plastic instability in equibiaxial deformation of sheet metals

plastic instability which results in the localization and failure of sheet metals has been studied in equibiaxial stretching of commercial purity aluminium sheet. attention was focussed on two main industrial conditions of the material. these were representative of the materials that could be processed by the commercial batchannealing process (slow heating, long time) or by a continuous annealing process (fast heating, short time). the instability strains were experimentally measured and compared with those predicted by different instability criteria. the results have been analyzed on the basis of the mechanical characteristics of the material under different conditions. it is concluded that pressure instability can not be used as the extreme limit of forming and thus there is a considerable amount of useful deformation after instability strain.