Reducing damage to turbomachinery blades due to high cycle resonant excitation is an important engineering challenge facing the engine design community. To minimize peak resonant stresses, designers attempt to reduce the forcing function and increase structural and aerodynamic damping. For higher frequency modes in particular, understanding of blade damping has been a challenge, due in part to a lack of detailed experimental measurements. The focus of this paper is on the description and validation of a novel experimental technique for the measurement of damping in turbomachinery blades.
The technique utilizes a continuously powered piezoelectric actuator embedded within the structure of the blade to excite a range of structural modes, while monitoring the structural response by the use of strain gage transducers. A novel data reduction technique is developed in order to 1) minimize the contamination of the strain gage output from the piezoelectric high voltage electrical field and to 2) efficiently obtain an accurate measure of the critical damping ratio and resonant frequency. The range of applicability and practical limitations of the technique is explored through a detailed uncertainty analysis.
Three different case studies were investigated to show the application of the technique including: a cantilever plate, broach block mounted turbine blades, and rotating turbine blades mounted in a rotor disk. Two different blade geometries were analyzed using full-scale engine hardware in a high speed rotating blade-disk assembly. Future papers will present more detailed results obtained from numerous spin tests that have been completed to date using the technique presented here.