Investigating Dynamic Weakening in Laboratory Faults Using Multi‐Scale Flash Heating Coupled With mm‐Scale Contact Evolution

Abstract

Flash‐weakening models typically show good agreement with the total magnitude of weakening in high‐speed rock friction experiments, however deviations during the acceleration and deceleration phases, and at low and intermediate sliding velocities, remain unresolved. Here, we incorporate inhomogeneous mm‐scale normal stress evolution into a model for flash heating and weakening to resolve outstanding transient and hysteretic friction observed in laboratory experiments and to identify unique solutions to constitutive parameters. We conduced 37 rock friction experiments on Westerly granite using a high‐speed biaxial apparatus outfitted with a high‐speed infrared camera. We initiated velocity steps from quasi‐static rates of 1 mm/s to sliding velocities ranging from 300 to 900 mm/s and conducted both constant‐ and decreasing‐velocity tests following the velocity step. Two sliding surfaces geometries were used to control mm‐scale life‐times and rest‐times. Constant‐strength sliding is achieved within 2–3 mm of initiating the velocity step in all constant‐velocity experiments. Macroscopic surface temperature is inhomogeneous and increases with slip distance, velocity, and decreasing rest‐time. Weakening increases with sliding velocity and decreasing rest‐time. We combine thermal models with measured surface temperatures to constrain the evolution of local normal stress at the mm‐scale and incorporate this evolution into a flash‐weakening model that considers weakening at both the µm‐ and mm‐scale. The flash‐weakening model improves when the effects of mm‐scale wear processes are incorporated and multi‐scale weakening is considered, however some transient friction remains undescribed. Models will be advanced by further incorporating wear processes and by considering processes at the mm‐scale and above.