Study on concrete internal defect detection from a moving cart equipped with LDVs and a sound source

A noncontact acoustic inspection method using acoustic irradiation induced vibration and a scanning laser Doppler vibrometer can be used for long-distance, noncontact inspection of internal defects in the surface layer of concrete structures and other structures. Since this method uses flexural vibration in principle, it can be used as an alternative method to tapping inspection. Therefore, there has been an increasing demand for the realization of moving measurement. Then, an experiment was conducted to verify the feasibility of moving measurement by mounting multiple LDVs without scanning mechanisms and a sound source on a moving cart. From the experimental results, it was clarified that there is a possibility of moving measurement at low speed of about 2km/h by using cross-correlation processing of the emitted waveform and the received waveform.

I. Introduction

Noncontact acoustic inspection (NCAI) method using acoustic irradiation induced vibration and a high-sensitivity scanning laser Doppler vibrometer (SLDV) is a method for long-distance, noncontact inspection of internal defects (cracks and cavities) in the surface layer of concrete structures, using the same flexural resonance phenomenon as that used in the tapping inspection [1]–[22]. On the other hand, with the declining birthrate and aging population, there is a need for labor-saving and automation in inspection work. Therefore, there has been an increasing demand for the possibility of realizing moving measurement using the NCAI method, even at low speeds. However, an SLDV which have a mechanism to scan the measurement area in an area, are not suitable for moving measurement because they are vulnerable to vibration and are basically designed for fixed installation. Since the laser Doppler vibrometer (LDV) is a measurement device using the Doppler effect, if the laser beam can be incident perpendicularly to the surface to be measured, it may be possible to detect surface vibration caused by sound wave irradiation even while moving. Then, a fundamental verification experiment was conducted to see if moving measurement using the NCAI method is feasible using several LDVs without a scanning mechanism.