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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. |
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