Practical Challenges Testing Coupons with Residual Stresses from Cold Expanded Holes

Abstract

Cold expanding holes in metal components is widely used throughout the aerospace industry to extend fatigue life and damage tolerance. The beneficial residual stresses induced by hole cold expansion arrest or significantly retard the growth of fatigue cracks, and in doing so make fatigue or crack growth life prediction more difficult. Under low stress level∕high cycle fatigue testing, residual compressive stresses have been shown to increase the fatigue strength of the material leading to an “infinite life” and consequent “run-out” of test coupons at the test hole or changing the location of failure away from the test hole. While advantageous to aircraft operators, it makes statistical life prediction difficult because the incomplete data have to be censored, leading to a knock-down of the beneficial life attained. Conversely, increasing stress levels toward material yield stress; life improvement factors are more definitive and tend toward unity. This paper reviews the derivation of residual stresses induced by hole cold expansion and discusses benefits to the structure∕test coupon and how the residual stress field impacts fatigue life and damage tolerance∕durability under cyclic load conditions. Challenges observed when testing coupons that have been processed using hole cold expansion including modification of the crack behavior and its effect on material microstructure, relocation of crack initiation, dynamics of the redistribution of stress and the crack growth mechanism as well as scatter of results, are also discussed with a focus on aluminum materials.