Environment sensitive fracture of white zone of AI-Zn-Mg alloy weldments under cyclic loading conditions

Abstract

The environment sensitive fracture under cyclic loading of the white zone of commercial 7017-T651 aluminium alloy weldments was investigated. Fatigue crack growth ratesfor the white zone were obtained by utilising small arc type specimens over a wide range of frequencies from 0·01 Hz to 10 Hz. Fracture mode changes were observed to take place at certain crack velocities and their dependence upon the square root of the reciprocal of the loading frequency are shown to be consistent with an environment enhanced crack growth rate involving hydrogen diffusion ahead of the crack tip during each cycle. An enhancement of crack growth rates observed near the 1 Hz load cycling frequency has been attributed to crack tip solution modification owing to the motion of electrolyte in and out of the crack enclave during fatigue.