MICROMECHANICS OF ROCK FRICTION .1. EFFECTS OF SURFACE-ROUGHNESS ON INITIAL FRICTION AND SLIP HARDENING IN WESTERLY GRANITE

Frictional behavior of Westerly granite sheared in a rotary apparatus was observed up to 20 MPa normal stress. Before and after each experiment, surface roughness was measured with a profilometer. Macroscopic frictional properties were observed to evolve toward steady state through two distinct stages denoted initial slip and slip hardening. Total slip was less than 200-mu-m, but some experiments were extended to several centimeters. The instant a shear load was applied, a finite shear stiffness was observed which decreased nonlinearly with displacement. Initial stiffness was roughness dependent: the smoother samples supported equal or higher shear stresses than rough samples at any given displacement. In the shear stress-displacement data, a distinct yield point was observed which was roughness and normal load dependent Samples with rougher surfaces and/or higher normal loads required longer displacements to reach a yield point. The yield point marked the start of the slip-hardening stage during which the shear strength of the surfaces continued to increase at a rate that was roughness and normal load dependent. Rougher surfaces exhibited higher rates of slip hardening than smooth, and over much greater slip distances. During slip hardening, normal closure continued, but at a diminished rate with increasing displacement. The closure rate and slip hardening were correllated: rougher surfaces exhibited higher rates of slip related closure which produced increased asperity interlock and, consequently, increased friction. Beyond 100 to 300-mu-m displacement, slip hardening and closure nearly ceased, beginning a third stage in which friction is nearly steady state. At steady state, friction of the rough surfaces (0.60 to 0.65) was greater than that of the smooth (0.50 to 0.55).