Development of a Material-Testing Machine for Study of Friction: Experimental Analysis of Machine Dynamics and Friction of Rock

Abstract

Experimental investigation of the coefficient of sliding friction in rock at normal stress and sliding velocity of typical earthquakes (10–100 MPa and 0.01–1 m/s) is necessary to develop velocity-dependent constitutive relations useful for earthquake rupture simulations. The velocity dependence of rock friction is best explored in parametric studies of interfacial friction by imposing step-like changes in velocity and measuring the frictional force, but such experiments are technically challenging. We present a testing-machine incorporating a prototype loading system designed to achieve high accelerations (up to 100 g) in test samples at earthquake conditions. A 3-degree-of-freedom model representing dynamics and vibrations of the machine is developed to assess machine–sample interactions and the capability to achieve step-changes in velocity. In addition, the prototype loading-system is instrumented with several sensors and operated in order to validate model analyses. Several preliminary experiments on test-samples that display different behaviors were conducted with the prototype system to document machine-sample interactions. The results show that for an impulse-type loading condition associated with dramatic weakening of test-samples, undesirable vibrations of the system can be significant. The dynamic model is modified to identify and treat the source of vibrations accurately, and is used to improve the design of the ultimate loading-system to minimize vibrations and best emulate load paths appropriate to earthquake slip. Finally, a set of sliding friction experiments on rock is presented and the weakening micro-mechanism is investigated. The results suggest that flash weakening at asperity contact points can be a dominant weakening mechanism in earthquake slip.