Magnesium has reasonable intrinsic cost and exceptional specific strength and stiffness, making it attractive for vehicle structural application. Its adoption in wrought form is limited by room-temperature formability issues related to complex plastic behavior caused by its HCP crystal structure and the presence of twinning. In order to potentially exploit these characteristics in designing a novel forming operation, an efficient material model suitable for finite element implementation would be helpful.
Such a model has been constructed based on three phenomenological deformation modes: S (slip), T (twinning) and U (untwinning), corresponding respectively to in-plane tension, initial in-plane compression and initial tension following compression. A von Mises yield surface with initial non-zero back stress was employed to account for plastic yielding asymmetry. The model was formulated with combined isotropic and nonlinear kinematic hardening. Texture was quantified using a weighted discrete probability density function of c-axis orientations. Starting from an assumed perfectly basal-textured sheet, in-plane compression causes reorientation of c-axes toward the compression direction by twinning, and subsequent tension returns them to a sheet normal direction by untwinning. The orientation of c-axes evolves with twinning/untwinning deformation using explicit rules incorporated in the model. The proposed model was implemented in ABAQUS/Standard through UMAT. Constitutive parameters were calibrated from in-plane tension/compression and reversal tests. Simple shear tests were simulated and compared with experiments with good agreement obtained. Tests under a complex loading path, such as orthogonal compressions, tension following orthogonal compression and tension following biaxial compression, were also simulated.