Particle shape transforms the driving of shear stress in granular materials

Particle shape has been regarded as one of the key factors in determining the macroscopic shear strength of granular materials. Despite great efforts, the underlying mechanism of the dependence of shear strength on particle shape remains elusive, especially why the increasing tendency of shear strength slows down and even disappears with ever-growing particle asphericity. This paper presents a micro-mechanical study on the shape-dependent shear strength from the perspective of force transmission and microstructure. A series of simple shear tests are conducted on ellipsoidal assemblies with various aspect ratios using the discrete element method. By systematically analyzing the contribution of normal and tangential force to shear stress, we highlight the nonnegligible role of tangential force in mobilizing shear resistance of non-spherical granular systems. The tangential contact force contribution to the shear stress increases with particle asphericity and even exceeds the contribution of the normal contact force. The statistics of shear stress components at different scales indicate the two types of contact forces have completely different dependence on particle shape, which leads to the nonlinear increase in shear strength with particle asphericity. Based on the analysis of local structure, we then prove that the different shape dependence of the two types of contact forces originates from the shear-induced nematic ordering, and the shape-dependent shear strength is constrained by interparticle friction.