The yield locus of a prestrained material can be based on different systems of combined stress, such as biaxial tension and combined tension and shear. No matter what the combined stresses may be, however, the deviatoric yield stress is a function of only two variables, the characteristic index, which represents the stress ratio, and the stress direction, or the inclination of the principal stress axes. It is shown that, when tubular specimens are tested under combined torsion and axial tension, the results show the mixed effects of the characteristic index and the stress direction on the yield stress. In such tests, the two effects can, however, be separated if the strain vectors as well as the yield stresses are known. The theory is applied to several groups of experimental results on the yield locus of prestrained materials, including Taylor and Quinney’s results.

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