Predictive accuracy of crystal-plasticity analyses for three-dimensional equal-plastic work contours in a 6000 series aluminum alloy sheet

The accurate representation of yield surfaces is important in sheet metal-forming simulations. For effectively determining the yield-function parameters, numerical simulation of material testing using crystal-plasticity models has received attention for the numerical evaluation of the deformation behaviors of materials. Yield surfaces are typically evaluated under principal-stress states; however, three-dimensional representations have not been studied in detail. In this study, we studied the predictive accuracy of crystal-plasticity finite-element simulations for evaluating the contours of the equal plastic work of a 1.0-mm-thick A6022-T4 sheet in the σRD−σTD−τRDTD stress space, where RD and TD denote the rolling and transverse directions, respectively. Specifically, experiments and simulations of uniaxial tension tests, biaxial tension tests using cruciform specimens with arms aligned to the RD and TD (RD-Bi cruciform test) and at 45° and 135° from the RD (DD-Bi cruciform test), and simple shear tests were conducted to obtain stress plots of equal plastic work. The Barlat's anisotropic Yld2000-2d yield stress function was also used to represent the contours of equal plastic work. The predictive accuracy of the crystal-plasticity simulations for the non-negligible shear-stress conditions was comparable to that of the principal-stress conditions. The maximum error between the simulation and experimental results was 5.0 %, which was comparable to that between the experimental results and Yld2000-2d function. By contrast, the predicted anisotropic stress hardening of the subsequent curves of equal plastic work did not necessarily match the experimental results: the anisotropic hardening was less pronounced in the uniaxial tension and RD-Bi cruciform test but more pronounced in the DD-Bi cruciform test. The results obtained in this study are valuable for the comprehensive evaluation of the contour of equal plastic work for general plane stress condition and in the three-dimensional stress space of an A6022-T4 sheet and the predictive accuracy of numerical simulations of materials testing using crystal-plasticity models.