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.

A bounding surface model for anisotropically overconsolidated clay incorporating thermodynamics admissible rotational hardening rule

“…The experimental evidence of the critical state yield surface is scarce, although Coombs argues that the level of the anisotropy should vanish and thus non‐oblique yield surface is resulted in at the critical state. Recently, Dafalias and Taiebat dissected four rotational hardening rules that are often employed in modeling anisotropic hardening of clay and identified two‐key parameters that describe the rotational hardening: one is the equilibrium value of critical rotational angle that relates to the stress ratio and the other is the parameter that controls the pace of rotation. However, in a later paper, they proposed two versions of rotational hardening rules, ie, with and without inclination at the critical state, ie, the former denotes an oblique yield surface at the critical state, whereas the latter designates a yield surface with zero inclination at the critical state.…”

“…The simulated effective stress paths and stress‐strain responses agree well with the experimental counterparts. Some existing models are able to predict the correct peak strength for normally and lightly consolidated soils, but may overestimate the strength on the “dry” side with a greater OCR . Remarkably, similar to the performance of the isotropic model presented in Chen and Yang, adoption of the modified yield surface can overcome this drawback and yield promising results.…”

Improved rotational hardening rule for cohesive soils and definition of inherent anisotropy

“…Following some earlier formulations,() the improved RH for the PPS rule evokes dependency on both the volumetric ( ) and deviatoric ( ) plastic strain increments. This is by no means a universally accepted proposition, as several RH rules have been proposed() that involve only . However, since anisotropy evolution has been linked to the evolution of not only the volumetric strain but also to the deviatoric stress, the present RH rule is a rational proposition.…”

“…Since these pioneering undertakings, several RH rules, possessing varying degrees of sophistication, have been proposed for cohesive soils. ()…”

“…Motivated by some detailed studies of RH rules for cohesive soils,() a thorough investigation of such rules recently identified areas for improvement to existing formulations. Consequently, an improved RH rule was developed.…”