Effects of interphase material properties in unidirectional fibre reinforced composites

A three-dimensional (3D) micromechanical study has been performed in order to investigate local damage in unidirectional (UD) composite materials with epoxy resin under transverse tensile loading. In particular the effect of different mechanical properties of a 3D interphase within the hexagonal array RVE have been considered and effects of thermal residual stress arising during the curing process have been accounted for in this study. To examine the effect of interphase properties and residual stress on failure, a study based on the temperature-dependent properties of matrix and interphase and a stiffness degradation technique has been used for damage analysis of the unit cell subjected to mechanical loading. Results indicate a strong dependence of damage onset and its evolution from the different interphase properties within the RVE (representative volume element). Moreover, predicted mechanical properties, damage initiation and evolution are also clearly influenced by the presence of residual stress. Numerical results and experimental data (in the literature) have also shown an interesting agreement.     

Micromechanical modeling of interface damage of metal matrix composites subjected to off-axis loading

A three dimensional micromechanics based analytical model is presented to investigate the effects of initiation and propagation of interface damage on the elastoplastic behavior of unidirectional SiC/Ti metal matrix composites (MMCs) subjected to off-axis loading. Manufacturing process thermal residual stress (RS) is also included in the model. The selected representative volume element (RVE) consists of an r × c unit cells in which a quarter of the fiber is surrounded by matrix sub-cells. The constant compliance interface (CCI) model is modified to model interfacial de-bonding and the successive approximation method together with Von-Mises yield criterion is used to obtain elastic–plastic behavior. Dominance mode of damage including fiber fracture, interfacial de-bonding and matrix yielding and ultimate tensile strength of the SiC/Ti MMC are predicted for various loading directions. The effects of thermal residual stress and fiber volume fraction (FVF) on the stress–strain response of the SiC/Ti MMC are studied. Results revealed that for more realistic predictions both interface damage and thermal residual stress effects should be considered in the analysis. The contribution of interfacial de-bonding and thermal residual stress in the overall behavior of the material is also investigated. Comparison between results of the presented model shows very good agreement with finite element micromechanical analysis and experiment for various off-axis angles.     

Modeling the Response of 3D Textile Composites under Compressive Loads to Predict Compressive Strength

The compression response of 3D woven textile composites (3DWC) that consist of glass fiber tows and a polymer matrix material is studied using a combination of experiments and finite element based analyses. A previous study reported by the authors consisted of an experimental investigation of 3DWC under high strain rate loading, Pankow, Salvi, Waas, Yen, and Ghiorse (2011). Those experimental results were explained by using the finite element method to analyze the high rate deformation response of representative volume elements (RVEs) of the 3DWC, Pankow, Waas, Yen, and Ghiorse (2012). In this paper, the same modeling strategy is used to examine the quasi-static, compressive deformation response of 3DWC. The effect of using different numbers of the textile repeat unit architecture in the RVE, on the predicted compression strength, is examined. The transitions in failure modes that are seen in experiments are seen to be captured by the model that is presented here.     

Fracture analysis of epoxy/SWCNT nanocomposite based on global–local finite element model

A global–local multiscale finite element method (FEM) is proposed to study the interaction of nanotubes and matrix at the nanoscale near a crack tip. A 3D FE model of a representative volume element (RVE) in crack tip is built. The effects of the length and chirality of single walled carbon nanotube (SWCNT) in a polymer matrix on the fracture behavior were studied in the presence of van der Waals (vdW) interaction as inter-phase region. Detailed results show that with increasing the weight percentage of SWCNT, fracture toughness improves. Three situations of nanotube directions with respect to crack are considered. Results show that bridging condition has minimum stress intensity factor. In addition, it can be seen that the crack resistance improves by increasing the length and chirality for all kinds of nanotubes. Finally, epoxy/SWCNT 10 wt.% has lower stress intensity factor compared to epoxy/halloysite 10 wt.% in similar loading state.     

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.     

Real microstructure based micromechanical model to simulate microstructural level deformation behavior and failure initiation in DP 590 steel

Dual phase (DP) steels having a microstructure consists of a ferrite matrix, in which particles of martensite are dispersed, have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility. In the present work, a microstructure based micromechanical model is developed to capture the deformation behavior, plastic strain localization and plastic instability of DP 590 steel. A microstructure based approach by means of representative volume element (RVE) is employed for this purpose. Dislocation based model is implemented to predict the flow behavior of the single phases. Plastic strain localization which arises due to incompatible deformation between the hard martensite and soft ferrite phases is predicted for DP 590 steel. Different failure modes arise from plastic strain localization in DP 590 steel are investigated on the actual microstructure by finite element method.     

Damage analysis of fiber reinforced Ti-alloy subjected to multi-axial loading—A micromechanical approach

Effects of initiation and propagation of interface damage on the elastoplastic behavior of a unidirectional SiC/Ti metal matrix composite (MMC) subjected to multi-axial loading are studied using a three-dimensional micromechanics based analytical model. Effects of manufacturing process thermal residual stress (RS) are also included in the analysis. The selected representative volume element (RVE) consists of an r × c unit cells in which a quarter of the fiber is surrounded by matrix sub-cells. The constant compliance interface (CCI) model is used to model interfacial debonding and the successive approximation method together with Von-Mises yield criterion is used to obtain elastic-plastic behavior. Failure modes during multi-axial tensile/compressive loading in the presence of residual stresses are discussed in details. Results revealed that for more realistic predictions both interface damage and thermal residual stress effects should be considered in the analysis. Comparison between results of the presented model shows very good agreement with available finite element micromechanical analysis and experiment for uniaxial loading. Also, results are extracted and interpreted for equi-biaxial including transverse/transverse and axial/transverse and equi-triaxial loading.     

Orientation-dependent compression/tension asymmetry of short glass fiber reinforced polypropylene: Deformation,damage and failure

Short glass fiber reinforced polypropylene (sgf-PP) is increasingly employed in structural components which are subjected to a variety of loading conditions including tensile, compressive and bending loading modes. Since typical industrial components exhibit a wide range of fiber orientation distributions, their mechanical response to these loading conditions is also highly anisotropic. In this paper, the compression/tension asymmetry in the stress–strain behavior of sgf-PP is investigated from a macroscopic engineering and a micro-mechanisms of deformation and failure point of view for specimens with varying, precisely defined fiber orientations. Furthermore, we performed volume strain measurements and two-cyclic tests. We used the results to deduce the onset of damage due to cavitational mechanisms under tension and compared this to the onset of deviation of the tensile from the compressive stress–strain behavior. The results showed a good correlation for specimens with high fiber orientation, whereas for specimens with low fiber orientation results deviate due to the high deviatoric matrix volume strain contribution.     

Effects of interphase properties in unidirectional fiber reinforced composite materials

A micromechanical study has been performed to investigate the mechanical properties of unidirectional fiber reinforced composite materials under transverse tensile loading. In particular, the effects of different properties of interphase within the representative volume element (RVE) on both the transverse effective properties and damage behavior of the composites have been studied. In order to evaluate the effects of interphase properties on the mechanical behaviors of unidirectional fiber reinforced composites considering random distribution of fibers, the interphase is represented by pre-inserted cohesive element layer between matrix and fiber with tension and shear softening constitutive laws. Results indicate a strong dependence of the RVE transverse effective properties on the interphase properties. Furthermore, both the damage initiation and its evolution are also clearly influenced by the interphase properties.     

Effects of particle clustering on the tensile properties and failure mechanisms of hollow spheres filled syntactic foams: A numerical investigation by microstructure based modeling

Particle clustering originated from manufacturing process is thought to be one of the critical factors to the mechanical performance of hollow spheres filled syntactic foams. Although experimental evidence provides a qualitative understanding of the effects of particle clustering on the mechanical properties of syntactic foams, a quantitative assessment cannot be made in the absence of an appropriate micromechanical modeling strategy. In this study, three-dimensional microstructures of syntactic foams with different degrees of particle clustering were reconstructed based on random sequential adsorption (RSA) method. Three-phase finite element models considering the progressive damage behavior of the microsphere–matrix interface were accordingly developed by means of representative volume element (RVE) to quantitatively investigate the effects of particle clustering on the tensile properties and failure mechanisms of syntactic foams. The simulation results indicate that the elastic behavior of syntactic foams is insensitive to the degree of particle clustering, but the strength properties as well as the failure mechanisms are significantly influenced by the degree of particle clustering. From the micromechanical viewpoint, the clustered regions containing higher concentration of microspheres than the average volume fraction would serve as crack initiation sites due to stress concentration, and consequently lead to a negative effect on tensile strength, fracture strain, and interfacial damage of syntactic foams.     

热残余应力对考虑微观孔隙碳纤维增强环氧树脂复合材料横向拉伸性能的影响

为研究由于材料固化产生的热残余应力对碳纤维增强环氧树脂复合材料横向拉伸性能预测结果的影响,发展了一种基于摄动算法的纤维和孔洞随机分布代表性体积单元（RVE）生成方法,建立更加接近真实材料微观结构的RVE模型。通过施加周期性边界条件,并赋予组分（纤维、基体和界面）材料本构关系,进而实现温度和机械荷载下模型的热残余应力和损伤失效分析。从结果中发现,材料固化过程会在纤维之间产生残余压应力,在模型孔隙周围产生沿加载方向的残余拉应力。所建立不含孔隙RVE模型的失效均是由于界面脱黏引起,材料固化在纤维之间产生的残余压应力会增加模型的预测强度。含有孔隙的RVE模型失效起始于孔隙周围的基体中,而材料固化在模型孔隙周围产生的热残余拉应力对含孔隙RVE模型预测的失效强度有降低作用。对于具有不同孔隙尺寸的RVE模型,模型的失效强度随着孔隙尺寸的增加而不断降低,但是热残余应力减弱了孔隙尺寸对模型预测结果的降低作用。对于具有不同长宽比椭圆形孔隙的RVE模型,热残余应力增加了孔隙长宽比对模型强度的降低作用。      

Interface damage of SiC/Ti metal matrix composites subjected to combined thermal and axial shear loading

A three-dimensional micromechanical finite element model is developed to study initiation and propagation of interface damage of unidirectional SiC/Ti metal matrix composites (MMCs) subjected to combined thermal and axial shear loading. Effects of various important parameters such as manufacturing process thermal residual stress, fiber coating and interface bonding are investigated. The model includes a representative volume element consists of a quarter of SiC (SCS-6) fibers covered by interface and coating, which are all surrounded by Ti-15-3 matrix. Appropriate boundary conditions are introduced to include effects of combined thermal and axial shear loading on the RVE. A suitable failure criterion for interface damage is introduced to predict initiation and propagation of interface de-bonding during shear loading. It is shown that while predictions based on perfectly bonded and fully de-bonded interface are far from reality, the predicted stress–strain curve for damaged interface demonstrates very good agreement with experimental data.     

Transverse stiffness properties of unidirectional fiber composites containing debonded fibers

First-principles micromechanics modeling for the determination of transverse stiffness properties of a unidirectional fiber composite with fiber–matrix interfacial debonding is presented. The composite has a packing arrangement of a periodic square array of fibers, but contains randomly distributed debonded fibers. The finite element method is employed for the exact treatment of local microscopic stress and strain fields in a representative volume element of the composite material, and of the nonlinear problem of separation and contact of fiber and matrix at debonded interface. The randomness of the distribution of debonded fibers is dealt with by means of the Monte Carlo method, and the composite stiffness properties are found as ensemble average properties over a large number of representative volume elements. Bimodular behavior of the composite under transverse loading, i.e., different stiffnesses in tension and compression, is accurately captured.     

Micromechanical modeling of interface damage of metal matrix composites subjected to transverse loading

A three-dimensional finite element micromechanical model was developed to study effects of thermal residual stress, fiber coating and interface bonding on the transverse behavior of a unidirectional SiC/Ti–6Al–4V metal matrix composite (MMC). The presented model includes three phases, i.e. the fiber, coating and matrix, and two distinct interfaces, one between the fiber and coating and the other between coating and matrix. The model can be employed to investigate effects of various bonding levels of the interfaces on the initiation of damage during transverse loading of the composite system. Two different failure criteria, which are combinations of normal and shear stresses across the interfaces, were used to predict the failure of the fiber/coating (f/c) and coating/matrix (c/m) interfaces. Any interface fails as soon as the stress level reaches the interfacial strength. It was shown that in comparison with other interface models the predicted stress–strain curve for damaged interface demonstrates good agreement with experimental results.     

Multiscale approach with RSM for stress–strain behaviour prediction of micro-void-considered metal alloy

This paper presents the concept of using a representative volume element (RVE) in a multiscale approach to predict the macroscopic stress–strain behaviour of a cast SS316L specimen under tension up to the point prior to necking. RVE models with various micro-void spatial configurations were built, and the effects of micro-voids and strain rate on the material properties (e.g., yield strength, ultimate tensile strength (UTS), ultimate tensile strain and strain hardening coefficient) were analysed. The spatial configuration of the micro-voids inside the cast SS316L specimen was acquired by the X-ray CT scanning system and each micro-void in the gauge length part was converted into a matching RVE model in the finite element (FE) analysis. Response surface methodology (RSM) was employed to investigate the effect of RVE configurations, i.e., the size of the RVE and the shape and spatial location of the micro-voids, on the material properties (yield strength and UTS) of the cast SS316L specimen at the macroscopic level, and then the optimal levels of the RVE configuration were determined. The stress–strain curve from the simulation did show a good agreement with the experimental results and hence the proposed concept was verified.     

连续石墨纤维增强铝基复合材料准静态拉伸损伤演化与断裂力学行为

针对连续石墨纤维增强铝基(CF/Al)复合材料,采用三种纤维排布方式的代表体积单元(RVE)建立了其细观力学有限元模型,采用准静态拉伸试验与数值模拟结合的方法,研究了其在轴向拉伸载荷下的渐进损伤与断裂力学行为。结果表明,采用基体合金和纤维原位力学性能建立的细观力学有限元模型,对轴向拉伸弹性模量和极限强度的计算结果与实验结果吻合良好,而断裂应变计算值较实验结果偏低。轴向拉伸变形中首先出现界面和基体合金损伤现象,随应变增加界面发生失效并诱发基体合金的局部失效,最后复合材料因纤维发生失效而破坏,从而出现界面脱粘后纤维拔出与基体合金撕裂共存的微观形貌。细观力学有限元分析结果表明,在复合材料制备后纤维性能衰减而强度较低条件下,改变界面强度和刚度对复合材料轴向拉伸弹塑性力学行为的影响较小,复合材料中纤维强度水平是决定该复合材料轴向拉伸力学性能的主要因素。     

Experimental investigation on the compression behaviors of epoxy with carbon nanotube under high strain rates

The compressive properties of epoxy with different carbon nanotubes (CNTs) contents at quasi-static and high strain rates loading had been investigated via experiment to evaluate the compressive failure behaviors and modes at different CNTs contents and different strain rates. The results indicated that the stress train curves were strain rate sensitive, and the compressive stiffness, compressive failure stress of composites with various CNTs contents was increased with the strain rates and CNTs contents. The compressive failure stress and the compressive failure modes of the composites were apparently different as the change of CNTs contents.     

复合材料跨尺度失效准则及其损伤演化

提出了一种新的基于物理失效模式的复合材料跨尺度失效准则, 从细观层面分别对纤维和基体的失效模式进行了表征, 将纤维失效分为拉伸失效和压缩失效, 将基体失效分为膨胀失效和扭曲失效。建立了相应的失效准则及损伤演化方法。通过正方形和六边形的代表体积单元(RVE)模型, 计算了宏观应力到细观应力的机械应力放大系数和热应力放大系数。以IM7/5250-4复合材料拉伸试验作为算例对失效模型进行了验证。计算结果与试验结果吻合较好, 表明跨尺度失效准则能够准确预测复合材料层合板的破坏。