Abstract: Yield functions based on different hardening models are introduced according to Hill48 yield criterion. A function, which can be used to identify the parameters of kinematic hardening model and mixed hardening model, is derived. The values of parameters are obtained by fitting the function with data of uniaxial tension curve and uniaxial tension curves based on three hardening models are obtained with these parameters. The results show that the above approach can accurately deter-mine the diameters of hardening models. The approach to cal-culate the forming limit based on kinematic hardening model and mixed hardening model is advanced. Forming limit dia-grams and forming limit stress diagrams are obtained under simple loading path based on three hardening models according to diffuse instability theory, Hill localized instability theory, groove instability and plane strain floating instability theory. From the diagrams, it can be found that hardening model can influence forming limit diagrams and forming limit stress dia-grams obviously. Therefore, hardening rule in sheet metal forming process should be identified in order to select proper hardening model to predict the forming limit.

Key words: Forming limit diagram (FLD), Forming limit stress diagram (FLSD), Hardening model, Sheet metal forming