On the constitutive modelling of twip steels and its application to sheet metal forming simulations

Over the past years, high manganese austenitic TWIP (twinning induced plasticity) steels have received much attention due to their extraordinary ductility at high tensile strength. Due to the twinning induced plasticity effect, the forming behavior is different compared to conventional sheet metals. For this reason, the development of a constitutive framework which can be applied to large plastic deformations is still an ongoing process. In this work, a constitutive framework for the accurate description of TWIP steels under large plastic deformations is proposed. The well-known physically-based Bouaziz-Allain approach has been considered as a base model for the current work. This one-dimensional model computes monotonic uniaxial tensile stress-strain curves on the basis of the evolution o f the dislocation density and the twin volume fraction. For practical applications like the simulation of sheet metal forming processes the model was substantially enhanced in the current work: The originally one-dimensional formulation was extended to a three dimensional and anisotropic formulation. Further an approach to take stress dependent hardening behavior into account was included. Alternative formulations for the description of kinematic hardening have been realized. The proposed model has been implemented into commercial finite element software LS-DYNA and PAM-STAMP. The calibrated model was validated using experimental data obtained from typical sheet metal forming experiments like bulge test and cup-drawing tests. These results were compared with both the experimental data and numerical simulations performed using a standard material model.