Controlling the life-time and rest-time of asperity contact populations to investigate the temperature and stress distribution in flash-weakened frictional surfaces in granite

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

Published August 15, 2019, SCEC Contribution #9726, 2019 SCEC Annual Meeting Poster #163

For frictional sliding in rock at rates >0.01 m/s, the coefficient of friction (µ) can be reduced by flash heating at asperity contacts and by the associated rise in average temperature of the sliding surface. Using a high-velocity (~1 m/s), high-acceleration biaxial apparatus with a high-speed infrared camera and a double-direct-shear sample configuration, we quantify µ and the distribution of temperature on Westerly granite surfaces (70 cm^2 area) as a function of slip. We demonstrate that for flash-weakening (1) the reduction in friction during sliding along rough rock surfaces depends on the local surface temperature distribution at mm-scale contacts, and (2) the temperature distribution varies with contact lifetimes, cumulative slip, and changes in the distribution of local normal stress (σL) on contacts resulting from plowing and wear. Experiments were conducted at a macroscopic normal stress (σn) of 9 MPa, and after sliding at 1 mm/sec we employed a velocity step (VS) from 1 to 900 mm/s to achieve ~30 mm of slip at high speed. Sliding surfaces were ground to control contact life-times (LT; 8.3 mm of slip for a LT of 9.2 ms), and rest-times (RT) in three geometries characterized as LT<RT, LT=RT, and LT>RT. For all geometries, steady-state µ is achieved after 4 mm of slip following the VS. As expected, surface temperature and magnitude of weakening increase with a reduction in RT. We use a 1-D heat conduction model incorporating thermal properties of Westerly granite and average steady-state values for sliding velocity (V) and µ to determine heat-generation rate during contact LT, and model expected temperatures for σL ranging from 0.5-14 times the applied σn. Comparing the models to IR thermographs we determine the distribution of σL. In early stages of sliding, the majority of the total normal load is carried by a few small (~ 1 mm diameter) contacts with σL equal to or exceeding 6*σn. As sliding progresses, mm-scale contact area increases and contacts with lower σL form. At the end of sliding, the majority of the load is carried by mm-scale contacts with σL below 6*σn. Decreasing RT generates higher mm-scale contact temperatures and greater weakening. In our experiments having fixed LT and RT, an increase in mm-scale contact area by wear and ploughing with decreased σL is compensated by the general increase in local temperature with cumulative slip to maintain a constant, steady-state µ during high velocity sliding.

Key Words
rock friction, flash weakening, temperature weakening

Citation
Barbery, M. R., Chester, F. M., & Chester, J. S. (2019, 08). Controlling the life-time and rest-time of asperity contact populations to investigate the temperature and stress distribution in flash-weakened frictional surfaces in granite. Poster Presentation at 2019 SCEC Annual Meeting.

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