Fatigue Crack Retardation in PM Aluminum Alloys AA8022 and AA509l

An important goal of fatigue research is the prediction of fatigue crack propagation rate under service loading conditions. Intermittent high peak tensile loads can cause crack retardation in subsequent load cycles. The mechanisms responsible for crack retardation must be characterized in each alloy or class of alloys so that appropriate predictive models can be developed and utilized. The objective of this investigation was to identify the mechanisms responsible for crack retardation in PM aluminum alloys AA8022 and AA5091 using an elastic-plastic fracture mechanics approach. The results of single tensile overload tests and computer simulations with FASTRAN, a plasticity-induced closure model, indicate that plasticity-induced closure is the primary cause of retardation in AA8022 at both low and high R in air at room temperature. Both plasticity and roughness-induced closure contribute to crack retardation in AA5091.