3D residual stresses around cold expanded holes in a new creep resistant aluminium alloy

Hole cold expansion is used frequently to enhance the fatigue endurance of components. The cold expansion process introduces compressive residual stress at the periphery of the hole and tensile stress further away. To understand how these residual stresses enhance the fatigue life, it is essential that we have an accurate; description of the three dimensional distribution of the stresses. In this paper we describe the results of experimental studies and finite element analyses used to determine detailed residual stress distributions in a new creep resistant alloy. The near surface residual stresses at the mandrel entrance and exit faces were measured using an X-ray diffraction technique. The results show significant differences between residual stresses at the two faces. The residual stresses are also shown to vary as a function of position around the hole. To take account of this variation a new approach, called the Garcia-Sachs method, was used to measure residual stress averaged through the thickness of the component. For finite element simulations, a combined hardening material model was used to reproduce the Bauschinger effect exhibited by the aluminium alloy. Using this new model, excellent agreement was found between predicted and measured near surface residual stresses. Finally there is also good agreement between the measured mid-thickness residual stress and the FE predictions.