Evaluation of fatigue specimens using EMATs for nonlinear ultrasonic wave detection

It is suggested that the waveform of the received ultrasonic wave is slightly different from that of the incident ultrasonic wave when the displacement of an ultrasonic wave and the opening width of a crack are almost same. Thus, as regarding the incident ultrasonic wave, its harmonic frequency components change as it penetrates the crack. Since, the nonlinearity of a solid material is very small compared to that of a liquid medium with a high nonlinear efficiency when used as a coupling medium, we applied an electromagnetic acoustic transducer (EMAT), which does not require a coupling medium. In addition, we tried to develop the EMAT that could alternately drive S-0-Lamb wave and fundamental Shear Horizontal (SH0)-plate wave to detect any nonlinearity in an ultrasonic wave. We actually tested the performance using the fatigue specimens we fabricated. As a result, we observed that the harmonic components changed when we used the specimen with a specific loading condition and a specific ultrasonic mode. This is indicated that the harmonic components detection using the trial EMAT could also provide useful information on the degree of damage to any structures or any materials.