Microscopic analysis of the nonlinear stiffness of granular materials at small-to-medium strain

The nonlinear stiffness of dense and loose granular materials at small-to-medium strain is investigated using the three-dimensional discrete element method (DEM). The threshold strain of the nonlinear elasticity of granular materials, i.e., the Y2 surface in the kinematic yielding surface framework, is obtained from the simulated drained triaxial tests. The analytical stress-force-fabric relationship is used to quantitatively examine the evolution of fabric anisotropy. Results show that the after reaching Y2 surface, the contact normal anisotropy begins to increase from a steady low level, and evident slippage begins to occur at strong contacts. The threshold strains of the contact normal anisotropy and strong contact slippage increase linearly with the confining pressure, which is consistent with the threshold strain of the nonlinear elasticity of granular materials. The creation of new contacts and disruption of existing contacts during loading are investigated. A three-stage micromechanism of the observed nonlinear stiffness behavior is proposed.