Modeling the effects of prior exfoliation corrosion on fatigue life of aircraft wing skins

This paper presents a preliminary analytical model, which was developed to evaluate the effects of prior exfoliation corrosion on the residual fatigue life of wing skins. A literature review was completed on the effects of exfoliation corrosion and fatigue interaction and the most significant conclusions are presented in this paper. A number of coupons with fasteners were cut from naturally exfoliated upper wing skins fabricated from 7178-T6 alloy, and the maximum depth of exfoliation corrosion was determined using an ultrasonic inspection technique. These coupons were tested to failure using constant amplitude compression dominated loading and the fracture surfaces were examined to determine the cracking mechanisms. A three-dimensional (3D) finite element (FE) model associated with a ‘soft inclusion’ technique was developed to determine the local stress/strain distribution around the exfoliated fastener hole. The model included the local geometry change due to material loss, the contact effects between the fastener and the wing skin as well as the predominately compressive loading. It was found that the local stress around the cracking sites did not increase significantly with the current levels of exfoliation. A life prediction was then carried out based on the 3D FE and soft inclusion model, and the prediction agreed reasonably with the test.