Dependence of prediction model of forming limit strains on forming method and mechanical properties of sheet metals

Taking into account the deformation characteristics of sheet metal forming under punch or fluid pressure by means of the states of tension-compression strains or biaxial tension strains, the possibility of development of the tensile instable deformation to localized necking deformation was analysed using the principle of minimum strain energy. It was found that this is impossible when the sheet metals are deformed by means of fluid pressure in the biaxial tension mode, and possible in other deformation conditions, but the possibility of breaking before the localized necking occurs due to the insufficient deformation ability of materials should be noted. Therefore, the choice of either localized necking-type model or breaking-type model for predicting the forming limit strains must be determined in the light of actual conditions.