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Turbine Blade High Cycle Fatigue Testing

Turbine Blade High Cycle Fatigue Testing

High cycle fatigue testing is a critical component of aircraft engine turbine blade qualification. It is becoming increasingly important with power generation turbine blades. Depending on the location of the blade, turbine blade failure in an aircraft engine can cause catastrophic damage as the blade causes secondary failures in other blades. In fact, turbine blade failure has been the cause of several crashes of jet aircraft and highlights the importance of turbine blade high cycle fatigue testing.

Since the 1980s, aftermarket part manufacturers have proliferated due to the demand from airlines to reduce costs and find alternative sources other than the OEM (Original Equipment Manufacturer) for replacement parts. PMA (Parts Manufacturer Approval) is a term used by the FAA (Federal Aviation Authority) for approval to manufacture aircraft parts such as turbine and compressor blades installed in FAA certified aircraft. With that approval, rigorous testing is requisite to demonstrate the PMA part equal to or better than the OEM part.

High cycle fatigue testing on a turbine blade is performed with the SignalStar Vector using the Sine Resonance Search and Dwell software. The base of the turbine blade is attached to a fixture on an electrodynamic shaker. For testing of smaller blades, with higher resonance frequencies, a piezoelectric shaker may be used. Usually, an accelerometer is placed on the fixture as a reference measurement and a non-contact sensor, typically a laser displacement sensor, is used to measure the response of the tip of the blade. To simulate the environment inside the engine, the turbine blade may be heated.

Swept sine is used to measure the magnitude and phase of the transfer function between the reference accelerometer on the fixture and the displacement of the tip of the blade as the shaker sweeps through the blade resonance. A resonance search is done on the swept sine data to determine the resonance frequency, phase, and Q. This information is used to excite the blade at its resonance frequency while controlling the displacement of the tip of the blade during the resonance dwell.

During a resonance dwell, the phase is measured between the accelerometer on the fixture and the laser displacement sensor on the blade tip. This phase measurement is compared with the phase found in the resonance search done on the swept sine data. If the measured phase deviates from the resonance phase, the dwell frequency is automatically shifted until the phases match. This “phase-tracked” technique ensures that the blade is excited at its resonance frequency throughout the duration of the test.

SignalStar Vibration Controllers have proven over and over again to be the only system on the market that can accurately and reliably control the very high Q resonances generated by aircraft turbine blades. Learn more about SignalStar Vibration Controllers.

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