Crack nucleation and stage I propagation in high strain fatigue—II. mechanism

Abstract In order to establish the properties of the grain boundaries at which cracks nucleate in high strain fatigue, groups of grains adjacent to the free surface of polycrystalline copper have been analysed by X-ray and trace techniques prior to testing, and subsequently observed during testing to check the nucleation sites. It has thus been found that vulnerable boundaries are associated with highly misoriented grains, the dominant slip systems of which are directed over large slip distances at the intersection of the boundary with the surface. The role of deformation incompatibility in crack nucleation is discussed and the nature of Stage I growth (intergranular) in high strain fatigue is described and defined in terms of bulk slip in the grains adjacent to the crack.