The Subsequent Yield Surfaces after Preloading under Combined Axial Load and Torsion

The subsequent yield surfaces after preloading along various strain paths of straight lines with or without a corner are obtained for an aluminium alloy whose stress-strain relation is not determined uniquely by the effective stress-strain value of Mises-type. Tests are made by subjecting thin-walled tubular specimens to combined axial load and torsion. The shapes of the yield surfaces are discussed in the deviatoric stress plane. Yield criterion is derived experimentally as a function of the length and the direction of the prestrain path. The criterion can express a rounded corner in the preloading direction, the flattening at the opposite side of the preloading, and the distortion caused by the difference of flow stress values between axial load and torsion, appearing in the yield surfaces obtained by experiment.