Methods of Determining a Damping Ratio From a Simple Impact Test

Abstract

Numerous tests have been performed in the nuclear industry to determine damping ratios for the analysis of steam generator tube bundles. Several methods have been used to analyze the response data, including the logarithmic decrement method, the half-power point method, and curve-fitting methods. Theoretically, the methods give the same damping results; however, in practice, the results may be appreciably different. Each of these methods presents certain difficulties in determining a damping ratio. For the half-power point method, the difficulties include selecting a sampling rate, setting a time span, and the use of a window to truncate the data. The difficulties associated with the logarithmic decrement method include selecting the initial and final peaks and selecting an appropriate frequency. In both cases, most of the difficulties arise from the need for subjective decisions by the engineer or technician. Blind curve-fitting methods have been shown to underestimate the actual damping. A practical comparison of these three methods was performed on damping data from a simple impact test. While the half-power point method and curve-fitting methods are useful tools to validate damping results, a logarithmic decrement method that averages the damping over a number of cycles appears to provide the most consistent and realistic damping results for SG tubes.