Predicting of plastic instability and forming limit diagrams

An analytical method is presented for predicting forming limit diagrams achieved during sheet metal forming operations for sheets having planar isotropy. This method is based on the three-phase deformation idealization developed by Johns and Gillis. The Johns–Gillis and Pishbin–Gillis model were restricted to the Hill yield criterion which is not suitable for aluminum alloys (R<1). The present work used the Hosford criterion that is widely used for materials with R<1, in conjunction with the power-law, the Tian–Zhang and the Vocé hardening equations. Results from this analysis are compared with the experimental data for AA3105 and AA8011 aluminum alloys. The results indicate good prediction of limit strains for the two alloys when the Vocé and the Tian–Zhang equations are applied.