裂尖大应变细观断裂研究

本文用反映空穴形核成长的Gurson本构方程来描述裂尖区域材料在大应变情形下的力学特性，并进一步考虑了空穴演变对材料杨氏模量的影响。文中用上述本构方程分别结合弹塑性大应变有限元方法对平面应变I型裂纹问题作了计算，分析了裂尖应力分布、裂尖形状变化和裂尖空穴演变过程，并与用Prandtl-Reuss本构方程教育处的结果作了比较。     

高温结构蠕变裂尖拘束效应

如何对蠕变裂纹扩展寿命进行准确预测和评价是高温结构完整性评定、寿命设计和运行维护中需要解决的核心问题.基于宏观单参数C?的蠕变断裂行为的评价方法,未有效纳入裂尖拘束效应的影响,因而其评价结果过于保守或非保守.目前国内外还未建立起有效纳入裂尖拘束效应的高温结构蠕变寿命评价的理论体系和技术方法,还没有纳入蠕变拘束效应的高温结构完整性评定规范.本文综述了作者近年来在高温蠕变断裂拘束效应方面的研究工作.主要包括:裂尖拘束对材料蠕变裂纹扩展行为的影响及机理;蠕变裂尖场和拘束参数R的定义和影响因素;载荷无关的蠕变拘束参数R? 的提出及其应用基础;承压管道表面裂纹的拘束参数R? 解及纳入裂尖拘束的蠕变寿命评价方法;试样与管道轴向裂纹蠕变裂尖拘束的关联;基于裂尖等效蠕变应变的面内与面外蠕变裂尖拘束的统一表征参数Ac的研究;材料拘束相关的蠕变裂纹扩展速率的建立;宽范围C? 区蠕变裂纹扩展速率及其拘束效应的数值预测;材料拘束对焊接接头蠕变裂纹扩展行为的影响及机理等.这些研究为建立纳入裂尖拘束效应的高温部件的蠕变裂纹扩展寿命评价方法奠定了理论和技术基础.论文对后续拟开展的工作也进行了展望.     

界面裂纹无摩擦接触裂尖应力场

基于哈密顿原理，通过分离变量及共轭辛本征函数展开法，解析地球解界面裂纹无摩擦接触裂尖的扇形域方程，从另一条途径研究了界面裂尖应力场的特征。     

裂尖大变形条件下的约束研究

通过面内应力比Tx和离面约束Tz在塑性区分布规律的深入分析，发现约束在塑性区不同区域的影响是不同的，在裂尖的钝化区，面内应力比Tx起重要作用，而在钝化区外的塑性区内，离面约束Tz起主要作用，在J－Q理论的基础上引入离面约束Tz可以很好地描述钝化区以外的应力应变分布，在钝化区内引入面内应力比Tx可以对裂尖前方的应力应变进行大变形分析和描述，研究表明综合两个约束的共同作用，可以很好地描述大变形条件下裂尖前方的应力分布。     

基于扩展有限元法的裂尖场精度研究

扩展有限元方法基于单元分解的基本思想,通过引入位移加强函数来表征裂纹的不连续性和裂尖的奇异性。在裂尖加强单元与常规单元之间有一层混合单元,当对裂尖特定区域进行加强时,混合单元个数相应增加,混合单元个数与计算精度存在一定联系。本文提出一种正方形裂尖加强区域的选择方式,可得到较单个加强和圆形加强精度更高、更稳定的计算结果。对于不同长度的裂纹,表征裂尖场奇异性所需的裂尖加强范围存在较大差异,以正方形裂尖加强方式进行计算,得到了不同裂纹长度下最优的加强尺寸。     

裂尖超钝化:实验、理论与数值模拟

本文报道了在改性聚丙烯裂尖平面应变段发生超钝化的实验发现。利用环向冷拉模型,可说明裂尖超钝化的形成取决于冷拉传播速率与加载率之比。裂尖超钝化可使材料韧性,尤其是冲击韧性的改善,达到一个新的水平。本文在超弹性-粘塑性各向异性损伤的有限变形本构模型下进行了详尽的数值计算。模拟结果成功地再现了裂尖超钝化的基本特征。     

单边裂纹通电瞬间裂尖处应力场的复变函数解

本文应用复变函数中的Schwarz-Christoffel变换方法，在具单边裂纹的导电薄板通电瞬间温度场复变函数解的基础上，推导出用复变函数表示的应力场的表达式，并且给出算例，通过理论计算得知；当对具有单边裂纹的导电薄板通入适当密度的电流时，裂尖处温度急剧升高并熔化便裂尖变钝。同时，在裂尖周围形成了有利于遏制裂纹扩展的压应力场，有效地防止了裂纹沿其主方向和其它方向延伸。从理论上证明了电磁热效应在裂尖处产生高温形成焊口的同时，压应力场的形成是遏制裂纹扩展的主要因素之一。理论计算结果与实验结果比较吻合，为这一止裂方法的应用打下了理论基础。     

动态起裂韧性测试过程的三维分析

本文应用线性弹性动脉有限元分析方法，对利用ＨＯＰＫＩＮＳＯＮ压杆技术测试材料动态起裂韧性的试验过程进行了三维数值计算，求得了加载波入，加载点位移，试样裂尖动脉应力强度因子，裂尖附近点应变历程以及材料动态起裂韧性值，并与实验－数值方法所得的各结果进行了比较分析。     

脉冲放电止裂后裂尖处纳米尺度下的力学性能测试

应用扫描探针显微镜SPM(scanning Probe Microscopy)，对经过脉冲放电止裂后的40Cr模具钢裂尖处的组织进行了纳米尺度下的力学性能测试。测试结果表明：脉冲放电不仅使裂纹尖端钝化，同时使围绕裂尖处组织的细观硬度和细观杨氏弹性模量得到了提高，与宏观观测和分析规律相符，纳米尺度下微观力学性能测试结果为脉冲放电裂纹止裂技术的工程应用提供了微观尺度下的证据。     

绕裂尖白亮层的形成及其止裂作用分析

应用电磁热效应对带有裂纹的金属导体通入脉冲电流进行止裂，在裂纹尖端处熔化形成钝化焊口的同时，围绕裂尖的很小范围内形成一白亮层．分析表明：白亮层的形成对裂纹的扩展起到了屏蔽和防止应力腐蚀的作用，进一步提高了钝化止裂的效果．     

Remarks on the energy release rate for an antiplane moving crack in couple stress elasticity

This paper is concerned with the steady-state propagation of an antiplane semi-infinite crack in couple stress elastic materials. A distributed loading applied at the crack faces and moving with the same velocity of the crack tip is considered, and the influence of the loading profile variations and microstructural effects on the dynamic energy release rate is investigated. The behavior of both energy release rate and maximum total shear stress when the crack tip speed approaches the critical speed (either that of the shear waves or that of the localized surface waves) is studied. The limit case corresponding to vanishing characteristic scale lengths is addressed both numerically and analytically by means of a comparison with classical elasticity results.     

陶瓷基复合材料基体唇形裂纹失效规律研究

建立横向拉伸载荷下的唇形裂纹数学模型,采用复变函数的方法,通过保角映射,推导了唇形裂纹尖端应力场和位移场的解析解,建立了唇形裂纹的应力强度因子准则和最大能量释放率准则,结合算例分析陶瓷基复合材料基体唇形裂纹的几何参数、外载荷和纤维分布对失效准则的影响规律.结果 表明,(1)裂纹尖端应力场和位移场的解析解与有限元计算结果进行对比,验证了方法的有效性;(2)相较于Griffith裂纹和椭圆裂纹,基于唇形裂纹的失效准则对裂纹尖端的敏感性更高,适用于预测脆性陶瓷基体裂纹的扩展;(3)对于不同几何参数的唇形裂纹,采用最大能量释放率准则的基体裂纹的扩展速率要大于应力强度因子准则.     

压电圆柱形夹杂对邻近裂纹三维应力场的影响

研究了压电复合材料薄板中压电圆柱形夹杂与邻近宏观钝裂纹间的相互作用。重点分析了外加电场，裂尖与压电圆柱形夹杂间韧带长度对裂尖三维应力场的影响。计算结果表明：在不同的外加电场作用下，压电体不仅能改变裂尖张开应力的大小，还能改变其分布。所得结果对进一步探讨线弹性介质中裂纹的启裂控制有参考价值。     

Analytic solutions to crack tip plastic zone under various loading conditions

Based on stress field equations and Hill yield criterion, the crack tip plastic zone is determined for orthotropic materials and isotropic materials under small-scale yielding condition. An analytical solution to calculating the crack tip plastic zone in plane stress states is presented. The shape and size of the plastic zone are analyzed under different loading conditions. The obtained results show that the crack tip plastic zones present “butterfly-like” shapes, and the elastic–plastic boundary is smooth. The size of the plastic zone for orthotropic composites is less at the crack tip for various loading conditions, compared with the case of isotropic materials. Crack inclination angle and loading conditions affect greatly the size and shape of crack tip plastic zone. The mode I crack has a crucial effect on the plastic zone for mixed mode case in plane stress state. The plastic zone for pure mode I crack and pure mode II crack have a symmetrical distribution to the initial crack plane.     

Boundary element analysis of cracked film–substrate media

This study evaluates the stress behavior of a cracked film–substrate medium by applying the multi-region boundary element method. Four problems addressed herein are the crack tip within a film, the crack tip terminating at the interface, interface debonding, and the crack penetrating into the substrate. The multi-region boundary element method is initially developed and, then, the stress intensity factors or the energy release rates are evaluated according to the different stress singularities of the four considered problems. These results indicate that the stress intensity factors or the energy release rates of the four problems rely not only on the different elastic mismatches and crack lengths, but also on the thickness ratio of the film and the substrate.     

考虑应力松驰复合型裂纹顶端塑性区分析

Ｓｈｉｈ［１］应用奇异单元，获得了不考虑应力松驰小范围屈服条件下复合型裂纹尖端塑性区形状。Ｚ．Ｚ．Ｚｕ等［２］采用Ｒｉｃｅ［５］给出的裂纹尖端应力关系式，利用有限元分析获得了不考虑应力松驰下复合型裂纹尖端塑性区，本文基于静力学中内力与外力平衡条件，用线弹性的全场解代替局部解，给出了考虑应力松驰下复合型裂纹尖端塑性区边界方程，获得了考虑应力松驰下的任意方向的塑性区尺寸及塑性区形状     

可压缩理想塑性体中逐步扩展裂纹顶端的弹塑性场

本文利用理想塑性固体平面应变问题的基本方程,分析了可压缩理想塑性体中逐步扩展裂纹顶端的弹塑性场,得到了关于应力的渐近场,分析了弹性卸载区的演变过程和修正的中心扇形区的发展过程,预示了出现二次塑性区的可能性,弹性可压缩性的影响明显表现在经典的中心扇形区必需加以修正,垂直于板面方向的应力偏量不再为零,而且随着新裂纹面的形成,裂纹前方的均匀应力场和紧连着的修正的中心扇形区的应力偏量将发生变化,这种变化是由于垂直于板面方向的应力偏量发生变化造成的。     

Photoelastic analysis of the stress field surrounding a fatigue crack

A photelastic analysis was carried out on plane polyester specimens containing a fatigue crack, in order to study the effect of plastic yielding around the crack tip on the elastic stress distribution in the vicinity of the crack. In general, results were in good agreement with values calculated for the case of a sharp-tipped crack. However, very near the crack tip, principal stresses obtained experimentally were slightly lower than calculated stresses, probably due to the bluntness of the fatigue crack. Also lines of constant stress tended to move behind the crack tip, in contrast with the calculated stresses, which occurred further forward over the field of investigation.     

各向异性压电材料平面裂纹的耦合场分析

用Stroh方法分析了各向异性压电材料电导通型裂纹问题的耦合场。结果表明,裂纹面上的切向电场强度和法向电位移均为常数,在裂纹尖端有由弹性场的耦事作用产生的奇异电导通裂纹模型中的静电场对裂纹尖端扩展的能量释放率不作贡献。     

Mode II dynamic fields near a crack tip growing in an elastic-perfectly-plastic solid

An asymptotic solution is given for Mode II dynamic fields in the neighborhood of the tip of a steadily advancing crack in an incompressible elastic—perfectly-plastic solid (plane strain). It is shown that, like for Modes I and III (Gao and Nemat-Nasser, 1983), the complete dynamic solution for Mode II predicts a logarithmic singularity for the strain field, but unlike for those modes which involve no elastic unloading, the pure Mode II solution includes two elastic sectors next to the stress-free crack surfaces. This is in contradiction to the quasi-static solution which predicts a small central plastic zone, followed by two large elastic zones, and then two very small plastic zones adjacent to the stress-free crack faces. The stress field for the complete dynamic solution varies throughout the entire crack tip neighborhood, admitting finite jumps at two shock fronts within the central plastic sector. This dynamic stress field is consistent with that of the stationary crack solution, and indeed reduces to it as the crack growth speed becomes zero.