Establishing a new Forming Limit Curve for a flax fibre reinforced polypropylene composite through stretch forming experiments

In developing an understanding of the failure in natural fibre reinforced polymer composites, the failure limits of this class of the material system are required. It is found that the conventional Forming Limit Curve is not suitable to predict the failure initiated in the natural fibre composite as principal strains cannot differentiate the strain on the flax fibres and the polypropylene matrix. This study proposes a new Forming Limit Curve for the composite which expresses limiting fibre strain as a function of forming mode depicted by the ratio of minor strain to major strain. The new Forming Limit Curve, along with the Maximum Strain failure criterion have been successfully implemented in FEA simulations, and numerical simulations suggest that the former is more accurate. The current work provides an innovative method to predict the onset of failure in natural fibre composites, which can be applied in composite forming and structural design.     

Numerical study of the effects of reinforcement/matrix interphase on stress-strain behavior of YAl2 particle reinforced MgLiAl composites

For the potential influence produced by the reinforcement/matrix interphase in particle reinforced metal matrix composites (PMMCs), a unit cell model with transition interphase was proposed. Uniaxial tensile loading was simulated and the stress/strain behavior was predicted. The results show that a transition interphase with both appropriate strength and thickness could affect the failure mode, reduce the stress concentration, and enhance the maximum strain value of the composite.     

Deformation gradient tensor decomposition for representing matrix cracks in fiber-reinforced materials

A new method is presented for the representation of matrix cracks in continuum damage mechanics (CDM) models for fiber-reinforced materials. The method is based on the additive decomposition of the deformation gradient tensor into ‘crack’ and ‘bulk material’ components, analogous to the additive strain decomposition of the smeared-crack approach. The potential improvements to the accuracy of CDM models that utilize the presented method are demonstrated for a single element subjected to simple shear deformation and for a unidirectional open-hole tension specimen. The presented method avoids load transfer across matrix cracks and eliminates the prediction of spurious secondary failure modes that occurs when conventional strain-based CDM models are used in geometrically nonlinear finite element analyses involving large shear deformations.     

A parametric study of fiber volume fraction distribution on the failure initiation location in open hole off-axis tensile specimen

We present a model to study the effect of the spatial distributions in fiber volume fraction on the failure initiation location in the open hole off-axis tensile. These variations are introduced with a micromechanical enhancement dehomogenization process. Good agreement is obtained between our predicted failure locations and experimental results by considering a failure criterion based on the effective shear strain in the matrix. The predicted failure angle distribution is in good agreement with the experimental results when the variability in the fiber volume fraction is included in the simulations.     

C/C复合材料销钉准静态和动态剪切性能

根据复合材料销钉剪切试验的需要,设计剪切试验装置。利用电子万能试验机和落槌冲击试验系统完成C/C复合材料销钉在准静态和动态加载工况下的面内剪切力学性能试验,并通过SEM试验系统分析其剪切失效模式和失效机制。结果表明:C/C复合材料销钉抗剪切强度具有明显的应变率效应,随着加载速率的增加,其抗剪切强度显著提高;C/C复合材料在不同加载速率下失效模式不同,准静态加载工况下纤维与基体严重剥离,纤维束丧失整体承载能力,其破坏过程表现出“伪塑性”失效特征;动态加载工况下纤维与基体未发生明显剥离,纤维束整体承载,其破坏过程表现为“脆性”失效特征。C/C复合材料在不同加载速率下剪切失效模式的不同可归结为内部缺陷扩展的应变率效应。      

Strain-rate-dependent failure criteria for composites

The objective of this study was to characterize the quasi-static and dynamic behavior of composite materials and develop/expand failure theories to describe static and dynamic failure under multi-axial states of stress. A unidirectional carbon/epoxy material was investigated. Multi-axial experiments were conducted at three strain rates, quasi-static, intermediate and high, 10⁻⁴, 1 and 180-400 s⁻¹, respectively, using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. A Hopkinson bar apparatus and off-axis specimens loaded in this system were used for multi-axial characterization of the material at high strain rates. Stress-strain curves were obtained at the three strain rates mentioned. The measured strengths were evaluated based on classical failure criteria, (maximum stress, maximum strain, Tsai-Hill, Tsai-Wu, and failure mode based and partially interactive criteria (Hashin-Rotem, Sun, and Daniel). The latter (NU theory) is primarily applicable to interfiber/interlaminar failure for stress states including transverse normal and in-plane shear stresses. The NU theory was expressed in terms of three subcriteria and presented as a single normalized (master) failure envelope including strain rate effects. The NU theory was shown to be in excellent agreement with experimental results.     

Dynamic micromechanical modeling of textile composite strength under impact and multi-axial loading

Micromechanical finite element modeling has been employed to define the failure behavior of S2 glass/BMI textile composite materials under impact loading. Dynamic explicit analysis of a representative volume element (RVE) has been performed to explore dynamic behavior and failure modes including strain rate effects, damage localization, and impedance mismatch effects. For accurate reflection of strain rate effects, differences between an applied nominal strain rate across a representative volume element (RVE) and the true realized local strain rates in regions of failure are investigated. To this end, contour plots of strain rate, as well as classical stress contours, are developed during progressive failure. Using a previously developed cohesive element failure model, interfacial failure between tow and matrix phases is considered, as well as classical failure modes such as fiber breakage and matrix microcracking. In-plane compressive and tensile loading have been investigated, including multi-axial loading cases. Highly refined meshes have been employed to ensure convergence and accuracy in such load cases which exhibit large stress gradients across the textile RVE. The effect of strain rate and phase interfacial strength have been included to develop macro-level material failure envelopes for a 2D plain weave and 3D orthogonal microgeometry.     

Improving the prediction of tensile failure in unidirectional fibre composites by introducing matrix shear yielding

A Monte Carlo simulation is established to predict the failure strain of unidirectional fibre composites. The effect of matrix shear yielding of a high performance epoxy resin is introduced into the model through load sharing factors between the fibres adjacent to fibre-break(s). Strain concentration factors (SCF) of fibres are obtained using Finite Element Methods (FEM) in a three dimensional multi-fibre unit cell containing one, two and three adjoining fibre-break(s). The tensile strains of the surviving adjacent fibres are intensified as a function of their distances from the fracture. A statistical simulation is carried out to predict the failure strain of a single layer of unidirectional (UD) fibre composites with the thickness of the fibre ineffective length. Using the weakest link theory, the ultimate failure strain of a real size UD composite is predicted.     

A study of the mechanical properties of short natural-fiber reinforced composites

The degree of fiber–matrix adhesion and its effect on the mechanical reinforcement of short henequen fibers and a polyethylene matrix was studied. The surface treatments were: an alkali treatment, a silane coupling agent and the pre-impregnation process of the HDPE/xylene solution. The presence of Si–O–cellulose and Si–O–Si bonds on the lignocellulosic surface confirmed that the silane coupling agent was efficiently held on the fibres surface through both condensation with cellulose hydroxyl groups and self-condensation between silanol groups.

The fiber–matrix interface shear strength (IFSS) was used as an indicator of the fiber–matrix adhesion improvement, and also to determine a suitable value of fiber length in order to process the composite with relative ease. It was noticed that the IFSS observed for the different fiber surface treatments increased and such interface strength almost doubled only by changing the mechanical interaction and the chemical interactions between fiber and matrix.

HDPE-henequen fiber composite materials were prepared with a 20% v/v fiber content and the tensile, flexural and shear properties were studied. The comparison of tensile properties of the composites showed that the silane treatment and the matrix-resin pre-impregnation process of the fiber produced a significant increase in tensile strength, while the tensile modulus remained relatively unaffected. The increase in tensile strength was only possible when the henequen fibers were treated first with an alkaline solution. It was also shown that the silane treatment produced a significant increase in flexural strength while the flexural modulus also remained relatively unaffected. The shear properties of the composites also increased significantly, but, only when the henequen fibers were treated with the silane coupling agent. Scanning electron microscopy (SEM) studies of the composites failure surfaces also indicated that there is an improved adhesion between fiber and matrix. Examination of the failure surfaces also indicated differences in the interfacial failure mode. With increasing fiber–matrix adhesion the failure mode changed from interfacial failure and considerable fiber pull-out from the matrix for the untreated fiber to matrix yielding and fiber and matrix tearing for the alkaline, matrix-resin pre-impregnation and silane treated fibers.     

Two-dimensional strain-based interactive failure theory for multidirectional composite laminates

A 2-D strain-based interactive failure theory is developed to predict the final failure of composite laminates subjected to multi-axial in-plane loading. The stiffness degradation of a laminate during loading is examined based on the individual failure modes of the maximum strain failure theory, and a piecewise linear incremental approach is employed to describe the nonlinear mechanical behavior of the laminate. In addition, an out-of-plane failure mode normal to the laminate is also investigated to more accurately predict the failure of multidirectional laminates. The theoretical results of the failure model presented are compared with the experimental data provided by the World-Wide Failure Exercise, and the accuracy of the model’s predictive capabilities is investigated.     

Time and temperature dependence of carbon/epoxy interface strength

Characterization of fiber/matrix interface is essential for the understanding of long-term properties of fiber reinforced composite materials. In this research, time and temperature dependence of carbon/epoxy interface strength were investigated. Unidirectional specimens were tested under tensile load up to failure, at various temperatures and testing speeds. The failure modes were identified as matrix dominant failure or interface dominant failure. A unit-cell model was considered to evaluate the stresses at the microscopic level and identify the critical points of highest stresses. Time and temperature dependent stress-concentration factor and thermal residual stress at the critical points were calculated using viscoelastic FEA. The micro stresses at the critical points were found to be properly represented by a bilinear curve with the interface dominant failure mode associated with the horizontal portion of the curve, suggesting that the interface strength is independent of time and temperature.     

In situ fibre fracture measurement in carbon-epoxy laminates using high resolution computed tomography

High resolution Synchrotron Radiation Computed Tomography (SRCT) has been used to capture fibre damage progression in a carbon-epoxy notched [90/0]_s laminate loaded to failure. To the authors knowledge this provides the first direct in situ measurement of the accumulation of fibre fractures for a high performance material under structurally relevant load conditions (i.e. fractures within the bulk of an essentially conventional engineering laminate). A high level of confidence is placed in the measurements, as the failure processes are viewed internally at the relevant micromechanical length-scales, as opposed to previous indirect and/or surface-based methods. Whilst fibre breaks are the dominant composite damage mechanism considered in the present work, matrix damage, such as transverse ply cracks, 0° splits and delaminations, were also seen to occur in advance of extensive fibre breaks. At loads where fibre break density levels were significant, splitting and delamination were seen to separate the central 0° ply in the near notch region from the 90° plies. Fibre breaks were initially observed in isolated locations, consistent with the stochastic nature of fibre strengths. The formation of clusters of broken fibres was observed at higher loads. The largest clusters observed consisted of a group of eleven breaks and a group of fourteen breaks. The large clusters were observed at the highest load, at sites with no prior breaks, indicating they occurred within a relatively narrow load range. No strong correlation was found between the location of matrix damage and fibre breaks. The data achieved has been made available online at www.materialsdatacentre.com for ongoing model development and validation.     

Creep failure time prediction of polymers and polymer composites

A theoretical approach for the prediction of creep rupture time of polymers and polymer composites is analyzed in the present work. This analysis takes into account the viscoelastic path at small strains and the viscoplastic path at higher stresses. The calculation of the rate of creep strain is based on a thermally activated rate process, while the emergence and growth of plastic strain, with increasing creep time, is also taken into account. When the accumulated strain attains values, high enough to lead to failure, its slope versus time exhibits an abrupt change. At this specific time, the creep rate function in respect to time appears a minimum. The creep failure time is defined as the time where the creep rate takes its minimum value. The model has been tested for various types of polymeric materials, as well as for polymer composites. Once the model parameters are estimated from short time creep strain data, then it was proved to successfully predict the creep failure time at a variety of stress levels, for all material types examined.     

A novel predictive model for mechanical behavior of single-lap GFRP composite bolted joint under static and dynamic loading

Mechanical connection of composite is critical due to its complicated meso-structure and failure mode, which has become a bottleneck on reliability of composite material and structure. Although many researches on composite bolted joints have been carried out, the theory and experiment on mechanical behavior of such a joint structure under dynamic loading were rarely reported. Here, we propose a novel predictive model for quasi-static and dynamic stiffness of composite bolted joint by introducing the strain rate dependent elastic modulus into the mass spring model. Combined with the composite laminate theory and Tsai-Hill theory, the present model was capable of predicting the strain rate dependent stiffness and strength of the composite bolted joint. Quasi-static and impact loading experiments were carried out by Zwick universal hydraulic testing machine and split Hopkinson tension bar, respectively. The stiffness and strength predicted by our model showed good accordance with the experiment data with errors below 12% under quasi-static loading and below 30% under impact loading. The results indicated that under impact loading, stiffness and strength of the composite bolted joint were significantly higher than their quasi-static counterparts, while the failure mode of the joint structure trended towards localization which was mainly bearing failure. Among various lay-up ratios studied, the optimal lay-up ratio for quasi-static and dynamic stiffness was 0:±45:90 = 3:1:1.     

Rate-dependent phenomenological model for self-reinforced polymers

The visco-elastoplastic nature of self-reinforced polymers (SRPs) implies that their mechanical behaviour depends on strain rate. Such dependence, when significant, must be taken into account in order to predict the impact response of these materials. In this paper, the strain rate dependence of the mechanical behaviour of a self-reinforced polypropylene (SRPP) and a self-reinforced poly(ethylene terephthalate) (SRPET) is determined and constitutively modelled. To do this, stress–strain curves corresponding to constant strain rates are deduced for each material by using a characterization method presented and validated in previous works. The strain rate dependence of the stress–strain response is quantified based on the ‘strain rate sensitivity coefficient’, defined by G’Sell and Jonas for their material model for semi-crystalline polymers. Such dependence is found to be higher in the SRPET than in the SRPP and, moreover, in both materials it depends on strain. Finally, a modified phenomenological constitutive model based on the G’Sell–Jonas one is proposed. The results show that the modified model improves the prediction of the original model reproducing accurately the rate-dependent behaviour of both SRPs.     

Matrix failure in composite laminates under compressive loading

The failure envelope of the matrix in composite laminates under compressive loads has not received much attention in literature. There are very little to no experimental results to show a suitable failure envelope for this constituent found in composites. With increasing popularity in the use of micromechanical analysis to predict progressive damage of composite structures which requires the use of individual failure criteria for the fibre and matrix, it is important that matrix behaviour under compression is modelled correctly.In this study, off-axis compression tests under uniaxial compression loading are used to promote matrix failure. Through the use of micromechanical analysis involving Representative Volume Elements, the authors were able to extract the principal stresses on the matrix at failure. The results indicated that hydrostatic stresses play an important role in the failure of the matrix. Thus, Drucker–Prager failure criterion is recommended when modelling compressive matrix failure in composite structures.     

Micro-mechanical analysis of the effect of ply thickness on the transverse compressive strength of polymer composites

A micro-mechanical model is used to study the effect of ply thickness on constrained 90° plies subjected to transverse compressive loading (in situ effect). For cross-ply sublaminates with conventional, standard-thickness 90° plies, failure is dominated by fibre–matrix interface cracking and large localised plastic deformation of the matrix, forming a localised band in a plane that is not aligned with the loading direction. Ultra-thin plies show a dispersed damage mechanism, combining wedge cracking with ply fragmentation/separation. Moreover, a transverse crack suppression effect is clearly observed. To the authors’ knowledge, it is the first time an in situ effect in transverse compression has been identified. When comparing the results of the micro-mechanical model with the predictions from analytical models for the in situ effect, the same trends are obtained. These results also show that, for realistic ply thicknesses, these analytical models can be considered fairly accurate.     

Macroscopic analysis of interfacial properties of flax/PLLA biocomposites

This study presents results from a study of the mechanical behaviour of flax reinforced Poly(l-Lactic Acid) (PLLA) under in-plane shear and mode I interlaminar fracture testing. Slow cooling of the unreinforced polymer has been shown to develop crystalline structure, causing improvement in matrix strength and modulus but a drop in toughness. The in-plane shear properties of the composite also drop for the slowest cooling rate, the best combination of in-plane shear performance and delamination resistance is noted for an intermediate cooling rate, (15.5 °C/min). The values of G_Ic obtained at this cooling rate are higher than those for equivalent glass/polyester composites. These macro-scale results have been correlated with microdroplet interface debonding and matrix characterization measurements from a previous study. The composite performance is dominated by the matrix rather than the interface.