Defect assessment procedure based on ductile tearing process

Strain hardening, crack growth resistance and nonuniform load distribution have significant effects on the ductile tearing process. A defect assessment procedure incorporating with the above features is proposed in the present paper, with the final appearance of “design curves” to facilitate engineering applications.     

Fatigue damage evolution in silicon films for micromechanical applications

In this paper we examine the conditions for surface topography evolution and crack growth/fracture during the cyclic actuation of polysilicon microelectromechanical systems (MEMS) structures. The surface topography evolution that occurs during cyclic fatigue is shown to be stressassisted and may be predicted by linear perturbation analyses. The conditions for crack growth (due to pre-existing or nucleated cracks) are also examined within the framework of linear elastic fracture mechanics. Within this framework, we consider pre-existing cracks in the topical SiO₂ layer that forms on the Si substrate in the absence of passivation. The thickening of the SiO₂ that is normally observed during cyclic actuation of Si MEMS structures is shown to increase the possibility of stable crack growth by stress corrosion cracking prior to the onset of unstable crack growth in the SiO₂ and Si layers. Finally, the implications of the results are discussed for the prediction of fatigue damage in silicon MEMS structures.     

变分法在含边裂纹正交各向异性板中的应用

文中给出了含边裂纹正交各向异性板问题的特征函数展开式，然后利用变分法及边值条件确定展开式中的未知系数，从而可求得裂端应力强度因子。该方法即可应用于应力边界值和位移边界值问题，又适应混合边界值问题。数值计算结果证实了该方法的有效性。     

弹塑性平面Ⅰ型裂纹应力场的一个摄动解析解

本文利用摄动方法,得到了幂硬化材料平面Ⅰ型裂纹端应力奇异场的一个解析表达式,并与HRR数值结果进行了比较。分析表明:当硬化指数在[1,∞)变化时,应力场的结构形式不发生变化,为三角函数的线性组合。在一定的幂硬化指数变化范围内,解析解是数值解的很好近似,对应力分量σ_(θθ)和σ_(vθ),这一特点尤为突出。该解析解形式简洁,明了,可为弹塑性断裂的工程应用提供方便。     

LY12铝疲劳裂纹在冲击载荷下的演化研究

对LY12铝合金在低周疲劳条件下的裂纹情况和随后进行的动态拉伸条件下裂纹的发展给予了观察和统计分析，发现裂纹的累积数密度分布在损伤演化过程中保持指数形式，用NAG模型对实验结果进行分析，得出该材料裂纹演化发展方程的各种参数。     

The displacement discontinuity method in the analysis of open cracks

The application of the displacement discontinuity numerical technique to the solution of some problems of fracture mechanics is demonstrated in the hypothesis of homogeneous and elastic material. The fracture is supposed to be free from traction and is represented by a set of constant displacement discontinuity elements, except for two parabolic elements, located at each crack tip, in order to simulate the singularity of the solution near the crack tips. On the basis of the stress and displacement field determined by the displacement discontinuity method, the stress intensity factors for mode I and II are computed according to the method of the displacements. Three examples are provided to verify the validity of the formulation.

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Sommario Lo scopo del presente lavore è di illustrare l'applicazione del metodo numerico della Displacement Discontinuity alla soluzione di alcuni problemi di meccanica della frattura, nell'ipotesi di materiale omogeneo ed elastico. La frattura è supposta aperta ed è rappresentata da una linea di elementi a discontinuità di spostamento costante, con l'eccezione di due speciali elementi parabolici, ubicati agli apici, al fine di simulare la singolarita' del campo tensionale. Sulla base del campo degli sforzi e degli spostamenti cosi determinati, vengono ricavati i fattori di concentrazione degli sforzi in modo I e II mediante il metodo degli spostamenti. Vengono inoltre riportati tre esempi di calcolo, effettuati al fine di verificare la validità del procedimento proposto.

     

Interfacial crack between elastic half-spaces under harmonic loading

A system of boundary integral equations that allows evaluating the displacement and stress fields for an interfacial crack under harmonic loading is presented. Expressions for the integral kernels are obtained. A numerical solution for a penny-shaped crack between steel and aluminum half-spaces under normally incident compression-rarefaction wave is given __________ Translated from Prikladnaya Mekhanika, Vol. 43, No. 8, pp. 47–56, July 2007.     

Modélisation de fissures elliptiques internes par hybridation de fonctions de poidsEmbedded elliptical cracks modeling with hybridization of weight functions

A method improving the evaluation of the stress intensity factor by hybridization of two weight functions is presented and applied to embedded elliptical cracks under various loadings. The hybridization consists in using one or the other function in the zone of the crack where it is the most efficient. The delimitation of the two zones is achieved after optimizing the axes ratio and the curvature radius of the crack. During this optimization we seek to reduce the effects of the singularities present in the weight functions as well as to take better into account the influence of the ellipse curvature. To cite this article: B. El Khalil Hachi et al., C. R. Mecanique 334 (2006).     

Stress intensity factors and crack deviation conditions in a brittle anisotropic solid

For arbitrary anisotropy in the linear manifold of singular solutions generating square-root singularities of the crack tip stress, a special basis is introduced that possesses the same properties as in the isotropic case and provides simple integral representations for the attributes of the energy fracture criterion, in particular, the conditions of crack deviation from a straight path. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 3, pp. 98–107, May–June, 2005.     

停歇对疲劳裂纹扩展超载迟滞效应影响的研究

本文用 16 Mn和 A 537 两种普通低合金钢试件,较系统地研究了停歇对疲劳超载迟滞效应的影响。研究表明,疲劳载荷中的停歇对超载迟滞效应有明显影响,其影响的程度大小与停歇时的载荷 P_H,停歇时间 t_H 等因素密切相关;不但可能使试件的疲劳寿命减少,也可能使试件的疲劳寿命增加。在工程结构疲劳设计、分析及应用超载迟滞效应时应予以注意。