During high-cycle fatigue and even more during ultra-high-cycle fatigue, most of the lifetime of a material is spent in the crack initiation phase. Although macroscopically mostly or even purely elastic material behavior is observed, locally small-scale plastic yielding can occur in regions of increased internal stresses. During such local reversed plastic deformation, damage caused by irreversible dislocation slip accumulates and finally leads to the nucleation of a crack-like defect, which is smaller than the grain size. Such microstructurally short cracks can grow and finally cause the failure of the material. Thus, it is important to understand and properly model the mechanisms and criteria for crack initiation, which are strongly influenced by accumulation of irreversible deformation.
In this work, micromechanical modeling is applied to predict crack initiation within a realistic microstructure. The micromechanical method is based on three main constituents: (i) generation of material microstructures in form of representative volume elements (RVE) (ii) implementation of a crystal plasticity model capturing the relevant mechanisms of cyclic plastic deformation, and (iii) definition of a reliable fatigue indicator parameter that describes the damage evolution during cyclic deformation. All these steps need to be validated against experiment to allow us to study fatigue crack initiation and the early stages of short crack growth. In this paper, the background of micromechanical fatigue models is introduced and the method is applied to study fatigue behavior and to predict S-N curves and lifetimes of polycrystalline microstructures.