SIMULATION OF FATIGNE CRACK INTIATION AND PROPAGATION BASED UPON DISLOCATION DYNAMICS

(1)疲劳裂纹起裂：通过位错动力学模拟，考虑了在循环施加剪切应力作用下，边缘位错从表面逸出的运动，从而研究了疲劳裂纹起裂行为。结果表明，由于相同数量的位错以相反的符号交替地从表面逃逸，因此不存在仅由活动位错形成的船阶。然而，在材料中随机分布的固位错存在的情况下，滑移阶跃的形成是由于固位错的应力场干扰了可动位错的行为。此外，在多重滑移面情况下，发现在固位错存在的情况下，每一滑移阶的扩展都会产生疲劳裂纹。(2)疲劳裂纹扩展：基于裂纹尖端位错的发射和移动，通过计算机模拟研究了加载历史和应力比对疲劳裂纹扩展行为的影响。模拟结果表明，上述循环的应力强度范围与裂纹停止扩展的阈值之间呈幂函数关系。通过Walker关系还发现，仿真得到的阈值与应力比密切相关。从上述两个参数不仅控制最大应力强度，而且还控制裂纹张开应力强度水平，而裂纹张开应力强度水平是由对残余位错的屏蔽作用程度决定的角度解释了这些关系的存在。

(1) Fatigue Crack Initiation : The behavior of fatigue crack initiation was considered through the dislocation dynamics simulation based upon the movement of edge dislocations escaping from the surface under a cyclic applied shear stress. The result showed that no ship step was formed only by movable dislocations because the same number of dislocations with the opposite sign alternately escaped from the surface. However, the formation of a slip step occurred under the existence of sessile dislocations randomly distributed in the material since the stress field of the sessile dislocations disturbed the behavior of movable dislocations. Furthermore, in the case of plural slip planes, it was found a fatigue crack can be produced by the growth of each slip step under the existence of the sessile dislocations.(2) Fatigue Crack Propagation : The influence of the loading history and stress ratio on the behavior of fatigue crack propagation was investigated through the computer simulation based upon the emission and movement of dislocations from the crack-tip. The result of the simulation showed that is a power relation between the stress intensity range in the preceding cycles and the threshold value where is the crack stops propagating. It was also found that the threshold value obtained by the simulation is closely connected with the stress ratio through the Walker relation. The existence of those relations was explained from the viewpoint that the above two parameters control not only the maximum stress intensity but also the crack opening stress intensity level which is determined by the degree of the shielding effect to the residual dislocations.