Strength differential in Al---Li alloy 8090

Abstract

Cyclic strength differential (SD) data have been derived from the variation of the tensile and the compressive stress amplitude with elapsed cycles during low cycle fatigue (LCF) for the quaternary Al---Li alloy 8090 in the T8E51 condition. LCF test specimens were machined out of the rolled plate such that the direction of stressing was along the longitudinal (L), L+45° and long transverse (LT) directions. The analysis of cyclic stress amplitude data corresponding to the half-life revealed an SD effect at all strain levels in the L+45° and LT directions. In the case of the L direction, at strain levels below 8.5 × 10⁻³, the SD is more than compensated by the Bauschinger effect resulting from the prestretch. The alloy exhibits a similar SD behaviour under monotonic loading conditions. The magnitude of cyclic SD at intermediate strain amplitudes in all the three test directions decreases upon cycling, which behaviour can be attributed to the relaxation of prestretch-related residual stresses leading to a decrease in σ_T only in the L direction and σ_C only in the L+45° and LT directions. As microstructural features vary in the differently heat-treated conditions, a comparison has been made of the monotonic SD data in the stretched and aged (T8E51) condition with those in the solution-treated, in the peak-aged (T6) and in the overaged (T7) conditions.