Remarks on Coaxiality in Fully Developed Gravity Flows of Dry Granular Materials

Abstract

The Coulomb-Mohr condition is quite widely accepted as a basis for determining the stress at flow or failure of a granular material, but the question of how correctly to express the equations which govern the flow behaviour still has no generally agreed answer. A brief review was given in Spencer (1982). In particular there are differing views as to whether the principal axes of the stress and the rate-of-deformation are or are not coincident. The requirement that these axes should coincide (the coaxiality assumption) is often stated as a condition for material isotropy, but this is true only under restricted constitutive assumptions. Coaxiality is a consequence of assuming that the deformation-rate is derived from a plastic potential which is an isotropic function of the stress. However coaxiality does not require the existence of a plastic potential.