Verification of Cold Working and Interference Levels at Fastener Holes

Cold working (Cx) and interference fit fasteners are well known to effectively impart favorable residual stresses (RS) and improve structural fatigue life around fastener holes. Heretofore, there has been no reliable means of measuring the RS or the effectiveness of Cx in the immediate vicinity of fastener holes, so reliance was placed on Cx process controls. In addition, the effects of various levels of Cx on remaining fatigue life have not been quantified. Accordingly, design credit for Cx is seldom taken and its favorable effects are not accounted for in structural fatigue life analyses. The Proto Team solution to this problem has two elements: A portable NDI/E system for quantitatively measuring in a production environment the levels of RS imparted by Cx and a reliable life prediction methodology that can exploit the output surface RS data. Proto has recent experience using advanced x-ray diffraction (XRD) technology to nondestructively measure RS fields around Cx holes and will demonstrate that capability concurrently during fatigue cycling of a representative coupon set with Fatigue Technology Inc. UDRI will introduce the RS/fatigue cycling data into finite element analyses and current life prediction models and correlate the life prediction output with fatigue test results.