Laboratory observations of frictional sliding of individual contacts in geologic materials

Abstract

This paper gives the results of grain-to-grain sliding friction experiments on several naturally occurring geologic materials and two manufactured materials. The materials include quartz sands with mean grain size $d_{ave}$ ranging from 0.14 to 3 mm, crushed and ball milled gneiss with $d_{ave} = 14-20$ mm, magnesite (limestone) with $d_{ave}=2$ mm, and Caicos ooids with $d_{ave}=0.44$ mm. The reference materials include manufactured glass beads ($d_{ave} = 0.30$ and 1.0 mm) and spheres of a synthetic material (Delrin, $d_{ave} = 19.05$ and 5.08 mm). The experiments involved normal loads $F_{N}$ that ranged from 0.46 to 20 N, depending on material, and the subsequent application of an increasing shear force at a loading rate of 1 $\hbox {Ns}^{-1}$. This work contributes to the goal of providing high-fidelity contact models for use in discrete element simulations of naturally occurring granular materials. The results presented here provide a picture of shear force-displacement behavior up to and through the onset of macroscopic sliding. For natural materials with relatively rough surfaces, the coefficient of friction $\mu$ ranged from 0.1 to 0.9 at normal force levels $F_{N} < 10$ N, but tended to converge to a narrower range (0.24–0.62) at higher $F_{N}$ levels. Grains with low surface roughness (glass beads, synthetic material and the 3-mm-diameter sand), on the other hand, exhibited a trend of decreasing $\mu$ with increasing $F_{N}$, with terminal values in the range of 0.1–0.2 for $10 \le F_{N} \le 20$ N. This behavior is explained in terms of the relationship between the normal force and the true area of contact. Additionally, observations of free sliding observed under cyclic shear loading are reported.