Effects of periodic overloads on the corrosion-fatigue behaviour of 7075-T651 aluminim alloy and HSLA steel

The corrosion-fatigue behaviour of 7075-T651 Aluminum Alloy and high strength low alloy steel (CSA G40.16M grade 400) subjected to periodic overloads was examined. The aluminum alloy is typically used in aerospace structural components such as the wing spars of aircraft; the HSLA steel is used as reinforcing bars in concrete structures.; Axial fatigue specimens were subjected to a loading spectrum that consisted of a periodic overload of yield magnitude followed by a number of smaller cycles at high R ratio: two hundred cycles in aluminum and fifty cycles in steel. The specimens were fatigue tested while they were fully immersed in an aerated and re-circulated 3.5%-wt NaCl simulated seawater solution. A separate set of fatigue tests was also performed to evaluate the aluminum in a salt-fog environment.; The results for the corrosion-fatigue testing were then compared to data obtained for the same overload spectrum applied in laboratory air. A damage analysis showed that the presence of the corrosive environment accelerated the damage accumulation rate to a greater extent than that observed in air. This resulted in a drastic reduction in the fatigue strength of the material when it was simultaneously subjected to overloads and a corrosive environment. The reduced fatigue-life of these materials was due to a combination of effects: premature crack initiation due to pitting corrosion on the surface of the specimens; anodic dissolution at the crack tip; and hydrogen embrittlement. For practical purposes, the endurance-limit of both of the materials disappears under these conditions.