Finite Element Simulation of Residual Stresses in Cold-expanded Plate

In the aerospace industry, the fastener holes are widely present because various mechanical parts need to be connected by rivets or bolts. These holes experience external forces which cause a tensile stress on the surface resulting in fatigue cracks and eventually fatigue failure of the structure. To alleviate this problem, cold expansion method has been used to improve the fatigue behavior of structures by inducing compressive residual stresses around the holes by forcing a tapered mandrel through the fastener hole that is slightly smaller than the mandrel. In this study, finite element simulations had been carried out for cold expansion process in order to determine the residual stress fields around two cold-expanded holes by varying the centre distance between two holes and also the thickness of the material. Moreover, the effect of through thickness position has also been investigated by considering the residual stresses distribution at different positions such as at the entrance face, at the middle of the thickness and at the exit face of the material. Aluminium alloy 7075-T6 was used as the material for the model simulated in ANSYS. The results showed that as the distance between the adjacent holes increased, the residual stresses decreased. Moreover, the simulation also highlighted the effect of the plate thickness and the through thickness position on the residual stresses distribution. The compressive residual stresses increased as the plate thickness increased. Meanwhile, the results at the mid-thickness were higher than the results at the entrance and exit faces.