Critical corrosion pit depth for fatigue crack initiation in 2024-T3, 6061-T6 and 7075-T6 aluminum alloys

Scope and method of study. Study the effects of corrosion pitting of 2024-T3, 6061-T6 and 7075-T6 aluminum alloys in 1.6 mm (0.063 inch) sheet thickness and determine the critical pit depth required for a fatigue crack to initiate. The corrosion pitting was accomplished using a 3.5% NaCl solution with hydrogen peroxide added as an accelerant. The pitting specimens were fatigue cycled in one-way bending until failure and the cycles to failure recorded. The specimens were examined with a metallurgical microscope, using the focus method of ASTM G46 to determine pitting depth.; Findings and conclusions. The presence of corrosion significantly reduces the fatigue life of the alloys tested as compared to the fatigue life of pristine (un-corroded) specimens. The average pitting depth required to initiate fatigue cracks was determined to be 2-4 mum (0.00008--0.00016 inches). This pitting depth corresponded with typical grain sizes at the surface and cracks began to initiate once pitting had penetrated about 50% of the grain thickness. Ratchet marks were noted on the fracture surface of the failed specimens, indicating multiple fatigue initiation sites; making it a useful indicator of fatigue crack nucleation. The effects of laser cutting aluminum sheet is detrimental to the fatigue life as compared to specimens manufactured using standard machining; however, the effects of corrosion pitting reduced the life of the laser cut 2024-T3 specimen to a much lesser degree that the machine cut 2024-T3 specimens, indicating that laser cutting is not necessarily bad.