Exciting news! We're transitioning to the Statewide California Earthquake Center. Our new website is under construction, but we'll continue using this website for SCEC business in the meantime. We're also archiving the Southern Center site to preserve its rich history. A new and improved platform is coming soon!

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

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

Submitted 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 ┬Ám- 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)