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 04/07/2024
Project Abstract
 Our multi-year continuation project investigates 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 both micrometer- and mm-scale contacts that thermally weaken 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. Experiments document inhomogeneous temperature distribution characterized by mm-size hot-spots representing high load bearing, true areas of contact. We confirm that true area of contact increases linearly with macroscopic normal stress. Additionally, we find that hot-spot patterns are similar in successive experiments on the same sliding surfaces. From characterization of mineral distributions and surface roughness of machined sliding surfaces, we conclude that mineral distribution and surface heights contribute to mm-scale hot-spot development. Using tests with different sliding velocity histories, and with shaped surfaces that dictate contact history, we show that the conventional (steady-state) flash-weakening model does not completely capture the observed weakening behavior in experiments. We have developed a multiscale model (mm- and micrometer-scale) to better simulate processes at the mm-scale that involve shearing thin layers of wear product, and localized contact slip at the microscopic scale.
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 3. Mineral distribution and surface heights of the machined sliding surfaces of westerly granite are characterized prior to conducting high-speed sliding experiments to understand the influence of mineral grains and surface roughness on the size and location of hot spots developed during high-speed sliding. a) A Micro XRF spectrometer is used to determine elemental concentrations on the machined sliding surface to map the distribution of the dominant minerals (feldspar, quartz and biotite) exposed at the surface. A laser profilometer, with spot size of ~50 µm and resolution of 1 µm, is used to characterize the surface heights of the same machined surface. The machined ridges (yellow-red) and grooves (aqua-blue) are apparent. Numbered boxes highlight the visual correlation of mineral type and the local surface height. b) Surface height distributions indicate a greater probability of local highs on quartz grains than on feldspar grains. c) Surface-height profiles, both parallel and perpendicular to slip, show two dominant scales of roughness, a longer wavelength roughness associated with the machined grooves and ridges, and the smaller wavelength associated with the surfaces on the ridge tops and groove bottoms. The amplitude to wavelength ratios are similar for the two scales of roughness, and similar to that of observed for natural fault slip-surfaces.