Influence of residual stresses on the fatigue crack growth from surface anomalies in a nickel-based superalloy

Abstract : The present study evaluates the influence of initial residual stresses surrounding scratch or dent type surface anomalies on the fatigue crack growth of Direct Aged (DA) Inconel 718 alloy. A heat treatment has been used to at least partly relax these residual stresses in order to evaluate their effect on the fatigue crack initiation and propagation starting from scratches and dents. In the presence of compressive residual stresses under the surface anomaly, the first stages of crack propagation are slowed down. The evolution of the crack front shape is also strongly influenced by the residual stress field and leads to the formation of an “ear-shaped” crack front for dents and initiation in the corners for scratches. The initiation fatigue life is short for both type of surface anomalies and a competition between the stress concentration and the initial microstructural state was noted for the control of this stage of the fatigue life. These results highlight the need to characterize the residual stress field resulting from the introduction of the surface anomaly in order to predict accurately the fatigue life of DA Inconel 718 alloy components containing scratches and dents. A preliminary finite element simulation is developed in this study to address this issue.