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Fatigue damage in a metallic material during very high cycle fatigue can strongly be correlated to the microstructure. This paper provides a review and a discussion on the micro damage behaviours in a nickel-base alloy during very high cycle fatigue using microplasticity and material mechanics. The results show that cyclic plastic deformation in this material can occur very locally even with an applied stress that is much lower than the yield strength. The fatigue damage occurs mainly at grain or twin boundaries because of local impingement and interaction of slip bands and these boundaries. The crystallographic properties, Schmid factors and orientations of grain and boundaries play very important roles to the fatigue damage. Subsurface fatigue crack initiation in the matrix is one of very high cycle fatigue mechanisms. Twinning and detwinning can also occur during the very high cycle fatigue process.

A fine grained area around a subsurface fatigue crack origin can usually be observed on fracture surface of a metallic material after very high cycle fatigue. This paper provides a fundamental study on the mechanisms to form this fine grained area using a martensitic stainless steel and advanced analysis instruments. The results show that the formation of a fine grained zone is a local behaviour. It is only a few micrometres in the transversal direction (cross section) and one micrometre in the longitudinal direction (crack propagation direction). High plastic deformation such as localized dislocation slip bands can be observed in this fine grained area. They interact with grain boundaries and cause the formation of damage by impingement cracking. The results indicate that occurrence of cyclic localized plastic deformation during very high cycle fatigue near the subsurface defect leads to the formation of fine grained area.