Analysis of forming limits using ductile fracture criteria

The application of the ductile fracture criteria for the prediction of the forming limit diagram (FLD) of sheet metal was investigated. The limit strains for the FLD were determined by substituting stress and strain values obtained from the finite element simulation of out-of-plane formability test into the ductile fracture criterion. Fracture predictions for the left side of the FLD were quite successful. However, the predictions were not successful for the right side of the FLD. It is concluded that the ductile fracture criteria could not be used directly to predict the FLD. New or modified form of the ductile fracture criteria should be explored. As an additional test the limit strains for the FLD were determined at 20, 25, 30, 35, 40, and 45% thickness strains from the finite element simulations of out-of-plane formability experiment. The predicted FLD curves were compared against the experimental and analytical curves. The shapes of the curves at left side of the FLD were found to be in accord with the experimental and analytical curves, but they did not agree at the right side of the FLD.