Short Surface Crack Growth of High-Strength Aluminum Alloy in Corrosion Fatigue.

Abstract

The short surface fatigue crack growth behavior of a high-strength aluminum alloy, 7075-T651, was examined in deaerated aqueous 3.5% NaCl. The growth rate of long cracks in corrosion fatigue was almost the same as the rate in air when the stress intensity range was smaller than 7 MPa√(m). In this region, the growth rate of short surface cracks was higher than that of long cracks in the same environment. Since the da/dN-ΔK relation of short surface cracks in air was located between the da/dN-ΔK relation and da/dN-ΔKeff relation of long cracks, the difference in crack closure may be responsible for the acceleration of short crack growth in air. Since the da/dN-ΔK relation of short surface cracks in corrosion fatigue was located left of the da/dN-ΔKeff relation of long cracks, and the value of ΔK is always greater than ΔKeff, the acceleration of short surface crack growth in corrosion fatigue cannot be explained solely by the difference in crack closure. The observations of fracture surface by scanning electron microscopy showed that the difference in the crack growth mode was also responsible for the acceleration.