含平行币状裂纹复合材料的强度准则

对于含平行币状裂纹材料表现出来的横观各向同性,引入Walpole's基来表示和计算.通过构造能量密度支函数来定义复合材料的宏观应力,在非线性均质化方案中采用了基于能量原理的修正割线法,考虑了静水压力对偏应变的影响.研究了含微裂纹材料弹性刚度张量的三种解法:稀疏解法、MT法和PCW法,获得了含平行币状裂纹材料具有Drucker-Prager型式的宏观强度准则,研究结果表明,材料的弹性模量比和宏观摩擦系数随着裂纹的体积分数增加而减小,并且裂纹外形比越小,稀疏解法、MT法和PCW法对材料的弹性模量比和宏观摩擦系数预测曲线差异越大.     

考虑孔隙及微裂纹影响的混凝土宏观力学特性研究

混凝土是一种典型的多孔介质材料,孔隙分布错综复杂,孔径尺寸跨越微观尺度和宏观尺度,对混凝土弹性模量及强度等力学参数产生巨大影响.认为混凝土是由骨料、孔隙及砂浆基质组成的三相复合材料,采用Monte Carlo 法将孔隙、微裂纹及微缺陷与骨料颗粒随机投放在砂浆基质中.根据三相球模型及中空圆柱形杆件模型得到含孔材料的有效力学性质,并推导得到含孔材料的等效本构模型.建立含孔隙混凝土试件的细观单元等效化力学模型,对二级配含孔隙混凝土试件在单轴拉伸及压缩条件下的反应进行了非线性分析.结果表明:孔隙、微裂纹的存在对混凝土宏观弹性模量、强度及残余强度等力学性质都有很大影响,在对混凝土宏观力学特性分析及研究混凝土损伤断裂时不应忽略其影响.     

基于共焦点椭球构型的复合材料有效性质预测

介绍了共焦点椭球构型,给出了基于该构型对空间任意取向复合材料模量的解析计算公式,并将其同Mori-Tanaka(MT)法、Ponte-Castaneda-Willis (PCW)方法以及Hashin-Shtrikman(HS)界限进行了比较。数值结果显示,基于该构型的预测处于MT法和PCW法所预测的值之间,并且与MT法所预测的值接近。此外,还对纤维不同的角度平均方法对有效性质的影响做了讨论。     

币形裂纹纤维桥联效应力学分析

本文基于Ｃａｓｔｉｇｌｉａｎｏ＇ｓ定理和界面剪滞模型，得到了含界面相效应的复合材料币形裂纹纤维桥联增韧和裂纹张开位移控制方程。并按照第二类Ｆｒｅｄｈｏｌｍ积分方程的迭代解法给出其数值结果。为便于分析界面相参数对增韧效果等影响，寻求了该控制方程的近似解，对近似解进行了误差估计。在此基础上得到了界面剪切模量、裂纹长度、界面厚度、纤维半径，纤维体积分数以及材料性质等参数对币形裂纹桥联效应的影响。     

梯度复合材料裂纹扩展路径和起裂载荷的有限元分析

为了模拟功能梯度材料(FGM)在工程应用中可能会出现的断裂问题并计算相应的开裂载荷,通过编写用户自定义UEL子程序将梯度扩展单元嵌入到ABAQUS软件中模拟功能梯度材料的物理场,并编写交互能量积分后处理子程序计算裂纹尖端的混合模式应力强度因子(SIF),采用最大周向应力准则编写子程序计算裂纹的偏转角,并模拟了裂纹扩展路径,计算了裂纹的起裂载荷。讨论了材料梯度参数对裂纹扩展路径以及起裂载荷的影响规律。通过与均匀材料的对比,验证了功能梯度材料断裂性能的优越性。研究表明:外载平行于梯度方向时,垂直梯度方向的初始裂纹朝着等效弹性模量小的方向扩展,且偏转角在梯度指数线性时出现峰值,并随着组分弹性模量比的增加而变大;当外载和初始裂纹均平行于梯度方向时,材料等效弹性模量和断裂韧性的增加或者梯度指数的减小都导致起裂载荷变大。     

币形裂弦纤维桥联效应力学分析

本文基于Ｃａｓｔｉｇｌｉａｎｏ’ｓ定理和界面剪滞模型，得到了含界面相效应的复合材料币形裂纹张开位移控制方程，并按照第二类Ｆｒｅｄｈｏｌｍ积分方程的迭代解法给出其数值结果，为便于分析界面相参数增韧效果等影响，寻求了该控制方程的近似解，对近似解进行了误差估计，在此基础上得到了界面剪切模量，裂纹长度，界面厚度，纤维半径，纤维体积分数以及材料性质等参数对币形裂纹桥联效应的影响。     

温度和应力比对航空铝合金疲劳裂纹扩展规律的影响及其机理

针对7475-T761航空铝合金中心开裂(MT)试样进行了不同温度、不同应力比条件下的一系列疲劳裂纹扩展试验,得到了相应试验条件下的疲劳裂纹扩展数据与规律,讨论了应力比、环境温度对疲劳裂纹扩展行为的影响,并利用扫描电镜(SEM)观测分析了疲劳断口。结果表明:7475-T761铝合金疲劳裂纹扩展速率随应力比、温度的增加而增加;消除裂纹闭合效应影响后,相同温度不同应力比下的da/dN-ΔKeff可由同一拟合公式描述;高温时弹性模量和材料抗拉强度的下降以及裂纹表面氧化导致裂纹扩展速率较快;对比不同条件下稳定扩展区疲劳条带宽度验证了试验分析结论。     

孔隙率变化规律及其对混凝土变形过程的影响

混凝土内部随机分布的微裂纹和孔洞等细观缺陷影响其破坏机理及宏观力学性能,且孔隙率随着外荷载的变化而不断变化。该文中将含缺陷(微裂纹、各种空隙等)混凝土复合材料简化为空心球力学模型,基于弹性力学理论推导并获得了混凝土当前孔隙率与材料初始孔隙率及体应变之间的定量关系;推导并得到了含孔隙混凝土的有效弹性模量、有效泊松比及峰值应变等与孔隙率的定量关系,进而得到了单轴加载条件下含孔隙混凝土细观单元的等效多折线本构关系模型。最后,采用细观单元等效化力学模型,研究了单轴加载(拉伸和压缩)情况下不同孔隙率混凝土材料的破坏过程及宏观力学性能,探讨了孔隙率变化规律及其对混凝土变形过程的影响。     

含穿透裂纹岩石的高频疲劳特性研究

该文以高强度白水泥为模拟材料,对含预置不同数目穿透裂纹的岩石进行了单轴压缩试验以及高频疲劳单轴压缩试验研究。该文充分考虑了裂纹对岩石试块宏观力学性能的影响,描述了裂纹扩展的规律,得到了含穿透裂纹的岩石材料的S-lgN曲线和考虑裂纹相互作用的疲劳方程。     

40Cr钢韧脆转变区域的一种细观随机模型

针对４０Ｃｒ钢提出孤细观模型认为；在统计意义上，材料的韧性断裂为空穴机制，临界空穴扩张比参数可以作为其判据；材料的脆性断裂可以用内嵌币状裂纹的脆性断裂模型来模拟；在韧脆转变区内，这两种机理并存并相互竞争。     

Influence of interfacial microcracks on the elastic properties of composites

A model is established to quantify the influence of interfacial microcracks on the elastic properties of a particulate composite using a combination of theoretical and finite element analysis. A unique way to construct physical models which could accommodate both crack size and crack density is proposed. Based on energy principles, the influence of a dilute concentration of interfacial microcracks is first studied. The case of a finite concentration of microcracks is solved subsequently by combining the dilute concentration solutions and the differential scheme. Both cases agreed well with existing composite theories for the limiting condition of complete decohesion. The final model predicts the effective elastic properties as functions of both crack size and microcrack density.     

Some reflections on the Mori-Tanaka and Ponte Castañeda-Willis methods with randomly oriented ellipsoidal inclusions

Summary For a two-phase isotropic composite consisting of an isotropic matrix and oriented isotropic ellipsoidal inclusions, Mori-Tanaka's (MT) [6] method and the more recent Ponte Castañeda-Willis (PCW) [1] method are perhaps the only two methods that deliver explicit results for its effective moduli. An attractive feature of the MT method is that it always stays within the Hashin-Shtrikman [3] bounds, while the novel part of the PCW approach is that it has a well defined microstructure. In this paper, we made a comparative study on these two models, for both elasticity and their applications to plasticity. Over the entire range of inclusion shapes, the PCW estimates are found to be consistently stiffer than the MT estimates. An investigation of the possibility of a PCW microstructure for the MT model indicates that the MT moduli could be found from the PCW formulation, but this would require a spatial distribution that is identical to the oriented inclusion shape. Such a requirement implies that the underlying two-point joint probability density function is not symmetric, and thus it is not permissible. One is led to conclude that, unlike the aligned case, the MT model cannot be realized from the PCW microstructure with randomly oriented inclusions.     

Simulation of crack growth in ductile materials

During crack growth of real materials, the total energy released can be partitioned into elastic and dissipative terms. By analyzing material models with mechanisms for dissipating energy and tracking all energy terms during crack growth, it is proposed that computer simulations of fracture can model crack growth by a total energy balance condition. One approach for developing fracture simulations is illustrated by analysis of elastic-plastic fracture. General equations were derived to predict crack growth and crack stability in terms of global energy release rate and irreversible energy effects. To distinguish plastic fracture from non-linear elastic fracture, it was necessary to imply an extra irreversible energy term. A key component of fracture simulations is to model this extra work. A model used here was to assume that the extra irreversible energy is proportional to the plastic work in a plastic-flow analysis. This idea was used to develop a virtual material based on Dugdale yield zones at the crack tips. A Dugdale virtual material was subjected to computer fracture experiments that showed it has many fracture properties in common with real ductile materials. A Dugdale material can serve as a model material for new simulations with the goal of studying the role of structure in the fracture properties of composites. One sample calculation showed that the toughness of a Dugdale material in an adhesive joint mimics the effect of joint thickness on the toughness of real adhesives. It is expected, however, that better virtual materials will be required before fracture simulations will be a viable approach to studying composite fracture. The approach of this paper is extensible to more advanced plasticity models and therefore to the development of better virtual materials.     

Models of Interface Separation Accompanied by Plastic Dissipation at Multiple Scales

Two continuum mechanical models of interface fracture for interfaces joining materials where at least one undergoes plastic deformation are reviewed and examined critically. The embedded process zone model (EPZ model) has an adhesive zone, characterized by a work of separation and an interface strength, embedded within a continuum model of the adjoining materials. The SSV model imposes an elastic, plasticity-free layer of prescribed thickness between the interface and the surrounding elastic-plastic continuum. Crack advance requires the work of separation to be supplied by the local elastic crack tip field. The objective of each model is to provide the relation between the macroscopic interface toughness (the total work of fracture) and the work of separation. Under steady-state crack growth, the total work of fracture is the work of separation plus the work of plastic dissipation, the latter often far exceeding the former. It will be argued that each model has its own domain of validity, subject to the accuracy of conventional continuum plasticity at small scales, but neither is able to capture the dramatic trends which have been observed in macroscopic toughness measurements stemming from alterations in interface bonding conditions. A unified model is proposed which coincides with the two models in their respective domains of validity and provides a transition between them. Interface separation energy and interface strength (the peak separation stress) each play a central role in the unified model. Strain gradient plasticity is used to illustrate the effect of plastic deformation at the micron scale on the link between interface and macroscopic properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fracture of a brittle particulate composite

A study was made of the fracture process in a model brittle composite containing nickel spheres in a glass matrix. The macroscopic fracture characteristics of the system were determined by fracture surface energy, fracture strength and elastic modulus measurements. The microstructures of the composites were defined using quantitative microscopy and the fracture process was studied by the technique of ultrasonic fractography. This procedure traced changes in the crack front configuration and local crack velocity as the crack interacted with the array of nickel particles.     

Effect of material plasticity on fatigue crack propagation under complex stress state

The maximum energy release rate criterion, i.e., G _max criterion, is commonly used for crack propagation analysis. However, this fracture criterion is based on the elastic macroscopic strength of materials. In the present investigation, a modification has been made to G _max criterion to implement the consideration of the plastic strain energy. This criterion is extended to study the fatigue crack growth characteristics of mixed mode cracks in steel pipes. To predict crack propagation due to fatigue loads, a new elasto-plastic energy model is presented. This new model includes the effects of material properties like strain hardening exponent n, yield strength σ_y and fracture toughness and stress intensity factor ranges. The results obtained are compared with those obtained using the commonly employed crack growth law and the experimental data.     

Homogenized mechanical behavior of composite micro-structures including micro-cracking and contact evolution

The aim of this paper is to study the effects of micro-cracking on the homogenized constitutive properties of elastic composite materials. To this end a novel micro-mechanical approach based on homogenization techniques and fracture mechanics concepts, is proposed and an original J-integral formulation is established for composite micro-structures. Accurate non-linear macroscopic constitutive laws are developed for a uniaxial and a shear macro-strain path by taking into account changes in micro-structural configuration owing to crack growth and crack face contact. Numerical results, carried out by coupling a finite element formulation and an interface model, are applied to a porous composite with edge cracks and a debonded short fiber-reinforced composite. The composite micro-structure is controlled by the macroscopic strain and the micro-to-macro transition, settled in a variational formulation, is obtained for three types of boundary conditions, i.e. linear displacements, uniform tractions and periodic fluctuations and anti-periodic tractions. The accuracy of the determined macroscopic constitutive properties to represent the failure characteristics of locally periodic defected composites is also investigated in terms of energy release rate predictions, by comparisons between a direct analysis and homogenization approaches. Results highlight the dependence of the macroscopic constitutive law for a micro-structure with evolving defects on both the macro-strain path and the type of boundary conditions and the capability of the proposed model to provide a failure model for a composite material undergoing micro-cracking and contact.     

硫化锌热冲击试验与裂纹间距预报

重点研究了硫化锌热冲击开裂机理和热冲击裂纹间距、深度的预报。10mm厚硫化锌试块的燃气急热试验表明:裂纹间距随热冲击能量的增大而减小,热冲击过程中加热面最先出现非贯穿裂纹,停止加热后,裂纹贯穿试件。结合传热和热强度仿真分析,获得了热冲击过程中试件的瞬态温度场和应力场。基于材料性能的损伤演化理论,以裂纹间距和深度为变量,利用最小能量原理,获得了热冲击裂纹间距的理论预报方法,预测结果与试验吻合较好,进而分析了断裂韧性、热胀系数、材料初始模量对裂纹间距、裂纹深度的影响。该文的研究对深入理解硫化锌的热冲击失效机制,对其改性和研制具有重要意义。     

加层压电层硬币型裂纹断裂分析

研究粘接着弹性层的压电层内硬币型裂纹的断裂问题。压电层与弹性层均为横观各向同性材料,r轴方向无限长,z轴方向有限厚度。压电层沿z轴方向极化。考虑电不导通裂纹表面条件,利用Hankel积分变换将问题化为求解积分方程组,导出了场强度因子与能量释放率的表达式。给出了数值计算结果,并分析了弹性层厚度对场强度因子与能量释放率的影响。