No abstract

Maintenance for concrete structures such as buildings, bridges, and tunnels is necessary, because it is thought that a lot of them show deterioration. As a periodic inspection, a hammering test is the most popular method. However, it has several problems. One of the problems is that it is difficult to inspect the places where people cannot reach. Therefore, non contact inspection methods have been developed. As a non-contact inspection method, we propose a system consisting of a high-power directional sound source and a scanning laser doppler vibrometer (SLDV). In this method, an air-borne sound wave is used for the excitation of a concrete wall, and then the vibration velocities on the concrete wall are measured two-dimensionally by the SLDV. From the vibration velocity, defective parts can be detected. In this paper, we describe two types of experiment on the feasibility of our proposed method. In these experiments, concrete wall test pieces, which have artificial defects, are used. From the experimental results, we confirmed the effectiveness of our proposed method as a non contact inspection method for concrete structures.

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.

This paper presents a new interferometric measurement technique which makes it possible to determine the spatial refractive index distribution inside asymmetrical transparent objects. The suggested set-up, consisting of mirror galleries for producing several projections, a holographic beam coupling system including a method for suppressing possible object self radiation, and only one camera in combination with a differential interferometer, is described in detail. Advantages and properties of the numerical tomographic reconstmtion method developed for this experimental set-up are discussed. Results of test experiments demonstrate that these concepts work in practice. and further results indicate the general usefulness of the technique.

A sound wave vibration using a scanning laser Doppler vibrometer are used as a method of exploring and imaging an extremely shallow underground. Flat speakers are used as a vibration source. We propose a method of distinguishing a buried object using a response range of a frequencies corresponding to a vibration velocities. Buried objects (plastic containers, a hollow steel can, an unglazed pot, and a stone) are distinguished using a response range of frequencies. Standardization and brightness imaging are used as methods of discrimination. As a result, it was found that the buried objects show different response ranges of frequencies. From the experimental results, we confirmed the effectiveness of our proposed method.