Effects of Stress Ratio and Oxygen Concentration on Short Surface Crack Growth of High-Strength Aluminum Alloy in Corrosion Fatigue.

Abstract

The fatigue crack growth behavior of a high-strength aluminum alloy, 7075-T651, was examined in deaerated and aerated 3.5% NaCl solution, in addition to in air. The relation between stress intensity range and crack growth rate in the same environment depended on stress ratio. For long cracks, the crack growth rate was always higher at higher stress ratio, while it was sometimes lower at higher stress ratio for short surface cracks. The effect of oxygen concentration was not observed for either long cracks or short surface cracks. The growth rate of long cracks in corrosion fatigue was almost the same as that in air when the stress intensity range was smaller than 7 MPam1/2. In this region, however, the growth rate of short surface cracks in the solution was higher than that in air, and the growth rate of short surface cracks was higher than that of long cracks in the same environment. Although the difference in crack closure was considered to be responsible for the acceleration of short surface crack growth in air, the acceleration of short surface crack growth rate in corrosion fatigue could not be explained solely by the difference in crack closure. The difference in chemical environment within the crack should be considered.