MECHANICS OF A FATIGUE CRACK NUCLEATION MECHANISM

The mechanics of the build-up of large local plastic strains at the free surface of a polycrystal under fatigue loading is shown. Two closely located thin slices in a most-favourably-oriented crystal located at a free surface of the polycrystal are assumed to have an initial resolved shear stress of a few psi of opposite sign. Under cyclic loading, the two thin slices slide in opposite directions, one during the forward loading and the other during the reversed loading. Based on the dependency of slip on the resolved shear stress, the increase of local plastic strain in the slices with cycles of loading is calculated by applying the analogy between plastic strain and external force. The calculated local plastic shear strain in both slices (one positive and the other negative) reaches 100 per cent at the free surface in a few hundred cycles. These large plastic shear strains clearly cause the start of an extrusion or intrusion and the nucleation of a fatigue crack. © 1969.