Investigation of transient and hysteretic flash-weakening behavior observed in high-speed friction experiments

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

Published August 15, 2020, SCEC Contribution #10726, 2020 SCEC Annual Meeting Poster #145

Friction, µ, during seismic slip reflects deformation over a range of length-scales, but fundamentally arises from physical and chemical processes operating at µm-scale contact junctions. In past SCEC research we documented inhomogeneous flash-temperatures, T, at the mm-scale with thermal imaging during high-speed, rock-on-rock friction experiments. By machining groove patterns in sliding surfaces to control mm-scale contact life-time, LT, and rest time, RT, we found that µ-weakening at constant sliding-velocity, V, is consistent with the steady-state model for flash weakening, where µ depends on V at µm-scale contact junctions and on the local mm-scale surface-T distribution. Through thermal modeling we determined the spatial variation of local normal stress and of local µ on the sliding surfaces. Given the different time-frame for heating and cooling of µm- and mm-scale contacts during sliding, we hypothesize that observations of transient µ in the lab, particularly during decelerating phases of slip, reflect the local surface-T dependence on the slip- and thermal-history of contacts at mm and larger length scales. More recently, we conducted a series of high-speed experiments on grooved surfaces in a double-direct shear configuration that achieves ~30 mm of high-speed slip at a macroscopic normal stress of 9 MPa. IR thermographs of the sliding surfaces at 300 Hz record the evolution of surface-T distribution during and after sliding. This series of experiments employed two different contact LT/RT histories, steps to three different velocities between 900 and 300 mm/s, and several different V history scenarios of a constant V followed by a final stage of sliding at decreasing V to arrest over slip distances of 1 to 30 mm. The experiments document an increase in macroscopic frictional weakening from an increase in average T and peak surface-T associated with an increase in V, or with a decrease in contact RT. During the decreasing V stages, µ increases as V decreases displaying hysteretic behavior and occurrence of stick-slip events in which the stress drop decreases with decreasing V. The data display a variety of friction behaviors and surface-T distributions that we will analyze through thermo-mechanical modeling to test the hypothesis above, as well as to determine if the conventional flash-weakening model can reproduce all observed behaviors using a single set of material-property and µm-scale contact values (e.g., contact dimension and strength).

Key Words
rock friction, flash weakening, temperature weakening

Citation
Barbery, M. R., Chester, F. M., & Chester, J. S. (2020, 08). Investigation of transient and hysteretic flash-weakening behavior observed in high-speed friction experiments. Poster Presentation at 2020 SCEC Annual Meeting.

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