NUCLEATION OF FATIGUE CRACKS IN A LOW-ALLOY STEEL UNDER HIGH-CYCLE FATIGUE CONDITIONS AND UNIAXIAL LOADING

A basic understanding of the mechanism of fatigue-crack nucleation in a high-strength steel, under true high-cycle conditions, is developed from microstructural observations of rotating-bending fatigue specimens and theoretical results for the stress concentrations at inclusions and holes. The results show that for true high-cycle fatigue (fatigue life > 106 cycles), where the applied stresses are too low to cause localised cyclic-plasticity at the site of inclusions, crack nucleation does not occur while the inclusions remain undamaged and firmly bonded to the matrix. Fatigue-crack nucleation is found to occur only after either the progressive debonding or local fatigue damage of alumina inclusions has led to the formation of holes at the specimen surface: the fatigue cracks being nucleated by a highly localised cyclic plasticity effect at points of maximum stress intensity on the hole boundary. The inclusion/matrix debonding mechanism is therefore not an essential preliminary to the nucleation of fatigue cracks, under true high-cycle conditions. Manganese sulphide inclusions and cementite particles are shown to have virtually no influence on the mechanism of fatigue-crack nucleation under the present high-cycle conditions. For the case of manganese sulphide this is a direct result of the high axial-elongation of the inclusions and the consequent low stress concentration under axial loading: under more general states of fatigue loading manganese sulphide inclusions are unlikely to remain ineffective. © 1979.