Hierarchy of the Metal Fatigue Mechanisms Based on the Physical Mesomechanics Methodology

Mechanisms of fatigue crack nucleation and propagation in metals are considered on the basis of the physical mesomechanics methodology. Evolution in the metal behavior is presented in the direction of increasing scale levels of evolution with growing cyclic equivalent stress or strain energy density. Differences in damage accumulation in the surface layer and internal volumes on the three scales corresponding to very high-cycle, high-cycle and low-cycle fatigue regions are analyzed. The subsurface crack initiation mechanisms leading to origins in the form of a smooth facet or fine-granular area are shown. Both types of origins are created under vortex flow of plastic deformation. Differences in the fatigue crack nucleation on the meso- and macroscales are discussed and illustrated. Fracture features of ductile metals in the ultralow-cycle fatigue region, which is a transition region from cyclic to monotonic static loading conditions, are considered.