Operational Modal Analysis of a Rectangular Plate Using Noncontact Acoustic Excitation

Abstract

Output-only modal analysis has been extensively developed in the past decades and widely used especially when the input is unknown or difficult to measure. This paper presents a noncontact experimental technique for measuring the modal parameters of a rectangular aluminum plate using only the output data, with the intention to apply the technique to turbine blades. Operational modal analysis (OMA) is employed to analyze an aluminum plate with free boundaries in the frequency range from 0 to 15000 Hz, which can be an operation frequency range of a turbine blade, under acoustic excitation in the form of white noise in a direction of interest. A single-point laser vibrometer and a free-field microphone are used to simultaneously measure the responses of the plate in a noncontact manner, with the microphone measurement serving as the reference. A cross-power spectral density (CPSD) acquired using the microphone measurement as the reference is shown to be equivalent to that using a laser vibrometer measurement as the reference as long as the natural frequencies of the test structure that are of interest are sufficiently high but lower than a certain value. A measurement method for in-plane vibration modes is also proposed and applied by shining the laser from the vibrometer with a certain incident angle. The natural frequencies and mode shapes of the out-of-plane and in-plane modes of the plate are measured. The experimental natural frequencies and mode shapes of the plate are compared with those calculated using commercial finite element software to demonstrate the validity of the experimental technique. Experimental modal analysis (EMA) is also performed on the plate using an impact hammer and the laser vibrometer to validate the experimental results from OMA. The maximum error between the measured and calculated natural frequencies of the plate is 1.53% for the first 18 elastic modes, including 16 outof-plane plane modes and two in-plane modes. The Modal Assurance Criterion (MAC) values between the corresponding mode shapes of the plate are all above 93%.