SIMULATION OF FATIGNE CRACK INTIATION AND PROPAGATION BASED UPON DISLOCATION DYNAMICS

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(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.