In this paper,modal frequency estimation by using autocorrelation functions in both the time and frequency domains for structural\r\ndiagnostics is discussed.With popular structural health monitoring methods for periodic inspections such as with the ââ?¬Å?hammering\r\ntest,ââ?¬Â hearing is very useful for distinguishing differences between structural conditions. Hearing detects pitch and tone, and it is\r\nknown that the auditory process is related to wave periodicity calculated fromautocorrelation functions. Consequently, on the basis\r\nof the hammering test, modal frequencies can be estimated by autocorrelation, the same as hearing. In this paper, modal frequencies\r\nwere estimated by using autocorrelation for constant structural health monitoring under a nonstationary noise condition. First,\r\nfundamental modal frequencies were estimated by using the autocorrelation of the time domain which was inspired by pitch\r\ndetection of hearing. Second, higher modal frequency compositions were also analyzed by using autocorrelation in the frequency\r\ndomain as with tones discrimination. From the results by conducting scale-model experiments under unknown nonstationary\r\nnoise conditions, periods of fundamental modal frequency were derived by using periods histogram of autocorrelation functions.\r\nIn addition, higher modal frequency estimation under nonstationary noises was also discussed.
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