Anatomy of rotational hardening in clay plasticity

Abstract

Rotational hardening is a constitutive feature of anisotropic clay plasticity models that allows rotation of the yield and plastic potential surfaces in stress space in order to simulate, more realistically than isotropic models, the material response under various loading conditions. The anatomy of some well-known rotational hardening rules, as well as a novel proposition with certain advantages, is performed in relation to the following characteristics: (a) the conditions under which rotational hardening yields a unique critical state line in e–p space; (b) the restrictions on model parameters necessary to prevent excessive and unrealistic rotation of the yield and plastic potential surfaces; and (c) the analytical description of the response of rotational hardening to loading at constant stress and strain-rate ratios, including K0 consolidation, and to undrained shearing. The anatomy will reveal strong and weak features, and corrections for the latter are proposed. Special attention is given to calibration of the rotational hardening rules parameters based on easily available experimental data, such as the K0 value. Simulations of experimental data under drained and undrained loading after isotropic and K0 consolidation at various OCRs for associative and non-associative flow rules illustrate the simulative capabilities of the rotational hardening rules considered.