A unified approach to classical plasticity, including metal plasticity, geomaterials, and
crystal plasticity, is presented. A distinctive feature of this approach is that the basic
constitutive elements (yield criterion, flow rule, consistency condition, and hardening
rule), instead of being assumed on a phenomenological basis or deduced from ad hoc
principles, are obtained directly from the stationarity of the energy. The plastic
continuum is regarded as a particular micromorphic continuum, and its energy has
the form resulting from a homogenization procedure introduced in the theory of
structured deformations. This form of the energy requires an additive decomposition
of the deformation gradient, in place of the multiplicative decomposition usually
adopted in finite plasticity. It is shown by examples that many of the models
adopted in classical plasticity can be obtained from ad hoc specifications of the
energy.