Determination of forming limit stress diagram for formability prediction of SPCE 270 steel sheet

Abstract

The aim of this research is to apply numerical Finite Element Method (FEM) for determining Forming Limit Stress Diagram (FLSD) of sheet steel of grade SPCE 270. As a failure criterion for formability prediction in sheet metal forming process, the conventional Forming Limit Diagram (FLD) is often used. The FLD is a strain based criterion, by which the principal strains at failure are evaluated. Many investigations showed that the FLD is dependent on forming history and strain path. However, the stress based criterion does not confirm this dependency. This criterion is more robust against any changes in the strain path occurring in a forming process. To determine the stress based criterion or the Forming Limit Stress Curve (FLSC), the FLD was initially determined by the Limiting Dome Height (LDH) test. Afterwards, the LDH test was simulated by FEM using the FLD data as a failure criterion. Calculated major and minor stresses were used to construct the FLSC. Finally, an industrial automotive part was taken in order to validate the applicability of the FLD and FLSD criterion. The investigations confirmed that the FLD is insufficient for evaluating parts being manufactured in complex forming process with strain path changes. Nevertheless, the results exhibited that the FLSD is a more precise tool for characterizing formability of steel sheet.