AbstractThe environment sensitive fracture under cyclic loading of the white-zone of commercial 7017-T651 alloy weldments has been investigated. The Klc value for the white-zone was determined by using small cylindrical circumferentially grooved specimens as proposed by Stark & Ibrahim and a value of 22MNm-3/2 was established Fatigue crack growth rates for the white-zone were obtained by utilising small arc-type specimens, similar to the one described in ASTM E399, over a wide range of frequencies from 0.01Hz to 10Hz. The enhanced growth rates in acidified (pH3) salt-chromate environment compared with dry air are not compatible with a simple superposition model as applied to 7079-T651 alloy. An enhancement of crack growth rates observed near the 1Hz 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. Fracture mode changes have been 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 diffusion of hydrogen ahead of the crack tip during each cycle. Striations observed on the fatigue fracture surface indicate a discontinuous crack propagation mechanism with the ACPD technique providing evidence of cracking initiating ahead of the crack tip, remote from the environment , with subsequent growth there and link up with the original crack. Such a model is consistent with indirect evidence that hydrogen accumulates at the region of maximum stress ahead of the crack tip facilitating cracking to initiate there. Acoustic emission results also point to such a mechanism. Crack closure effects have also been demonstrated to take place at the high frequency and low stress intensity ranges.
An interesting further observation was that for test frequencies above 10Hz fatigue cracks grew in a direction almost parallel to the loading axis invariably into the heat affected zone as soon as the environment was introduced, with the presence also of severe crack branching effects.
|Date of Award||Mar 1992|