Superplastic behaviour of sheet materials under biaxial stresses

Abstract

The deformation of a superplastic zinc-aluminium eutectoid alloy is examined by subjecting thin circular diaphragms to one-sided hydrostatic pressure at elevated temperature. The geometry, the stress, the strain, and the strain-rate are determined at various points covering the whole specimen and at various stages of the forming process. One distinctive feature of this study is the use of the curvatures of the thin shell for the stress analysis. It is shown that the ratio of the two principal stresses at a point in the formed shell is related in a simple way to the ratio between the principal curvatures at that point. In fact the ratio between the principal curvatures is also a measure of the deviation of the local geometry from the spherical shape. The rheological properties of the material under this biaxial stress system are presented in triangular coordinates. The nature of the stresses, strains and strain-rates at the dome and the rim of the bulge are discussed in detail. It is shown that the strain rates vary widely in the material at different regions. The biaxial stress is analysed into as train proportional and a strain-rate proportional component which represent respectively, the quasi-solid and the quasi-liquid behaviour of the superplastic material.