Experimental investigation of the sliding-velocity dependence of friction in rock is necessary to understand slip instability and earthquake phenomena, but a capability to conduct sliding friction experiments at high normal loads and at earthquake slip rates (1 m/s) must be developed. Velocity-dependent friction is ideally studied by imposing step-wise changes in rate of sliding. Velocity steps to earthquake slip rates must occur in a few milliseconds and requires high accelerations, so feedback control is not applicable because the rise time of actuators that can be used for our application is large relative to the rise time of the required velocity response. For this reason, we analyze the capability of a high-speed loading system with high load capacity to impose a velocity step, followed by constant rate of sliding, using only several preset passive-control variables. Assuming representative transient-friction response of rock test-samples, preset passive-control variables of the loading system may be obtained such that the deviation from the ideal velocity-step is minimized. A parametric exploration of the preset passive-control variables and test-sample behavior is used to define the machine workspace, and to optimize the key design characteristics of the loading system, such that the desired load paths for a range of expected sample behaviors may be achieved.

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