Traditionally, the validation of three dimensional constitutive formulations (i.e. theories of plasticity) has been carried out using biaxial testing. The most widely used method for biaxial testing is the combined tension-torsion loading of thin-walled cylindrical specimens. Unfortunately, the results obtained in the past, using the incremental theory to model tension-torsion experiments involving large strains and non-proportional loading paths, are not always in agreement with observations. Two possible sources of error lie in: (i) the particular objective rate chosen for the constitutive equation, and (ii) the kinematic hardening model used to account for material anisotropy. In this study, it is demonstrated that an appropriate choice of objective stress rate can lead to improved correlation between analytical and experimental results even with the use of a simple kinematic hardening law. The evaluation is carried out using non-proportional tension-torsion loading of a thin tube. The purpose of this paper is to review the objective E-rate formulation against alternative rate formulations and demonstrate its advantage in problems involving elastic-plastic and non-proportional loading, through the finite deformation solution of tension followed by torsional loading of a thin tube. Details of the analytical thin tube solution of non-proportional tension torsion loading generalized to finite deformation plasticity is presented along with comparison of results to experiments.

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