| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 102 === This thesis aims to investigate the behavior of R-wave across the surface-opening cracks in concrete, and to detect the depth of the crack based on R-wave propagation characteristics. First, using numerical analysis method to study the characteristics of the wave propagation in concrete containing surface-opening cracks, and to understand the disturbance signals caused by the arrivals of stress waves and their arrival times. The parameters including the depth of the crack (d)、the contact time of impact (tc)、the distance between the impact and the receiver are considered to investigate the disturbance signals caused by the impact-genertaed stress waves.
The first disturbance in the waveform at the receiver behind the crack must be caused by the arrival of the P-wave diffracted from the crack tip and the arrival time can be used to determine the crack depth. However, the energy of the diffracted P-wave is very small. This study further observes how the subsequent arrivals of R-waves with different wavelengths affect the waveform and investigates the feasibility of applying the R-wave to crack detection. Numerical results ahow that there are three possible travel paths for P-wave and R-wave: (1) P-wave diffracted from the tip of the crack travels to the receiver; (2) R-wave with long wavelength travels directly through the crack to the receiver; (3) R-wave with short wavelength travels along the surface of the crack to the receiver. Thus, besides the time-of-flight diffracted P-wave method, the R- wave with relatively high energy can be used to detect the crack depth by observing the change in the waveform due to various R-wave wavelengths. In addition, the wavelet transform technique is adopted to obtain the time-frequency spectra. In the time domain waveform, it is hard to identify the arrival of R-wave with short wavelength propagating along the surface of the crack (the third travel path). However, in the time-frequency spectra, one is able to identify the arrival of R-wave following the third travel path because there is high energy distribution at the theoretical arrival time and the frequency moves to a higher one. Finally, experimental study is carried out on a concrete specimen containing surface-opening cracks to verify the numerical results.
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