The shape of extrusions and intrusions and initiation of stage I fatigue cracks

Scanning electron microscopy (SEM) combined with local milling by focused ion beam (FIB) was used to study the true shape of the relief developed on the surface of polycrystalline 316L steel cycled with constant plastic strain amplitude. Localization of the cyclic strain to persistent slip bands results in early formation of persistent slip markings. The evolution of the early relief formation is reported using the observation on two perpendicular surfaces. The observed configurations of extrusions and intrusions are discussed and compared with predictions of the latest models of fatigue crack initiation. The prominent role of intrusions in fatigue crack initiation is emphasized.