In Situ X-Ray Radiographic Study of Stress Corrosion Cracking in AA2024-T3

Despite the many studies that have addressed stress corrosion cracking (SCC) in high-strength aluminum alloys, the separate contributions of stress and electrochemistry to the SCC process is still a matter of dispute. Conventional fracture mechanics approaches constrain SCC to a single crack per specimen, and the interaction of multiple stress corrosion cracks in a finite plate is unclear. In this study, a new nondestructive evaluation approach was developed to investigate SCC in AA2024-T3 (UNS A92024). A microfocal x-ray radiography technique was used to image multiple intergranular SCC cracks in situ. A modified ASTM G49 stressing jig was used to apply a fixed tensile displacement to a thin sheet sample, and a novel electrochemical cell containing flowing 1 M sodium chloride (NaCl) was attached to the edge of the sample. Potentiostatic polarization was applied at a potential that promoted intergranular corrosion. The initiation and growth of multiple intergranular stress corrosion cracks were characterized using transmission microfocal x-ray radiography. The kinetics of intergranular stress corrosion cracking growth was found to be in good agreement with the results of a completely different technique, the foil penetration method, which reflects the validity of the new approach. Interestingly, in many experiments the deepest crack at the beginning of the experiment was found to slow and stop growing, and was then surpassed by another crack that eventually penetrated through the sample. The competition between multiple intergranular stress corrosion cracks is not explained by the theory of mechanical fracture in inert environments. The possible mechanisms underlying this competition between cracks are discussed.