A physically based fatigue model for prediction of crack initiation from persistent slip bands in polycrystals

In many engineering applications, fatigue is the dominant failure mechanism governing the life of a component. Thus, many studies have focused on this phenomenon, although there is a need for a model that addresses fatigue based on the material’s microstructure, specifically the energetics of the grain boundaries (GBs) and persistent slip bands (PSBs). Our approach is to model the energy of a PSB structure and use its stability with respect to dislocation motion as our failure criterion for fatigue crack initiation. The components that contribute to the energy of the PSB are identified, namely the stress field resulting from the applied external forces, dislocation pile-ups and work-hardening of the material is calculated at the continuum scale. Further, energies for dislocations creating slip in the matrix/precipitates, interacting with the GBs and nucleating/agglomerating within the PSB are computed via molecular dynamics. The results of our simulations on the stability of a PSB produce the correct fatigue crack initiation trends for the grain size, grain orientation, character of the GB, precipitate volume fraction and applied strain. From this information, we see that distinct GBs act as strong barriers to slip and increase the fatigue strength of the material.