On the Retardation in Fatigue Crack Growth Rate due to an Overload; a Review

For over forty years it has been known that the rate of fatigue crack growth at a constant load amplitude can be significantly reduced by the application of an overload. There has been a continuing interest in the mechanism responsible for the reduction in growth rate after an overload, and such factors as the compressive residual stress field in the overload plastic zone and crack closure have figured importantly in many explanations. Critical experiments have shown that the phenomenon is largely related to the overload plastic zone in the plane-stress, surface region of a planar specimen such as the compact tension specimen. The results of careful experimentation indicate that the relaxation of the compressive stress within the overload plastic zone as the fatigue crack penetrates this zone gives rise to increased crack closure in the wake of the crack tip, and as a result to a reduced level of ĢK_eff and consequently to a reduced crack growth rate. Recent experiments have shown that this mechanism is operative at high R values as well as at lower values of R. An attempt at a semi-quantitative analysis of the effects of an overload on the rate of fatigue crack propagation will also be described.