Mixed mode fatigue crack growth in the commercial AlLi alloy, 8090

Abstract

Crystallographic fatigue crack propagation along planar slip bands has been recognized to occur over a particularly wide range of conditions in Al-Li alloys due to the presence of coherent, shearable [delta][prime] (Al[sub 3]Li) precipitates, in conjunction with the strong underlying crystallographic texture often developed in these materials during secondary processing. Such Stage 1 propagation typically produces a highly tortuous crack path during conventional mode 1 fatigue testing. While the net crack growth direction remains parallel to the nominal mode 1 direction, crack tortuosity has been generally recognized as beneficial to fatigue resistance, as both local mixed mode crack-tip loading conditions and roughness induced closure may significantly reduce the driving force for crack extension (i.e. crack shielding'' occurs). Recent investigations of commercial Al-Li plate in the damage tolerant condition have however shown that co-planar Stage 1 crack propagation along preferred [111]-plane orientations may give rise to sustained macroscopic crack deviation from the nominal mode 1 growth direction. Such crack growth raises a number of important questions with regard to conventional, nominally mode 1 based structure lifting and fatigue testing methods when applied to these materials. Work has therefore been carried out to characterized fatigue crack growth during sustained co-planar Stage 1more » propagation in a commercial Al-Li alloy, with particular reference to mixed mode loading conditions.« less