Introduction
Turbine engines are the power sources of aircrafts. The reliability of turbine engines is crucial when ensuring aircraft safety. Failure analysis of turbine engines reveals the catastrophic damage a turbine blade failure can cause to the engine, depending on the location of the blade. Turbine blade fatigue testing can help engineers investigate and improve the material and design of turbine blades.
Turbine blades work under harsh conditions for extended periods of time. The constant subjection to mechanical strain and high temperatures may result in the accumulation of fatigue, which gradually causes deformation and even cracks.
Therefore, it is necessary to simulate the operating environment of turbines blades in the lab to assess if a blade can withstand long-term vibration and high temperature.
Blade Types
Aircraft turbine structures usually include compressor blades and turbine blades.
The compressor blades of aircraft engines are generally longer and operate at room temperature. The larger the blade, the lower the first resonance frequency, and it usually takes a longer time to test before its failure.
The turbine blades are in the combustion chamber and operate at high temperatures during operation. The simulation ground test must heat the unit under test for fatigue testing.
Test System
The environmental testing system consists of three parts:
Vibration control: shaker, power amplifier, controller, fixture, blade (UUT), data acquisition system
Sensors: laser displacement sensor, strain gage
Heating system: heat insulation device, heating device.
Displacement Sensor Selection
The measurement of the displacement of a blade tip must be taken with a contactless sensor so that its characteristics do not change. A laser displacement sensor is the best option for this application. According to how far the laser sensor is placed and the maximum displacement to be measured, the following specifications must be considered.
Measuring range: the maximum peak-peak displacement that can be measured. It should be greater than the movement of UUT but not too much greater to maintain a proper resolution of the measurement.
Start of measuring range: the minimum distance from the sensor to the target measured.
Midrange: the midpoint of the measuring range. It is the optimal distance between the sensor and the UUT.
End of measuring range: the maximum distance from the sensor to the target measured.
Measuring rate: should be larger than double the frequency of the movement to allow the software to detect the correct frequency.
Analog output: a must-have feature allows users to send a displacement signal to the Spider.
The sensitivity of a laser displacement sensor is determined by its measurement range and its analog output range. Crystal Instruments tested a micro-epsilon optoNCDT-1900 series sensor.
Fixture Selection
A blade fixture should be designed according to its blade size and the insert pattern of the shaker.
Option 1: two screws tightening
Option 2: option 1 with an enclosure and one screw added
Option 3: option 1 with one screw added on top
Option 4: three screws tightening
Find Minimum Fixture Torque
Test is performed in the ambient temperature.
Install the blade under test and fixture on the shaker.
Set the accelerometer on the shaker table as the control.
Laser displacement sensor points to the blade tip.
Compute the transfer function between the accelerometer and displacement sensor (look for resonance frequency).
Each blade under test is subject to different torque on the fixture.
Perform multiple sine sweeps (at low level, 5~10 g) to find the resonance frequency. According to the specification of the blade, select a control level to avoid damaging the blade under test.
For each blade, run multiple sine sweeps at the same level with different torque.
Record the resonance frequency and torque (reading from the torque wrench).
Test 3~5 blades
Plot Resonance frequency (Hz) vs. torque (N*m or in*lb)
The plot shows the resonance frequency becomes constant. The minimum torque corresponding to the constant resonance frequency is the minimum fixture torque.
Users should find the minimum fixture torque before performing a turbine blade fatigue test.
Crystal Instruments offers complete systems for turbine blade fatigue testing, including modular vibration controllers, upgraded software, wireless and high channel data acquisition, and strain measurement. Users can monitor tests remotely on EDM Cloud.