Precise Measurement of Natural Frequencies and Mode Shapes of Cantilever Thin Cylindrical Shell

This research combined theory with experiment to investigate on precise measurement method of natural frequencies and mode shapes of Cantilever Thin Cylindrical Shell (CTCS). First of all, transfer matrix method and finite element method were introduced for CTCS to solve its inherent characteristics, which assist in deep understanding of vibration characteristics of shell structure. Secondly, major influences on precise measurement of modal parameters of CTCS were experimentally analyzed and the corresponding problem-solving techniques were proposed for improving test accuracy. Then, four different excitation devices were set up to measure natural frequencies of CTCS, such as hammer, electromagnetic exciter, piezoelectric ceramic exciter and vibration shaker. By comparatively studying on test results as well as advantages and disadvantages of each excitation method, it can be concluded that the combination of vibration shaker as excitation source and laser Doppler vibrometer for response test has the highest test accuracy of natural frequencies of CTCS. Next, in order to overcome the limitations in traditional modal testing method, a laser rotating canning method was adopted to accurately measure mode shapes of CTCS, and laser rotating scan system was established by clarifying test principle of this method, including laser Doppler vibrometer, Vibration shaker, DC power supply, DC geared motor, mirrors and etc. After that, test procedure as well as the key techniques, e.g., how to ensure that sufficient excitation energy was used to excite CTCS at resonance state and how to obtain mode shapes by data processing technique were described in detail. Finally, the reliability of this method had been validated by comparing the resulting mode shape with the one obtained by the other methods. The results showed that it can provide a very dense measurement of the mode shapes data of CTCS in a shorter amount of time than traditional test methods.