PROPAGATION OF CURVED FATIGUE CRACKS UNDER VARIABLE AMPLITUDE LOADING

Abstract. In this work, a two-step methodology to predict fatigue crack propagation in generic 2D structures is extended to variable amplitude loading histories, modeling crack retardation effects. Two complementary pieces of software have been developed to implement this methodology. In the first program, self-adaptive finite elements (FE) are used to calculate the fatigue crack path and the stress intensity factors along the crack length, at each propagation step. The computed values are then exported to the second program to predict the propagation fatigue life of the structure by the local approach, considering overload-induced crack retardation effects. To validate the methodology, fatigue crack growth experiments under variable amplitude (VA) loading are performed on 40mm-wide compact tension C(T) test specimens made of cold-rolled SAE 1020 steel, some of them modified with 7mm-diameter holes specially positioned to curve the crack path. FE predictions under constant amplitude (CA) loading indicated that depending on the hole position the crack could either curve its path and grow toward the hole (“sink in the hole ” behavior) or just be deflected by the hole and continue to propagate after missing it (“miss the hole ” behavior). Specimens presenting each of the behaviors are machined and tested under both CA and VA loadings, and the results are compared to crack growth predictions. The multiple overload effect on curved cracks is also studied, using 200mm-wide eccentrically-loaded single edge crack tension specimens ESE(T) made of SAE 4340 steel