Loading conditions in the Very High Cycle Fatigue (VHCF) regime occur in many modern technical components, like micro-electro mechanical systems. Unlike conventional fatigue, the crack initiation process dominates the fatigue-lifetime of a material. To understand and quantify the mechanisms leading to plastic irreversibility in this regime, 3D discrete dislocation dynamics simulations of single grains, representing surface grains were performed under cyclic loading. In these simulations, the role of the grain shape, grain orientation, crystallography and loading on the dislocation evolution is studied. A comparison between fcc and bcc materials is done, in order to understand the role of the friction stress for screw dislocations in bcc, leading to an anisotropy in the glide of screw dislocations.
To quantify irreversible changes different measures have been calculated: (i) the accumulated plastic strain; (ii) the number of prismatic dislocation loops formed during cycling; (iii) the number resp. type of dislocation junctions; (iv) the number of cross-slip events and (v) the surface roughness. For low stress amplitudes, the dislocation structure is often found to stabilize. For such systems, a stability analysis is performed by introducing a new dislocation and further cycling. This analysis shall allow to extrapolate the changes to larger number of cycles , which is not possible by direct numerical simulation.