Condition based maintenance for blanket cooling pipes by using electromagnetic acoustic technique

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For the case of the sample thickness longer than the wave length, defects were positioned between a transmitter and a receiver separated by a distance of 200mm. In order to facilitate the propagation of the ultrasonic waves uniformly over the whole inspection region, the propagation angle of the ultrasonic wave was varied by varying the transmission frequency. Next, the waveform from a defect was compared with reference waveform data for the case of no defects. The results showed that the differential signal had a maximum amplitude at 500KHz and 700KHz in the case of defects in the front surface and at 560KHz for defects at the bottom surface. We found that the transmission frequency yielding maximum single amplitude was equal to the propagation direction of the ultrasonic wave.For the case of the sample thickness nearly same as the wave length, the amplitude of the waves varied with the defect position when the Electromagnetic Acoustic Transducers (EMATs) swept above the defect while keeping the distance between the transmitter and the receiver constant. As a result, the value of the thickness reduction evaluated from the gradient of fitting line was obtained within maximum error of 10 %, for 3mm and 5 mm of the thickness reduction.In order to obtain high transmission amplitude, new design of achieved EMAT with a magnetism coil was proposed. For all types of the magnetism coils, the current driven EMAT the high amplitude performance. Especially, in the case of two stacked magnetism coils of 1.0 mm in width for the current driven EMAT, the obtained amplitude was 1.8 times as large as that of the magnet type EMAT. This indicates that the ratio of signal to noise would be considerably improved in the high frequency region.