Yield condition and propagation of Lüders' lines in tension–torsion experiments on poly(vinyl chloride)

Abstract We have derived from the Eyring theory of non‐Newtonian flow a yield condition which is valid for an arbitrary state of stress. Experimental data obtained in simple axial compression tests show the influence of the hydrostatic stress on the yielding of poly(vinyl chloride). This fact confirms the proposed condition and disproves the von Mises criterion. Tension–torsion tests performed on thin tubes lead to results which fit our condition fairly well. The pattern of Lüders' lines appearing on the surface of thin tubes subjected to simple tension, simple shear, and tension–torsion are parallel to the direction where the value of the normal stress is equal to the hydrostatic stress.