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This paper presents a fabric tensor-based bounding surface model accounting for anisotropic behaviour (e.g. the dependency of peak strength on loading direction and non-coaxial deformation) of granular materials. This model is developed based on a well-calibrated isotropic bounding surface model. The yield surface is modified by incorporating the back stress which is proportional to a contact normal-based fabric tensor for characterising fabric anisotropy. The evolution law of the fabric tensor, which is dependent on both rates of the stress ratio and the plastic strain, rules that the material fabric tends to align with the loading direction and evolves towards a unique critical state fabric tensor under monotonic shearing. The incorporation of the evolution law leads to a rotational hardening of the yield surface. The anisotropic critical state is assumed to be independent of the initial values of void ratio and fabric tensor. The critical state fabric tensor has the same intermediate stress ratio (i.e. b value) and principal directions as the critical state stress tensor. A non-associated flow rule in the deviatoric plane is adopted, which is able to predict the non-coaxial flow naturally. The stress–strain relation and fabric evolution of model predictions show a satisfactory agreement with DEM simulation results under monotonic shearing with different loading directions. The model is also validated by comparing with laboratory test results of Leighton Buzzard sand and Toyoura sand under various loading paths. The comparison results demonstrate encouraging applicability of the model for predicting the anisotropic behaviour of granular materials. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical study on a hybrid fabric evolution law for modelling anisotropic behaviour of granular media. In the hybrid evolution law, the rate of a contact normal-based fabric tensor is related to the rates of both stress ratio tensor and plastic strain. Assumptions and principles that were adopted for the development of the fabric evolution law are presented and discussed at first. Its accuracy is then examined by comparing with discrete element modelling (DEM) results under proportional loading and experimental data under complex loading and unloading processes. It is found that fabric evolution at low stress ratios is closely related to the stress-rate driven term of the hybrid law, while the strain-rate driven term dominates at high stress ratios. The hybrid evolution law satisfies the uniqueness requirement of fabric at the critical state by introducing an 'attractor' concept. Overall, fabric evolutions predicted by the hybrid law show a close agreement with DEM simulation results and experimental data. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the effects of the intermediate stress ratio, i.e., b -value (b = (σ 2 − σ 3)/(σ 1 − σ 3)), on the contact normal-based fabric evolution of granular material, are incorporated into an extant hybrid fabric evolution law. The new evolution law is validated by Discrete Element Method (DEM) simulation results under monotonic shearing with different b -values. Predictions of the proposed generalized fabric evolution law agree well with the DEM simulation results. This evolution law can be widely used for constitutive modelling of granular materials, considering the effects of b -value in a general geomechanical three-dimensional stress space. [ABSTRACT FROM AUTHOR]