A 20 Kilohertz Optical Strain Gage
The endurance limit for steels is traditionally taken at one million cycles, but modern machinery such as gas turbines and high-speed train wheels can expect lifetimes of one billion cycles or more. Further, microdevices such as digital light projection mirrors and RF switches operate at high frequencies and last for long times. It is possible, in both cases, to cycle test specimens at 20 KHz and produce one billion cycles in 14 hours or so. However, force and strain measurement are difficult.
Research at the University of Michigan uses an ultrasonic generator coupled to a variable cross-section ‘horn’ with a test specimen at the end, which is excited to resonance at 20 KHz. The specimen has an hourglass shape, and the vibrating system produces a node and maximum strain at its center. The specimen can be heated to 1000°C by an RF coil around it. The specimen initially has two foil strain gages mounted on it, and these are used to correlate strain with power levels of the ultrasonic generator at low strains. Tests are run at higher power levels, but the strain gages do not survive. There is a need for an accurate strain measurement, and DARPA (Defense Advanced Research Projects Agency) has sponsored the development of an optical system at Johns Hopkins to be installed on the Michigan test machine.