The relation between the resistance of fatigue crack propagation and the cyclic softening characteristics in a ship structural steel is investigated numerically by using Crystalline-Elastic-Plastic F.E. (CPFE) analysis in order to clarify the microscopic mechanisms of the fatigue strength improvement of FCA steel developed by Konda et al. (2001). The degree of the resistance is evaluated by microscopic-crack closure level and Crack-Tip Opening Displacement(CTOD). The closure level is determined by detecting the load at which the stress changes from compression to tension near the crack tip. As results, followings are found; 1) The in-house CPFE code which has the capability to perform the fatigue crack closure analysis for a cyclic softening metallic material has been developed. 2) For conditions chosen, the accumulated sum of slip within the plastic wake in the cyclic softening material becomes larger than that in the ordinary material, and a lower stress region appears directly behind the the crack tip. 3) For conditions chosen, the maximum compressive stress on the crack surface plane at the minimum load appears in front of the crack tip in the ordinary material while it appears behind the crack tip in the cyclic softening material. The stress changes more steeply near the crack tip in the cyclic softening material than do that in the ordinary material. 4) For conditions chosen, it is found that the crack opening level increases and CTOD decreases when the cyclic softening occurs. This means that the propagation rate of a fatigue crack in the cyclic softening material becomes smaller considerably compared with that in the ordinary material.
