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This paper describes a robust testing method for the experimental determination of the vibration damping loss factor of composites. This procedure is being proposed as a result of a detailed experimental program that was undertaken to characterize the damping loss factor of glass and graphite-epoxy composite materials. During the experimental testing to determine the damping loss factor of the composites, it became evident that the amplitude of vibration of the test beam had a pronounced effect on the calculated loss factor determined using the half-power bandwidth method. Calculated loss factors were significantly reduced if the tip displacement amplitude versus time were lower than 0.001 in. (0.0025 cm) for more than 25% of the data set. To alleviate this problem, a robust testing method was developed. In this procedure, a set of 2048 displacement versus time data points are recorded. This displacement versus time data is then partitioned into 512 data point intervals. A Fourier transform is performed on each of these data sets and loss factor determined using the half-power bandwidth method. The loss factor as a function of the maximum tip displacement within each data set is plotted. A linear least squares fit is performed on the data, with an extrapolation being made to zero displacement. The extrapolation to zero displacement effectively reduces the extraneous losses, providing a more robust testing protocol for material characterization. This testing method should provide increased accuracy and precision for the determination of the damping loss factor of materials.
Associate director, Center for Composite Materials, and research associate professor of mechanical engineering and Material Science Program, University of Delaware, Newark, DE
David Taylor Research Center, Ship Materials Engineering Department, Annapolis, MD
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