Corrosion Pit to Fatigue Crack Transition Methodology for AA7075-T651 Aluminum Alloy

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 greatly help the aircraft community. This paper presents the development of a standardized fatigue test method for the determination of 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 several sub-protocols (1) a pitting protocol to create a mechanically small ( 200 micron diameter) corrosion pit on the hole edge of the fatigue specimen (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 (3) a dcPD fatigue test method applying a unique 10-4-6 marker band sequence to measure the fatigue crack growth and (4 ) a crack growth life prediction protocol using AFGROW. AFGROW simulations are used to predict the crack shape evolution during the test. New stress intensity factor (SIF ) solutions for small finite width plates with different crack shapes are being developed and validated with the pit-crack test sample for more accurate crack growth predictions.