Combined effects of contact friction and particle shape on strength properties and microstructure of sheared granular media

Abstract

We present a systematic numerical investigation concerning the combined effects of sliding friction and particle shape (i.e., angularity) parameters on the shear strength and microstructure of granular packings. Sliding friction at contacts varied from 0 (frictionless particles) to 0.7, and the particles were irregular polygons with an increasing number of sides, ranging from triangles to disks. We find that the effect of local friction on shear strength follows the same trend for all shapes. Strength first increases with local friction and then saturates at a shape-dependent value. In contrast, the effect of angularity varies, depending on the level of sliding friction. For low friction values (i.e., under 0.3), the strength first increases with angularity and then declines for the most angular shapes. For high friction values, strength systematically increases with angularity. At the microscale, we focus on the connectivity and texture of the contact and force networks. In general terms, increasing local friction causes these networks to be less connected and more anisotropic. In contrast, increasing particle angularity may change the network topology in different directions, directly affecting the macroscopic shear strength. These analyses and data constitute a first step toward understanding the joint effect of local variables such as friction and grain shape on the macroscopic rheology of granular systems.

Shape or friction? Which of these characteristics drives the shear strength in granular systems?

“…The shear strength first increases for all shapes, but it may decline, saturate or continue to increase at larger angularity depending on the value of the local friction. This behavior is rather unexpected, and suggests a complex interplay at the microscopic scale [30]. Furthermore, our numerical results clearly highlight that attempting to represent noncircular/non-spherical shapes by hindering the rotations in assemblies of circular/spherical particles [31,32] must be reconsidered.…”

“…Each numerical sample is composed of 10 000 particles, geometrically deposed and randomly oriented in a rectangular box with frictonnless walls. We prepare 11 packings, each of them composed of particles having the same number of sides n s : one packing of disks and 10 packings with n s ∈ [50, 30,20,10,8,7,6,5,4,3]. In addition, shape properties were described by the angularity, α = 2π/n s .…”

Experimental investigation of granular friction behaviors during reciprocating sliding

“…Granular friction is vital to the ubiquitous packing and flow phenomena in nature and industrial production, such as geographical faults [1,2], powder mixing and densification [3], and granular flow lubrication [4,5]. The coefficient of friction (COF) is expressed as the tangential friction force divided by the total normal force and is typically used to describe the shear strength of a granular system [6][7][8][9]. Currently, granular friction research is associated with two experimental schemes, as follows: (1) the surface-sliding scheme, where the shearing unit is sliding against the surface of the granular system [10][11][12][13][14][15]; (2) the inside-sliding scheme, where the shearing unit is sliding inside the granular system [16][17][18].…”

Particle dynamics in two-dimensional point-loaded granular media composed of circular or pentagonal grains

“…Recent studies of granular media composed of grains of a variety of shapes have shown that non-spherical/noncircular grains, which feature additional rotational resistance at the grain-grain interaction scale [9] , better represent measurable bulk parameters like shear strength [10][11][12][13], relative viscosity [14] in sheared suspensions, and jamming packing fraction [15] observed in real systems.…”