The effect of varying strain-ratio on the hydraulic bulging behaviour of aluminium sheet

Annealed commercial-purity aluminium sheet was cold-rolled up to 32%, and the effect of this treatment on the strain-ratio (r) in various directions in the sheet plane was evaluated. Up to approximately 16% cold reduction ro, r454 and r901 remained approximately constant, while the average strain-ratio, r, showed no change. At cold reductions in excess of 16% ro, and rgo, fall steadily, while the fall in r4s, is less pronounced. Specimens were then 'electromarked' with an array of 0.1 in. dia. circles and bulged, using a pvc 'punch' technique. Plots of natural thickness strain (e) vs. bulge height (h max) show that, for a given height, the strain distribution is more even for an annealed material than for a cold-worked one, due to the effect of work-hardening. The relationship between polar thickness strain and uniaxial uniform elongation (eu) shows a discontinuity at about 10% eu and a further plot of h and r against eu suggests that this is associated with the change in strain-ratio. Thus, bulge height increases linearly with increasing uniform elongation at a constant strain-ratio, but in a more complex fashion with varying strain-ratio. Increased r gives decreased € at the pole, producing a more even strain distribution over the bulge.