Characterizing the distribution of temperature ad normal stress on flash-heated granite at seismic slip rates

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

In Preparation September 28, 2020, SCEC Contribution #10819

At seismic slip rates, flash weakening can significantly reduce the coefficient of friction, and the magnitude of weakening increases with surface temperature. To quantify the distribution of flash temperature, high-speed double-direct shear experiments were conducted on Westerly granite blocks using velocity steps from 1 mm/s to 900 mm/s at 9 MPa normal stress. We employed a high-speed infrared camera to measure surface temperatures on the moving block during sliding, and utilized a novel sliding-surface geometry to control the mm-scale contact history. Following the initial weakening upon the velocity step, the blocks slide at a constant coefficient of friction. Surface temperatures are inhomogeneously distributed across the sliding surface, and increase with displacement. We employ a one-dimensional thermal model with conventional flash-weakening models incorporating a surface temperature-dependence, and informed by the controlled mm-scale contact history, to determine the local normal stress distribution at the mm-scale. Early contacts are characterized by local normal stresses exceeding 40 times the applied normal stress. As sliding progresses, the local normal stress at the locally hottest contacts decrease and contact area increases, leading to local normal stresses ranging from 2-6 times the applied normal stress on most contacts by 30 mm of slip. Increases in surface temperature, which would decrease the coefficient of friction, are buffered by increases in contact area and decreases in local normal stress, attributed to wear processes. Treatments of flash heating can be advanced by incorporating improved characterization of the state of the sliding surface at the mm and larger scales during sliding.

Barbery, M. R., Chester, F. M., & Chester, J. S. (2020). Characterizing the distribution of temperature ad normal stress on flash-heated granite at seismic slip rates. Journal of Geophysical Research: Solid Earth, (in preparation).