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-offreedom 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.

Characterizing the Distribution of Temperature and Normal Stress on Flash Heated Granite Surfaces at Seismic Slip Rates

“…We suggest that with knowledge of surface temperatures and of the LT and RT of mm-scale contacts, which effectively provide critical constraints on the thermal state of the surface, determinations of temperature and contact properties will be more robust. Our calculations of normal stress distribution in the last few millimeters of HV sliding show greater stress concentration on mm-scale contacts than inferred by Saber (2017), primarily because of his uncertainty in the LT of contacts for conventional flat-ground surfaces. Nonetheless, Saber (2017) showed that hysteretic friction behavior during acceleration and deceleration could be better matched by using the observed surface-temperature distribution in coupled thermomechanical models of flash-weakening.…”

“…Our calculations of normal stress distribution in the last few millimeters of HV sliding show greater stress concentration on mm-scale contacts than inferred by Saber (2017), primarily because of his uncertainty in the LT of contacts for conventional flat-ground surfaces. Nonetheless, Saber (2017) showed that hysteretic friction behavior during acceleration and deceleration could be better matched by using the observed surface-temperature distribution in coupled thermomechanical models of flash-weakening. Experiments employing different velocity histories, different mm-scale contact LT and RT, and thermal imaging show great promise in enhancing our ability to perform more complete tests of the flash-weakening model for rock-on-rock friction.…”

“…The HSB employs a hydraulic cylinder accompanied by a gas accumulator to apply a constant σ n across the sliding surface interface of each block during experiments. A pneumatic actuator equipped with a hydraulic damper was used to achieve velocity steps (VS) from quasi-static sliding rates to sliding rates up to 1 m/s, within 2 mm of d. After reaching the target velocity (V t ), constant-velocity sliding (within ±10% of the V t ) is maintained for up to 3.5 cm of d (see Saber et al, 2014Saber et al, , 2016 for a full description of the HSB).…”

Large-area photogrammetry based testing of wind turbine blades

“…strain gages, string potentiometers, and accelerometers) that provide information at a number of discrete points. [1][2][3][4]. However, the complex dynamic response of these sizable blades cannot be quickly or easily measured using a limited set of mounted accelerometers.…”

Frictional properties of simulated fault gouges from the Groningen gas field and implications for induced seismicity.

“…Another approach is the development of a direct-shear testing machine that can impose a high velocity slip pulse, allowing for dm displacements, where the geometrical effects (slip-and velocity gradients) that affect traditional rotary shear experiments, are eliminated. Saber et al (2016) developed such a machine, allowing for displacements up to 4 cm. In addition, finding a way to probe temperature and pore fluid pressure locally within the actively slipping zone would significantly improve our capability of constraining the mechanisms responsible for dynamic weakening at (sub) seismic slip velocities, which is necessary in order to accurately incorporate these effects in numerical modelling of fault reactivation and any dynamically generated seismic wave field.…”