Yield Loci of Anisotropic Sheet Metals.

Abstract

Stress-strain curves in uniaxial tension, uniaxial through-thickness compression, biaxial tension and plane-strain compression were obtained for four sheet metals: aluminum-killed steel, aluminum 3003-0, copper-110 and brass-260. Bulge testing was used to produce biaxial tension; the radii of curvature at the top of the dome were measured with a projected fringe technique using a holographic grating; and also with the spherometer designed by Duncan and Johnson. The radii measured with the first technique were in most cases lower than those measured with the second technique. The strains were measured from photographs of a printed grid. The uniaxial compression tests, which were made on stacks of discs cut from the sheet metals and lubricated with Teflon tape, gave in most cases curves very close to, but slightly lower than the bulge tests. For copper-110 the uniaxial compression curve was slightly higher than the curve from bulge test at low strains. The plane-strain compression curves were obtained by indenting a single sheet thickness, lubricated with either Teflon tape or a preparation of molybdenum disulfide and vaseline. The yield stresses obtained from the stress-strain curves, were compared to the yield stresses predicted by three proposed yield criteria. Hill's "old" yield criterion described well the yield locus of aluminum-killed steel, but it was inadequate for aluminum 3003-0, copper-110 and brass-260. Hosford's modification of Hill's "old" criterion, using exponents 6 (BCC metals) or 8 (FCC metals), definitely improved the predictions of the yield loci of aluminum 3003-0 and copper-110 with respect to Hill's "old" criterion. Hill's "new" yield criterion can be used with materials, like brass-260, that have biaxial-to-uniaxial stress-ratios greater than unity together with strain-ratios less than unity. It was the only one that could predict the yield locus of brass-260. Both Hill's "new" and Hosford's yield criteria gave similar results for copper-110.