Effect of plastic strain and ductile damage on elastic modulus of multiphase steel and its impact on springback prediction

Abstract

The springback behavior of cold formable steel of grade DP 1000 is assessed experimentally and numerically. Bending tests according to the VDA test specification are interrupted at three characteristic roller displacements, and the unloading characteristics are investigated. The tests are simulated with four different material models: i.) elastic plastic simulation with constant elastic modulus, ii.) elastic plastic simulation with plastic strain-dependent elastic modulus, iii.) damage mechanics simulation with constant elastic modulus, iv.) damage mechanics simulation with plastic strain-dependent elastic modulus. To provide the required input data for these simulations, the effect of plastic strain on the elastic modulus is studied based on uniaxial tensile tests, whereas the possible effect of ductile damage evolution on springback properties is numerically captured by the modified Bai-Wierzbicki model. The studies reveal that consideration of plastic strain effects on the elastic modulus adds a significant amount of accuracy to the numerical simulations, whereas the consideration of ductile damage does not improve the simulation results as much. This observation has to be related to the fact that steel DP 1000 is characterized by late damage initiation with rapid subsequent damage accumulation.