Transformation relation between limit strain and limit stress in sheet metal forming

Strain-based FLD is strongly affected by the strain path and has brought much inconvenience to the study of forming limit under complex strain paths. While FLSD, which is stress-based and independent of strain path, has become the hotspot in this field. Transformation formulas are given on the base of analysis of limit strain and limit stress. Transformation between FLD and FLSD under different strain paths were obtained according to diffuse instability theory, groove instability theory and plane strain floating instability theory. The result shows that the FLSD under different strain paths are almost the same curve according to the same instability theory and its more convenient to use FLSD as the criterion of forming limit.