点焦点型電磁超音波センサによる探傷とスリット近傍の局所振動

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.