 This condition represents the so-called 'loss of ellipticity' of the differential equations governing the rate equilibrium.The state-of-the-art of the research on shear bands is that the phenomenon is well understood from the theoretical  and experimental  point of view and available constitutive models give nice qualitative predictions, although quantitative predictions are often poor.Moreover, great progresses have been made on numerical simulations, so that shear band nucleation and propagation in relatively complex situations can be traced numerically with finite element models, although still at the cost of a great computational effort.