The effects of the thickness, lubricant, and temper of metals on pure stretchability of aluminum sheets have been studied. The pure stretchability of the sheet metals was markedly deteriorated by decreasing both the thickness in every lubricant and the temper of metal used in this experiment. The optimum frictional coefficient, which gives a maximum of the critical forming depth, was found in this experiment, and it was confirmed to change with the thickness and temper of metal. The thickness dependence of the critical forming depth was analytically calculated by means of Hill’s diffuse neck criterion, Yamaguchi’s method based on M-K theory and the criterion proposed by Gotoh. The result calculated by Yamaguchi’s method was comparatively in good agreement with the experiment in soft aluminum sheet and the result by Gotoh’s method did so in the half-hard one.

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