| This thesis focuses on the principles of springback for 6022-T4 aluminum sheets, using a special draw-bend test with a range of sheet tensions and tool radii. To model the anisotropic yielding of 6022-T4 sheet, Barlat '91, '96 and 2000 yield functions were implemented into Abaqus/Standard through user material subroutines. A nonlinear kinematic hardening model with multiple back stresses was constructed to closely reproduce the reversed strain hardening behavior of sheet metals. The new material constitutive models are as accurate as the previous work based on two-surface plasticity, but they have simpler mathematical forms and require fewer model parameters.; The mechanics of the persistent anticlastic curvature were studied by draw-bend experiments, finite element analysis and an elastic plate-bending theory. The transverse cross-section shape was dictated by a dimensionless parameter, beta, which depends on the specimen geometry, tool radius and sheet tension. The rapid decrease of springback angle as the front sheet tension approaches yielding is correlated to a critical values of beta, above which the retained anticlastic curvature is small and hence has little impact on the springback angle. However, the anticlastic curvature built up during the forming step persisted after unloading when beta is less than 10--15, and thus greatly reduced the final springback.; In order to quantify the time-dependent springback phenomenon and infer its physical basis, several aluminum alloys were draw-bend tested under conditions promoting the time-dependent response. The time-dependent springback angles are approximately linear with log(time) for times up to a few months, after which the kinetics become slower and saturation is reached in approximately 15 months. Residual stress-driven creep and anelasticity are discussed as the possible sources of the time-dependent springback. For 6022-T4, qualitative agreement was obtained using a crude finite element model, with creep laws derived from constant load creep tests. For the second possibility, novel anelasticity tests following a reverse loading path were performed for 6022-T4 and DQSK steel. Based on the experiments and simulations, it appears that anelasticity is unlikely to play a large role in long-term time-dependent springback of aluminum alloys. |
