Experimental Considerations for Determining the Damping Coefficients of Hard Coatings

This paper attempts to characterize the various methods that may be used in defining the vibration features of a plate dimensionally proportioned to represent a turbine compressor blade. The comparison is specifically devoted towards certain damping properties of a titanium plate coated with magnesium aluminate spinel (mag spinel). Two different coating thicknesses were tested: $0.13\mathrm{mm}\left(.005\mathrm{in.}\right)$ per side and $0.25\mathrm{mm}\left(.010\mathrm{in.}\right)$ per side [total thicknesses of $0.25\mathrm{mm}\left(.010\mathrm{in.}\right)$ and $0.51\mathrm{mm}\left(.020\mathrm{in.}\right)$ ]. In order for the reader to appreciate the experimental variations possible in this undertaking, the writers define various tests performed. Dynamic ping tests were conducted on all specimens to determine their resonance frequencies. Laser vibrometry was used to determine the mode at each resonance frequency. Damping ratios were determined through the use of sine sweeps. It is important that damping characteristics be measured very precisely. Thus, in the approach using a sine sweep, one must make sure that the rate of sweep is sufficiently slow to evaluate the proper frequency response function. A discussion of the proper rate is provided. This project demonstrated that, in order to represent the basic damping properties of a coating, one must consider the rate of the dynamic load application. The usual method associated with the damping coefficient evaluation is referred to as the half-power method. The true value of the damping coefficient is directly associated with the rate of experimental power versus frequency.