In this paper, a reliable a posteriori error estimator for a model problem of elastoplasticity with linearly kinematic hardening is derived, which satisfies some (local) efficiency estimates. It is applicable to any discretization that is conforming with respect to the displacement field and the plastic strain. Furthermore, the paper presents hp-finite element discretizations relying on a variational inequality as well as on a mixed variational formulation and discusses their equivalence by using biorthogonal basis functions. Numerical experiments demonstrate the applicability of the theoretical findings and underline the potential of h- and hp-adaptive finite element discretizations for problems of elastoplasticity.
Bammer et al. (2025) studied this question.