考虑裂纹闭合效应的岩石损伤本构关系

岩石中的预存裂纹只有在一定的法向压应力即裂纹闭合应力的作用下才可能闭合，其闭合过程与其方位和外加应力场有关，并且，即使对于裂纹已经完全闭合的岩石，如果裂纹闭合应力不同，则岩石的应力应交关系也不相同。本文建立了考虑裂纹闭合效应的岩石细观损伤力学模型，分析了裂纹闭合应力对岩石损伤演化过程和应力应变关系的影响。数值结果表明裂纹闭合应力显著地改变岩石的应力应变关系，表现为随裂纹闭合应力的增加，岩石的轴向应变变化较小，侧向应变和体积应变则大为增加。     

基于裂纹闭合效应的高载迟滞预测模型修正

裂纹闭合效应通常是导致I 型裂纹扩展在高载作用下发生迟滞效应的主要因素之一．本文采用汽车薄板QSTE340TM 材料，针对不同应力比，高载比条件下疲劳裂纹扩展行为进行了实验研究．论文通过断面分析，针对各参数对裂纹闭合效应的具体影响进行了分析讨论，认为裂纹作用区域随裂纹扩展而动态变化，从而提出了一种对有效应力强度因子幅的修正方法．通过在原有模型中引入幂函数形式的动态变量α，表征裂纹闭合效应的作用比例随裂纹长度的动态变化，取得了较好的效果．     

疲劳裂纹塑性闭合特性的数值分析方法

塑性诱导裂纹闭合是导致裂纹闭合发生的主要机理之一.利用弹塑性有限元法模拟含中心裂纹矩形板试件的疲劳裂纹扩展,并确定疲劳裂纹张开、闭合应力水平.通过计算,考察应力比R、裂纹长度、最大应力强度因子Kmax等对疲劳裂纹张开闭合应力的影响规律.论文阐述了所采用的裂纹扩展模拟方法及确定裂纹张开和闭合应力的原理.采用了等K加载方式,即在裂纹扩展中裂尖应力强度因子的最大值Kmax保持不变(给定R比,最大应力σmax随裂纹长度变化).分析了两种Kmax水平下R比分别为0.3,0,-0.5和-1.0共8种载荷工况.结果表明,对各个载荷工况,用瞬时最大应力σmax正则化的裂纹张开、闭合应力水平σop/σmax和σcl/σmax与裂纹长度无关.等K循环加载比等幅循环加载更有利于分析影响裂纹闭合水平的因素和闭合效应对疲劳裂纹扩展的影响规律,为建立基于裂纹闭合效应的疲劳裂纹扩展规律模型提供了一种新的思路.     

渗碳淬硬层疲劳裂纹闭合模型

本文在Newman裂纹闭合模型的基础上,以裂纹面附加残留位移为主要线索,综合考虑了渗层组织、强度、初始残留应力分布及残余奥氏体转变对裂纹闭合行为的影响,提出了一个渗碳硬层疲劳裂纹闭合计算模型。文中利用该模型对疲劳裂纹扩张过程中卸载后裂纹面接触应力的分布、渗层裂纹闭合效应进行了成功地计算;借助于实测的渗碳层da/dn—ΔK_(eff)关系,较好地预测了渗层裂纹扩张速率的变化规律;并在定量分析残余奥氏体转变、初始残留应力分布对闭合行为的影响方面进行了有益的尝试。     

含微裂纹弹性体的应力应变关系

本义建立了考虑裂纹闭合和裂纹表面摩擦影响的含微裂纹弹性体的应力应变关系，给出了柔度张量增量的显式表达式。对于二维平面应力和平面应变状态，给出了等效工程弹性系数。数值计算结果表明，裂纹闭合和裂纹面摩擦对裂纹体的应力应变关系和等效工程弹性系数有重要影响。     

随机载荷下疲劳裂纹的闭合和扩展估算模型

本文分析了裂尖前,后方塑性区对闭合性能的影响,提出从裂尖塑性钝化量和尾迹区残余塑性变形两个方面来确定裂纹面的残余变形,并讨论了压缩载荷对闭合应力的影响,由此建立了一个疲劳裂纹闭合模型,然后通过模型“有限记忆”性质的假定将它应用到随机加载情况。用此模型对铝合金2219——T851受飞机谱载荷的CCT平面应力试件进行了疲劳寿命估算,估算值与实验结果接近。     

含微裂纹弹性体的应力应变关系

本义建立了考虑裂纹闭合和裂纹表面摩擦影响的含微裂纹弹性体的应力应变关系，给出了柔度张量增量的显式表达式。对于二维平面应力和平面应变状态，给出了等效工程弹性系数。数值计算结果表明，裂纹闭合和裂纹面摩擦对裂纹体的应力应变关系和等效工程弹性系数有重要影响。     

考虑材料循环塑性的疲劳裂纹扩展模拟

提出了一种考虑材料循环塑性性能的研究疲劳裂纹扩展与闭合行为的有限元模拟方法．对所选用的循环塑性本构关系进行了基本实验检验．探讨了在疲劳裂纹扩展有限元分析中网格尺寸的影响，给出了网格优化准则．研究了在循环硬化条件下考虑裂纹合效应时裂纹面张开廓形、裂纹尖端应力、应变场和正反向塑性区的演变规律．对于循环硬化和不同循环应力比Ｒ等因素对裂纹张开应力水平的影响也作了考察     

爆炸应力波与裂纹作用实验研究

为研究爆炸应力波与裂纹相互作用机理,利用透射式爆炸动态焦散线光学实验系统研究了预制水平静态裂纹和切缝药包炮孔爆破产生的水平运动裂纹受正入射爆炸动载作用后动态特性的变化规律。结果表明:正入射爆炸应力波与静止裂纹作用时,爆炸应力波P波使得裂纹先闭合后张开,S波在裂纹壁面形成波浪状散斑上下交替向外扩展;运动裂纹尖端应力场对静止裂纹的起裂和扩展有重要影响。后爆孔爆炸应力波对先爆孔产生的水平定向运动裂纹尖端动力学特性影响显著。当爆炸应力波与运动裂纹同向时,P波使得裂纹扩展速度和应力强度因子KI^d先减小后增大,S波促进了裂纹的扩展,波与裂纹作用之后,裂纹扩展速度增大;当爆炸应力波与运动裂纹反向时,P波抑制了运动裂纹的扩展,波与裂纹作用之后,裂纹扩展速度和应力强度因子KI^d均逐渐降低。     

任意多孔多裂纹板的裂纹闭合接触分析

基于摩擦接触问题的数学规划解法，采用各向异性体平面弹性理论中的复势方法，建立了含多椭圆孔及裂纹群有限大各向异性板，在任意载荷作用下裂纹闭合或局部闭合问题的有效分析方法。通过在可能闭合的裂纹边界引入互补变量函数并将其展成Fourier级数形式，以Faber级数为工具，应用保角映射技术和最小二乘边界配点法，导出无卸载情况下裂纹面摩擦接触的线性互补模型，并通过算例验证了方法的有效性。数值结果表明，由于采用级数解描述板应力场和位移场，该方法具有较高的计算精度和效率，便于研究裂纹闭合对应力强度因子等断裂参数的影响。     

狭长缺口根部萌生的小裂纹扩展分析

根据伴随着小裂纹延展过程中不同力学参数相互作用的分析研究基础之上,我们就疲劳小裂纹的扩展特性进行了分析,根据不扩展裂纹性质对小裂纹的长度范围进行了讨论,探讨了狭长缺口根部萌生的小裂纹扩展速率, 并研究了不扩展裂纹形成机理,推演出不扩展裂纹长度计算方法,发展出一种预测小裂纹扩展行为的方法,借助这种方法可以实现裂纹萌生寿命的预测,与疲劳设计相结合可以进一步提高构件安全性与可靠性．不扩展小裂纹的长度可以通过计算定量得到,它与材料的疲劳极限、裂纹门槛值及缺口尺寸等参数紧密相关．以理论数据为依据,可以设计出针对一定材料及尺寸的试样,使其在一定荷载条件下不扩展裂纹长度达到宏观可观测的范围,进而降低小裂纹测量方面的技术难度,进一步验证不扩张裂纹形成机理．     

缺口根部塑性区和拉应力场对循环压缩疲劳裂纹止裂长度的影响

本文采用激光显微像面全息和散斑照像分离技术检测了双缺口试件在不同压缩应力条件下缺口根部塑性区的形状及大小,并采用有限元分析法计算了相同应力下的塑性区和裂端应力场.结果表明:用显微像面全息法测量塑性区的大小及形状简单易行,且有比较满意的工程精度. 对在不同压缩循环加载条件下测得的瘦劳裂纹止裂长度与所测压缩塑性区相比较表明:疲劳裂纹止裂长度和塑性区大小之间不存在简单的对应关系.止裂长度主要由缺口根部在循环压缩卸载过程中所产生的拉伸应力区控制.     

缺口根部和裂纹尖端残余应力的X射线法测定

X射线法用于缺口根部和裂纹尖端等徽区的残余应力测试的先决条件是解决缩小光束直径、提高衍射束的强度和准确设置试样等技术问题.在X射线衍射仪上借助于自行设计制造的限束对光装置和侧倾对中附件,成功地测定了缺口根部半径为1mm的喷丸残余应力场和板形试样压-压周期载荷下裂纹尖端的残余拉应力场.     

A reexamination of plasticity-induced crack closure in fatigue crack propagation

The crack closure concept is often used to consider the R-ratio and overload effects on fatigue crack growth. The presumption is that when the crack is closed, the external load produces negligible fatigue damage in the cracked component. The current investigation provides a reassessment of the frequently used concept with an emphasis on the plasticity-induced crack closure. A center cracked specimen made of 1070 steel was investigated. The specimen was subjected to plane-stress mode I loading. An elastic–plastic stress analysis was conducted for the cracked specimens using the finite element method. By applying the commonly used one-node-per-cycle debonding scheme for the crack closure simulations, it was shown that the predicted crack opening load did not stabilize when the extended crack was less than four times of the plastic zone size. The predicted opening load was strongly influenced by the plasticity model used. When the elastic–perfectly plastic (EPP) stress–strain relationship was used together with the kinematic hardening plasticity theory, the predicted crack opening load was found to be critically dependent on the element size of the finite element mesh model. For R = 0, the predicted crack opening load was greatly reduced when the finite element size became very fine. The kinematic hardening rule with the bilinear (BL) stress–strain relationship predicted crack closure with less dependence on the element size. When a recently developed cyclic plasticity model was used, the element size effect on the predicted crack opening level was insignificant. While crack closure may occur, it was demonstrated that cyclic plasticity persisted in the material near the crack tip. The cyclic plasticity was reduced but not negligible when the crack was closed. The traditional approaches may have overestimated the effect of crack closure in fatigue crack growth predictions.     

On fracture toughness of orthotropic metallic materials under cyclic loading

AVT-6 titanium alloy rolled sheet, which is initially isotropic and subject to kinematic hardening, is used as an example to analyze the effect of kinematic hardening on crack growth resistance under uniaxial cyclic loading. Crack growth resistance is characterized by the number of cycles to failure, critical crack length, and critical stress intensity factor. The subject of study is plane specimens with an edge notch. It is shown that prestrain changes the anisotropy of the specimens, which is determined as the ratio of the crack growth resistance in the rolling direction to that in the transverse direction. The crack path under a load applied at an angle to the axes of anisotropy is studied     

Prediction models of fatigue crack closure and growth under random loading

The effects of plastic zones both in front of and behind crack tip on crack closure have been analysed. The total residual deformations of crack surfaces involve two parts, that is, the amount of plastic blunting of crack tip and the residual deformation in the wake of the tip. This paper presents a fatigue crack closure model in which the influences of compressive load on closure stress are discussed. The model is applied to random loading conditions by the assumption of limited memory properties. The fatigue lives are predicted using the proposed crack growth model for CCT plane stress specimen cut from 2219-T851 aluminum alloy under flight spectrum loadings, and the prediction values agree with the test results.The project was supported by the natural science foundation of China.     

A Study on the Evaluation of the Fracture Process Zone in CCNBD Rock Samples

The safety of many civil and mining concrete and rock structures including pre-existing crack networks is fundamentally affected by the mechanical behaviour of the material under static and cyclic loading. In cyclic loading case, cracks can grow at a lower load level compared to the monotonic case. This phenomenon is called fatigue due to subcritical crack propagation and depends on the behaviour of the fracture process zone (FPZ). This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to investigate their mode-I (tensile) fracture toughness response to static and cyclic loading and relevant FPZ. The FPZ and fracture toughness response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damage mechanisms in front of the chevron crack. A maximum reduction of the static fracture toughness (K _IC ) of 42 % was obtained for the highest amplitude increasing cyclic loading test. Detailed scanning electron microscope (SEM) examinations were performed on the surfaces of the tips of the chevron notch cracks, revealing that both loading methods cause FPZ development in the CCNBD specimens. When compared with monotonic FPZ development, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle breakage under cyclic loading, while smooth and bright cracks along cleavage planes were formed under static loading. Further, the SEM images showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix because of both intergranular and transgranular subcritical fracturing.     

A STUDY OF CRACK CLOSURE IN ELECTRIC-FIELD-INDUCED FATIGUE IN FERROELECTRIC CERAMICS

Considering the influence of the domain switching near the tip of a crack and apply-ing the idea of multiscale singularity fields in piezoelectric fracture,we have obtained an empiricalcriterion for the crack closure.Based on the domain switching in the electric yield region,referringto Yang's results on the small scale yield model for the electrical fatigue crack,a model of thecrack closure during electric-field-induced fatigue is developed to analyze the crack growth.Interms of the model we have obtained the formula of the rate of the crack growth under cyclicelectric loading.Finally we compare the theoretical predictions with the results given by Cao andEvans experimentally.It should be pointed out that the model proposed is empirical and needsto be verified by more experimental results.     

随机超载下疲劳裂纹扩展寿命的计算

针对在基本循环载荷上加入随机超载序列的疲劳裂纹扩展问题，应用裂纹闭合的概念考虑超载的迟滞效应，将延迟时间描述成纯离散型马尔可夫过程。对相应的柯尔莫哥洛夫－费勒微积分方程进行了分步求解，结合疲劳断裂分别出现在基本循环峰载作用时和超载作用时两种情况，计算出不同可靠度下裂纹的扩展寿命，并研究了超载大小和发生强度对扩展寿命的影响。