分子动力学模拟含不同初始缺陷单晶镍的断裂行为与应力演化

断裂是一个跨尺度复杂的物理过程，对宏观尺度的断裂行为已有深入的研究和发展，然而对微观尺度的断裂行为及断裂过程中应力场的变化缺乏深入的理解。本文通过分子动力学模拟，研究了具有不同初始缺陷(尖锐裂纹、钝裂纹和孔洞)的单晶镍的断裂行为和应力分布特征。结果表明，不同的初始缺陷导致了不同的断裂机制、断裂强度和抗断裂性能。含初始孔洞的单晶镍样品有最高的断裂强度和最强的抗断裂性能，这与孔洞扩展过程中堆积层错的形成密切相关。其次是含初始钝裂纹的样品，在裂纹扩展过程中出现由100]超位错发射引起的裂尖钝化；含尖锐裂纹的样品表现为脆性断裂，裂尖原子没有出现微结构的变化，其强度和抗断裂性能最低。此外，不同的初始缺陷也会导致断裂过程中应力分布的变化，对含有尖锐裂纹的脆性断裂试样，高应力(拉伸应力、平均应力和米塞斯应力)总是出现在扩展裂纹的裂尖。而对于含有钝裂纹或孔洞的韧性断裂试样，高应力不仅分布在裂尖，也分布在位错发射和堆积层错形成的区域，在裂纹/孔洞扩展之前，应力随着加载时间的增加而迅速增加，而一旦裂纹或孔洞开始扩展，应力增加非常缓慢或几乎不增加，但拉伸应力值始终大于平均应力和米塞斯应力值。这表明，在I型...

Molecular dynamics simulation of fracture behavior and stress evolution of single crystal nickel with different initial defects

Fracture is a complex physical process across several different length scales.The fracture behavior at the macro-scale has been deeply studied and developed.However,there is a lack of in-depth understanding of the fracture behavior at the nanoscale and the change of stress field in the process of fracture.In this paper,the fracture behavior and stress distribution characteristics of single crystal nickel with different initial defects(sharp crack,blunt crack,and void) are studied by molecular dynamics(MD) simulation.The results show that different initial defects induce different fracture mechanisms,fracture strengths and fracture resistance.The single crystal nickel sample with a void has the highest fracture strength and fracture resistance,which is closely related to the formation of stacking faults in the process of void propagation.The sample with a blunt crack is next in magnitude of these properties,which is related to the crack tip blunts caused by 100] super-dislocations emission.The sample with a sharp crack shows a brittle fracture,and the atoms at the crack tip have no microstructure changes.This sample has the lowest fracture strength and fracture resistance.Moreover,different initial defects also cause the change of stress distribution during the fracture process.The high stresses(tensile stress,mean stress,von Mises stress) always occurs at the crack tip of the growing crack for the cleavage fracture sample with a sharp crack.While for the ductile fracture sample with a blunt crack or void,the high stresses occurs not only at the crack tip but also in the region of plastic deformation(dislocation emission and stacking faults formation).Before crack/void propagation,the stresses increase rapidly with the increase of loading time,and then almost stop increasing once the crack/void begins to propagate,and the value of tensile stress is always larger than that of mean stress and von Mises stress,which implies that the material failure at nanoscale under I loading condition is controlled by the tensile(normal) stress.