In this paper, the linear perturbation theory is used to study material
instability in a classical elasto-plastic model. In this framework, the well
known Rice criterion of bifurcation into localized modes is found to be a
limiting case corresponding to unbounded growth of perturbation. In the
first part, derived expression of the critical plastic modulus is
numerically plotted versus the spherical coordinates of the potential normal
to the localization band in order to describe the whole space. In the second
part, conditions of occurrence of the other types of instability are
established, namely divergence and flutter types of instability, and
modelling details influencing them checked. These conditions are also
relative to the parameter of growth of perturbations which can be plotted
versus the plastic modulus considered as the loading parameter. When several
modes of instability are possible, an hierarchy is established.