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

Cold expansion is an effective technique of inducing favorable compressive residual stresses around fastener holes essentially vital for improving fatigue performances of aircrafts. The benefit is caused by the magnitude and distribution of the compressive stress field. Stress gradients are entirely contingent on 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. Recent method of Combined Step Drilling-Fourier Series Solution ‘ATÖzdemir Method’ was employed to appraise the continual redistributions of residual hoop stresses at the side of the hole subjected to cyclic load. Some results were compared with that of diffraction methods and Artificial Neural Network (ANN) modeling where, close similarities in stress distributions were confirmed. It is clear that during early fatigue, material nearby the hole is dynamically hardening; in response compressive residual stresses along the bore are gradually increasing till the onset of cracking. Short fatigue cracks mostly initiate from the mandrel entrance side, where subsequently residual hoop stresses begin to relax considerably.