低作用应力引起的溶质在晶界非平衡偏聚或贫化

提出了关于应力时效 (高温及作用应力 )引起溶质原子的晶界非平衡偏聚或贫化机理的一个新模型。此模型基于如下假设 :晶界相对于完整晶体而言在强度上是弱化区。当多晶体受到一个在弹性范围内的作用应力时 ,晶界优先变形。若压应力作用时 ,晶界作为源发射空位 ;若拉应力作用时 ,晶界作为阱吸收空位。应力引起的过饱和空位与溶质原子形成复合体 ,此复合体在基体中的扩散速率远高于溶质原子。作用应力对晶界溶质偏聚或贫化的影响 ,是由复合体的扩散和溶质原子的反向扩散之间的平衡所决定。根据此模型 ,应力时效过程中存在一个临界时间 ,在此时间 ,应力引起的晶界偏聚或贫化程度将达到一个极大值。此模型已由 Shinoda T等所作的磷在钢中的应力时效实验和 Misra R D

Crack branching mechanism of rock-like quasi-brittle materials under dynamic stress

The cracking patterns of a thin sheet with a pre-existing crack subjected to dynamic loading are numerically simulated to investigate the mechanism of crack branching by using the FEM method.Six numerical models were set up to study the effects of load,tensile strength and heterogeneity on crack branching.The crack propagation is affected by the applied loads,tensile strength and heterogeneity.Before crack branching,the crack propagates by some distance along the direction of the pre-existing crack.For the materials with low heterogeneity,the higher the applied stress level is and the lower the tensile strength of the material is,the shorter the propagation distance is.Moreover,the branching angle becomes larger and the number of branching cracks increases.In the case of the materials with high heterogeneity,a lot of disordered voids and microcracks randomly occur along the main crack,so the former law is not obvious.The numerical results not only are in good agreement with the experimental observations in laboratory,but also can be extended to heterogeneity media.The work can provide a good approach to model the cracking and fracturing of heterogeneous quasi-brittle materials,such as rock,under dynamic loading.

Dynamic Crack Propagation Analysis Using Scaled Boundary Finite Element Method

The prediction of dynamic crack propagation in brittle materials is still an important issue in many engineering fields. The remeshing technique based on scaled boundary finite element method(SBFEM) is extended to predict the dynamic crack propagation in brittle materials. The structure is firstly divided into a number of superelements, only the boundaries of which need to be discretized with line elements. In the SBFEM formulation, the stiffness and mass matrices of the super-elements can be coupled seamlessly with standard finite elements, thus the advantages of versatility and flexibility of the FEM are well maintained. The transient response of the structure can be calculated directly in the time domain using a standard time-integration scheme. Then the dynamic stress intensity factor(DSIF) during crack propagation can be solved analytically due to the semi-analytical nature of SBFEM. Only the fine mesh discretization for the crack-tip super-element is needed to ensure the required accuracy for the determination of stress intensity factor(SIF). According to the predicted crack-tip position, a simple remeshing algorithm with the minimum mesh changes is suggested to simulate the dynamic crack propagation. Numerical examples indicate that the proposed method can be effectively used to deal with the dynamic crack propagation in a finite sized rectangular plate including a central crack. Comparison is made with the results available in the literature, which shows good agreement between each other.