Factors controlling crack velocity in 7000 series aluminium alloys during fatigue in an aggressive environment

Abstract

Results obtained for the corrosion fatigue of the aluminium-zinc-magnesium alloy 7017-T651 in seawater at frequencies from 0.1 to 70 Hz are examined on the basis of existing models for predicting crack growth rates. The enhanced growth rates in seawater, compared with dry air, are not compatible with the simple superposition model that may be applied to 7079-T651, unless some secondary process produces several orders of magnitude increase over the stress corrosion rates observed under static loading. The changes in fracture mode observed at certain critical crack velocities, and their dependence upon the square root of the reciprocal of the frequency of loading, are shown to be consistent with an environment-enhanced crack growth rate involving diffusion of hydrogen ahead of the crack tip during each fatigue cycle. Examination of the reported frequency dependence of crack growth rates during corrosion fatigue in steels and other aluminium alloys indicates a need for further work to identify the rate-determining steps for crack propagation during corrosion fatigue. Theories proposed to date are found to be inadequate.