纤维排列方式对单向纤维加强复合材料弹性常数的影响

本文应用GMC方法计算了纤维在规则排列和随机排列时单向纤维加强复合材料的总体弹性系数，结果表明，在一定纤维体积比范围内，虽然纵向弹性模量不受纤维排列方式的影响，但是复合材料的横向弹性模量和横向剪切模量受纤维排列方式的影响较大，所以，在考察复合材料的横向弹性常数时，应该考虑纤维排列方式的影响，在相同纤维比的情况下，正方形排列和正方形对角排列所得的弹性常数是上下界，而六角形排列和随机排列的结果位于以上两者之间，当纤维比较小时（小于20％），纤维的排列方式对各弹性常数均无明显影响，本文的结果在工程应用中有指导意义。     

含大量随机分布细小纤维的单向纤维增强复合材料横向弹性性能参数计算的内嵌区域模型法

利用有限元方法求取单向纤维增强复合材料的横向弹性性能参数的计算模型包括三维模型、两维平面应变模型、单胞模型等等.由于单胞模型仅仅适用于纤维规则排列情况.在纤维随机分布且纤维大小亦为随机时,单向纤维增强复合材料横向弹性性能参数必须通过对于复合材料块体的计算才能获得.同时在随机分布纤维的数量增大时,三维模型和二维平面应变模型的计算量急剧增加,模型的处理能力不强.该文提出一种利用内嵌区域模型来计算含大量随机大小、随机分布细小纤维的单向纤维增强复合材料块体的横向弹性性能参数的方法,有效降低了计算量.在较低的计算费用下,能够快速获得单向纤维增强复合材料的横向弹性性能参数.     

各向异性介质波动问题的时域边界元法及实验验证

对前已建立的各向异性介质波动问题的时域边界元计算模型进行了实验验证．用单向纤维增强光弹性复合材料模拟正交异性介质，用冲击加载，加载方向与纤维方向分别成0°，90°及45°角度，进行了正交异性动态光弹性实验及动态应变测量，并同时对该模型进行了时城边界元计算．将时域边界元方法计算出的应力分量代入正交异性动态光弹性的动态应力-光性定律，得到双折射条纹级数随时间的变化曲线，将其与动态光弹性实验的结果进行比较；此外，由动态电测获得的应变响应曲线推算出应力时程；与时城边界元计算出的应为响应曲线也进行了比较．两种情况下，时域边界元的计算成果均与实验成果吻合较好，从而证明该各向异性介质波动问题的时域边界元计算模型具有较好的精度及稳定性，可用于各向异性介质的动态问题特别是波传播问题的分析研究．     

INVESTIGATION ON FATIGUE HYSTERESIS LOOPS MODELS OF FIBRE-REINFORCED CERAMIC-MATRIX COMPOSITES

纤维增强陶瓷基复合材料初始加载到疲劳峰值应力时, 基体出现裂纹, 纤维/基体界面发生脱粘. 在疲劳载荷作用下, 纤维相对基体在界面脱粘区往复滑移使得陶瓷基复合材料出现疲劳迟滞现象. 建立了纤维陶瓷基复合材料疲劳迟滞回线细观力学模型, 采用断裂力学方法确定了初始加载纤维/基体界面脱粘长度、卸载界面反向滑移长度与重新加载新界面滑移长度, 分析了4种不同界面滑移情况的疲劳迟滞回线. 假设正交铺设与编织陶瓷基复合材料疲劳迟滞回线主要受0°铺层、轴向纱线内纤维/基体界面滑移的影响, 预测了单向、正交铺设与编织陶瓷基复合材料在不同峰值应力与不同循环的疲劳迟滞回线, 与试验结果吻合.     

纤维增强陶瓷基复合材料疲劳迟滞回线模型研究

纤维增强陶瓷基复合材料初始加载到疲劳峰值应力时, 基体出现裂纹, 纤维/基体界面发生脱粘. 在疲劳载荷作用下, 纤维相对基体在界面脱粘区往复滑移使得陶瓷基复合材料出现疲劳迟滞现象. 建立了纤维陶瓷基复合材料疲劳迟滞回线细观力学模型, 采用断裂力学方法确定了初始加载纤维/基体界面脱粘长度、卸载界面反向滑移长度与重新加载新界面滑移长度, 分析了4种不同界面滑移情况的疲劳迟滞回线. 假设正交铺设与编织陶瓷基复合材料疲劳迟滞回线主要受0°铺层、轴向纱线内纤维/基体界面滑移的影响, 预测了单向、正交铺设与编织陶瓷基复合材料在不同峰值应力与不同循环的疲劳迟滞回线, 与试验结果吻合.      

SMA纤维复合材料超弹性滞回特性研究

本对SMA纤维复合材料的轴向超弹性滞回特性进行理论分析，首先采用三相同心圆柱体模型和多胞模型计算材料的宏细观力学性能及其纤维轴向应力应变响应曲线。超弹性滞回特性的数值计算则采用平均抛物线插值法，最后给出环氧树脂/Ni-44.8wt%Ti纤维复合材料的具体数值结果。     

界面脱粘对陶瓷基复合材料疲劳迟滞回线的影响

采用细观力学方法对脆性纤维增强的陶瓷基复合材料拉-拉疲劳载荷下应力-应变迟滞回线进行了研究,将拉梅公式与库仑摩擦法则相结合分析了界面脱粘区以及粘结区复合材料细观应力场.根据卸载与重新加载时纤维相对基体滑移机制,分析了加卸载纤维轴向应力分布,结合断裂力学界面脱粘准则确定了初始加载界面脱粘长度ls、卸载界面反向滑移长度y以及重新加载界面滑移长度z',讨论了界面脱粘能和界面摩擦系数对初始界面脱粘、卸载界面反向滑移、重新加载界面滑移以及加卸载迟滞回线的影响.并与Pryce-Smith模型和试验数据进行对比表明:该文模型与试验曲线吻合的较好.     

单向纤维增强复合材料的载荷集中因子和裂纹扩展统计理论

本文对单向纤维增强复合材料裂纹扩展统计理论的两个主要论据进行了研究.1.由于纤维断裂引起的应力重分布的计算:文中采用通常的剪滞模型,进行了弹塑性形变理论分析和弹性分析,弹性分析的结果与已有的J.M.Hedgepeth 的结果相符.2.裂纹扩展模型的探索:这里舍弃沿用Gücer-Gurland.Rosen 链式模型,提出了逐渐扩大断裂层的模型,参照A.S.Argon 等人的方法进行了统计分析,得到了单向纤维增强复合材料拉伸强度的计算公式.     

纤维排列方式对复合材料总体粘弹性常数的影响

对于金属基或高分子聚合物基复合材料，在特定情况下会表现出明显的粘弹性特性。本文采用Riemann—Liouville形式的分数阶导数模型描述基体的粘性特性，通过渐进均匀化方法给出了预测纤维加强复合材料整体本构关系的解析表达式，给出应用于基体具有Makris粘弹性关系的具体形式。最后，考察了圆截面纤维正方形排列和对角排列时的总体粘弹性弹性常数随纤维比的变化曲线。结果表明，这类复合材料仍具有粘弹性特性，其整体粘弹性本构关系的弹性部分综合了纤维弹性和基体弹性的贡献，粘性部分来自基体粘性的贡献，复合材料具有和基体相同的粘性系数和分数阶。为分析微结构特征对整体特性的贡献，须求解两类局部问题。在相同纤维体积比情况下，正方形排列的总体弹性系数大于正方形对角排列，而粘性常数相反。     

考虑滞弹性的蠕变—塑性交互本构模型

基于CCP模型和对滞弹性描述的结合,统一考虑了滞弹性、蠕变和塑性耦合。通过对304不锈钢高温下的非比例循环加载蠕变响应进行的分析,验证了所用方法的有效性。     

Inelastic behavior of metal-matrix composites at elevated temperature

A set of constitutive relations are given for the prediction of the overall behavior of fiber-reinforced materials, in which both matrix and fiber constituents are thermoelastic-thermoviscoplastic materials. The temperature-dependent response of the composite to a given loading is determined from the set of equations by an incremental procedure in time. The theory is applied to investigate the overall response of unidirectional graphite thermoelastic fibers reinforcing an elastic-thermoviscoplastic aluminum matrix, to thermal and mechanical loadings. The residual microstresses and microstrains which develop in the composite when it is cooled from its curing temperature, are investigated. The effect of this residual field on the overall behavior of the composite when it is subjected to various types of mechanical cyclic loadings is studied.     

Viscoelastic response of a unidirectional composite containing two viscoelastic constituents

Viscoelastic response of a unidirectional aramid fiber-reinforced epoxy was measured. Procedures to measure all five time-dependent material properties necessary to describe behavior of a transversely isotropic continuous-fiber unidirectional lamina were implemented. The Iosipescu shear method was used to measure in-plane and interlaminar shear viscoelastic response. Applicability of the Schapery single-integral nonlinear viscoelastic constitutive model to describe time-dependent mechanical behavior of a laminated composite material containing two viscoelastic phases was explored. Linear and nonlinear viscoelastic parameters for this two-viscoelastic constituent composite were measured and data summaries are presented in the paper. The time-dependent behavior of this two-viscoelastic constituent composite material was found to be complex, but the Schapery nonlinear viscoelastic model did adequately fit the response of such a composite to uniaxial applied loads.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Effects of matrix inelasticity on the overall hysteretic behavior of TiNi-SMA fiber composites

The effects of the inelastic deformation of the matrix on the overall hysteretic behavior of a unidirectional titanium–nickel shape-memory alloy (TiNi-SMA) fiber composite and on the local pseudoelastic response of the embedded SMA fibers are studied under the isothermal loading and unloading condition. The multiaxial phase transformation of the SMA fibers is predicted using the phenomenological constitutive equations which can describe the two-step deformation due to the rhombohedral and martensitic transformations, and the inelastic behavior of the matrix material using the standard nonlinear viscoplastic model. The average behavior of the SMA composite is evaluated with the micromechanical method of cells. It is observed that the inelastic deformation of the matrix due to prior tension results in a compressive stress in the matrix after unloading of the SMA composite and this residual stress impedes the complete recovery of the pseudoelastic strain of the SMA fibers. This explains that a closed hysteresis behavior of the SMA composite is no longer observed in contrast with the case that an elastic behavior of matrix is assumed. The predicted local stress–strain behavior indicates that the cyclic response of matrix is crucial to the design of the hysteretic performance of the SMA composite under the repeated loading conditions.     

Anisotropic wave propagation and its applications to NDE of composite materials

This paper utilized anisotropic wave propagation theory to measure the elastic constants of a unidirectional fiber-reinforced composite specimen. For plane waves propagating in the composite specimen, the deviation of the propagational directions between the energy and phase velocities were measured. It is found that in such a sample, the deviations may be as large as 60 degrees. The measured energy velocities were transformed to the phase velocities of the plane waves by employing a numerical scheme. It is demonstrated that the elastic constants of a unidirectional fiber-reinforced composite can be determined by conducting ultrasonic experiments in two principal symmetry planes.     

Modeling rate-independent hysteresis in large deformations of preconditioned soft tissues

A phenomenological model is proposed for characterizing rate-independent hysteresis exhibited by preconditioned soft tissues. The preconditioned tissue is modeled as an isotropic composite of a hyperelastic component and a dissipative (inelastic) component. Specifically, the constitutive equations are hyperelastic in the sense that the stress is determined by derivatives of a strain energy function. Inelasticity of the dissipative component is controlled by a yield function with different functional forms for the hardening variable during deformation loading and unloading. The constitutive equations proposed in this paper are simple. In particular, they depend on only seven material constants: three controlling the response of the elastic component and the remainder controlling the response of the dissipative component. More importantly, the material constants can be determined to match rather general loading and unloading behavior. It is observed that the hysteretic response of the model compares well with experimental data for passive uniaxial loading/unloading of Manduca muscle. Moreover, the present model treats partial loading and reloading of preconditioned tissue as elastic–plastic response, which is different from the treatment of pseudo-elastic models used in the literature.     

Effect of initial flaw shape on crack extension in orthotropic composite materials

An analytical study is presented showing the effects of the notch tip geometry on the location and direction of crack growth from an existing notch in a unidirectional fibrous composite modelled as a homogeneous, anisotropic, elastic material. Anisotropic elasticity and the normal stress ratio theory are used to study crack growth from elliptical notches in unidirectional composites. Sharp cracks, circular holes, and ellipses are studied under far-field tension, and shear loading. Limited comparisons are made showing good correlation with experimental results.     

Higher-order theory for periodic multiphase materials with inelastic phases

An extension of a recently-developed linear thermoelastic theory for multiphase periodic materials is presented which admits inelastic behavior of the constituent phases. The extended theory is capable of accurately estimating both the effective inelastic response of a periodic multiphase composite and the local stress and strain fields in the individual phases. The model is presently limited to materials characterized by constituent phases that are continuous in one direction, but arbitrarily distributed within the repeating unit cell which characterizes the material's periodic microstructure. The model's analytical framework is based on the homogenization technique for periodic media, but the method of solution for the local displacement and stress fields borrows concepts previously employed by the authors in constructing the higher-order theory for functionally graded materials, in contrast with the standard finite-element solution method typically used in conjunction with the homogenization technique. The present approach produces a closed-form macroscopic constitutive equation for a periodic multiphase material valid for both uniaxial and multiaxial loading. The model's predictive accuracy in generating both the effective inelastic stress-strain response and the local stress and inelastic strain fields is demonstrated by comparison with the results of an analytical inelastic solution for the axisymmetric and axial shear response of a unidirectional composite based on the concentric cylinder model and with finite-element results for transverse loading.     

Application of fractional exponential hereditary kernels in the nonlinear theory of viscoelasticity

It is proved that fractional exponential hereditary kernels of nonlinear viscoelasticity can be used to evaluate creep strains and stress relaxation. A nonlinear theory of viscoelasticity with time-independent nonlinearity described as a nonlinear curve of instantaneous elastoplastic deformation is used. The calculated results are validated against experimental data on the viscoelastic deformation of laminated and unidirectional fibrous composites and their components under the conditions of constant stresses, complete unloading, incremental loading, pure torsion, and constant strains     

Experimental investigation of dynamic punch tests on isotropic and composite materials

An experimental investigation is conducted on the two-dimensional punch problem for isotropic materials and unidirectional fiber-reinforced composite materials under quasi-static and impact loading. Singular stresses are generated in the specimen near the punch corners, and the stress intensity factorK _Iis introduced to describe the singular stress field. Laser interferometry is used to measure in-plane stresses (transmission mode) and out-of-plane displacements (reflection mode) and then estimate the stress intensity factor. In the dynamic case, a high-speed photography technique was employed to capture the transient response of the specimen and measureK(t) just after the impact. In all the cases, a good agreement between the measurements ofK and theoretical predictions was found.