Dynamic Weakening of Sliding Friction and the Influence of Gouge Development

Monica R. Barbery, Frederick M. Chester, Judith S. Chester, & Omid Saber

Submitted August 24, 2016, SCEC Contribution #6878, 2016 SCEC Annual Meeting Poster #032

Previous experiments demonstrate that as sliding velocity approaches seismic slip rates, the coefficient of sliding friction (u) typically reduces significantly and may reflect flash heating of surface contacts. Experiments further demonstrate differences in the weakening behavior of bare rock and gouge-lined surfaces across different rock types. We conducted quasi-static (QS) experiments and high-speed velocity-step (VS) experiments on ground surfaces of granite (Westerly) and quartzite (Sioux) using a double direct shear (DDS) configuration, with sliding area of 75cm2, to investigate the effects of gouge generation and accumulation on the frictional weakening behavior. High-speed infra-red imagery was used to measure sliding surface temperatures. QS experiments were conducted at 6-8 MPa normal stress and with sliding velocities of 1 mm/s for 36 mm of displacement. VS experiments were conducted at 7-9 MPa normal stress and achieved steps from 1 mm/s to 1 m/s at high acceleration (100g) over a small displacement (2 mm), and with sustained high-speed sliding for 30 mm. Successive experiments were conducted without disassembling the blocks or disturbing the sliding surfaces to generate and accumulate gouge for cumulative displacements up to 0.5 m. For granite at QS rates, u strengthened from ~0.75 to 0.8 within ~10 mm of displacement. For quartzite, u weakened from ~0.65 to 0.6 within a typical weakening distance, dc, of ~3 mm. In VS tests, locally high temperatures were observed correlating to corrugated structures within the gouge. For VS granite tests, an abrupt decrease in u from ~0.7 at quasi static slip rates to 0.5 at m/s slip rates was observed over a dc of ~3 mm, consistent with rotary shear experiments conducted at similar conditions. With the accumulation of gouge on the sliding surface, dc progressively increases to ~2 cm. In contrast, VS tests on quartzite show an abrupt increase in u, from ~0.65 to 0.7 within 1 mm of slip, followed by gradual weakening for the duration of high-speed sliding. As quartz gouge accumulated on the sliding surface, similar behavior is observed, but with a slightly greater magnitude of strengthening. The quartzite results are unlike those reported in the literature for quartzite at similar conditions. Gouge is generated at a greater rate in quartzite when compared to the granite, and is particularly abundant during the initial stage of slip. The different behavior of quartzite is currently under further investigation.

Key Words
rock mechanics, rock friction, dynamic weakening, gouge

Citation
Barbery, M. R., Chester, F. M., Chester, J. S., & Saber, O. (2016, 08). Dynamic Weakening of Sliding Friction and the Influence of Gouge Development. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)