Experimental assessment of the redistribution of 3D residual stresses during early fatigue at split-sleeve cold expanded reamed A/C fastener holes

Abstract

Cold expansion is an effective technique of inducing favorable compressive residual stresses around fastener holes which are essentially vital for improving fatigue performances of aircrafts. The benefit is derived through the magnitude and distribution of the compressive stress field. Stress gradients are entirely contingent on the type of the cold expansion, local geometry of the hole, and characteristics of the metallic structure. During cyclic loading, however, initial residual stresses do not remain stable. In the present work, specimens with 4% split-sleeve cold expanded and reamed holes were cycled at the fatigue limit for short periods. A recent method of Combined Step Drilling-Fourier Series Solution 'AT Özdemir Method' was employed to appraise the continual redistributions of residual hoop stresses on the side of the hole subjected to cyclic load. Some results were compared with those of diffraction methods and Artificial Neural Network (ANN) modeling, where close similarities in stress distributions were confirmed. It is clear that the material nearby the hole is dynamically hardening during early fatigue; in response, compressive residual stresses along the bore are gradually increasing until the onset of cracking. Short fatigue cracks are mostly initiated from the mandrel entrance side, where, subsequently, residual hoop stresses begin to relax considerably.