Stress distribution and effective stress intensity factor of a blunt crack after dislocation emission

The stress fields induced by a dislocation and its image dislocations around a narrow elliptic void are formulated. Based on the solution, the stress distribution and effective stress intensity factor of a blunt (elliptic) crack were calculated under mode I constant loading. The results show that a dislocation-free zone (DFZ) is formed after dislocation emission. There exists a second stress peak in the DFZ except a stress peak at the blunt crack tip. With an increase in the applied stress intensity factor Kla or the friction stress T, of the material, the DFZ size and the peak stress at the crack tip decrease, but the peak stress in the DFZ and the effective stress intensity factor Klf presiding at the crack tip increase. Because of dislocation shielding effects, shielding ratio Kla/Klf increases with increasing Kla, but it decreases with increasing Tf.     

法向压缩载荷对II型裂纹扩展的影响规律研究

压剪共同作用下裂纹是地下岩体中广泛存在的,为了研究法向压缩载荷对剪切型即II型裂纹扩展的影响规律,利用砂岩制作了单边裂纹试件,通过岩石剪切实验系统对不同法向压缩载荷作用下的裂纹进行了剪切实验研究,结果表明法向压缩载荷对裂纹的起裂及扩展起到抑制作用,随着裂纹法线压缩载荷的增大,裂纹起裂角逐渐减小;同时,得出法向压缩载荷与II型裂纹极限应力强度因子之间的关系,随着法向压缩载荷增大,岩石II型裂纹极限应力强度因子也随之增大。运用ABAQUS模拟软件,对不同压缩载荷作用下压剪裂纹的扩展进行模拟研究,得出了裂纹尖端II型应力强度因子随压缩载荷的变化规律。     

Crack initiation ahead of piled—up of dislocations emitted from a mode Ⅱ blunt crack

In situ tensile tests in a transmission electron microscope (TEM) show that dislocations emitted from a mode II crack tip will form a inverse piled-up group after equilibrium or a double piled-up group when they meet a obstruction, e.g., grain boundary or second phase. A microcrack can initiates in front of the piled-up group of dislocations. Micromechanics analysis shows that dislocations emitted from a mode II blunt crack tip can form a inverse piled-up or double piled-up group, depending upon the applied stress intensity factor K_(lla), lattice friction stress τ_ f and the distance of the obstruction from the crack tip L. The maximum normal stress in front of the double piled-up group which is located at the direction of α = -64° increases with the increase in the stress intensity K_(lla) and the obstruction site L, and the decrease in the friction stress τ_f When it increases to equate the cohesive strength, a microcrack will initiate in front of the piled-up group.     

裂纹尖端场一种新的奇异元研究

提出构造奇异单元的一种新方法,引入8节点四边形单元,通过退化为6节点三角形单元,从而构造出表达裂纹尖端的奇异性的奇异元;给出了应用ANSYS计算应力强度因子的具体实施步骤,计算结果与理论值进行对比,表明该方法的可行性与可靠性.     

I型运动裂纹面受双重及冲击载荷作用下的位错分布函数

通过复变函数论的方法,对I型运动裂纹面受双重载荷、瞬时冲击载荷作用下的位错分布函数问题分别进行研究．采用自相似函数的方法可以获得运动裂纹的应力、位移、应力强度因子及位错分布函数的解析解,应用该法可以迅速地将所论问题转化为Riemann-Hilbert问题,并可以相当简单地得到问题的闭合解．     

Analytical study of subcritical crack growth under mode I loading to estimate the roof durability in underground excavation

The long-term stability of the roof is particularly important in designing underground rock structures. To estimate the durability of roof strata in underground excavation, a computation scheme of subcritical crack growth is proposed in this study. By adopting the proposed method, the potential collapse location of strata is derivable in accordance with a static model, the durability of roof strata can be estimated, a dynamic time step control strategy is achieved to balance the accuracy and speed of computing, and the initial crack size of rock can be estimated. In addition to the above, a mechanical model of underground excavation with non-uniformly distributed loads and partially yielded foundation is presented as the prototypical case. A set of case studies is carried out that showcase a power correlation between applied stress and roof durability. The allowable applied tensile stress for a 100-year life cycle is about 76% of the tensile strength. By using the proposed subcritical crack growth computation scheme, the roof stability in an underground excavation can be identified not only from the spatial view but also from the temporal perspective.     

任意裂纹面荷载作用下界面断裂分析

为研究裂纹面上作用的荷载对裂纹稳定性的影响,本文基于比例边界有限元方法提出裂纹面作用有任意方向、任意大小面荷载的界面应力强度因子求解模型.界面裂纹具有复数形式的应力奇异性指数,在任意裂纹面荷载作用下其奇异应力场更为复杂.应用本模型,径向的位移和应力可解析求解,无需网格细分即可自动反映裂尖的应力奇异性.裂纹面上的任意荷载首先可分解成平行于裂纹面以及垂直于裂纹面的分量,并进一步分解成有限项幂函数的和.对每个幂函数荷载解析求解,基于线性叠加原理获得结构在全部荷载作用下的解.该模型对各向同性材料和各向异性材料均适用.文中通过板承受裂缝面荷载时的应力强度因子求解的多个算例对该模型进行了验证和应用,对板的几何尺寸和双材料参数进行了敏感性分析,并应用于重力坝坝踵界面裂缝在水压力作用下的应力强度因子求解.     

镍基单晶合金板裂纹尖端塑性区分析

裂纹尖端塑性区对研究裂纹开裂扩展具有重要意义.应用Hill屈服准则和考虑拉-剪耦合的修正Hill屈服准则,对镍基单晶合金板裂纹尖端塑性区进行了分析.得到了Ⅰ型、Ⅱ型和Ⅰ、Ⅱ复合型裂纹尖端塑性区.对镍基单晶合金板内裂纹尖端塑性区与各向同性材料板内裂纹尖端塑性区进行了比较.讨论了拉-剪耦合、Ⅰ、Ⅱ复合型裂纹复合比及温度对裂...     

双孔切槽爆破裂纹扩展的动焦散实验

应用爆炸加载动态焦散线测试系统,研究了不同切槽方式下,双孔同时起爆时贯穿裂纹的扩展行为及裂纹尖端应力强度因子的变化情况.实验结果表明：炮孔间两切槽方向优先扩展的主裂纹,裂纹尖端并未直接相遇,而是发生偏转并移向异方已经形成的裂纹.裂纹扩展速度先减小,振荡变化后逐渐升高到峰值,随后减小,振荡变化后直至裂纹止裂.应力强度因子KI由初始时的最大值迅速减小,反复振荡后,又逐渐增大至第2个峰值,之后开始减小.定性的分析了应力波与裂纹尖端相互作用机理.     

一类弹粘塑性材料II型扩展裂纹的尖端场

为了研究粘性效应作用下的II型扩展裂纹尖端场,假设扩展裂纹尖端的粘性系数与塑性应变率的幂次成反比,通过量级匹配表明应力和应变均具有幂奇异性,奇异性指数由粘性系数中等效塑性应变率的幂指数唯一确定.引入Airy应力函数,求出了裂纹尖端应力和应变场的控制方程.选取适当的特征参数,给出了边界条件,对控制方程通过双参数打靶,进行了数值计算,求得了裂纹尖端的应力应变场.分析与计算结果表明粘性效应是裂纹尖端场的一个重要因素.     

裂尖损伤对弹塑性断裂特征量的影响

依据Lemaitre损伤演化模型，建立了裂尖塑性区内损伤材料的损伤演化，并对金属材料裂纹尖端损伤以及弹塑性断裂有关特征量的影响进行了分析．     

压应力对铝合金长短疲劳裂纹尖端应力影响有限元分析

应用弹塑性有限元方法,研究压应力对铝合金长短疲劳裂纹尖端应力场、塑性区的影响.分别建立两个具有长短中心穿透裂纹高强铝合金板的有限元模型,进行拉压加载模拟分析.结果表明,压应力对铝合金长短疲劳裂纹尖端应力有显著影响,相同的应力强度因子条件下,在一拉-压加载周期,当拉应力减小到零时裂纹尖端应力不为零,裂纹尖端应力对裂尖的挤...     

倾斜疲劳裂纹在混合型应力下张开及滑移特性

对于存在裂纹的机械零件或者构件,它很有可能会受到来自不同方向的作用力, 因此有必要研究裂纹在混合型应力条件下的传播特性. 该文使用具有不同倾斜角度的裂纹--这些裂纹是在应力比R=0的Ⅰ型循环应力下制作的,通过测量裂纹的开口方向和滑移方向的位移量,计算了裂纹端部Ⅰ型应力强度因子(KⅠ)mes和Ⅱ型应力强度因子(KⅡ)mes.裂纹的倾斜角度是指载荷方向与裂纹间的夹角.结果表明,对于存在压缩残余应力的疲劳裂纹,当倾斜角度较大,即β=60°时,应力强度因子的实验值(KⅠ)mes小于理论计算值(KⅠ),但此时实验值(KⅡ)mes与理论值(KⅡ)基本一致.另一方面,对于倾斜角度较小β=45°的疲劳裂纹,其(KⅠ)mes减少的同时,由于裂纹上下表面相互接触,从而导致(KⅡ)mes同时下降.     

保角映射方法在各向异性板断裂分析中的应用

采用各向异性体平面弹性理论中的复势方法,引用适当的保角变换,研究各向异性板中穿透性直线裂纹的平面弹性问题。借助应力边界条件推出应力函数的表达式,得到Ⅰ型裂纹尖端附近的应力强度因子、应力场及位移场的解析解.     

岩石裂纹小范围屈服时声发射总数与应力强度因子关系的研究

根据裂纹尖端微裂纹密度理论,假定声发射总数与裂纹尖端的显微裂纹总长度成正比,推导了出了岩石裂纹稳定扩展时声发射总数与应力强度因子之间的关系式。将并一些试验结果与理论推导进行了比较,结果表明;理论推导与试验结果大体一致。     

Elastodynamic stress intensity factor due to three-dimensional concentrated transient loading on the faces of a crack

The dynamic stress intensity factor for a semi-infinite crack in an otherwise unbounded elastic body is analyzed The crack is subjected to a pair of suddenly applied point loads on its faces at a distance l away from the crack tip The solution of the problem is obtained by superposition of the solutions of two simpler problems. The first of these problems is Lamb' s problem, while the second problem considers a half space with its surface subjected to the negative of the normal displacement induced by Lamb's problem in the range x>0. The latter is solved by means of integral transforms together with the application of Weiner-Hopf technique and Cagniard-de Hoop method. An exact expression is derived for the mode I stress intensity factor as a function of time for any point along the crack edge. Some features of the solution are discussed.     

基于流场复势的空间裂纹止裂应力强度因子分析

选择带有圆形埋藏裂纹的金属构件作为研究对象。在向金属构件通入脉冲电流时,可以将金属构件内电流绕圆盘裂纹的流动比作在流体力学平面势流中流体绕平板流动的情况,给出电流流动的复势函数,进而推导出电流密度,据此推求热源功率,最后确定带有圆形埋藏裂纹的金属构件脉冲放电瞬间的热应力强度因子,结合只有外载荷作用下的金属构件裂纹尖端的机械载荷应力强度因子,给出外载荷作用下带有埋藏裂纹金属构件脉冲放电瞬间的综合应力强度因子。     

静载下连杆裂解裂尖塑性区及其对裂解质量的影响

对静载条件下连杆裂解过程进行了数值分析,将连杆裂尖附近塑性区形状与理想状态进行了对比,定性给出了连杆断裂前裂纹区应力应变状态,并据此分析了不同位置的断裂状态。结果表明,连杆厚度方向不同的层面处于不同的平面应变与平面应力状态,裂尖塑性区大小也不同,启裂首先发生在塑性区较早达到变形极限的平面应变区。裂尖塑性区的差异造成启裂点散布、裂纹扩展不同步,扩展路径不同,极易造成裂纹分叉、交汇异常等问题,并引起撕裂、崩角、断裂面台阶、夹屑、外缘爆口等裂解缺陷,从而影响连杆裂解加工质量。     

A simple method of determining stress intensity factor KI from isochromatic fringe loops

A new photoelastic method of obtaining mode I stress intensity factor (SIF) is presented. The method considers the influence of far field stress, σ _ox , on the value of SIF. The only information needed for K _I calculation is the area between isochromatic fringe loops. The method is examined by two kinds of specimen in different load cases. Experimental results show that it is quite simple and of high precision. Project supported by the National Natural Science Foundation of China Synopsis of the first author Chen Feng, associate professor, born in October 1949, worked on experimental fracture mechanics in Sweden from 1990 to 1995 as a visiting scholar. Major research fields include perturbation and weight function method and its application, experimental mechanics, rock fracture mechanmics.     

A SIMPLE METHOD OF DETERMINING STRESS INTENSITY FACTOR K_I FROM ISOCHROMATIC FRINGE LOOPS

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