Comparison of two CDM-based models for necking and fracture limits of metal sheets under hot stamping conditions

A unified viscoplastic constitutive model based on continuum damage mechanics (CDM) has been recently developed for the prediction of forming limit curves (FLCs) and fracture forming limit curves (FFLCs). In this CDM model, two damage variables which are strain path dependent (so called strain-based model) have been introduced to model the necking and fracture limits in sheet metal forming. In this study, the two damage variables have been modified by replacing the strain components with stress components to model stress path dependence (so called stress-based model). Then the two sets of CDM models have been analysed and compared in respect to their computational accuracy and efficiency. For this purpose, all the material constants in the models have been calibrated using the recently published data of 22MnB5 boron steel sheet under hot stamping conditions. Subsequently, the two models together with the calibrated material constants have been implemented into the commercial software ABAQUS using a user subroutine VUMAT, and applied to biaxial tests for the computation of both the necking and the fracture limit strains at hot stamping temperatures. Computational results show that there is little difference between the two CDM models with respect to the computed limit strain values, but the strain-based CDM model is more time-efficient.