Investigation of residual stresses in a sleeve coldworked lug specimen by neutron and x-ray diffraction

Sleeve coldworking (SCW) is a mechanical process used in the aircraft industry to strengthen fastener holes of structural parts. By cold-expanding the holes, compressive residual stresses and a high dislocation density are introduced around the holes, the effect of which is to counteract the initiation and propagation of fatigue cracks and thus increase the fatigue life of the parts. The knowledge of residual stress due to SCW is therefore crucial for assessing the fatigue properties of a treated part. In this study, residual stresses were investigated, by employing neutron and X-ray diffraction methods, in a lug specimen that was sleeve coldworked and fatigued. The specimen had been used for testing the influence of the SCW process on fatigue life and crack propagation behaviour under constant amplitude or variable amplitude cyclic loading. X-ray diffraction has been widely used to determine surface residual stresses in engineering components. Neutron diffraction measurements, on the other hand, probe in-depth stresses, owing to the penetration of the neutron beam into most engineering materials. By using both techniques, we were able to investigate the surface as well as the in-depth distribution of residual stress in the specimen