LIMIT STRAIN PREDICTIONS FOR OUT-OF-PLANE STRETCHING OF SHEET METALS

An investigation is performed to predict the limits to sheet metal str etchability in different out-of-plane experiments. Based on the time-i ndependent flow theory, the analysis includes the effects of strain ha rdening and normal anisotropy and utilizes different yield functions w ith the assumption of isotropic hardening. The limit strains, determin ed either from the condition of bifurcation at maximum pressure for hy drostatic bulging, or from the analysis of flow localization in hemisp herical punch stretching, depend on the type of the test. For a given test, however, the plot of the limit strains in the usual epsilon1 - e psilon2 diagram does not exhibit a marked sensitivity to the yield fun ction utilized. This behaviour contrasts with the predictions of insta bility models assuming in-plane stretching.