Scales of Metal Fatigue Failures and Mechanisms for Origin of Subsurface Fracture Formation

Three fatigue regimes in accordance with three scale levels are considered and mechanisms of fatigue crack origination subsurface discussed. Surface hardening procedure can be used for the transition area of the crack origination from surface to subsurface in High-Cycle-Fatigue (meso-scale level) regime. In this case subsurface cracking characterized the same fatigue curve that was constructed for the Very-High-Cycle-Fatigue (micro-scale level) regimes due to the same mechanism of metals cracking. This effect was considered for Al-and Fe-based alloys. Subsurface crack origination can be received because of introduction of residual compressive stresses in the bulk material in the Low-Cycle-Fatigue (macro-scale-level) regime. The unified or master fatigue curve can be constructed for all scale levels for metals if subsurface crack origination will be realized because of hardening procedures for meso-and macro-scale levels. Subsurface crack origination in Very-High-Cycle-Fatigue regime for Ti-based alloy in the case of torsion is also discussed and three scale levels for fatigue regimes are demonstrated.