微结构弹性损伤模型

本文从微结构理论出发,用相对变形描述微结构之间的微空隙和微裂纹引起的材料损伤的影响,得到用损伤张量描述各向异性损伤的微结构弹性损伤模型.化简该损伤模型可得到目前公认的几种主要损伤模型.对于混凝土材料的单向拉、压的基本受载情况的计算结果与D.krajcinovic的平面裂纹矢量损伤模型的计算结果及实验数据吻合较好,特别在失稳后阶段对实验结果也有较好描述.     

基于局域分析的疲劳短裂纹群体演化随机模型

采用局域裂纹数密度描述金属材料中不同局部区域的疲劳短裂纹群体损伤的发展情况通过考虑在不同局域存在的材料性质的随机涨落及局部损伤对损伤总量发展的影响，建立了局域裂纹数密度演化随机方程对方程数值求解从而模拟了材料的疲劳短裂纹损伤过程结果显示出主裂纹出现的随机性，并讨论了裂纹总数与最大裂纹尺度在统计意义上的演化特征     

湿式铜基粉末冶金摩擦材料黏着损伤研究

通过离合器试验台研究了铜基粉末冶金摩擦材料的摩擦学性能和黏着损伤特性;采用扫描电子显微镜(SEM)和能谱仪(EDS)分析了表面损伤特征和成分变化.研究了湿式铜基粉末冶金摩擦材料黏着损伤形成发展历程和损伤机理.结果表明:在高于临界功率密度的试验条件下,出现黏着损伤.黏着损伤初期摩擦材料表面出现大量长度200μm~1mm的剥落坑;黏着损伤进展期摩擦片表面形成点状、连续分布撕裂坑;黏着失效期对偶片表面出现显著的过铜现象,扭矩曲线波动异常.摩擦材料发生黏着失效前,在低于临界功率密度的条件下,摩擦性能无明显改变,具有可回复性,仍可继续使用.     

复杂应力下脆性岩石材料的微裂纹损伤特性

基于无限大体深埋平片状椭圆形裂纹的变形场及其扩展条件,分别推导了脆性岩石材料内部具有任意空间取向的单个张开型或闭合型椭圆形微裂纹及其扩展引起的附加柔度张量;考虑微裂纹系统对材料变形的影响,引入了概率密度函数,得到了任意应力状态下脆性岩石材料的宏细观损伤模型;分析了椭圆形微裂纹的短长轴比率对材料损伤的影响.计算结果显示:材料损伤随着短长轴比率的增大而增大,短长轴比率对材料损伤的影响会随着载荷的增大而提高.将本文模型应用于混凝土的单轴拉伸和花岗岩的单轴压缩,结果表明本文模型能够对实验现象给予很好地解释.     

一种基于材料细观特征量的PBX拉伸强度理论模型

为给塑性黏结炸药(PBX)的力学强度设计提供支撑、探索材料细观特征量与材料强度之间的定量规律,应用微裂纹扩展区理论,将PBX炸药的单轴拉伸过程中力学响应特征的变化归结为扩展裂纹取向角度的增加,将扩展裂纹最大取向角与拉伸强度相关联,构建了基于材料细观特征量的拉伸强度理论模型,并采用不同温度的单轴拉伸实验验证了该理论模型的有效性。研究表明:该拉伸强度理论模型可以实现对PBX炸药拉伸强度与炸药微裂纹密度、颗粒/黏结剂界面性能以及颗粒/黏结剂体系的表观杨氏模量、泊松比等细观特征量之间关系的定量描述。     

平面撞击对Ti6Al4V合金结构的影响

采用一级轻气炮对Ti6Al4V合金进行对称与非对称正碰撞冲击,回收冲击样品,并进行电镜扫描和X射线衍射分析。结果表明:随着冲击速度的增加,样品发生不同程度的变形,材料内部产生微裂纹并导致材料发生损伤,而损伤进一步促进了微裂纹的产生、扩展;由X射线分析结果可知,冲击使材料的晶面发育更加完整,并在材料冷却过程中出现一种亚稳态结构,在一定程度上可提高冲击韧性。 更多还原     

高温疲劳表面短裂纹的分形研究

对2.25Cr-1Mo合金钢进行了高温疲劳实验，采用中断试验与复膜技术相结合的方法观测了试样表面疲劳短裂纹萌生，扩展及合体的演化过程，并用数据格子方法研究了含裂纹表面的分形特征。结果表明：疲劳短裂纹的萌生扩展行为具有分形特征，可以用分形理论加以描述；分形维数随着疲劳过程的继续而稳定地成比例增加；分形维数可以作为把握材料总体损伤状态的参数，可作为寿命评价的依据。     

准脆性材料损伤破坏的细宏观联结分析

准脆性材料在外力作用下不仅产生应力与变形,而且还会内部缺陷或微裂纹随着变化和出现宏观裂缝扩展.根据材料的微细观测,通过固体力学原理建立模型分析其微裂纹与宏观裂缝之间的关联.材料中宏观裂缝端部通常存在损伤区,且在该区内存在许多相对微小缺陷或微裂纹,从而使得材料局部变形增加而力量减弱.若把该损伤带视为裂缝一部分,那么在该虚拟裂缝两岸上将存在分布黏聚力;虚拟裂缝两岸的相对位移将是微裂纹区变形及扩展的综合体现.为此,对包含有细观裂纹的代表性体积单元进行力学计算,然后将其嵌入到宏观裂缝端部损伤带的变形计算中,以探寻固体失效的细观与宏观尺度力学分析的联结,并与实测数据相对比.     

压缩载荷作用下岩石的细观损伤和塑性研究

建立岩石微裂纹扩展的细观力学模型,研究了岩石的细观损伤和塑性性质.压缩载荷下微裂纹尖端翼裂纹稳定扩展表征岩石的细观损伤,采用应变能密度准则求解复合型断裂的翼裂纹扩展长度,微裂隙统计的二参数Weibull函数模型反映绝对体积应变对微裂纹分布数目影响,进而用翼裂纹扩展所表征的应力释放体积和微裂纹数目来表示含有微裂隙的岩石损伤演化变量;宏观塑性屈服函数采用Voyiadjis等的等效塑性应变的硬化函数,反映了塑性内变量对硬化函数的影响;建立岩石模型的本构关系及其数值算法,并用回映隐式积分算法编制了模型的本构程序.分析弹塑性损伤模型的围压对岩石损伤的影响,并从围压和短微裂隙长度等因素分析模型的岩石的损伤和宏观塑性特性.     

基于M积分的脆性材料微缺陷等效损伤面积/体积表征

随着脆性材料在工程中的广泛应用,对脆性材料中微缺陷进行统一的损伤水平标定,具有重要的科学研究和工程应用价值. 本研究提出一种基于M积分的材料等效损伤面积/体积标定方法, 以具有相同M积分值的圆孔面积或球孔体积来标定 复杂微缺陷构型的损伤水平,从而实现不同类型微缺陷真实损伤水平的统一表征. 首先,基于Lagrangian能量密度函数推导 了M积分定义式,并简述了M积分的物理意义,基于域积分方法实现二维/三维M积分的数值计算. 随后,提出了基 于M积分的材料缺陷等效损伤面积/体积标定方法,以圆孔面积/球孔体积来标定复杂微缺陷材料系统的损伤水平. 最后, 针对单轴拉伸载荷作用下的二维/三维脆性体含不同缺陷构型,具体计算了椭圆孔、裂纹以及双裂纹、双孔洞、裂纹和孔洞干 涉等复杂缺陷构型情况下的等效损伤面积/体积,并详细分析了缺陷之间的干涉效应及影响因素. 本研究旨在基于M积分等 效方法量化脆性材料中各类微缺陷造成的损伤程度,实现损伤等级标定,有益于工程材料及结构的损伤容限设计及完整性评估.      

单轴拉伸条件下脆性岩石微裂纹损伤模型研究

利用断裂力学、损伤力学和均匀化原理,对脆性岩石单轴拉伸条件下的力学特性进行分析,建立了脆性岩石的微裂纹损伤本构模型.首先对岩石内部微裂纹的统计分布规律进行分析,给出了理论分析过程中微裂纹分布的假设条件,在此基础上,参考已有研究成果,得到含细长微裂纹脆性岩石有效弹性参数的计算公式.然后,对岩石内部单一微裂纹进行断裂力学和损伤力学分析,得到了扩展裂纹尖端的应力强度因子计算公式,在一定微裂纹断裂扩展准则和断裂扩展速率的假设基础上,利用积分原理,得到了岩石整体的损伤变量和损伤演化方程,由此建立单轴拉伸条件下脆性岩石的微裂纹损伤本构模型.最后,通过一花岗岩的单轴拉伸试验结果对微裂纹损伤本构模型进行了验证.     

Asymptotic homogenization analysis for damage amplification due to singular interaction of micro-cracks

The paper investigates the overall damage amplification effect due to micro-crack interaction in a framework of two-scale modeling. A homogenization method based on asymptotic expansions is employed to deduce the macroscopic damage equations. The damage model completely results from energy-based micro-crack propagation laws. We consider a locally periodic microstructure with periods containing pairs of micro-cracks separated by small ligaments. The asymptotic solution in the ligament region allows the study of the effect of micro-crack interaction on the effective coefficients. The local macroscopic response expresses the collective coalescence of a periodic microstructure with interacting micro-cracks. We show that the slope of the homogenized coefficients is inversely proportional to the square root of the distance between the tips of the interacting micro-cracks, accounting for the singularity in the stress fields as the micro-cracks approach each other. This leads to damage amplification as the result of the interaction of micro-cracks.     

A two-scale damage model with material length

The Note presents the formulation of a class of two-scale damage models involving a micro-structural length. A homogenization method based on asymptotic developments is employed to deduce the macroscopic damage equations. The damage model completely results from energy-based micro-crack propagation laws, without supplementary phenomenological assumptions.We show that the resulting two-scale model has the property of capturing micro-structural lengths. When damage evolves, the micro-structural length is given by the ratio of the surface density of energy dissipated during the micro-crack growth and the macroscopic damage energy release rate per unit volume of the material.The use of fracture criteria based on resistance curves or power laws for sub-critical growth of micro-cracks leads to quasi-brittle and, respectively, time-dependent damage models. To cite this article: C. Dascalu, C. R. Mecanique 337 (2009).     

考虑微裂纹相互作用的的岩石微观力学弹塑性损伤模型研究

本文建立基于微裂纹扩展的岩石弹塑性损伤微观力学模型。用自洽方法考虑裂隙间相互影响,压缩载荷下微裂纹尖端翼裂纹稳定扩展表征岩石的微观损伤,基于应变能密度准则用Newton迭代法求复合型断裂的翼裂纹扩展长度,并采用微裂隙统计的二参数Weibull函数模型反映绝对体积应变对微裂纹分布数目影响,进而用翼裂纹扩展所表征的应力释放体积和微裂纹数目来表示含有微裂隙的岩石损伤演化变量;宏观塑性屈服函数采用Voyiadjis等的等效塑性应变的硬化函数,反映塑性内变量对硬化函数的影响;建立岩石的弹塑性损伤本构关系及其数值算法,并用回映隐式积分算法编制了弹塑性损伤模型的程序。从围压和微裂隙长度等因素分析弹塑性损伤模型的岩石的损伤和宏观塑性特性。     

Micro-crack tip fracture of commercial grade aluminum under mixed mode loading

In situ tensile tests were made in a scanning electron microscope (SEM) to investigate the deformation and micro-fracture in the immediate vicinity of a micro-crack tip in commercial pure aluminum with large-size crystal. Examined are the slip line field, stress intensity factor, strain energy density factor and crack tip opening displacement (CTOD) for mixed mode loading. Blunting and sharpening effects are observed. The latter is controlled by localized slip while the former by uniformed slip of the operating slip system with the highest crack tip Schmid factor. The operating slip system depends on the crystallographic orientation of crystal containing micro-cracks.The damage and fracture take place in the blunted region and depend on the coarsening and spacing of uniformed slip lines. The mixed mode micro-crack propagates along the direction where the voids grow and coalesce into the micro-crack. The direction also depends on the orientation of the applied loading. This suggests that the formation of macro-fracture mechanics could be applied. In particular, the minimum strain energy density criterion is suitable for determining the direction of micro-crack instability in the mixed mode. The in situ data were used to yield a nearly constant critical, minimum strain energy density factor for onset of micro-cracking.     

应用微相关技术研究裂端弹塑性变形

测量裂端附近区域的弹塑性变形，对于研究材料的损伤或微裂纹的产生，获得材料早期的损伤信息，为预防宏观裂纹的产生及扩展具有十分重要的意义。本文用A3钢材料制作边缘带裂纹的显微拉伸试件，试件表面进行了金相处理。试件是用由计算机控制的显微拉伸台进行加载试验。在加载过程中，利用环境扫描电镜对裂端附近的显微结构进行原位观察以及扫描记录。应用数字散斑相关技术对扫描电子显微镜记录的显微结构图进行分析处理，求出裂端附近的弹塑性变形。借助扫描电子显微镜，不仅可以定性观察应力集中区微裂纹的萌生及扩展过程，还可以结合数字散斑相关技术，对微区内的弹塑性变形进行测试，是研究微区域内材料变形的一个有效手段。     

Evolution of fabric tensors in damage mechanics of solids with micro-cracks: Part I – Theory and fundamental concepts

The evolution of fabric tensors based on micro-crack distributions is formulated based on sound thermodynamic principles. In Part I of this work, the exact definition of fabric tensors based on micro-crack distributions is presented. This definition is seen to incorporate both the orientation and length of a micro-crack. In this regard, the micro-crack distribution is assumed to be radially symmetric, i.e. symmetric about a line through the origin.The equations of thermodynamics are employed in order to derive the exact evolution equations of the fabric tensors defined in the first part. In this regard, a thermodynamic force that is associated with the fabric tensor is defined and utilized in the derivation of the evolution equations. The application of the theory to the case of uniaxial tension is derived in Part II (companion paper) of this work.     

A penny-shaped cohesive crack model for material damage

A novel micromechanics based damage model is proposed to address failure mechanism of defected solids with randomly distributed penny-shaped cohesive micro-cracks (Barenblatt–Dugdale type). Energy release contribution to the material damage process is estimated in a representative volume element (RVE) under macro hydrostatic stress state. Macro-constitutive relations of RVE are derived via self-consistent homogenization scheme, and they are characterized by effective nonlinear elastic properties and a class of pressure sensitive plasticity which depends on crack opening volume fraction and Poisson’s ratio. Several distinguished features of the present model are compared with Gurson model and Gurson–Tvergaard–Needleman (GTN) model, showing that the proposed model can better capture material degradation and catastrophic failure due to cohesive micro-crack growth and coalescence.