Constitutive modeling of structured granular materials with anisotropic grain skeleton and cement bonds

Structured granular materials exhibit strongly anisotropic mechanical behaviours resulting from the directional properties of their microstructural components such as grain shape, cement topology and contact orientation. Here, a fabric-enriched continuum breakage-damage framework is proposed to examine and simulate the evolution of anisotropy in granular rock with evolving internal structure. Fabric tensors for both grains and cement are embedded in the expression of the elastic free energy potential. To reflect the influence of anisotropic strain energy storage on inelastic mechanisms, such free energy function is then incorporated within a continuum breakage-damage formulation. The performance of the model is evaluated against experimental data for a high porosity granular rock tested at different orientations of its bedding planes. It is shown that the proposed model can accurately predict the yielding and stress–strain responses by accounting for the microstructure of cement and grains. Parametric analyses indicate that the evolution of the degree and the orientation of anisotropy is controlled by the independent fabric tensors of the two solid phases (i.e., grains and cement), as well as by the competition between grain crushing and cement disintegration. As a result, the model is able to capture naturally that a higher volume fraction of cement can enhance the stiffness and augment the softening behavior of the granular rock.