Effect of Galvanic Corrosion on Crack Propagation of Cold Expanded Holes in AA2024-T3

A search for the balance between weight reduction and durability of a structure has always been a popular topic within the aerospace industry. Among numerous researches to improve a structure’s fatigue life, processes on mechanically fastened joints have been a topic of focus as it is naturally a discontinuity in the material leading to a local stress concentration under loading. Cold expansion is a well-known technique with a wide range of applications in both new and repaired structures. The benefits include the extension of fatigue life and a delay in crack growth without addition of weight. However, localized galvanic corrosion may be induced, since cold-worked holes are often fastened to dissimilar materials. While cold expansion on holes with initial cracks has shown to obtain the same benefits in previous research, another step must be taken to simulate realistic applications in aggressive environments. In this research, a relationship between crack propagation and secondary crack initiation will be established for AA2024-T3 cold worked holes subjected to cyclic loads to determine the impact on fatigue life of joints in presence of saline solution, including Stress Intensity Factor (SIF) adjustments in the Linear Elastic Fracture Mechanic (LEFM) equations to account for those effects. Galvanic corrosion of a steel fastener/aluminum plate assembly will be investigated assuming the presence of cracks in the aluminum plates. The specimens will be prepared to include an open hole with enough distance to allow cold expansion of commercial interest and will contain induced fatigue flaws. The cracks will be monitored in-situ with a digital microscope following pre-established cycle milestones. A special setup will be designed to accommodate the specimen with fastener bolt in presence of saline solution in a fatigue test machine