| Summary: | 碩士 === 朝陽科技大學 === 營建工程系碩士班 === 92 === Fourier transform is frequently used in analyzing the frequency content of a transient signal. The drawback of Fourier transform is that the time- domain characteristic cannot be shown simultaneously. Wavelet transform is one of the time-frequency dual transforms that preserves both frequency content and time information of a transient signal. The length of both time window and frequency window can be varied in wavelet transform. As a result, the time resolution is better in high frequency band while the frequency resolution is better in low frequency band.
A typical problem in the processing of impact-echo signals arises from peak detection based solely on frequency spectrum. Current study aims to improve the feature identification of impact-echo signals obtained from buried objects in concrete. Rebars, steel tubes, and PVC tubes of different diameters at various depths from the concrete surface are tested.
Numerical simulations are carried out based on models constructed using the finite element method. The received signals, both experimental and simulated, are analyzed using both fast Fourier transform and continuous wavelet transform. Multiple back-wall reflections can be easily detected from the wavelet coefficient plots (WCPs). Low-frequency signals due to rebar at a greater distance is clearer shown in the WCP than those due to rebar at a smaller distance from the side wall of concrete specimens. Such findings provide a more effective way to pick up rebar signals using the impact-echo method.
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