two layer-up composite laminates were investigated by dynamic mechanic analysis. Static flexural modulus was also measured by control force mode in dynamic mechanic analysis. A new and simple temperature-dependent model both for the dynamic storage modulus and static flexural modulus was developed. This model depends only on one parameter which has specific physical meaning. The model prediction showed excellent agreement with our own experimental results over the full range of transition region. Furthermore, comparing our model’s prediction and other experimental data of many types of composites, which were studied by other researchers, we point to the fact that our model can be applied to many different polymer matrix composites.
The present study investigates the effect of stacking sequence on the failure loads (strength) and modes of pinned-joints glass-fiber reinforced epoxy composite laminates. Specimens with [0/90]2S, [15/-75]2S, [30/-60]2S and [45/-45]2S stacking sequences were investigated both experimentally and numerically. A series of ASTM tests were performed on unidirectional [8]0 glass-fiber reinforced epoxy composite laminate to determine the properties of the single lamina that was needed for the finite element analysis. A 3D progressive damage model was built with the aid of ABAQUS software, failure criteria and property degradation rules to simulate the problem. The results showed that the [0/90]2S laminate has the highest ultimate strength. The minimum bearing and ultimate strength was observed for [30/-60]2S laminate. Loading the specimens up to the ultimate value lead to shear-out failure mode for [0/90]2S, [15/-75]2S and [30/-60]2S stacking sequences, while specimens with [45/-45]2S stacking sequence are characterized by bearing failure mode. The experimental and numerical results agree well with a maximum Euclidean error norm of 8.57%.
This article reports an experimental investigation on failure responses of single lap double serial fastener joints in glass fiber/epoxy composite laminates when subjected to low-temperature environment. The results of experiments, implemented at five different low-temperature levels ranging from 0°C to -40°C, were evaluated in comparison with room temperature tests. Joints exhibited relatively higher load-carrying capacities with increased stiffness by decreasing temperature. In order to examine tightening torque effects at each temperature condition, bolts were fastened under M = 6 Nm and M = 0 Nm (finger tightened) torques. As expected, a greater amount of bearing load could be carried by the joints with pre-tightened fasteners. Furthermore, any reduction in temperature is observed to lift the effectiveness of tightening torque on the joint strength. Regardless of the temperature exposed, bearing mode, the most desirable failure type in mechanically fastened joints was monitored as the main failure mode.
In this study, fatigue behavior of polypropylene fiber reinforced concrete has been studied under constant and variable amplitude loading. Crack length was measured during flexural fatigue test under three loads. Accordingly, damage curves were determined as function of stress levels. The results ascertained that the presence of 1 wt% polypropylene fibers considerably increases resistance against fatigue crack growth under constant amplitude loading. Under variable amplitude loading, it was found that the damage curve approach predicts the fatigue life with 13–15.8% estimation error, whereas linear model predicts the fatigue life with 36.8–56.5% estimation error.
This paper presents an experimental study of the quasi-static and fatigue behaviour of a three-dimensional braided carbon/epoxy composite. The study involves a three-dimensional braided carbon preform and composite samples produced at 3TEX Inc. on their 576-carrier rotary braiding machine. The first part of the paper describes the preform and composite sample fabrication procedures, the fiber volume fraction determination and the porosity evaluation using micro-computed tomography three-dimensional observation. The second part is devoted to experimental study of the quasi-static tensile response of the material, including the acoustic emission monitoring and the microscopic damage detection. The third part is dedicated to the fatigue tensile–tensile behaviour illustrated by the fatigue life curve and the micro-computed tomography images of the damage imparted after different number of cycles. The fourth part presents results of the quasi-static tensile tests of preliminarily cyclically loaded specimens. These results provide significant insight into the influence of the damage imparted during fatigue loading on the subsequent quasi-static behaviour.
In this article, alumina nanoparticles were co-deposited within Ni–P electroless coating on mild steel samples and then isothermal heat treatment was done at 400°C for 1 h. The size distribution of nanoparticles was evaluated by transmission electron microscopy and the concentration of alumina reinforcements in Ni–P matrix was determined using field emission scanning electron microscopy and image analysis software. The phase transformation of coatings was analysed by X-ray diffraction and differential thermal analysis. Also, mechanical properties of coatings were evaluated by microhardness and indentation tests. The results showed that mechanical properties of Ni–P–Al2O3 nanocomposite coatings strongly influences by dispersion and/or precipitation hardening mechanisms.
This study addresses the issue of structural damage identification and location in carbon fiber reinforced polymer plates using electrical measurements. Electrical resistance tomography is presented as a method for structural damage localization in composite parts. A set of electrodes is fixed on the edges of the part and combinations of DC current injections and voltage measurements are applied to the system. The change of voltage between different times in the part’s service life (e.g. start and degraded) are monitored. These sets of measurements are used as input to inversely calculate conductivity maps for the complete composite part and thus indirectly assess its structural health. Such processes are inherently ill-posed. Data post-processing approaches are proposed here to diminish this uncertainty and to conclude to an optimally converge solution of the inverse problem. To assist the process, a material-originating mathematical constraint is introduced. The method is applied on carbon fiber reinforced polymer plates for different damage modes. Experimental recordings show that the analysis of electrical fields allows detecting the presence of damage. Discontinuities as small as 0.1% of the inspected area can be sensed. The proposed data post-processing techniques were applied and conductivity maps were calculated. The results show that using these techniques locating damage is possible with less than 10% error. Material-based constraints greatly enhance the prediction of the data post-processing techniques. It is believed that by overcoming certain implementation issues, electrical resistance tomography could evolve in the direction of a non-destructive evaluation or a structural health monitoring technique for composite structures.
The morphological and physical properties of poly(lactic acid) composites reinforced by woven hemp fabrics pre-treated with enzyme and ammonia were investigated in this study. During the preparation procedure of the poly(lactic acid)/hemp composites, enzyme and ammonia treatments were applied to the surface of woven hemp fabrics in order to improve the interfacial adhesion between hemp fibers and poly(lactic acid) resin. The mechanical and morphological properties of poly(lactic acid)/hemp composites were examined via Instron measurement and scanning electron microscopic observation. Furthermore, the thermal shrinkage and flame-retardant properties of the composites were analyzed to understand the effect of the pre-treatment.
Braided composite actuators are pressure-driven muscle-like actuators capable of large displacements as well as large blocking forces. Braided composite actuators can also exhibit a large change in effective stiffness through simple valve control when the working fluid has a high bulk modulus. This is due to the stiff fiber reinforcement of the braided sleeve and the high bulk modulus of the fluid resisting the volume change when a load is applied in the closed-valve condition. Several analytical models have been previously developed that capture the geometrical and material nonlinearities of the composite actuator, the compliance of the inner liner, and entrapped air in the fluid. This article focuses on inter yarn compaction in the fiber sleeve, which is shown to reduce the effective closed-valve stiffness. In this article, a new analytical model that considers inter fiber yarn compaction, fiber extension and entrapped air effect as well as the material and geometric nonlinearities is developed. Analysis and experimental results demonstrate that the new compaction model can improve the prediction of the response behavior of the actuator.
In this article, the AlMgB14–carbon Nanotubes (CNT) composites were synthesized by means of the mechanical alloying (MA) and field activated and pressure assisted in-situ synthesis (FAPAS). The microstructure and component of the synthesized metallic ceramics were observed and determined using the X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM). Further investigations on the mechanical properties of the composites showed that adding CNTs could not only increase the hardness of AlMgB14, but also improve the fracture toughness. It was found that the addition of 0.5% CNTs could achieve the best reinforcing and toughening effect. The microstructure analysis on the composites with an addition of 0.5% CNTs revealed the mechanism for such effects, i.e., the addition of CNTs could refine the grain, promote the formation of network microstructure, and confine the crack deflection along the interface between the CNTs and AlMgB14. Furthermore, the high quality, uniform dispersion, integrity and compatibility with matrix of the CNTs were the main factors that improved the fracture toughness of AlMgB14-CNTs composites dramatically.
The proposed research aims to manufacture, fabricate and evaluate the 5-layer ‘through-the-thickness’ angle-interlock composite structure from Kevlar. Results show that composites developed from angle-interlock fabric, exhibit different and far better tensile properties in the weft direction as compared to the warp direction. It is much harder to break the fibres in the weft direction than in the warp direction for this particular construction of Kevlar-based composite. This behaviour of composites can lead to better products while developing advanced textile-based composite structures.
TiO2 and TiO2/SiC composites are synthesized through chemical route followed by solid-state sintering at 1450°C. The X-ray diffraction pattern shows the peaks of TiO2 and SiC for the compositional ratio up to 60:40 whereas no peak corresponding to TiO2 has been observed for the SiC percentage of 50 and 60. It is confirmed from the micro-Raman and Fourier transform infrared spectra that TiO2 has got diffused into the SiC surfaces and interfaces in the composites having higher percentages of SiC. Maximum hardness has been observed for the TiO2 and SiC composition of 50:50. No optical transition has been observed for the composites with TiO2: SiC ratio equal to or below 1.5. The increase in absorption percentages with increasing concentration of SiC indicates that these composites can be utilized as photo-catalytic active material in visible light. The electrical resistivity measurements clearly show the increase in conductivity with increase in SiC percentage up to 50% in TiO2/SiC composites. All these TiO2/SiC composites are found to have carrier concentration in the order of 1016–1018/cm3 at room temperature and hence can be proven as good ceramic semiconductors. It is observed that the insulating nature of TiO2 gets transformed to the semiconducting nature by the addition of SiC. The electrical, mechanical and optical properties are also found to be strongly dependent on the SiC percentages.
This article concerns the effect of pristine, carboxylic and amino-functionalised carbon nanotubes used as filler in epoxy-based nanocomposites. The amount of carbon nanotubes was within the range 0.2–0.8 wt%. Their mechanical properties were investigated by means of flexural strength and resilience tests. The carbon nanotubes lead to an improvement of ductility and mechanical strength compared to the neat epoxy resin in the order: amino > carboxylic > pristine. The results of morphological, calorimetric, rheological and electrical conductivity tests suggest that this improvement is due to a good dispersion of the filler in the matrix and it occurs especially with low filler amounts (0.2–0.4 wt%) of amino carbon nanotubes. In the nanocomposites realised with amino-functionalised carbon nanotubes there is an interphase that creates a weak interfacial interaction between the filler and the epoxy resin. The thermal stability as well as the electrical conductivity of resin, is not appreciably improved after the addition of either functionalised carbon nanotubes.
Naturally woven coconut sheath, a new natural fiber available in the form of woven mat is used as fiber reinforcement in polyester matrix. The hybridization effect of montmorillonite (MMT) on naturally woven coconut sheath/polyester composite has been investigated experimentally for free vibration characteristics using impulse excitation technique (IET). In the present study, naturally woven coconut sheath/clay-reinforced hybrid polyester composites were fabricated using a compression molding machine. The effect of organoclay addition (1, 2, 3 and 5 wt%), alkali (ATC) and silane treatment (STC) of the coconut sheath surface on free vibration characteristics were analyzed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques were used to characterize the morphology of coconut sheath and structure of clay dispersed polyester. The dynamic mechanical analysis (DMA) and Fourier transform infrared (FTIR) analysis were carried out to analyze the effect of surface topology on chemically treated coconut sheath. The mechanism of chemical modifications on natural frequencies and the resultant improvement in the temperature dependence mechanical properties have also been reported. Enhanced vibration properties are observed up to 3 wt% of clay dispersion in hybrid composites, then it gets reduced for higher clay (>3 wt%) content. Modal damping of the hybrid composite is influenced by the addition of nanoclay and the surface treatments of the coconut sheath. Experimentally obtained natural frequencies are in good agreement with analytical and numerical results.
This article reports the results of a study related to the reinforcement of mortar-based composites with newly developed polyethylene/polypropylene blended fibers. The study showed the hybrid fibers to have a mechanical/reinforcing effect in the cement-based matrix. Mechanical strengths improvements, as high as 40 ± 2%, were obtained for composites with a fiber length of 24 mm and a fiber volume fraction of 2.9%.
The mechanical enhancement was associated with a strong mechanical interlocking between the fibers/cement matrix interfaces promoted by the fibers’ surface ridges and fibrillation ability. Macro/microstructure and morphological observations showed a post-cracking ductility improvement of composites by the hybrid (bridge effect) fibers, also evidenced a multi-cracking mechanism.
An unsymmetric laminated composite having bi-stability is interesting as a morphing structure because it can sustain large deformation without energy supply and the deformed state is stable. Especially, inducing different curvature values at each stable state is important for the practical application of unsymmetric laminates. In this study, we propose a method for tailoring the curvature of unsymmetric cross-ply laminates by curing the composite on a curved tool plate. The results show that the final normal curvature tensor of unsymmetric laminates is the summation of the tool plate curvature tensor and the normal curvature tensor of the laminates.
Uniform and bead-free polyvinyl alcohol (PVA) and its nanocomposite fibers filled with different loadings of CdSe@ZnS quantum dots (QDs) were prepared by the electrospinning process. The incorporation of QDs into PVA solution lowered the viscosity of the system. The electrospinning parameters, including applied voltage, feed rate, and working distance were optimized to prepare high quality nanocomposite fibers. Increasing the voltage from 20 to 25 kV, the average diameter of PVA fibers reduced from ca. 284 to 164 nm (42.5% decrease) and the average diameter of PVA/QDs nanocomposite fibers decreased from ca. 365 to 240 nm (34.2% decrease). The PVA/QDs (5.0 wt%) nanocomposite fibers were examined under a fluorescent microscopy. The thermal stability was investigated by both thermogravimetric analysis and differential scanning calorimetry. Fourier transform infrared spectroscopy was utilized to characterize the functionality of the fibers and to investigate the interaction between PVA and inorganic additions. Unique fluorescent phenomenon was observed in the PVA fibers after incorporation of small amount of QDs.
Materials possessing both high stiffness and high damping would be beneficial in many structural applications. Composites that combine a stiff material, which usually has low damping, with a soft and lossy material have been proposed to engender both high dynamic modulus and high loss factor. In this article, we investigate the effective dynamic moduli and loss factors of Reuss and Voigt composites in response to a uniaxial harmonic load. The constituent materials are characterized by multiaxial viscoelastic models in the frequency domain. Using the viscoelastic correspondence principle, we derive formulae for Reuss and Voigt composites of infinite dimensions taking into account Poisson effects. We show that the effective loss factor of a Reuss composite is sensitive to the values of the Poisson's ratio and bulk loss factors of the constituent materials. Finally we simulate, using finite element analysis, the response of cylindrical Reuss composite rods of finite radius to an axial load. We demonstrate that the effective dynamic properties of these rods is highly sensitive to the ratio of the radius to the layer thickness of the composite.
This article presents an easy and inexpensive method to elaborate superhydrophobic surfaces. A superhydrophobic surface was prepared by spray coating a mixture of calcium carbonate particles, stearic acid and polymer latex suspensions on an aluminum substrate. The Taguchi L18 orthogonal array was used to optimize the fabrication process parameters, namely the percentage of stearic acid, the calcium carbonate particles to copolymer weight ratio, and the spray distance from the substrate. Through the Taguchi method, it was found that the percentage of stearic acid plays the most significant role in affecting the coating’s wettability. The optimal condition proposed by this method has been verified through additional experiments which showed an increase in contact angle up to 158°.
The permeability of the fabric preform is a critical input parameter for analyzing the liquid composite molding impregnation process. However, the permeability prediction is challenging due to its complex dependence on the fabric structure. In this paper, a novel analytical model is developed to predict the permeability of non-crimp fabric preform based on the relation between the pressure drop and geometric parameters of the microchannel cross section. The model takes into account four structural parameters including the width, the height, the semi-major axis length of the ellipse section of fiber bundle and the distance between fiber bundles. The permeability of the unit cell is calculated by the presented analytical model and the finite element simulation, respectively. The results show that the channels between fiber bundles play an important role in determining the fabric permeability. The structural parameters of the unit cell have important effects on the permeability. The new structure-related permeability model can accurately predict the permeability of the non-crimp fabric preform in a certain range.
In this study, experimental bond investigations through pullout and split tensile testing were carried out for the possible application of embedded fiber reinforced polymer laminates/plates as internal reinforcements as an alternative form of reinforcement to fiber reinforced polymer rods. The effect of different surface treatments and two concrete compressive strengths on the bond behavior are the main parameters investigated. Results showed that the bond strength of the embedded carbon fiber reinforced polymer plates increased as the concrete compressive strength increased and that the surface condition of the embedded carbon fiber reinforced polymer plate enhanced the bond behavior. An in-depth visual examination of the test specimens after failure indicated that bond failure occurred in both the concrete and at the resin layer of the embedded carbon fiber reinforced polymer plate. In addition, the tensile bond strength of the concrete-fiber reinforced polymer interface for different surface conditions is determined. The failure mode and bond strengths of embedded carbon fiber reinforced polymer plates was found to depend on the relative shear strengths of the concrete and resin layer interfaces.
An acoustic emission technique is used to quantify and position microfracture events ahead of a growing opening mode crack in paper materials containing different amounts of added starch. A mechanical model based on gradient-enhanced elasticity, containing an intrinsic length parameter reflecting the fibre-based materials microstructure, is applied to analyse the results. It is found in experiments that the addition of starch increases the tensile strength of paper significantly while the level of onset of microfracture nucleation at the crack-tip is only slightly increased. It is also found that the height of the process zone (zone in which microfractures ahead of the crack predominantly take place), measured from the crack plane, decreases with increasing amount of starch. The experimental and analytical results suggest that adding cationic starch to paper reduces the material’s sensitivity to gradients in the stress and strain fields and making the fibre network material more ‘continuum-like’. The experimental observations are shown to be qualitatively in agreement with the numerical results and lend confidence to the applied model.
This work provides techniques to separately determine temperature and duration of a thermal pre-load on a polymer matrix composite with a focus on long-term load (max. 400 days) without massive polymer degradation. The aim is a non-destructive, rapid, robust and precise in-service method to characterize incipient heat damage. A commercially available composite 8552/IM7 is investigated. Infrared spectroscopy of the surface and bulk material traces thermal degradation of the polymer. A multivariate (chemometric) data analysis was performed. The reliability of the calculated values for time (±19 d) and temperature (± 12°C) is increased by including other parameters with various degradation velocities such as mass loss of the composite and color changes and binder degradation of a typical top coat. The residual strength of a composite with unknown thermal history can be predicted and thermal loads such as heating in an oven and hot air are compared.
A series of polyimide composites with various mass fractions of multi-walled carbon nanotubes (MWNTs) were prepared by in situ polymerization. To increase the chemical compatibility of carbon nanotubes with the polyimide matrix, MWNTs were treated with an acid mixture and sulfoxide chloride in turn. The modified MWNTs are dispersed homogeneously in the matrix while the structure of the PI and MWNTs are stable in the preparation process. The composite films hold preferable thermal stability the same as the pure PI. The composite films exhibited good thermomechanical properties. The storage modulus increased significantly by increasing MWNT content and decreasing the enhancement of temperature. The films’ glass transition temperature increased with enhancing of MWNTs frictions. The dielectric constants of the composites decrease with increasing frequency and increase sharply with the adding of MWNTs, which is favorable for practical use in anti-static materials and embedded capacitors.
A new mathematical model and its analytical solution for the analysis of the stress–strain state of a linear elastic beam cracked in flexure and strengthened with plates on its lateral sides is presented. Both the longitudinal and the transversal interactions at the side plate/beam interface are considered. Linear behaviour of the contact connection is assumed. The method is based upon the linearised planar beam theory of Reissner. The weakening of the beam induced by the flexural crack is modelled conventionally as a rotational spring. The suitability of the theory is demonstrated in a case presentation involving the comparison between analytical results of the present beam (one-dimensional) model, the experiments and the numerical results of a full three-dimensional solid model created in the LUSAS finite element analysis software. An excellent agreement between the results is observed and the proposed formulation is found to be accurate and reliable. Finally, the solution is employed in an engineering analysis, discussing the effects of the material and the geometric properties of selected characteristic cases of the observed beams on the static and kinematic quantities, including the boundary conditions of the side plates, the longitudinal and the transversal stiffness of the connection, the size of the cracks, the span of the beam, and the length and the stiffness of the side plates. For the cracked cantilever beam, a substantial effect of any of these parameters is found. In contrast, for the cracked two-span continuous beam, only the effect of the stiffness of the side plates and the effect of the length of the beam spans are noticeable.
Structural composites based on unidirectional E-glass and epoxy have been fabricated through resin film infusion. Low weight fractions of carbon nanofibers are dispersed in epoxy resin using a solvent-assisted ultrasonication process. Rheological characterization of carbon nanofiber-filled epoxy revealed that viscosity, and in turn processing characteristics of the resin remain almost unaffected as compared to the pristine resin system at elevated temperature of composite processing. Glass transition temperature of epoxy showed a considerable improvement with carbon nanofibers. Local flow of the modified resin through the embedded fabric plies in the resin film infusion process made sure that a uniform distribution of nanoparticles is achieved throughout the composite. Compressive strength of hybrid composites showed over 40% increase while interlaminar shear strength improved by 33% with carbon nanofibers at a loading fraction as low as 0.5 wt%.
An improved shear-lag model is developed in this paper to study the effects of interface roughness on the mechanical properties of unidirectional fiber-reinforced polymer composites with a staggered structure, in which the roughness is incorporated by establishing equilibrium equations for the fiber platelets with varying thickness along its axial direction. The stress transfer and effective Young’s modulus of composites are mainly investigated due to the influence of fiber’s surface roughness. Since the polymer matrix can be chosen as thermoplastic or thermosetting materials, a uniformly interfacial shear stress distribution due to the frictional transfer along fiber/matrix interfaces and a non-uniformly one due to the elastic transfer are analyzed, respectively. It is found that when the surface roughness becomes larger, fibers in the former will carry more tensile loads, while the tensile loads keep almost invariant in fibers and the shear stress reduces in matrix in the latter. Moreover, the effective Young’s modulus of composites will be enhanced due to increasing fiber’s surface roughness. However, the enhancing effect will gradually reduce with an increasing aspect ratio of fibers. The results should be very useful for the design of novel fiber-reinforced polymer composites, especially for those that needed interfacial modifications in order to improve the interfacial adhesion, for example, carbon-fiber reinforced polymer composites.
Carbon fabric reinforced epoxy and carbon fabric reinforced epoxy containing different weight fraction of silane-treated fly ash cenospheres filled composites were cast, sectioned, and subjected to three-body abrasive wear tests for evaluating the abrasive wear behavior. Mechanical characterization was done and a comparison was made between the different samples. Abrasive wear tests were performed on a rubber wheel abrasion tester under different loads, abrading distances using quartz and silica sand as abrasives. The results showed that both unfilled carbon fabric reinforced epoxy and fly ash cenospheres filled carbon fabric reinforced epoxy composites exhibit differing magnitudes of wear volume loss, it being highest for unfilled carbon fabric reinforced epoxy composite. The data trends point to the fact that the wear volume and specific wear rate decreases with increasing fly ash cenospheres loading in carbon fabric reinforced epoxy composites. It was found that silane-treated fly ash cenospheres could effectively reduce the wear rate especially under silica sand as abrasives. To explain these differences, the worn surfaces were examined using scanning electron microscope and the features thus observed were correlated with the selected mechanical properties.
Pulse-shaping techniques have been used for many years now in Kolsky bar testing of brittle materials. The use of pulse shapers allow the experimentalist to conduct high strain rate tests on brittle materials while ensuring that the sample will achieve a state of dynamic stress equilibrium before it fails, as well as to achieve a constant strain rate loading state for a large portion of the test. The process of choosing the appropriate pulse-shaper system has typically been one of trail-and-error, sometimes requiring many experimental trails to achieve optimal results. Advances in analytic modeling of Kolsky bar tests now make it possible, in an a priori fashion, to design a pulse-shaper system to produce a known constant strain rate experiment. This article describes the approach of coupling these analytic models to an optimization technique to quickly find a pulse-shaper system that will produce an experiment at a known constant strain rate. Experiments were conducted and the model predictions compared to resulting strain rate histories for a G10 material. Stress–strain curves for G10 are presented at three different strain rates in both the in-plane and out-of-plane loading configurations with respect to the laminate plys. The G10 material is not found to be rate sensitive in either its strength or failure properties.
The present work deals with the valorization of the lignin. The lignin is a by-product of the black liquor of the paper industry, which has a very complex composition structure. In this study, this lignin is obtained from the Alfa grass (Stipa Tenacissima L, also named Esparto grass). A composite material with polymer matrix (unsaturated polyester) reinforced with the lignin at various proportions has been elaborated and a comparison of its mechanical and physico-chemical characteristics to another type of composite material constituted of polymer matrix (unsaturated polyester) reinforced with Alfa fibers has been achieved. The characterization of these composites materials is based on tensile as well as thermal degradation tests under isothermal conditions. In order to explain the deviation from the linear profile (weight losses), a mathematical model has been used to show that the degradation energy is the same for all temperature ranges. This model allowed us to calculate the activation energy (48 kJ/mole.K), which corresponds to the process of off-gassing, break of macromolecular chains and weight loss.
In this research, the effects of crosslinking agent and clay content on the morphology, barrier and mechanical properties of ethylene vinyl acetate-organoclay nanocomposites prepared by solution method were studied. Dicumyl peroxide has been used as crosslinking agent. The morphology of the prepared nanocomposites was investigated using wide-angle X-ray diffraction and transmission electron microscopy. Wide-angle X-ray diffraction and transmission electron microscopyindicated that the prepared nanocomposites had predominantly intercalated morphologies. The obtained results of permeability tests showed that the permeability of ethylene vinyl acetate films dramatically decreases with addition of organoclay and dicumyl peroxide. Mechanical tests showed that tensile modulus and tensile strength of ethylene vinyl acetate increase with addition of organoclay. Furthermore, the mechanical properties of ethylene vinyl acetate nanocomposites significantly improved in presence of crosslinking agent (dicumyl peroxide).
Composites are being widely used because of their high strength and stiffness, low density and high formability for creating complex shapes. An all-composite joint is a structural connector known for its ability to reduce the weight and assembly cost while retaining a good load-carrying capability. Based on the material characteristics of unidirectional fiber composites used in composite joints, a modified maximum stress failure criterion, which is able to assess damage onset, propagation and final failure, is presented for unidirectional fiber composites. The stiffness and strength of four types of composite joints under tensile and bending loads are simulated by progressive damage models, involving a finite element analysis, failure criteria and a material degradation model. Numerical results from the application of this criterion in nonlinear element analysis show good agreement with experimental outcomes.
The broader goal of this research is to develop a commercially viable material system and manufacturing method to mass produce functional parts using selective laser sintering, a rapid manufacturing method, for electrostatic charge dissipation applications. The specific objective of this research is to produce and characterize polyamide 11/ nanographene platelets nanocomposites that have improved electrical conductivity for electrostatic charge dissipation applications and better thermal stability to be used in selective laser sintering manufacturing. Polyamide 11 and nanographene platelets were blended using industry size co-rotating twin-screw extrusion. Four batches were prepared containing 1 wt%, 3 wt%, 5 wt% and 7 wt% of nanographene platelets. Microsctrucre of nanocompoistes was studied using scanning electron microscopy. Thermal characterization of nanocomposites was conducted using thermogravimetric analysis at three heating rates 5, 20, 40°C/min. Electrical resistivity was measured using the Hioki Megaohmmeter Instrument four probe method. Mechanical characterization includes tensile, flexure, and Izod-impact properties. Flammability property was measured using UL94 test.
In this article, we investigate the strength characteristics of woven glass fiber reinforced polymer composite laminates subjected to tensile fatigue loading at cryogenic temperatures using the open hole specimens. Tension–tension fatigue tests were conducted on the open hole specimens of the woven glass fiber reinforced polymer laminates at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), and microscopic observations of damage around the hole were made on failed specimens. A numerical procedure based on the finite element method was then applied to evaluate the fatigue strength of the unnotched woven glass fiber reinforced polymer laminates using the experimentally applied load and the length of the hole edge damage zone. The obtained results were compared with the existing experimental data from the unnotched specimens. It was demonstrated that the presented combined numerical–experimental method was effective for the determination of the fatigue properties of the woven glass fiber reinforced polymer laminates at cryogenic temperatures.
In this study, the failure behaviors of composite plates with double-pin joints were investigated with experimental and numerical methods. The effects of joint angle, fiber orientation angle, and numerical modeling techniques on failure behavior were examined for composite plates consisting of epoxy matrix resin reinforced with woven glass fiber in four layers. The numerical study was performed in ANSYS 11.0 using two- and three-dimensional modeling techniques. Progressive failure analysis was performed by means of a subprogram. Tsai–Wu and Hashin failure criterion were used for two- and three-dimensional solutions, respectively. Material properties degradation rules were adopted for modeling the failure progress. As a result studies, a convergence ranging between 1% and 6% was obtained between the predicted and experimented failure loads. At the end of the study, it has been discovered that the failure loads, due to increment of the joint angle, cause decrease ranging between 16% and 43%.
This article considers the potential of boron as matrix-alloying element and gives perspectives about which content of boron is favorable to maximize the interfacial bonding and thermal conductivity of copper/diamond composites. The thermal conductivity of Cu–B/diamond composites is investigated both experimentally and theoretically. The thermal conductivity measurements show that the optimum boron content at 0.8 wt% has provided highest thermal conductivity of 538 W/mK, increases 190% compared to that of copper/diamond composite without boron addition. Theoretical models are used to understand the underlying thermal conductivity enhancement mechanisms of matrix alloying. It is found that the Hasselman–Johnson model combined with a modified Debye model considering the carbide thermal resistance can provide a satisfactory agreement to the experimental data.
The poly(ether ether ketone) micro/nanocomposite reinforced with N-(2-aminoethyl)-3-aminopropyl triethoxy silane treated micro- and nano-sized nickel and zirconia (0.5, 1, and 3 wt%) were prepared by melt mixing in a co-rotating twin screw extruder followed by test specimen fabrication in microinjection molding. The resulting nanocomposites with 3 wt% Ni and ZrO2 nanoparticles exhibit the maximum improvement in tensile and flexural strength as well as the modulus with respect to neat poly(ether ether ketone). The lowest specific wear rate of 17.6 x 10-4 mm3/N/m has been achieved with 3 wt% nano-Ni-filled composite in comparison to neat poly(ether ether ketone)’s value 191.5 x 10-4 mm3/N/m. The thermal stability of micro- and nano-particle reinforced poly(ether ether ketone) composites measured by thermogravimetric analysis found to be higher than the unfilled poly(ether ether ketone).
The elastic property mismatch between plies with different orientations induces stress concentrations near free edges. This free edge effect can cause early delamination of composite structures. Laminates with 15° and –15° plies are studied experimentally to highlight the free-edge effect and the induced micromechanism damage. Full field measurements under tensile loading are performed at macroscopic and mesoscopic scales on edges of the sample. Results show displacement gradients and strain concentrations near interlaminar interfaces. Residual displacement gradients are measured after unloading, which highlights local damage at interlaminar interfaces. Observations at the microscopic scale show that cracks appear at fibre/matrix interfaces and propagate between adjacent fibres along interlaminar interfaces. A comparison of the results obtained on different composites highlights the influence of mechanical properties and material microstructure on edge effects. The study of samples with dropped plies highlights the influence of the combination of both geometric and material singularities on edge effects.
The classic strength of materials theory considers out-of-plane stresses in beams subjected to pure bending to be negligible. This work undertakes a theoretical analysis of this proposition by basing it on the development of the Airy’s functions in polar coordinates. The purpose of this research is studying the effect of this kind of stresses in structures fabricated with non-isotropic materials. Specifically, multi-component sandwich beams subjected to pure bending and large deflections have been studied. An analytical model is proposed that allows these out-of-plane stresses to be calculated according to the curvature change and other design parameters of the specific structure. The completed model was used to calculate the out-of-plane stresses for different design configurations, in order to study their effects. Finally, results are compared to a finite element model.
Based on the theory of the first-order shear deformation, a new set of equilibrium equations is developed by the principle of Hamilton. Using the Giannakopoulos’ contact model, the expressions of the contact force and the central deflection for a functionally graded materials circular plate are obtained. By using the orthotropic collocation point method and Newmark method, the unknown variable functions are discreted in space domain and time domain, and the whole problem is solved by the iterative method synthetically. Numerical results show that the material compositions, geometrical parameters and the initial velocity of the striking ball have great effects on the nonlinear dynamic response of the functionally graded materials circular plate.
Thermal buckling and postbuckling behavior is presented for fiber-reinforced laminated plates subjected to in-plane temperature variation and resting on an elastic foundation. Two kinds of fiber-reinforced laminated plates, namely, uniformly distributed and functionally graded reinforcements, are considered. The material properties of fiber-reinforced laminated plates are based on a micromechanical model and are assumed to be temperature dependent. The governing equations are based on a higher order shear deformation plate theory that includes plate–foundation interaction and the thermal effect. Numerical illustrations are carried out for fiber-reinforced polymer matrix and metal matrix composite laminated plates without or resting on elastic foundations. The numerical results show that the buckling temperature as well as thermal postbuckling strength of the plate can be increased as a result of functionally graded fiber reinforcements. The results reveal that the effect of functionally graded fiber reinforcements on the thermal buckling and postbuckling strength of the plate with polymer matrix is more pronounced compared to the plate with metal matrix.