  Starting from known kinematic picture for plasticity, we derive a set of
dynamical equations describing plastic flow in a Lagrangian formulation. Our
derivation is a natural and a straightforward extension of simple fluids,
elastic and viscous solids theories. These equations contain the Maxwell model
as a special limit. We discuss some results of plasticity which can be
described by this set of equations. We exploit the model equations for the
simple examples: straining of a slab and a rod. We find that necking manifests
always itself (not as a result of instability), except if the very special
constant-velocity stretching process is imposed.
