In the previous studies, we developed the point-focusing electromagnetic acoustic transducer (PF-EMAT) for inspecting stainless steel piping, and found that the slit of 0.05 mm depth and artificially introduced stress corrosion cracking of 0.5 mm depth, being close to the weld, were well detectable at the frequency of 2.0 MHz. We also observed that the echo amplitude of SV wave from slits shows oscillation with nearly equal spacing as the slit depth increases. In this paper, we calculate the two-dimensional acoustic field using the finite element method (FEM) to understand this significant behavior. The FEM calculation reproduces the oscillatory amplitude change for the same condition as the experiment. We also calculated resonant frequencies of the two-dimensional models with slit defects and found that there are particular resonant frequencies in the band centered at the excitation frequency. At these frequencies, vibrational energy is highly concentrated to the region around the slits and a distinct node and antinode pattern occurs in the slit faces. We then conclude that the acoustic energy of the incident SV wave is focused at the slit mouth and a part of the energy is spent to excite this local vibration, making the echo amplitude decrease.

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This paper describes the non-destructive inspection method to detect a void inside the PC grout by using an impulse hammer and accelerometers. We have already proposed the non-destructive inspection method to detect a crack inside a large concrete structure such as a caisson by using an impulse hammer. The proposed method can detect a crack deep inside a large concrete structure because of using low-frequency vibration signals, which are generated by the impulse hammer. Moreover, this method can also use the impulse hammer’s output signal as well as the accelerometers output signal. Therefore, the proposed method can estimate the position and size of a crack by the beam forming method. In this paper, we describe the application of the proposed non-destructive inspection method using an impulse hammer to detect a void inside the PC grout, and we evaluated the proposed application method. To evaluate the performance of the proposed method, we experimented using specimens of a PC-bridge. Moreover, we analyzed the propagation of the impact wave in specimens by FDTD method. From the results of the experiment and analysis of impact elastic wave propagation, we realized that the proposed method could measure the influence of a void inside the PC grout.

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