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Development and Characterization of a High-Speed Material-Testing Machine, and Experimental Analysis of Frictional Flash Heating and Dynamic Weakening in Rock
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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.
Saber, Omid (2017). Development and Characterization of a High-Speed Material-Testing Machine, and Experimental Analysis of Frictional Flash Heating and Dynamic Weakening in Rock. Doctoral dissertation, Texas A & M University. Available electronically from