Subdifferential-based implicit return-mapping operators in computational plasticity

In this paper we explore a numerical solution to elastoplastic constitutive initial value problems. An improved form of the implicit return-mapping scheme for nonsmooth yield surfaces is proposed that systematically builds upon a subdifferential formulation of the flow rule. The main advantage of this approach is that the treatment of singular points - apices or edges at which the flow direction is multivalued - only involves a uniquely defined set of non-linear equations, similarly to smooth yield surfaces. This paper focuses on isotropic models containing: a) yield surfaces with one or two apices (singular points) on the hydrostatic axis, b) plastic pseudo-potentials that are independent of the Lode angle, and c) possibly nonlinear isotropic hardening. We show that for some models the improved integration scheme also enables us to a priori decide about a type of the return and to investigate the existence, uniqueness, and semismoothness of discretized constitutive operators. The semismooth Newton method is also introduced for solving the incremental boundary-value problems. The paper contains numerical examples related to slope stability with publicly available Matlab implementations. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim