Abstract: Transformation between limit strain and limit stress is evolves on the basis of plastic relationship between strain and stress. Theoretic FLSDs (Forming limit stress diagrams) are calculated under linear strain path, curvilinear strain path and compounded strain path according to diffuse instability theory, groove instability theory and plane strain floating instability theory. The result shows that FLSD is independent of strain path, and that the FLSD under different strain paths are almost the same curve according to the same instability theory. So it’s more convenient and practical to use FLSD as the criterion of forming limit.

Key words: FLD(Forming limit diagram), FLSD (Forming limit stress diagram), Sheet metal forming, Strain paths