Progress Report on Addition of a High-Speed Drive to High-Pressure, Rotary-Shear Apparatus

Terry E. Tullis

Published August 14, 2016, SCEC Contribution #6782, 2016 SCEC Annual Meeting Poster #030

My rotary-shear, high-pressure machine at Brown University has been operational for over 35 years. It allows unlimited slip on annular samples at confining pressures up to 1 GPa. Nitrogen or argon gas is used as the pressure medium in order to operate at elevated temperatures and to use electronic transducers inside the pressure vessel for measuring stress and displacement. The samples are jacketed by a composite jacket assembly consisting of O-rings to exclude the confining pressure from the sample and Teflon rings to separate the O-rings from the sample so they will not be torn apart by the large displacement and to transfer the confining pressure to the sample. Separate pore pressure systems access the top and bottom halves of the sample in order to allow fluid flow through the sample, for measuring permeability and for changing fluid composition during an experiment.
Rotation of the sample is driven by two systems. An electrohydraulic stepping motor provides unlimited displacements at over 7 orders of magnitude in speed from 0.001 microns/s to 10 mm/s, using two ~100:1 harmonic-drive speed-reducers, one of which can be bypassed. Riding piggy-back on the rotating table driven by this system is a rotary servo system with 9 degrees of rotation. This is used for rapid rotary servo control, allowing effective stiffening of the loading system, using as the feedback device a resolver internal to the pressure vessel mounted close to the sample’s sliding interface. The resolver has 24-bit resolution in one revolution, allowing a slip precision of 10 nanometers. The effective shear stiffness of the machine using the resolver and rotary servo is increased 37x over the natural stiffness to, for example, 1.8 MPa/micron at a normal stress of 25 MPa. This stiffness is only attainable at slip speeds below about 3 microns/s, due to the finite response time of the hydraulically actuated rotary servo.

I am in the process of adding a high-speed servo motor drive to the apparatus to allow rotation of samples at slip speeds up to ~4.5 m/s. This will allow study of high-speed friction weakening mechanisms at effective normal stresses similar to those at which crustal earthquakes occur. The motor will be positioned on the table rotated by the electrohydraulic stepping motor, taking the place of the present hydraulic rotary servo drive. This will allow continuous changes in speed of over 9.5 orders of magnitude. The motor is an ETEL torque motor capable of delivering over 100 kw of power. It can deliver 2250 Nm of torque at speeds up to 400 rpm (1 m/s for our samples) and, using field weakening, can rotate up to 1800 rpm, while delivering progressively lower torques. The torque capability will allow sliding samples with Byerlee friction values at normal stresses up to 100 MPa. The motor’s torque and speed capabilities nicely match the needs for our samples, meaning that no gearing is required, eliminating that as a source of backlash. A 27-bit Heidenhain absolute optical encoder will be mounted on the motor as the feedback device for a Siemens motor controller, allowing tight motor control to a precision corresponding to a sample slip of 1 nanometer. The controller can use my existing 24-bit resolver in a secondary feedback loop, allowing the same electronic stiffening of the apparatus as at present, but at speeds up to 4.5 m/s.
Once the motor is installed and operational, jacketing of the samples at high slip speeds will be the next challenge. Elevated temperatures due to frictional heating of the samples will require use of more refractory materials than the present Teflon rings and Viton O-rings. Kalrez O-rings are good to 300 degrees C, which, given their distance from the sample, may be sufficient for the short duration of high-speed slip to realistic slips, e.g. 10 m of slip at 2 m/s for 5 s. However, Teflon breaks down at 300 degrees C at room pressure and is closer to the sample so will get hotter faster than the O-rings. Graphite, molybdenum disulfide, and hexagonal boron nitride are refractory materials that may be suitable to replace the Teflon, either alone or in some composite geometry, perhaps involving Vespel, which is good to 500 degrees C. Only by fast sliding at high pressure, and some trial and error, will it be possible to develop a satisfactory refractory sliding jacket.

Key Words
high-speed, friction, experiments, apparatus

Tullis, T. E. (2016, 08). Progress Report on Addition of a High-Speed Drive to High-Pressure, Rotary-Shear Apparatus. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)