Harmonic Frequency Components Detection by a Guide Wave type Electromagnetic Acoustic Transducers [EMATs]

Abstract

When an ultrasonic wave is injected into a crack, if the width of the crack opening is of almost the same order as the displacement of the ultrasonic wave, the crack may be closed or opened. As a result, the waveform of the received ultrasonic wave is different from that of the incident ultrasonic wave. Therefore, regarding the incident ultrasonic wave, its harmonic frequency components change as it penetrates the crack. However, the nonlinearity of a solid material is very low compared to that of a liquid medium, therefore, a liquid medium with a high nonlinear efficiency had to be used as a coupling medium. We then attempted to apply an electromagnetic acoustic transducer (EMAT), which does not require a coupling medium. In addition, we tried to develop an EMAT that could alternately drive a Lamb wave (S₀-mode, A₀-mode) and a Shear Horizontal (SH₀) -plate wave to detect any nonlinearity in an ultrasonic wave. We actually tested the performance using fabricated fatigue specimens. As a result, we observed that the harmonic components increased when we used the specimen with a specific loading condition and a specific ultrasonic mode. This indicated that the harmonic component detection using the trial EMAT could also provide useful information on the damage to any structures or any materials