Study on detectable size and depth of defects in noncontact acoustic inspection method

Abstract

We study a noncontact inspection method for large-scale structures such as tunnels and bridges. This method involves the use of a high-powered sound source and a scanning laser Doppler vibrometer (SLDV). In our previous study, we proposed a tone burst wave method to improve the signal-to-noise ratio (SNR) of the measured result. Using this method, a defect that was difficult to detect using our previous method was detected. In this study, we examined the detectable size and depth of the defect by using a model wall with circular defects. The distance between the sound source and the concrete test piece was 5 m, and the output sound pressure was about 100 dB near the surface of the concrete test piece. As the transmitted wave, tone burst waves with different center frequencies from 500 to 7000 Hz were used. A conventional investigation by the hammer method was also simultaneously carried out for comparison and almost identical performance was confirmed. From the experimental result, we confirmed that the bending resonance frequency detected was proportional to the depth of the circular defect, and was in inverse proportion to the plane size (area) coincident to the analytical result for a circular plate. We also found that the vibration energy of the defect shows a strong dependency on its depth. Therefore, the possibility of defect depth estimation using the resonance frequency and the vibration energy ratio is expected. In the future, a practical investigation system that will replace the hammer method might be developed.