Rotor health monitoring and online damage detection are increasingly gaining the interest of aircraft engine manufacturers. This is due primarily to the need for improved safety during operation and for lower maintenance costs. Applied techniques for damage detection and health monitoring of rotors are essential for engine safety, reliability, and life prediction. NASA’s Aviation Safety Program provides research and technology products to help the aerospace industry improve aviation safety. The Nondestructive Evaluation (NDE) Group of the NASA Glenn Research Center’s Optical Instrumentation and NDE Branch is developing propulsion-system-specific technologies for detecting damage prior to catastrophe.
Currently, the NDE group is assessing the feasibility of utilizing real-time vibration data for detecting cracks in turbine disks. The data are obtained from radial blade tip clearances and shaft clearances measured by a variety of sensors. The spectrum of sensors ranges from capacitive, to eddy current, to microwave-based technologies, all of which are designed for the extreme environments encountered within turbine engines. Damage can be detected in a rotating disk because the development of a disk crack distorts the strain field within the component. This causes a small deformation in the disk’s geometry as well as a possible change in the system’s center of mass. The geometric change and the center of mass shift are determined by monitoring the amplitude and phase of the first harmonic (i.e., the 1× component) of the vibration data. Spin pit experiments and full-scale engine tests have been conducted while this vibration-based crack-detection methodology was being used to monitor for crack growth. Even so, published data are extremely limited, and the basic foundation of the methodology has not been studied fully. To address these limitations, the NDE group developed a theoretical model that can assess the feasibility of this disk crack-detection approach.
The vibration response obtained from the newly developed model provides a quantitative view of the method’s sensitivity. The model was characterized using numerical and experimental data that emulated the Glenn spin rig that will be utilized for verification tests. For this setup, it appears that monitoring the crack-induced shift in the center of mass is a feasible approach for identifying damage in a disk during operation. For the notched disk shown in the photograph, the model predicted an amplitude deviation on the order of 0.0005 in. at 15,000 rpm. The plots show the predicted response for multiple notch orientations. Further laboratory tests will be conducted to verify the data. The next steps involve modifying the vibration-based damage-detection methodologies to address more complex systems (e.g., multiple disks).
Image of notched disk emulated by the model.
Predictions of the amplitude and phase of the 1× vibration component for a disk located midspan on a flexible shaft. Results show various notch orientations in relation to the initial eccentricity.
Lastly, localized approaches, where wireless sensors “ride on the spinning disk,” are also being developed and studied as a complement to the global, vibration-based methodologies. An example of the local approach includes using piezoelectric patches (actuators/sensors) attached to the disk for conducting in situ ultrasonic and impedance-based measurements.
Ohio Aerospace Institute (OAI) contact: Dr. Andrew L. Gyekenyesi, 216-433-8155, Andrew.L.Gyekenyesi@nasa.govLast updated: October 16, 2007
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