Application of the forming limit stress diagram to forming limit prediction for the multi-step forming of auto panels

The strain-based forming limit diagram (FLD) is generally obtained on the linear (proportional) or near linear loading condition. So it often leads to erroneous assessments on the analysis of metal sheet formability under complex strain paths such as multi-step forming by the FLD criterion. In recent years, forming limit stress diagrams (FLSDs) have been intensively studied and have been considered being path-independent. In this study, it has been carried out that application of the FLSD to forming limit prediction for the multi-step forming of auto panels. Firstly, it was been performed that the forming limit tests of sheet metal for the SPECN steel sheet of thickness of 1.6 mm under the loading conditions considered include both proportional and non-proportional loading. Secondary, the forming limit stress diagram (FLSD) of the sheet metal is gained by the transformation relations between the strain state and stress state in deformation of sheet metal. Afterwards, implementation of the forming limit stress diagrams into finite element numerical simulations of rear hanger of auto panels multi-step forming has been established by DANAFORM program code, the FLSD is applied to forming limit prediction in the stamping finite element analysis. The verification experiment of the simulation results were executed in motorcar manufacturing factory, the predicted results agree well with those determined experimentally.