Strain Gauge Measurements Of Blade Resonance Using Eddy Current Excitation In A Vacuum Spin Pit

As part of an on going High Cycle Fatigue program related to gas turbine engines, which is sponsored jointly by the Naval Air Systems Command and the Air Force, unsteady strain gauge measurements were made on a 37.5 inch diameter titanium rotor in the Naval Postgraduate School, Turbopropulsion Laboratory vacuum spin pit. Vibratory excitation was produced by a number of evenly spaced magnets positioned around the rotor perimeter, which generated eddy currents in the blades and associated magnetic forces on the blade tips. A critical heating problem was experienced in initial tests, and instrumentation to monitor temperature of the metal passing within the magnetic field was installed. The first five runs following rework are reported. Correlations were made between temperature effects, magnet position, sweep rate, resonance amplitudes, and resonance frequency. An extensive evaluation was also performed of the strain data collection software, Data Physics Signal Calc 620.