Effect of working temperature on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present study, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) in the working temperature range from 23 °C to 90 °C was studied by experimental and constitutive methods. Monotonic and cycle loading-unloading single-lap-shear tests were employed to determine the interfacial properties of hybrid composites. The experimental results show that both interfacial shear strength and shear stiffness decrease with increasing working temperature. A regression function was adopted to evaluate the decaying degree of interfacial properties with increasing working temperature. The shear stress-displacement relationship under monotonic loading exhibits nonlinear behavior after an initial elastic region. The envelope lines of shear stress-displacement of hybrid composites under cyclic loading indicate that the nonlinearity in the curve is caused by the plastic deformation of polypropylene in the interphase region. A constitutive model was built to describe the nonlinear shear stress-displacement relation of hybrid composites at different working temperatures. A full suite of temperature-dependent plastic parameters in the model was obtained from cyclic loading-unloading tensile tests. The predicted shear stress–displacement curves agreed well with experimental results from different working temperatures. In addition, the failure mode of hybrid composites varied with working temperature.     

Experimental investigation on mechanical properties of unidirectional and woven fabric glass/epoxy composites under off-axis tensile loading

Mechanical properties of unidirectional (UD) and woven fabric glass/epoxy composites under off-axis tensile loading were experimentally investigated. A number of off-axis tests considering different fibre orientations were performed to study the character and failure mechanisms of the composite laminates. The experimental results indicated that both off-axis elastic moduli and strength degrade with increasing off-axis angle in all cases, and the woven fabric composites present nonlinear stress-strain behaviour under off-axial tension loading. The Tsai-Wu criteria used for failure analysis of the UD and woven fabric composites were compared and discussed, especially considering different values of interaction coefficient F₁₂. The prediction results demonstrated that the Tsai-Wu criterion can be used successfully to analyse failure properties of the woven fabric composites under multiaxial stress conditions, where the criterion with the modified coefficient F₁₂ obtained from the 45° off-axial tension tests is better and has higher accuracy. Finally, the specific failure modes were compared in the UD and woven fabric composites. The selected fracture surfaces were also observed by scanning electron microscopy (SEM), and the corresponding failure mechanisms of the woven fabric composites under off-axis tensile loading were identified.     

Strain rate effects on the mechanical response of polypropylene-based composites deformed at small strains

The mechanical properties and response of two polypropylene (PP)-based composites have been determined for small strains and for a range of strain rates in the quasi-static domain. These two materials are talc-filled and unfilled high-impact PP. Uniaxial tensile tests were performed at different strain rates in order to characterize the mechanical response and the strain rate effect. The experimental results showed that both unfilled and talc-filled high-impact PP were sensitive to strain rate and exhibited nonlinear behavior even at relatively low strains. SEM analysis was conducted to obtain a better comprehension of deformation mechanisms involved during loading by observations of the microstructure evolution. For each of these two materials, two existing modeling approaches are proposed. The first one is a three-parameter nonlinear constitutive model based on the experimental results. The second is a micromechanically based approach for the elastic-viscoplastic behavior of the composite materials. The stress-strain curves predicted by these models are in fairly good agreement with our experimental results. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 6, pp. 1051–1059. This article was submitted by the authors in English.     

A new viscoplastic model and experimental characterization for thermosetting resins

Resins as matrix materials for structural composites show nonlinear rate-dependent mechanical behaviors. In the present work, a new viscoplastic constitutive equation based on a potential function is proposed to predict the mechanical response of an epoxy matrix to any three-dimensional loading condition. The proposed potential function is a combination of the second and third invariants of the deviatoric stress tensor as well as the first invariant of the stress tensor, i.e. the hydrostatic stress. Series of tensile and shear constant-rate straining tests were performed on epoxy resin specimens up to the fracture. Under shear loading, the nonlinearity of the stress-strain curve and the rate dependency of the initial modulus and strength are more significant than that under tensile loading. The viscoplastic model parameters are derived from the experimental data, and the fracture patterns of the specimens under tensile and shear loadings are studied. Further, the model predictions are compared with a known rate-dependent model to show the accuracy of the presented model.     

Non-linear material characterization and numerical modeling of cross-ply basalt/epoxy laminate under low velocity impact

The low velocity impact behavior of basalt/epoxy composites, seen as an eco-friendly replacement of glass-epoxy composites, has not been studied systematically so far. Here, the elastic elasto-plastic properties, strengths, intralaminar and interlaminar fracture energies were determined. The intralaminar energies were determined using compact tension and compression tests. The elasto-plastic properties needed in the plastic potential were determined using off-axis test. These properties are used in Finite Element (FE) code with an elasto-plastic damage model developed earlier to simulate the impact response of cross-ply laminates basalt/epoxy laminates. Low velocity impact (LVI) experiments at 10 J, 20 J and 30 J are performed on these composites. The FE simulation is successful in capturing force, energy, deflection histories and damage zones showing a close match to the experiments. A comparison of impact force history and damage area (ultrasonic C-scan) of basalt-epoxy laminates with glass epoxy laminates having same volume fraction shows nearly similar peak forces but the major axis of the ellipsoidal damage zone was bigger in glass/epoxy laminates.     

Mechanical characterization of 3D angle-interlock Kevlar/basalt reinforced polypropylene composites

The present work reported the mechanical characterization of novel polypropylene (PP) composites reinforced with three-dimensional angle-interlock (3D-A) Kevlar/basalt fabrics. Two homogeneous fabrics with Kevlar (K3D) and basalt yarns (B3D), and a hybrid fabric (H3D) with a combination of both Kevlar and basalt yarns were produced. Three types of two layer 3D-A composites were manufactured using vacuum-assisted compression molding method. Static tensile and in-plane compression tests were carried out on the manufactured composites. The mechanical behavior of the three 3D-A composites was compared in terms of stress-strain response, elastic modulus, strength and failure strain. Influence of hybridization on the mechanical behavior of the 3D-A composites was also studied. Significant improvement in the tensile behavior of 3D-A homogeneous composites was observed due to hybridization. Meanwhile, there was no considerable improvement in in-plane compression behavior. The damage patterns for in-plane compression loading were examined through scanning electron microscopy (SEM) to explore the possible damage patterns such as matrix cracking, fiber failure, delamination and deformation. Numerical simulations were carried out using ABAQUS/Standard, by implementing a user-defined material subroutine (VUMAT) based on the Chang-Chang linear orthotropic damage model. Good agreement between experimental and numerical simulations was achieved in terms of damage patterns.     

On the degradation and stability of high abrasion furnace black (HAF)/acrylonitrile butadiene rubber (NBR) and high abrasion furnace black (HAF)/graphite/acrylonitrile butadiene rubber (NBR) under cyclic stress-strain

Acrylonitrile butadiene rubber (NBR) compounds filled with 40 phr of high abrasion furnace black (HAF) and HAF (20 phr)/graphite (20 phr) were experimentally investigated. The stress-strain curves of the composites were studied, which are described by applying Ogden's model. The effect of cyclic fatigue and hysteresis was also examined. The dissipation energy that indicates the vibration damping capacity for all samples was determined. A continuum damage model is used to investigate the fatigue damage behavior for elastomers. Experiments on the cyclic fatigue of a carbon-filled NBR rubber and carbon/graphite filled NBR rubber were conducted to determine the relation between the number of cyclic fatigue and the strain amplitude. The results indicate that the theoretical formula for the number of cyclic fatigue as a function of the strain amplitude, derived from the damage model, can describe experimental data for the prepared samples very well.     

Compression and tensile properties of self-reinforced poly(ethylene terephthalate)-composites

Tensile and compression properties of self-reinforced poly(ethylene terephthalate) (SrPET) composites has been investigated. SrPET composites or all-polymer composites have improved mechanical properties compared to the bulk polymer but with maintained recyclability. In contrast to traditional carbon/glass fibre reinforced composites, SrPET composites are very ductile, resulting in high failure strains without softening or catastrophic failure. In tension, the SrPET composites behave linear elastically until the fibre-matrix interface fails, at which point the stiffness starts decreasing. As the material is further strained, strain hardening occurs and the specimen finally fails at a global strain above 10%. In compression, the composite initially fails through fibre yielding, and at higher strains through fibre bending. The stress-strain response is reminiscent of an elastic-perfectly plastic material with a high strain to failure (typically over 10%). This indicates that SrPET composites are not only candidates as semi-structural composites but also as highly efficient energy absorbing materials.     

A theory of finite tensile deformation of double-network hydrogels

A continuum damage model was developed to describe the finite tensile deformation of tough double-network (DN) hydrogels synthesized by polymerization of a water-soluble monomer inside a highly crosslinked rigid polyelectrolyte network. Damage evolution in DN hydrogels was characterized by performing loading-unloading tensile tests and oscillatory shear rheometry on DN hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm). The model can explain all the mechanical features of finite tensile deformation of DN hydrogels, including idealized Mullins effect and permanent set observed after unloading, qualitatively and quantitatively. The constitutive equation can describe the finite elasto-plastic tensile behavior of DN hydrogels without resorting to a yield function. It was showed that tensile mechanics of DN hydrogels in the model is controlled by two material parameters which are related to the elastic moduli of first and second networks. In effect, the ratio of these two parameters is a dimensionless number that controls the behavior of material. The model can capture the stable branch of material response during neck propagation where engineering stress becomes constant. Consistent with experimental data, by increasing the elastic modulus of the second network the finite tensile behavior of the DN hydrogel changes from necking to strain hardening.     

Investigation on interlaminar delamination tendency of multidirectional carbon fiber composites

In this investigation carbon fiber reinforced laminates with different orientation layups are prepared and studied under tensile loading condition. Multiple strain measurement techniques, namely, resistive strain gauges, embedded optical sensors and digital image correlation are used to analyze stress-strain behavior simultaneously through the thickness of composite materials, and to determine the sequence of failure in different plies. Inconsistencies of strains measured through different methods is correlated with the tendency for interlaminar delamination, therefore demonstrating the ability of multi-instrument approach to describe damage progress through the thickness of multidirectional laminates. Complementary analysis through acoustic emission methods reveals that the angle of off-axis surface plies can influence the sequence of failure under tensile loading condition, and damage monitoring capabilities of acoustic emission system is directly affected by delamination tendency of surface plies. Remarkably, the delayed failure of off-axis plies is shown to be related to reorientation of these layer towards loading direction using infrared thermography method.     

Compressive-tensile fatigue behavior of cords/rubber composites

Cord/rubber composites are used to build complex structures which may be submitted to cyclic loads, sometimes leading to critical fatigue failure. The focus of this study is to investigate the cyclic compressive/tensile strain behavior of polyester, polyamide and hybrid polyaramid/polyamide cords. For that, the cords were embedded in rubber belts to be used in a specially designed rotating pulley equipment that allows monitoring and controlling of tensile force, frequency and strain level. All fatigue tests were performed using stress-control mode, and tensile residual strength of the cords was measured as a function of material type, number of cycles and compressive/tensile strain level. The results show that compressive and tensile cyclic strains decrease residual properties. Hybrid cords showed higher residual strength than polyester and polyamide cords when subject to high compressive strain or high number of cycles. Moreover, morphological evaluation indicated failure to be associated with microbuckling and extensive fibrillation.     

The effect of annealing on the elastoplastic response of isotactic polypropylene

Four series of tensile loading-unloading tests are performed on isotactic polypropylene in the sub-yield domain of deformations at room temperature. In the first series, injection-molded specimens are used as produced, whereas in the other series the samples are annealed for 24 h at 120, 140 and 160 °C, which covers the low-temperature region and an initial part of the high-temperature region of annealing temperatures. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer. The stress-strain relations are determined by five adjustable parameters that are found by fitting the experimental data. The effect of annealing is analyzed on the material constants.     

Effects of fabric architectures on mechanical and damage behaviors in carbon/epoxy woven composites under multiaxial stress states

The mechanical properties and damage evolutions of carbon/epoxy woven fabric composites with three different fabric architectures, including one plain weave and two twill weave patterns, are experimentally investigated under multiaxial stress states. In particular, the effects of weave patterns are investigated by monotonic and cyclic off-axis tension tests. Both elastic modulus and strength degrade remarkably with increasing off-axis loading angle, while Poisson's ratio is much higher than that measured from on-axis tests and increases with loading strain gradually. Different fabric architectures show limited effects on the modulus and strength under multiaxial stress states, and they are well predicted by transformation equation and Tsai-Wu failure criteria, respectively. However, significantly different failure behaviors are observed in three fabric composites, and microstructure observation shows that fabric architecture affects the stress concentration and the damage development. Smaller crimp ratio and compacted structure postpone the damage development but result in more abrupt failure under multiaxial stress states.     

Use of digital image correlation to measure large-strain tensile properties of ductile thermoplastics

This paper presents an experimental investigation of the mechanical behaviour of a talc and elastomer modified polypropylene compound subjected to large strains. 3D digital image correlation with two cameras and stereo-vision was used to determine full-field displacements during uniaxial tensile tests on specimens with rectangular cross-section. Local strains were derived from the displacement field and used to calculate the current cross-sectional area of the specimen during the whole loading process. Points on the true stress–strain curve (Cauchy stress versus logarithmic strain) were then calculated from the data. Volume dilatation was separated into elastic and plastic parts through tests where the specimens were unloaded after varying degree of stretching. The unloading events were also used to investigate damage evolution as function of plastic straining.     

Failure analysis of plain woven glass/epoxy laminates: Comparison of off-axis and biaxial tension loadings

An experimental study was focused on investigation of the failure properties of plain woven glass/epoxy composites under off-axis and biaxial tension loading conditions. Four fibre orientations (0°, 15°, 30° and 45° with respect to the load direction) were considered for off-axis tests and two biaxial load ratios for biaxial tests to study failure characteristics and mechanism. Four classical polynomial failure criteria - Tsai-Hill, Hoffman, Tsai-Wu and Yeh-Stratton - were analysed comparatively to predict off-axis and biaxial failure strength of the composites. For failure prediction of the plain woven composites under multiaxial tension loads, the Tsai-Wu criterion was modified by introducing an interaction coefficient F₁₂ obtained from 45° off-axis or biaxial tension tests and the Yeh-Stratton criterion was modified with the interaction coefficient B₁₂ = 0 or obtained from the biaxial tension test. The former criterion was found to have higher accuracy. Finally, according to macroscopic and microscopic studies, the failed specimens showed mostly distinct failure with a specific fracture orientation, mainly exhibiting fibre or fabric tensile fracture mode and a combination of matrix cracking and delamination, both in off-axis and cruciform samples.     

Glutaraldehyde treatment of bacterial cellulose/fibrin composites: impact on morphology, tensile and viscoelastic properties

Bacterial cellulose/fibrin composites were treated with glutaraldehyde in order to crosslink the polymers and allow better match of the mechanical properties with those of native small-diameter blood vessels. Tensile and viscoelastic properties of the glutaraldehyde treated composites were determined from tensile static tests and cyclic creep tests, respectively. Glutaraldehyde-treated (bacterial cellulose) BC/fibrin composites exhibited tensile strength and modulus comparable to a reference small-diameter blood vessel; namely a bovine coronary artery. However, the breaking strain of the glutaraldehyde-treated composites was still well below that of the native blood vessel. Yet a long strain hardening plateau was induced by glutaraldehyde treatment which resembled the stress–strain response of the native blood vessel. Tensile cyclic creep test indicated that the time-dependent viscoelastic behavior of glutaraldehyde-treated BC/fibrin composites was comparable to that of the native blood vessel. Covalent bonding between BC and fibrin occurred via glutaraldehyde, affording mechanical properties comparable to that of the native small blood vessel.     

Experimental and numerical investigation of yielding phenomena in a shape memory polymer subjected to cyclic tension at various strain rates

This paper presents experimental and numerical results of a polyurethane shape memory polymer (SMP) subjected to cyclic tensile loading. The goal was to investigate the polymer yielding phenomena based on the effects of thermomechanical coupling. Mechanical characteristics were obtained with a testing machine, whereas the SMP temperature accompanying its deformation process was simultaneously measured in a contactless manner with an infrared camera. The SMP glass transition temperature was approximately 45 °C; therefore, when tested at room temperature, the polymer is rigid and behaves as solid material. The stress and related temperature changes at various strain rates showed how the SMP yield limit evolved in subsequent loading-unloading cycles under various strain rates. A two-phase model of the SMP was applied to describe its mechanical response in cyclic tension. The 3D Finite Element model of a tested specimen was used in simulations. Good agreement between the model predictions and experimental results was observed for the first tension cycle.     

Cyclic viscoplasticity of high-density polyethylene/montmorillonite clay nanocomposite

Observations are reported on high-density polyethylene (HDPE) and nanocomposite, where HDPE matrix is reinforced with montmorillonite (MMT) nanoclay, in uniaxial cyclic tensile tests with various cross-head speeds ranging from 1 to 50 mm/min. Each cycle of deformation involves tension up to the maximal strain ?_max = 0.1 and retraction down to the zero stress. The study focuses on low-cycle deformation programs with N = 5 cycles in each test.A constitutive model is derived for the viscoplastic response of polymers at three-dimensional cyclic deformations with small strains. Given a strain rate and a maximum strain, the stress-strain relations involve eight material constants that are found by fitting the experimental data. Good agreement is demonstrated between the observations and the results of numerical simulation. It is shown that the rate of cyclic deformation affects the adjustable parameters in a physically plausible way.     

Rigidity analysis of polypropylene/vegetal fibre composites after recycling

Polypropylene/hemp or sisal fibre composites exhibit interesting recyclability [Bourmaud A, Baley C. Investigations on the recycling of hemp and sisal fibre-reinforced polypropylene composites. Polymer Degradation and Stability 2007;92(6):1034-45]. The obtained results prove that the tensile modulus of these polypropylene/vegetal fibre composites is well conserved with the number of reprocessing cycles. In this work, we investigated the relationship between the mechanical properties of the fibres and those of the composites by taking the influence of the recycling into account.In the first part of this study we carried out nanoindentation and tensile tests on vegetal fibres to obtain transversal and longitudinal Young's moduli. The experimental values were then introduced into micromechanical models, taking the aspect ratio changes into account, to estimate the stiffness of the PP/vegetal fibre injected composites before and after recycling. The first results show an interesting correlation between experimental results and model predictions; however a general underestimation of tensile stiffness of composites can be noticed.