A mixed experimental/numerical approach to sheet metal plastic modeling, with application to the prediction of the forming limit curves

An accurate description of anisotropic plasticity is required for obtaining reliable predictions in numerical simulations of sheet metal forming processes. An approach to material parameters identification is developed in this paper, which combines classical results obtained in uniaxial tension tests performed along different orientations from the rolling direction, and strain fields measurements obtained by an optical method on cruciform specimens tested in biaxial tension. Material behaviour is described using an 8-parameter anisotropic yield function with the assumption of isotropic hardening. Uniaxial tension tests are used to determine strain-hardening parameters and yield function parameters related to strain- and stress-anisotropy. Strain fields measured in the biaxial tests are used as input data in an optimization procedure that consists of fitting simulated fields with experimental ones in order to determine the material parameters describing the shape of the yield surface in the biaxial stretching range. The method is applied to the determination of the yield surfaces of IF steel and HSLA steel sheets. The identified material parameters also are introduced in an elasto-viscoplastic Marciniak-Kuczinski localization analysis for predicting the Forming Limit Curves (FLCs). The predictions are compared with experimental FLCs determined with a Bragard-type method.