Effect of single overloads in ductile metals: A reconsideration

Fatigue crack growth tests were carried out under constant-ΔK loading interrupted by a single overload (OL). The specimen material was a ductile austenitic CrNi alloy. Variables of the experimental program were the severity of the OL and the specimen thickness, the latter one in view of plane strain/plane stress aspects. The results confirm the delayed retardation phenomenon as abundantly observed for aluminium alloys. Fractographic observations were made with special attention to crack extension during the OL and base line cycles following the OL, a topic somewhat neglected in previous studies. Striation observations indicate an increased crack growth rate in a small number of cycles after the application of the OL. This is an unexpected new phenomenon, which could be made with innovative fractographic procedures. It is associated with crack tip blunting and crack tip opening displacements for which a model was proposed. The results should be of interest for fracture mechanics prediction models on fatigue crack growth under variable amplitude loading.