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.     

玻纤增强注塑件的均匀化弹性力学参数研究

基于均匀化方法,根据长玻纤增强聚丙烯(LGFR-PP)的微观特征,建立了非连续长玻纤增强复合材料的代表性体积单元(RVE),通过有限元方法模拟预测了复合材料的宏观等效弹性力学参数,与注塑样条拉伸性能测试结果进行了比较。研究表明,通过在玻纤两侧增加聚丙烯(PP)分布,所采用的RVE较传统连续纤维的有限元模型更为合理;当玻纤成单一取向时,玻纤增强聚丙烯为一种横观各向同性材料;改变玻纤取向与拉伸方向之间的角度,拉伸方向的等效模量先微幅减小,再迅速降低,而后趋于稳定。利用均匀化方法预测非连续长玻纤增强注塑件的等效弹性力学性能具有较高的工程可行性,能进一步为玻纤增强注塑件的结构服役性能分析提供科学依据。     

Predictions of elastic property on 2.5D C/SiC composites based on numerical modeling and semi-analytical method

In this paper, the predictions of elastic constants of 2.5D (three-dimension angle-interlock woven) continue carbon fiber reinforced silicon carbide (C/SiC) composites are studied by means of theoretical model and numerical simulation. A semi-analytical method expressing elastic constants in terms of microstructure geometrical parameters and constitute properties has been proposed. First, both the geometrical model of the 2.5D composite and the representative volume element (RVE) in both micro- and meso-scale are proposed. Second, the effective elastic properties of the RVE in 2.5D C/SiC composites are obtained using finite element method (FEM) simulation based on energy equivalent principle. Finally, the remedied spatial stiffness average (RSSA) method is proposed to obtain more accurate elastic constants using nine correction factor functions determined by FEM simulations, also the effects of geometrical variables on mechanical properties in 2.5D C/SiC composites are analyzed. These results will play an important role in designing advanced C/SiC composites.     

Automatic generation of random distribution of fibers in long-fiber-reinforced composites and mesomechanical simulation

An algorithm for the automatic generation of 2D representative volume element (RVE) of unidirectional long-fiber-reinforced composites (LFRCs) is presented in this paper. Both high fiber volume fraction and random fiber distribution are considered in the RVE. Two procedures which are named as global crisscrossing and local disturbing are included in this algorithm. Based on the model generated, mesomechanical analysis is carried out by using general finite element method (FEM) software ABAQUS. Firstly, the effect of the randomness of fiber distribution on the transverse modulus is investigated. Secondly, user subroutine to redefine field variables at a material point (USDFLD) in ABAQUS is used to simulate the damage behavior. A series of computational experiments are performed to evaluate the influence of mesh size on the ultimate load of the composites. The obtained results prove that the algorithm is capable of capturing the random distribution nature of these materials and the RVE produced could be used for predicting the damage onset and propagation of LFRCs.     

Micromechanics based analysis of randomly distributed fiber reinforced composites using simplified unit cell model

The geometry of the simplified unit cell (SUC) model [Aghdam MM, Smith DJ, Pavier MJ. Finite element micromechanical modeling of yield and collapse behavior of metal matrix composites. J Mech Phys Solids 48 (2000) 499–528] is extended to study effects of random fiber arrangement on the mechanical and thermal characterizations of unidirectional composites. The representative volume element (RVE) considered in the model consists of an r × c unit cells in which fibers are surrounded randomly by matrix cells. The presented model is general and can be used to predict the behavior of a fibrous composite subjected to thermal and mechanical, normal and shear, loading. The model also is capable of analyzing various combinations of these loading conditions such as off-axis test of unidirectional coupons. Both random and repeating fiber arrays can be considered in the model. Results for the overall thermal and elastic properties of a SiC/Ti metal matrix composite (MMC) show good agreement with both the finite element and other analytical models with repeating fiber arrays. Results of transverse properties also revealed that hexagonal array assumption for fiber arrangement is more realistic than square array assumption.     

Effects of Interphase Damage and Residual Stresses on Mechanical Behavior of Particle Reinforced Metal-Matrix Composites

Mechanical behavior of aluminum matrix composites reinforced with SiC particles are predicted using an axisymmetric micromechanical finite element model. The model aims to study initiation and propagation of interphase damage subjected to combination of thermal and uniaxial loading. Effects of manufacturing process thermal residual stresses and interphase de-bonding are considered. The model includes a square Representative Volume Element (RVE) from a cylindrical unit cell representing a quarter of SiC particle surrounded by Al-3.5wt.%Cu matrix. Suitable boundary conditions are defined to include effects of combined thermal and uniaxial tension loading on the RVE. An appropriate damage criterion with a linear relationship between radial and shear stresses for interphase damage is introduced to predict initiation and propagation of interphase de-bonding during loading. A damage user subroutine is developed and coupled to the finite element software to model interphase damage. Overall Stress-strain behavior of particulate metal-matrix composite by considering residual stresses is compared with experimental data to estimate interphase strength. Effects of thermal residual stresses in elastic, de-bonding and plastic zones of composite system are discussed in details. Furthermore, parametric study results show high influence of interphase strength on the overall mechanical behavior of composite material.     

Discrete element modelling of unidirectional fibre-reinforced polymers under transverse tension

The mechanical behaviour of unidirectional fibre-reinforced polymer composites subjected to transverse tension was studied using a two dimensional discrete element method. The Representative Volume Element (RVE) of the composite was idealised as a polymer matrix reinforced with randomly distributed parallel fibres. The matrix and fibres were constructed using disc particles bonded together using parallel bonds, while the fibre/matrix interfaces were represented by a displacement-softening model. The prevailing damage mechanisms observed from the model were interfacial debonding and matrix plastic deformation. Numerical simulations have shown that the magnitude of stress is significantly higher at the interfaces, especially in the areas with high fibre densities. Interface fracture energy, stiffness and strength all played important roles in the overall mechanical performance of the composite. It was also observed that tension cracks normally began with interfacial debonding. The merge of the interfacial and matrix micro-cracks resulted in the final catastrophic fracture.     

Progressive failure analysis of unidirectional fiber-reinforced polymers with inhomogeneous interphase and randomly distributed fibers under transverse tensile loading

The effect of damage due to interfacial debonding on the post initial failure behavior of unidirectional fiber-reinforced polymers subjected to transverse tension was investigated using numerical homogenization techniques based on the finite element method. Calculations were performed for unit cells containing fibers distributed at random over the transverse cross-section with inhomogeneous interphase layers. The mechanism of progressive failure was examined at both a global and a local level. A detailed analysis of the proposed micromechanics model revealed that it is able correctly to simulate the evolution of damage and to explain the softening mechanism. It was found that the post initial failure behavior of unidirectional lamina under transverse tension is mainly controlled by the interface strength and the interphase stiffness. The present study showed that local fiber array irregularities are a significant contributor to matrix cracking through local stress concentrations and the occurrence of localization.     

Multiscale modeling for mechanical properties of carbon nanotube reinforced nanocomposites subjected to different types of loading

Multiscale modeling was presented for the nonlinear properties of polymer/single wall carbon nanotube (SWNT) nanocomposite under tensile, bending and torsional loading conditions. To predict the mechanical properties of both armchair and zigzag SWNTs, a finite element (FE) model based on the theory of molecular mechanics was used. For reducing the computational efforts, an equivalent cylindrical beam element was proposed, which has the unique advantage of describing the mechanical properties of SWNTs considering the nonlinearity of SWNT behavior. For a direct evaluation of the rigidities of the proposed equivalent beam, the data obtained through atomistic FE analyses of SWNT were fitted to six different equations, covering the three types of loading for both armchair and zigzag configurations. The proposed equivalent beam element was then used to build a cylindrical representative volume element (RVE) using which the effects of the interphase between SWNT and the polymer on the mechanical properties of RVE could be studied. It was found that while the interphase has a small effect on the nanocomposite stiffness, the ratio of (SWNT length)/(RVE length) dramatically affects the nanocomposite stiffness.     

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。      

Effect of fiber arrangement on mechanical properties of short fiber reinforced composites

The present paper developed a three-dimensional (3D) “tension–shear chain” theoretical model to predict the mechanical properties of unidirectional short fiber reinforced composites, and especially to investigate the distribution effect of short fibers. The accuracy of its predictions on effective modulus, strength, failure strain and energy storage capacity of composites with different distributions of fibers are validated by simulations of finite element method (FEM). It is found that besides the volume fraction, shape, and orientation of the reinforcements, the distribution of fibers also plays a significant role in the mechanical properties of unidirectional composites. Two stiffness distribution factors and two strength distribution factors are identified to completely characterize this influence. It is also noted that stairwise staggering (including regular staggering), which is adopted by the nature, could achieve overall excellent performance. The proposed 3D tension–shear chain model may provide guidance to the design of short fiber reinforced composites.     

On numerical evaluation of effective material properties for composite structures with rhombic fiber arrangements

This paper deals with unidirectional fiber reinforced composites with rhombic fiber arrangements. It is assumed, that there is a periodic structure on micro level, which can be taken by homogenization as a representative volume element (RVE) for the composite, where the composite phases have isotropic or transversely isotropic material characterizations. A special procedure is developed to handle the primary non-rectangular periodicity with common numerical homogenization techniques based on FE-models. Due to appropriate boundary conditions applied to the RVE elastic effective macroscopic coefficients are derived. Results are listed and compared with other publications and good agreements are shown. Furthermore new results are presented, which exhibit the special orthotropic behavior of such composites caused by the rhombic fiber arrangement.     

双随机分布细观分析模型与复合材料性能预报

发展了一种细观力学有限元分析方法——拟真实的参数化双随机分布模型, 该模型综合考虑了纤维增强树脂基复合材料的真实微结构特点和纤维单丝综合力学性能测试结果的离散性特征, 模拟了复合材料中纤维排列和强度分布的随机性。借助移动窗口法研究了该参数化双随机分布模型的可靠性, 确定了其代表性体积单元的尺寸。基于能量法原理推导了单向复合材料的弹性模量预测公式, 结合能量法和渐进失效分析方法, 利用该细观力学有限元方法分别预测了单向纤维增强树脂基复合材料T300/5228的弹性模量和强度性能。数值模拟结果和大部分试验结果吻合良好, 表明发展的细观力学有限元方法能够较好地预测复合材料的力学性能。      

基于多尺度数值模型的复合材料各向异性热膨胀系数预测

依据复合材料内部纤维在基体内的排布规律及层合板铺层特性,基于多尺度方法,建立单层板和层合板代表性体积单元(RVE)模型,施加相应的边界条件,预测单层板的热膨胀系数和工程常数,进而预测复合材料层合板各向异性的等效热膨胀系数。通过与实验数据对比发现,基于正六边形单层板RVE模型预测的热膨胀系数,相比理论预测值,整体更接近实验值,其中预测的单向T300/5208碳纤维增强环氧树脂基复合材料、P75/934碳纤维增强环氧树脂基复合材料和C6000/Pi碳纤维增强环氧树脂基复合材料的横向热膨胀系数与实验结果的误差分别只有3%、1%和2%;采用单层板RVE预测的单向ECR/Derakane 510C玻璃纤维增强乙烯基酯树脂基复合材料的工程常数与实验值最大相差7.5%;层合板RVE模型预测的正交AS4/8552碳纤维增强环氧树脂基复合材料厚度方向的热膨胀系数与实验结果误差可以忽略,只有0.08%。最后以大型复合结构常用的正交铺层结构为研究对象,基于给出的单层板和层合板RVE模型预测了不同铺层复合材料烟道层合板的等效热膨胀系数,环向铺层比例对厚度方向的热膨胀系数影响较小。      

Failure response of fiber-epoxy unidirectional laminate under transverse tensile/compressive loading using finite-volume micromechanics

The transverse damage initiation and extension of a unidirectional laminated composite under transverse tensile/compressive loading are evaluated by means of Representative Volume Element (RVE) presented in this paper based on an advanced homogenization model called finite-volume direct averaging micromechanics (FVDAM) theory. Fiber, fiber-matrix interface and matrix phases are considered within the RVE in determining fiber-matrix interface debonding and matrix cracking. The simulated fracture patterns are shown to be in good agreement with experimental observations.     

Control of strength anisotropy of metal matrix fiber composites

Summary This paper examines theoretically the stress distribution around fiber breaks in a unidirectional reinforced metal matrix composite, subjected to axial loading when plastic yielding of the matrix is allowed to occur. The composites considered have a ductile interphase, bonding the matrix to the fiber. The likelihood of failure of a fiber adjacent to the existing broken fiber is considered. Detailed and systematic results are given for composites with a wide range of fiber volume fractions, Young's modulus of the fibers and the matrix, interphase properties and Weibull modulus for the strength of the fibers. The objective is the optimization of these material and geometric variables to ensure global load sharing among the fibers in the longitudinal direction, which will give the composite good longitudinal strength. Calculations are carried out for transverse loading of the composite to determine the effect of the ductile interphase on the yield strength. Characteristics of the ductile interphase are determined that will provide good longitudinal strength through global load sharing and a relatively high yield strength in the direction transverse to the fibers. This, in turn, will allow control of the strength anisotropy of uniaxially reinforced metal matrix composites.     

Mechanical behavior of carbon fiber reinforced polyamide composites

The purpose of this work is to compare tensile, compressive and interlaminar shear properties of different carbon reinforcement/polyamide composites obtained by interfacial polymerization and hot compression molding techniques. Two types of composite matrices were studied: polyamide 6 and polyamide 6/6, both reinforced by fabric and unidirectional carbon fibers. The effects of the fiber volume fraction and the matrix on mechanical properties were analyzed through tensile, interlaminar shear and compressive tests. In general, the results have shown a slight increase of the composite elastic modulus, tensile and compressive strength with the increase of carbon fiber content. The microscopic damage development within selected composites during the loading has been observed through optical and scanning electron microscope techniques and has shown that shear failure at the fiber/matrix interface has been mostly responsible for damage development, initiated at relatively low stress.     

界面相性态对纤维增强复合材料内应力传递的影响

本文用有限元法研究了具有基体裂纹的纤维增强复合材料内的应力传递问题。假设纤维与基体的界面为非理想的，文中运用“弹簧层”模型首先分析了在不同的组分弹性模量比、纤维体积含量与边界约束条件下，界面相性态对复合材料的应力传宾影响，然后进一步考察了在几种典型的损伤模式下界面附近的应力分布情况。     

孔隙率对碳纤维/尼龙6复合材料湿热性能影响的数值模拟研究

孔隙在复合材料制造过程中广泛存在,在湿热环境下孔隙的存在会改变应力场和水分场,进而影响复合材料的吸湿性能与力学老化性能。对碳纤维/尼龙6(Carbon fiber reinforced polyamide 6,CF/PA6)复合材料在不同温度浸水环境下吸湿老化后的力学性能测试,研究了温度与吸湿量对其力学性能的影响及强度与模量等力学参数的演化规律,建立吸湿参数与力学参数的关联函数。基于随机顺序吸附法算法(Random sequential adsorption,RSA),建立了纤维、界面和孔隙随机分布的代表性体积单元(Representative volume element,RVE)模型。在本构模型中引入依赖于吸湿量的退化因子,研究了孔隙含量对复合材料横向拉伸、压缩、剪切强度和模量的影响,揭示了湿热老化前后不同的失效机制。结果表明：在热湿老化前,由于应力集中,孔隙会导致复合材料力学性能下降,孔隙率含量每增加1%,横向拉伸强度降低6.4%;湿热老化后,基体吸湿塑化效应是复合材料力学性能降低主要因素,对应降低率为3.86%。     

孔隙微观特征对碳纤维/环氧树脂复合材料横向拉伸强度的影响

通过有限元方法研究了相同孔隙率下孔隙的分布、尺寸和形状等微观特征对碳纤维增强环氧树脂复合材料单向板横向拉伸强度的影响。首先使用Matlab对复合材料微观图像进行处理,提取孔隙的半径分布。然后通过C++编写多种孔隙随机分布算法,包括可以生成不同分布孔隙、不同尺寸孔隙以及不同形状孔隙的随机分布算法。最后通过Python参数化生成代表性体积单元（RVE）,用有限元方法研究相同孔隙率下孔隙的分布、尺寸和形状对碳纤维/环氧树脂复合材料单向板横向拉伸强度的影响。研究结果显示,孔隙率相同时,碳纤维/环氧树脂复合材料的孔隙形状对横向弹性模量的影响较大,孔隙尺寸和形状对横向拉伸强度有较大的影响。