New experiments for determining yield loci of sheet metal

Abstract

Two new methods for determining yield loci of sheet metal are described. In the cross tensile test, by varying longitudinal and transverse stress, the yield locus can be determined in the range of biaxial tensile stress. Starting from a specimen described by Kreiβig the test piece geometry was varied by finite element calculations and optimized for obtaining a large zone of homogeneous deformation and high strain before instability occurs. The optimized geometry was verified through photoelastic tests. In the inclined tensile test with suppressed lateral contraction, by varying the angle between the clamps and the direction of movement, states of stress ranging from pure shear to biaxial tensile stress can be obtained. Cross tensile tests were carried out in which the temperature was measured as a function of strain. After a small decrease due to thermoelastic cooling, the beginning of plastic deformation is indicated by the dissipation of deformation heat. By applying this principle, yield loci were determined for various metals. The results were compared with those obtained from uniaxial tensile tests assuming yield criteria by Tresca, v. Mises, Hosford-Backofen and Hill.