The initiation of fatigue cracks is an event whose very definition is strongly linked to the size scale of observation. For example, materials scientists are likely to consider the nucleation of flaws along persistent slip bands as the initiation stage of fatigue failure, whilst a mechanical engineer may associate the resolution of crack detection with the threshold for crack nucleation. Between this wide range of view-points lies a variety of failure mechanisms that are affiliated with the inception of microscopic flaws at grain boundaries, twin boundaries, inclusions, micro structural and compositional inhomogeneities, as well as microscopic and macroscopic stress concentrations. The differences in the approaches to fatigue crack initiation constitute the fundamental distinction between the fatigue design philosophies currently practiced in industry. From a scientific standpoint, developing a quantitative understanding of crack initiation processes must be regarded as one of the most important tasks.

In this chapter, attention is first directed at the mechanisms of fatigue crack initiation in nominally defect-free (unnotched) pure metals and alloys, and commercial materials. Models for fatigue crack initiation are described and their significance and limitations are pinpointed. Also addressed are the mechanisms by which fatigue cracks initiate ahead of stress concentrations under fully compressive cyclic loads. Continuum aspects of crack initiation based on stress–life and strain–life approaches are addressed in Chapters 7 and 8, respectively, where the initiation of fatigue cracks at stress concentrations under tension and tension–compression fatigue are treated.