  A simple phenomenological approach to metal plasticity, including the
description of the strain-induced plastic anisotropy, is considered. The
advocated approach is exemplified by a two-dimensional rheological analogy.
This analogy provides insight into modelling of nonlinear kinematic hardening
of Armstrong-Frederick type combined with a nonlinear distortional hardening.
In the previous publications of the authors, an interpolation rule between the
undistorted yield surface of a virgin material and the saturated yield surface
of a pre-deformed material was considered. In the current publication, a
somewhat more flexible approach is considered. Given a set of convex symmetric
key surfaces which correspond to different hardening stages, the form of the
yield surface is smoothly interpolated between these key surfaces. Thus, any
experimentally observed sequence of symmetric convex yield surfaces can be
rendered. In particular, an arbitrary sharpening of the yield locus in the
loading direction combined with a flattening on the opposite side can be taken
into account. Moreover, the yield locus evolves smoothly and its convexity is
ensured at each hardening stage.
