Characterization of the asymmetric evolving yield and flow of 6016-T4 aluminum alloy and DP490 steel

Abstract

6016-T4 aluminum alloy and DP490 steel were systematically tested under 24 proportional loading paths, including uniaxial tensile tests with a 15° increment, uniaxial compressive and simple shear tests with a 45° increment, and biaxial tensile tests using cruciform specimens. Cruciform specimens in the rolling/transverse and 45°/135° sampling directions were tested with seven and four different stress ratios, respectively. The normal and diagonal planes plastic work contours and the yield stresses under uniaxial tension and compression were measured to investigate the anisotropic yield. Meanwhile, the normal and diagonal planes directions of plastic strain rate and the rα-values under uniaxial tension and compression were characterized to confirm the plastic flow. Several existing asymmetric yield criteria under the associated and non-associated flow rules were comprehensively evaluated to describe the asymmetric plastic anisotropy of 6016-T4 aluminum alloy and DP490 steel. The results suggest that in the investigated yield criteria, the non-associated models can predict the tension and compression asymmetry of materials more accurately than the associated models, and the function of stress triaxiality can more effectively describe the asymmetric yield behavior than the first stress invariant. In addition, the pure shear stress states are helpful in assessing the validity and applicability of advanced asymmetric yield stress functions, and the inspection of diagonal plane plastic work contours containing more pure shear stress states should prioritized over that of normal plane plastic work contours. The evaluation of plastic potential functions should not only consider the prediction accuracy of the normal plane directions of plastic strain rate, but also further check the diagonal plane directions of plastic strain rate. Expressing mechanical properties as a function of equivalent plastic strain to calibrate parameters of the yield criterion allows the continuous capture of anisotropic evolution of the asymmetric yield surface and the changes in the asymmetric plastic potential surface.