裂纹动态起始问题的研究进展

力图就前人提出的冲击载荷下裂纹的起始判据进行较全面的综述．这些较著名的判据有：（１）动态应力强度因子判据；（２）动态Ｊ积分判据；（３）最小作用量判据；（４）极小作用时间判据等．简要介绍了近年来有关裂纹在冲击载荷（特别是短脉冲）下动态起始的一些重要实验和实验中所发现的一些重要结论．实验公认，一般对于小范围屈服而言，材料的动态断裂韧性随加载（应变）率的提高而减小，此时材料的断裂形式为解理型；而对于大范围屈服则韧性随加载率的提高而增大，此时材料的断裂形式为纤维型．特别指出，Ｂｒｏｗｎ大学的平板撞击实验表明，裂纹在起始时，观察到一个不再满足二分之一阶奇异性的“尖峰”，按Ｃｌｉｆｔｏｎ和Ｆｒｅｕｎｄ等人所给出的模型，在裂纹起始断裂瞬间，在裂纹顶端会突然形成一个小洞，该洞的半径作为一个参数等于二相粒子的间距．

THE ADVANCES OF STUDIES ON THE DYNAMIC INITIATION OF CRACKS

It is attempted by this paper to systematically review the existing dynamic initiation criteria. Some well -known dynamic initiation criteria are:(1) Dynamic stress intensity factor criterion; (2) Dynamic J-integral criterion;(3) Least action criterion; and (4) Minimum time criterion. Some recent important experiments and some important conclusions obtained from experimentsare briefly introduced. It is generally agreed by the experiments that the dy namic fracture toughness of materials under small-scale yielding (SSY) decreases with increasing strain rate, the fracture in this case is one of cleavage in nature, on the other hand, the dynamic fracture toughness of materials under large-scale yielding (LSY) increases with increasing strain rate, resulting in a fibrous fracture. It is emphatically pointed out that a sharp "spike" was recorded by the plate impact experiments conducted in Brown Uniyersity for the normalvelocity of the rear surface of the specimen at the onset of crack growth,which could not be predicted by the inverse square root singular stress field of the linear elastic fracture mechanics (LEFM). The crack initiation was modeled by Profs. Clifton and Freund et al. as the sudden formation of a very small hole at the crack tip. The radius of the hole, which appeared as a parameter in the solution, agreed reasonably well with the interparticle spacing.