The Effect of Corrosion Location Relative to Local Stresses on the Fatigue Life of Geometrically Complex, Galvanically Corroded AA7075-T6

Abstract

Aluminum components used in aerospace structures are commonly coupled with stainless steel fasteners. These through-hole geometries on the aluminum substrate cause a concentrated stress field. The high-stresses at the fastener sites can preferentially initiate coating damage allowing for moisture ingress, which can lead to the formation of a galvanic couple between the aluminum alloy and the stainless steel fastener. Corrosion damage is known to cause early initiation of fatigue cracks, thus severely reducing the total life of the component. This work aims to understand the relative impact and interaction of fastener hole geometry-induced stress concentrations and corrosion damage on the fatigue crack initiation behavior and total fatigue life. Specifically, by imparting various levels of corrosion severities at different locations within the macro-scale stress field, the relative impact of each on the fatigue process can be determined. This work demonstrated a dominant role of the macro-scale stress field on the ability of corrosion morphologies to initiate fatigue cracks. Specifically, crack formation was found to preferentially occur at high-stress regions in lieu of forming at lower-stress regions, regardless of corrosion severity, and corrosion severity in the through-hole had a significant, but nonpredictive, correlation with the total fatigue life. Critically, the findings of this work will inform the means by which coatings are evaluated and will serve as a controlled validation of experiments for fracture mechanics modeling.