Proportional combined loading tests using axial load and torque were conducted on thin-walled tubular specimens of annealed isotropic aluminum alloy 5056. The experimental results showed that the third invariant of stress deviator affects considerably the plastic deformation of the material; that is, an equivalent stress-equivalent strain curve for simple tension or compression appears higher than that for pure torsion and the plastic strain vector is generally noncoaxial with the deviatoric stress vector. These results were formulated in the form of a nonlinear tensor relation between stress deviator and plastic strain on the basis of the general relation between coaxial tensors. The tensor equation derived was found to represent well the experimental results.

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