A model was constructed for predicting fatigue-crack initiation based upon the material’s microstructure. In order to do so, the energy of a persistent slip band was monitored and an energy balance approach was taken, in which cracks initiate and the material fails due to stress concentration from a persistent slip band (with respect to dislocation motion). These persistent slip bands were able to traverse low-angle grain boundaries, thus belonging to clusters of grains. As a consequence of the ongoing cyclic slip process, the persistent slip bands evolve and interact with high-angle grain boundaries, the result of which led to dislocation pile-ups, static extrusions in the form of ledges/steps at the grain-boundary, stress concentration, and ultimately crack initiation. Hence, this fatigue model was driven by the microstructure, i.e. grain orientations, widely distributed grain sizes, precipitates, persistent slip bands with grain-boundary interactions, as well as the effect of neighbouring grains. The results predicted that cracks initiated near to twin boundaries from persistent slip bands spanning a single large grain with a favourable orientation or multiple grains connected by low-angle grain boundaries. Excellent agreement was found between model predictions and experimental data.

The Role of Grain Boundaries on Fatigue Crack Initiation – an Energy Approach. M.D.Sangid, H.J.Maier, H.Sehitoglu: International Journal of Plasticity, 2011, 27[5], 801-21