Rate-controlling Step in Stress Corrosion Cracking

Abstract

IT is usually accepted that crack initiation determines the time necessary for most stress corrosion failures. For example, Haynie and Boyd¹ have said that in aluminium alloys “The initiation stage may take months or years, while the propagation stage occurs in a matter of minutes or hours”. Furthermore, Brown² has stated that in some cases “most of the time required for a stress corrosion test is wasted on an irrelevant process” [crack initiation] and that “any attempt to measure kinetics of the stress corrosion cracking process by measuring the total time-to-rupture is thwarted by the overwhelming bias in terms of pit growth”. We would like to report conclusive experimental evidence that this is not the case for inter-granular stress corrosion failure of the high strength aluminium alloy 7075–T651. So far as we know, this is the first experimental observation that crack propagation, rather than crack initiation, may be the rate limiting step in stress corrosion failure. Hunter³ at about the same time as this work and Eckel⁴ earlier were able to show by optical microscopy that crack initiation occurs at least as early as 8 to 10 min. Using scanning electron microscopy, however, we were able to show that this initiation occurs even earlier, virtually instantaneously.