An attempt to quantify errors in the experimental modal analysis process

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Virginia Tech


Experimental modal analysis (EMA) techniques have become a popular method of studying the dynamic characteristics of structures. A survey of literature available reveals that experimental modal models resulting from EMA may suffer from inaccuracy due to a host of reasons. Every stage of EMA could be a potential source of errors - from suspension of the test structures, transduction to parameter estimation phase. Though time-domain methods are actively being investigated by many researchers and are in use, fast Fourier transform (FFT) methods, due to their speed and ease of implementation, are the most widely used in experimental modal analysis work.

This work attempts to quantify errors that result from a typical modal test. Using a simple beam with free-free boundary conditions simulated, three different modal tests are performed. Each test differs from the other chiefly in the excitation method and FRF estimator used. Using finite element models as the reference, correlation between finite element and experimental models are performed. The ability of the EMA process to accurately estimate the modal parameters is established on the basis of level of correlation obtained for natural frequencies and mode shapes. Linear regression models are used to correlate test and analysis natural frequencies. The modal assurance criterion (MAC) is used to establish the accuracy of mode vectors from the modal tests. The errors are further quantified spatially (on a location-by-location basis) for natural frequencies and mode shapes resulting from the EMA process. Finally, conclusions are made regarding the accuracy of modal parameters obtained via FFT-based EMA techniques.