Nonlinear Ultrasonic Wave Detection by Electromagnetic Acoustic Transducer [EMAT] for Guided Wave

摘要

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 or ultrasonic velocity changes as it penetrates the crack. However, the nonlinearity of a solid material is very small compared with that of a liquid medium, and 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 studied the conditions under which the EMAT could alternately drive a Lamb wave and a shear horizontal (SH)-plate wave to detect nonlinearity in an ultrasonic wave. Although we observed an increase in the higher frequency components when a Lamb wave was perpendicularly injected into a V-shaped slit, this was not observed when using the SH-plate wave. Conversely, when a Lamb wave and an SH-plate wave were injected in parallel into the V-shaped slit, the received signal characteristics of the SH-plate wave were not affected by the Lamb wave; the received signal characteristics of the SH-plate wave were monitored for any change. This is indicates that the new nonlinear ultrasonic wave detection method can also provide information on the direction of a crack.