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.