Initiation of Stage I Fatigue Cracks – Experiments and Models

Early fatigue damage has been studied in a number of model and structural polycrystalline materials using modern experimental techniques. Early initiation of fatigue cracks was found to be related to the localization of the cyclic plastic strain into persistent slip bands during cyclic loading. Internal dislocation structure of the persistent slip bands and also the evolution of the surface relief as surface persistent slip markings are documented. The model of the surface relief formation describing the formation of extrusions and intrusions is briefly described. The intrusions representing sharp surface crack-like defects play the principal role in the initiation of fatigue cracks.