AbstractDislocation accumulation caused by a crystallographic slip or plastic flow is one of the most critical factors for the growth of fatigue cracks. The dislocation density-based crystal plasticity method accounting for mobile and immobile dislocation densities has been widely used to evaluate crack behavior in polycrystals. However, the evolution of mobile and immobile dislocation densities in fatigue has not been carefully investigated, especially during the crack nucleation period. In the present study, a dislocation-based crystal plasticity model is employed and verified with experimental results. A representative domain with 39 grains is used to evaluate the dislocation density evolution in fatigue crack nucleation. The results indicate that mobile and immobile dislocation densities evolve at a decreasing rate in low plasticity and a constant rate in large plasticity for loading cases with constant strain amplitudes, whereas an increasing rate of dislocation density evolution is observed for loading cases with variable strain amplitudes. By analogy with cumulative plastic strain, a fatigue crack nucleation criterion is proposed, which is correlated well with the Coffin-Manson relationship. Based on a grain level analysis, a loading case with a low strain rate results in transgranular or mixed-mode (intergranular and transgranular) fatigue damage. In comparison, immobile dislocation density at a high strain rate mainly builds up at the grain boundary, indicating intergranular fatigue damage.

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