Effects of a non-coaxial flow rule on shear bands in viscous-plastic materials

A class of non-coaxial plasticity models is revisited in which the plastic deformation is carried by one (single slip) or two (double slip) slip systems that are inclined at ± (π/4+ν/2), 0 ≤ ν ≤ φ to the less compressive principal stress axis; ν is the angle of non-coaxiality and φ is the internal angle of friction of the material. The non-coaxial model is combined with a Newtonian viscous rheology to form a viscous-plastic model. The model is applied to model strain localisation in extension and compression on a time scale typical for deformation processes in the lithosphere. We would like to dedicate this article to the memory of Ioannis Vardoulakis who always took a keen interest in bifurcation theory and shear banding for granular materials.