  We prove the well--posedness of a dynamical perfect plasticity model under
general assumptions on the stress constraint set and on the reference
configuration. The problem is studied by combining both calculus of variations
and hyperbolic methods. The hyperbolic point of view enables one to derive a
class of dissipative boundary conditions, somehow intermediate between
homogeneous Dirichlet and Neumann ones. By using variational methods, we show
the existence and uniqueness of solutions. Then we establish the equivalence
between the original variational solutions and generalized
entropic--dissipative ones, derived from a weak hyperbolic formulation for
initial--boundary value Friedrichs' systems with convex constraints.
