Fatigue-crack propagation in a high-strength aluminum alloy

Abstract

Fatigue-crack propagation experiments were carried out within the framework of linear-elastic fracture mechanics to examine the effect of water and its constituents on the rate of crack growth in a high-strength aluminum alloy over a range of test temperatures from 295 to 380° K. Dehumidified high-purity (99.9995 percent purity) argon was used as an inert reference environment. The results showed that water accelerated the rate of fatigue-crack propagation by about a factor of 10 in this temperature range, whereas dry oxygen and dry hydrogen had a negligible effect. They confirm the findings of Hartman, and Bradshaw and Wheeler that the cause for the large increase in the rate of crack growth is the formation of hydrogen gas at high pressure in the region ahead of the crack tip, driven in by the reaction of water with the-freshly created aluminum crack surfaces, as suggested by Broom and Nicholson. The results showed further that fatigue-crack propagation in water, as well as in the dry environments, is controlled by thermally activated processes, with apparent activation energies that depend strongly on the crack-tip stress-intensity parameter, ΔK. The rate controlling process appears to be that associated with the creation of new crack surfaces in the range of crack growth rates 10−6 to 10−5 inch per cycle. The strong dependence of the apparent activation energy on ΔK suggests that a careful study of the kinetics of fatigue-crack growth and of the crack growth laws is in order. Such a study should incorporate both the mechanical and chemical variables involved.