New insights into the effect of interparticle friction on the critical state friction angle of granular materials

This letter examines the effect of interparticle friction (mu) on the critical state friction angle (phi(cs)) of granular materials via the discrete element method. A series of drained triaxial tests are conducted on multisphere ellipsoids. The results indicate that phi(cs) first increases with mu and then reaches a plateau when mu >= 0.3. Anisotropic analysis provides new insights into the underlying mechanisms of the phi(cs) evolution from two perspectives. First, the initial increase of phi(cs) results from the increases of contact normal anisotropy a(c), normal contact force anisotropy a(n) and tangential contact force anisotropy a(t); subsequently, the saturation of phi(cs) is a consequence of the decrease of a(n) being compensated by the increase of a(t). Second, the relationship between the stress ratio q/p and the contact normal anisotropy within the strong and nonsliding contacts a(c)(sn) is nearly linear for ellipsoids (i.e., q/p = ka(c)(sn)). Accordingly, the initial increase of phi(cs) is related to the increase of k and a(c)(sn) at low mu. The plateau of phi(cs) at high mu can be attributed to the decrease of a(c)(sn) being compensated by the increase of k.