Measurement of the three-dimensional residual stress distribution around split-sleeve cold-expanded holes

Cold expansion of fastener holes is an increasingly common way of improving the fatigue endurance of airframes. Although a number of methods of cold expansion are possible, the split-sleeve cold-expansion process is the most widely accepted and is widely used in the repair and manufacture stages of both military and civil aircraft. In the present work, the residual stress distribution around split-sleeve cold-expanded holes has been measured as a function of both position through the plate thickness and around the hole through novel use of the Sachs method. It is found that the residual hoop stresses being generated at the mid-thickness of the plate at orientation 90 degrees from the position of the split in the expansion sleeve. It is suggested that this variation must be taken into account both in the implementation of the technology and in any potential model used for the prediction of fatigue life of cold-expanded holes.