Standardized Test Method for Corrosion Pit-to-Fatigue Crack Transition for AA7075-T651 Aluminum Alloy

Abstract

Corrosion damage (pit) is a stress raiser that can have deleterious effects on the fatigue life of airframe structural components. A better understanding and method for modeling the corrosion pit to fatigue crack transition would advance the fidelity of aircraft structural integrity estimates and fleet management decision making. Here, the focus is on developing a standardized fatigue test method for investigating the transition of a corrosion pit to fatigue crack in aluminum alloy AA 7075-T651 specimens. The standardized test method requires the development and validation of two sub-protocols (1) a pitting protocol to create a corrosion pit less than 200 μm diameter at the intersection of the central hole bore and planar surface of sheet and (2) a spot welding protocol for attaching the direct current potential drop (dcPD) probes on either side of the corrosion pit for fatigue crack growth measurement. A dcPD fatigue test method coupled with a unique 10-4-6 marker load sequence is used to measure the fatigue crack growth. The crack shape evolution and crack growth life are predicted using AFGROW.