Shear strength and structures of highly polydisperse packings composed of pentagons

Duc-hanh Nguyen, E. Azéma, F. Radjai and P. Sornay

By means of extensive contact dynamic simulations, we present a detailed analysis regarding the combined effect of particle size and shape polydispersities (defined as size span and as the degree of irregularity of the particles) on the shear strength and structure of sheared granular media composed of pentagons. We find that the shear strength is independent of size span, but unexpectedly, it declines as shape polydispersity is increased. In contrast, the solid fraction is an increasing function of both size span and shape polydispersity revealing that the densest packings have the same shear strength as the loosest. At the scale of the particles and contacts, we analyze the connectivity, force transmission, friction mobilization as well as the associated anisotropies. We show that stronger forces are carried out by the largest particles which are propped by an increasing number of small particles. As a result, the non-variation of the shear strength with size span is shown to be due to the falloff of the geometrical anisotropy compensated by an increase of force and branch anisotropies. On the other hand, the increase of shape polydispersity induces that the sharp corners of irregular particles allow for deep contacts between neighbors that are unreachable for more regular particles. Thus, the geometrical anisotropy declines at large values of shape polydispersity, which explain the decreases of the shear strength.






Videos of the force-bearing particles during bi-axial test, at (s,delta)=(0.01,0) (a), (s,delta)=(0.9,0)(b), (s,\delta)=(0.01,1) (c) and (s,delta)=(0.9,1) (d)
The floating particles (i.e., particles with one or no contact) are drawn in white and normal forces are represented by the thickness of the segments joining the particles center.