Multiple necks around biaxially loaded holes in sheets

Experiments are described in which holes in sheets of 3 and 4 mm thick half-hard commercially pure aluminium are equibiaxially expanded by hydraulic bulging using an undersheet of 1 mm thick mild steel. Discrete radial necks are formed within the annulus (not at the bore), the number of which depends on the initial hole diameter/sheet thickness ratio. Prediction of the amount of deformation at the onset of neck formation requires the deformation history of elements to be followed. The number of necks depends approximately linearly on the ratio of the initial diameter of the hole to the sheet thickness. Further loading produces radial cracks within some (but not all) of the thinning necks.