Fatigue performance of 6261-T6 aluminium alloy – constant and variable amplitude loading of parent plate and welded specimens

Abstract

The work reported in this paper is a summary of the results obtained from several projects run over a five year period. A number of thrusts were encompassed in the project scope, which was to examine the fatigue strength of extruded 6261-T6 I-beams with centrally located, welded cover plates under constant amplitude (CA) and variable (two-level repeated block) amplitude (VA) loading. The CA part of the work examined the effect of cover plate geometry, specimen size, thermal and vibratory stress relief, and differences in fabrication quality on fatigue performance. A number of cases were identified which gave rise to cross-overs in performance in moving from shorter to longer fatigue lives. The aim of the VA part of the work was to identify situations which might give rise to fatigue lives shorter than predicted by linear damage summation models, such as Miner's rule. Two such cases were identified: specimens which had been subjected to vibratory stress relief; and specimens in which weld `quality' (as assessed by profile, heat affected zone hardness, extent and microstructure) varied. Explanations for these observations are proposed and supported by interpretation of a two-level block of small fatigue crack growth.