Saber, Omid (2017-06). Development and Characterization of a High-Speed Material-Testing Machine, and Experimental Analysis of Frictional Flash Heating and Dynamic Weakening in Rock. Doctoral Dissertation.
Thesis
Experimental investigation of dependence of sliding-friction on velocity is necessary to understand the physics of earthquakes. Velocity-dependent friction is ideally studied in experiments by imposing step-wise changes in sliding-rate. In this dissertation, a novel High-Speed Biaxial (HSB) testing machine capable of imposing steps from quasi-static (1 mm.s?1) to seismic (1 m.s?1) sliding-rates has been developed and characterized. The HSB can achieve steps to seismic sliding-rates in only a few milliseconds under loads as high as 0.5 MN. Herein, the dynamics of the hydro-pneumatic loading-system of the HSB is studied, and by developing an analytical model, feasibility of achieving the desired velocity-steps under different load-path scenarios is assessed. Moreover, the HSB prototype is instrumented and tested to validate the model analysis. Based on the experimental results, a model of vibrations is developed for the continuous loading-system. The model is used to identify and treat vibration sources in the HSB prototype, and a modified design is proposed to reduce vibrations in the ultimate testing machine. After development and verification of the HSB prototype, a series of sliding friction experiments were conducted to study dependence of friction on sliding-rate, slip history and normal-stress. At seismic sliding-rates, frictional heating can lead to dramatic frictional-weakening in rock. Here, I report on high-speed friction experiments for which flash-heated contacts are thermally imaged on rock samples. The thermographic images provide the first documentation of the geometry and spatial distributions of load-bearing contacts formed in rock during frictional sliding at seismic rates. The thermographs display a highly heterogeneous distribution of temperature and stress at millimetric scale. The maximum temperature observed in our experiments (500 ?C) is remarkably higher than average surface temperature calculated by other studies (100 ?C), which reflects the localization of stress to small portions of the contact surface. The observations indicate that, opposed to the original micro-scale flash-weakening model, flash-heating occurs in multiple length- and time-scales. Accordingly, a multi-scale flash-weakening model is proposed and developed, which can simulate the transient friction more accurately. The new findings can play a key role in understanding nucleation and propagation of earthquake ruptures in natural faults.
