韧性材料的几种断裂形式及判据讨论

考察几种韧性材料的复合型断裂试验过程，用传统强度理论思想认定启裂位置与启裂方向，通过对不同物理机制的断裂形式变化规律的研究，判定韧性材料的主要断裂形式有三类，以空穴成核、扩张、聚合为主导机制的正拉断，和以局部剪切带形成、发展为主导机制的两种不同类型的剪切断裂。局部大塑性变形的出现在材料内引起损伤，空穴的聚合或局部剪切带的出现仅发生在载荷达到某一临界值时。导致材料断裂发生的根本因素，是危险点上某应力参数达到了材料的断裂临界值，以此对不同断裂形式的断裂判据进行讨论，提出新的韧性断裂判据。

VARIOUS FRACTURE STATUS OF DUCTILE MATERIALS AND THEIR CRITERIA DISCUSSION

A variety of existing fracture criteria used to solve the problem of fracture of brittle material have been satisfied. However, when the criteria are applied to ductile material, the conclusions always do not coincide with the testing results. The experimental procedures of mixed mode fracture of several ductile materials will be investigated in this paper. Having determined the initial cracking position and direction of cracking growth according to the conventional strength theory, having recognized the variations of fracture caused by different physical mechanism, authors believe that the main ways of the fracture of the ductile material can be divided into three groups. They are the normal tension break and the two different types of shear fracture. Cavity nucleation, expansion and coalescing are the dominant mechanism of the normal tension. The shaping and the development of localized shear bands are the dominant mechanism of the two different types of shear fracture. Localized large plastic distortion will cause damage within the material. Authors also believe that the coalescing of cavity and happening of localized shear bands occur only when a certain load reaches a critical value. The fundamental factor, which causes the occurrence of cracking of material, is the stress parameter at the dangerous point has reached the critical fracture value of the material. Based on those phenomena listed above, several fracture criteria for different fracture statuses have been discussed. Then authors put forward the new criteria. To the normal tension break, at the place where the three dimension of the stress of the fracture is the maximum, the direction of the fracture of the danger point is the direction of the maximum tension stress, the tension stress is the main factor which causes the happening of fracture, while the other two main tension stresses are the subordinate factors. To shear fracture with the effect of cavity, at the place where the three dimension of the stress of the fracture is the maximum, the direction of the fracture is the direction of the maximum shear stress. The maximum shear stress is the main factor which cause the occurrence of the break, while the value of the three dimension of the stress is the secondary factor that affects the break. To the shear fracture without the effect of cavity, at the place where the three dimension of the stress of the fracture is the maximum, the direction of the fracture is the direction of the maximum shear stress. The maximum shear stress is the main factor that causes the material shear break, while the other factors have little effects on this type of shear break.