Abstract
High cycle bending fatigue tests were performed on sheet specimens of an Al, 5.0 wt.% Zn, 2.5 wt.% Mg alloy in laboratory air. Electron microscopic observations of the free surfaces of the alloy sheets were performed on specimens cycled to 0.1, 1, 67 and 100% of a predetermined number of cycles to crack initiation. These observations showed that low strain fatigue results in the generation of screw dislocations in the vicinity of the alloy free surface. As cycling continues, these screw dislocations form jogs which are subsequently pinched off to form sessile edge dislocation loops. It is suggested that crack initiation occurs at a critical dislocation loop density in very narrow slip bands, and that the initiation step is affected by a normal component of stress across the slip plane as a result of dislocation dipole interaction. This normal stress results in a localized cleavage type of failure associated with Stage I cracking in high strength precipitation hardened alloys.
Published Version
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