The discrete element method (DEM) is adopted to investigate the influence of the particle shape on the smallstrain stiffness and stiffness degradation of granular materials during triaxial compression tests. Clumped particles are used to simulate irregular granular particles. The simulation results show that a more irregular particle shape causes an increase in the initial stiffness at very small strains and more delayed stiffness degradation. The micromechanism is explored on the basis of the analytical stress-force-fabric relationship, which reveals that increased particle irregularity leads to higher relative contribution of the tangential force anisotropy to the deviatoric stress. The achievable slip ratio and the mechanical coordination number also increase with increasing particle irregularity, resulting in larger resistance to deformation. An equivalent spherical particle analysis method is proposed, which reveals that the irregularity of particle shapes significantly increases both the sliding resistance and the rotational resistance between two particles, resulting in greater stability in the contact network and thus contributing to higher macroscopic stiffness and slower stiffness degradation.

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.