SCEC Award Number 21095 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Experimental Investigation of Multi-scale Flash Weakening
Investigator(s)
Name Organization
Frederick Chester Texas A&M University Judith Chester Texas A&M University
Other Participants 1 Graduate student
1 Undergraduate student
SCEC Priorities 1d, 3c, 2d SCEC Groups FARM, Geology, SDOT
Report Due Date 03/15/2022 Date Report Submitted 03/18/2022
Project Abstract
 INTERIM REPORT: Our multi-year continuation project investigated flash heating during high speed frictional sliding in rock to better constrain and test the conventional model of flash weakening. Theoretical models treat flash heating at micrometer-scale contacts that thermally weakens the contacts and reduces friction at earthquake slip rates. The conventional model also treats the progressive rise in overall sliding surface temperature with displacement that further reduces macroscopic friction. To test the theoretical models, we have conducted high-speed friction experiments using a biaxial apparatus equipped with an IR camera to document flash temperature distributions on the sliding surfaces. Our unique data set allowed us to document the inhomogeneous temperature distribution, which is characterized by mm-size hot-spots comprising approximately 10-15% of the macroscopic area. Based on the conventional flash weakening model and thermal modeling of heated contacts, we documented the local normal stress magnitude at the mm-scale hot spots. Knowing surface temperature and normal-stress contact distributions has allowed us to test the conventional flash weakening model. Using tests with different sliding velocity histories, and with shaped surfaces that dictate contact history, we showed that the conventional (steady-state) flash-weakening model does not completely capture the observed weakening behavior in experiments. During this contract period we have developed a multiscale model (mm- and micrometer-scale) to better simulate the observed friction behaviors. In addition, we are finishing microstructural studies to better constrain mm-scale weakening processes and the effect of macroscopic normal stress on mm-scale contact distributions, which will allow us to advance the multiscale friction model.
Intellectual Merit The experimental rock deformation, microscopy and profilometry studies of this continuation project are designed to constrain the frictional properties of rock sliding at seismic rates as well as identify the underlying, multiphysics processes responsible for the observed macroscopic response. Such information is key to developing accurate mathematical descriptions of evolving fault strength and the effects of frictional heating and shear localization. The outcomes of this research are useful to inform physics-based earthquake models including numerical models of nucleation, propagation and arrest of earthquakes. This project has developed novel experimental techniques and unique data sets that can be used to test, constrain and refine theoretical models of friction, instability, and rupture propagation in fault zones.
Broader Impacts This project has supported training and learning of graduate student researchers, particularly within the SCEC group of researchers in our College. The project has broadened the participation of underrepresented groups, specifically by supporting research of two Ph.D. graduate students, both women, one of which is Hispanic-Latina. Both students have been and will continue to be involved in SCEC.
Exemplary Figure Figure 1. Comparison of model fits (dashed lines) to measured response (solid lines) of a sliding friction experiment with a step increase in sliding velocity of 0.7 m/s followed by a gradual decrease in velocity to arrest. Note the spontaneous stick-slip event after arrest at 25 mm slip. Best fit material parameters for the flash-weakening models are determined from a suite of experiments imposing a variety of velocity paths and maximum velocity. The multiscale contact model result is represented by long-dashed red lines and displays an improved fit to the experiment data and the spontaneous stick-slip event, whereas the conventional flash weakening model fails to display the stick-slip event