基于结构化变形驱动的非局部宏-微观损伤模型的真Ⅱ型裂纹模拟

TURE MODE Ⅱ CRACK SIMULATION BASED ON A STRUCTURED DEFORMATION DRIVEN NONLOCAL MACRO-MESO-SCALE CONSISTENT DAMAGE MODEL

Ⅱ型载荷作用下裂纹变形模式也为Ⅱ型的破坏问题称为真Ⅱ型破坏.准确定量地把握真Ⅱ型破坏的全过程是具有挑战性的问题.本文采用结构化变形驱动的非局部宏-微观损伤模型对真Ⅱ型破坏问题进行了模拟.根据结构化变形理论将点偶的非局部应变分解为弹性应变与结构化应变两部分,进而利用Cauchy-Born准则与结构化应变计算点偶的结构化正伸长量.在本文中,结构化应变取为非局部应变的偏量部分.当点偶的结构化正伸长量超过临界伸长量时,微细观损伤开始在点偶层次发展.将微细观损伤在作用域中进行加权求和得到拓扑损伤,并通过能量退化函数将其嵌入到连续介质-损伤力学框架中进行数值求解.进一步地,本文采用Gauss-Lobatto积分格式计算点偶的非局部应变,将积分点数目降低到4个,显著降低了前处理和非线性分析的计算成本.通过对Ⅱ型加载下裂尖应变场的分析揭示了采用偏应变作为结构化应变的原因.对两个典型真Ⅱ型破坏问题的模拟结果表明,本文方法不仅可以把握Ⅱ型加载下的真Ⅱ型裂纹扩展模式,同时可以定量刻画加载过程中的载荷-变形曲线,且不具有网格敏感性.最后指出了需要进一步研究的问题.

The fracture problem in which the crack deformation mode under mode Ⅱ loading is also mode Ⅱ is called the true mode Ⅱ fracture problem. It is challenging to accurately and quantitively capture the whole process of true mode Ⅱ fracture. In this paper, a structured deformation driven nonlocal macro-meso-scale consistent damage model is adopted to simulate the true mode Ⅱ fracture problem. The nonlocal strain of a material point pair is decomposed into elastic strain and structured strain based on the theory of structured deformation. Then the structured positive elongation quantity of the material point pair can be evaluated by using the Cauchy-Born rule and the structured strain. In the present paper,the structured strain is taken as the deviatoric part of the nonlocal strain. When the structured positive elongation quantity of a material point pair exceeds the critical elongation quantity, mesoscopic damage starts to emerge at the point-pair level. The topologic damage can be obtained by weighted summing of the mesoscopic damage within the influence domain, then it is embedded into the framework of continuum damage mechanics through the energetic degradation function bridging the geometric damage and energetic damage for numerical solution. Further, the Gauss-Lobatto integration scheme is adopted in this paper to evaluate the nonlocal strain of point pairs, which reduces the number of integral points to 4 and thus considerably reduces the computational cost of preprocessing and nonlinear analysis. The reason for adopting the deviatoric strain as structured strain is revealed based on the analysis of the strain field at the crack tip under mode Ⅱ loading. Numerical results for two typical true mode Ⅱ fracture problems indicate that the proposed model can not only well capture the crack deformation pattern of true mode Ⅱ cracks, but also quantitatively characterize the load-deformation curves without mesh size sensitivity. Problems to be further investigated are also discussed.