Аннотация:Processes of severe plastic deformation significantly change material properties. Therefore, it is reasonable to expect that conventional constitutive equations of plasticity theory are not applicable for severe plastic deformation processes. The main assumption made in the present paper is that the relative spin (the difference between the velocity spin and the spin of the principal axes of stress) should be included in constitutive equations to adequately describe severe plastic deformation processes. An experimental procedure to verify this assumption is proposed and partly carried out.