Multi-scale flash-weakening incorporating inhomogeneous normal stress in high-velocity friction experiments on granite

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

Published September 11, 2022, SCEC Contribution #12450, 2022 SCEC Annual Meeting Poster #159

Models for flash heating and weakening show good overall agreement with results from high-velocity friction experiments, however transient and hysteretic friction exists. We previously documented inhomogeneous surface temperature (T) on sliding surfaces during high-velocity friction experiments that reflected inhomogeneous local normal stress. Measured T was combined with thermal modelling to characterize the evolution of local normal stress (sl) with displacement (d). At the beginning of sliding, a few contacts with high sl carry the majority of the total load. With continued d, the load becomes more distributed and after 30 mm of d low sl contacts carry the majority of the total load. Here, we develop a model for flash-weakening that incorporates inhomogeneous sl. We conducted 37 double-direct shear rock friction experiments on samples of Westerly granite using a high-speed, high-acceleration biaxial apparatus and executed velocity steps from 1 mm/s to velocities (V) of 300-900 mm/s over ~2 mm of d. Following the initial acceleration, both constant V and decreasing V tests were performed. A high-speed IR camera was used to measure T on the moving block sliding surface during experiments. We used two sliding surface geometries featuring flat-topped ridges and grooves to control mm-scale contact life-times (LT) and rest-times (RT). LT were kept constant in both geometries and varied the RT from RT=LT and RT=LT/2. In all constant V tests, constant-strength sliding is achieved within 1-3 mm of initiating the velocity step; strength decreases as V increases and RT decreases. Macroscopic T is inhomogeneous across the sliding surface and increases with d, V, and decreasing RT. We combined a 1D thermal model informed by measured T with a multi-scale flash-weakening model that considers weakening at both the µm- and mm-scale. Using comparisons between model results and mechanical data from eight experiments with different peak V, V paths, and mm-scale LT-RT histories, we identify unique best-fitting parameter values to modeled experiments in addition to a single non-unique set of parameter values to all experiments. The flash-weakening model improves when both inhomogeneous local normal stress and multi-scale weakening are incorporated, however some transient and hysteretic friction remains undescribed. Flash-weakening models may be improved by further considering processes at the mm-scale and above and by incorporating the mechanical effects of wear processes.

Key Words
dynamic weakening, flash heating, rock friction

Citation
Barbery, M. R., Chester, F. M., & Chester, J. S. (2022, 09). Multi-scale flash-weakening incorporating inhomogeneous normal stress in high-velocity friction experiments on granite. Poster Presentation at 2022 SCEC Annual Meeting.

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