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Inspired by some recents developments in the theory of small-strain elastoplasticity, we
both revisit and generalize the formulation of the quasistatic evolutionary problem in
perfect plasticity for heterogeneous materials recently given by Francfort and Giacomini.
We show that their definition of the plastic dissipation measure is equivalent to an
abstract one, where it is defined as the supremum of the dualities between the deviatoric
parts of admissible stress fields and the plastic strains. By means of this abstract
definition, a viscoplastic approximation and variational techniques from the theory of
rate-independent processes give the existence of an evolution statisfying an energy-
dissipation balance and consequently Hill's maximum plastic work principle for an
abstract and very large class of yield conditions.