  This paper presents a theory for the behaviour of
isotropic-hardening/softening elastoplastic materials that do not have a
preferred reference configuration. In spite of important differences, many
ingredients of classical plasticity are present. Main features are: the elastic
and plastic responses are given by solutions of hypoelastic differential
equations, no decomposition of the deformation into elastic and plastic parts
is done from the start, the hardening rule is an outcome, and the principle of
material objectivity is obeyed. An important result is the existence of a limit
surface that divides the stress space into regions of hardening and softening
and is composed of equilibrium points of the differential equation of plastic
response.
