Evolution of yield surface in the 2D and 3D stress spaces

Initial and subsequent yield surfaces for 6061 aluminum, determined by a method of automated yield stress probing, are presented in the 2D (sigma(zz) - sigma(theta z)) and 3D (sigma(theta theta) - sigma(zz) - sigma(theta z)) stress spaces. In the (sigma(zz) - sigma(theta z)) space, yield surfaces at small pre-strains show the noses and unapparent cross effect. At larger pre-strains, they become ellipses with positive cross effect. In the (sigma(theta theta) - sigma(zz) - sigma(theta z)) space, the initial yield surface is not well described by von Mises yield criterion due to material anisotropy. The yield surfaces of various torsional pre-strains show obvious rotation around the sigma(zz) axis but they do not rotate when subjected to axial pre-strains. Therefore, the rotation behavior of yield surface is pre-strain path dependent. The rotation of yield surfaces in the 3D space is the emphasis of the present paper. Coupled axial-torsional behavior subjected to torsion after axial pre-strain are also presented for the same material that is used to determine the yield surfaces. This information is useful for verification of constitutive models. (C) 2010 Elsevier Ltd. All rights reserved.