Finite element analysis of forming limit of high-strength steel sheets using ductile fracture criterion

In this study, first, the limit strains of a few types of high-strength steel sheets under various strain paths from balanced biaxial stretching to uniaxial tension are examined by the Marciniak-type in-plane biaxial stretching test. They are compared with those derived from some criteria for ductile fracture. It is found that the fracture strains derived from the criterion proposed by Cockcroft and Latham give the best fit to the experimental results. Next, as fundamental 3-dimensional press forming processes, the square cup deep drawing and the T-shape forming of the high-strength steel sheet are analyzed by the finite element method combined with the ductile fracture criterion. The dynamic explicit finite element program LS-DYNA ver. 970 with membrane shell is used. The comparison with the experimental results demonstrates that the fracture initiation sites and the critical punch strokes are successfully predicted by the present approach.