Investigations into the fatigue enhancement provided by the hole cold expansion process using accurate 3D FEA simulations and fatigue testing

Abstract

Enhancing the fatigue performance of aging aircraft structures is of significant concern for military and civil operators worldwide. One such method involves cold expanding fastener holes to exploit residual compressive stresses in the region surrounding the holes. The beneficial effect derived from this process depends on the magnitude and distribution of the residual stress surrounding each hole, therefore accurate identification of residual stress profiles is critical to the evaluation of the life of aircraft structure containing cold expanded fastener holes. A 3-D Finite Element (FE) simulation of the hole cold expansion process was created. Advances in FE technology allowed this simulation to closely represent the physical expansion induced by the commercial split-sleeve process. The simulation included a post-expansion loading step that applied a remote tensile load to the cold expanded hole to estimate the interaction of the residual stresses and stress concentration of the open hole. The FE simulations indicated a significant 3-D variation of the residual stress field, with notable variations through the thickness of the specimen. The magnitude of compressive residual stress was lowest at the mandrel entry face for the cold expansion process. It increased to a maximum at the mid-plane of the specimen, before decreasing to an intermediate value at the mandrel exit face. A threshold value of remote tensile load was identified, below which the residual stress field remained compressive at the bore of the hole. Over this threshold, the application of a remote tensile load created tensile stresses at the fatigue critical plane on the bore of the hole. A constant amplitude fatigue testing programme was then conducted to ascertain the correlation of predicted residual stress fields and the fatigue properties of cold expanded fastener holes. For constant amplitude fatigue loading below the tensile threshold, small fatigue cracks initiated and then arrested, with no crack growth after 10 Million fatigue cycles. There was good correlation between the size and shape of these cracks and the residual stress field predicted by the FE simulation. For constant amplitude fatigue loading above the threshold, similarly shaped fatigue cracks initiate and continue to propagate. Fractographic analysis showed initial crack growth was from the mandrel entry face. Propagation within the zone of compressive residual stress zone was complex. Once cracks had passed through the residual compressive stress zone, they rapidly transitioned to corner cracks and then to through cracks, which propagated to failure.