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

对于样品厚度大于波长的情况，缺陷位于间隔200 mm的发射器和接收器之间。为了便于超声波在整个检测区域上均匀地传播，通过改变发射频率来改变超声波的传播角度。接着，将来自缺陷的波形与用于无缺陷的情况的参考波形数据进行比较。结果表明，对于表面缺陷，微分信号在500 KHz和700 KHz处有最大振幅，而对于底面缺陷，微分信号在560 KHz处有最大振幅。结果表明，产生最大单振幅的发射频率与超声波的传播方向一致;当样品厚度与波长接近时，当电磁超声换能器（EMAT）扫过缺陷上方时，当收发距离保持不变时，超声波的振幅随缺陷位置的变化而变化。结果表明，对于厚度减小3 mm和5 mm的情况，由拟合线的斜率估算的厚度减小值的最大误差为10%。对于所有类型的电磁线圈，电流驱动电磁超声换能器都具有高振幅的性能。特别是，在用于电流驱动EMAT的宽度为1.0mm的两个堆叠的磁性线圈的情况下，所获得的振幅是磁体型EMAT的振幅的1.8倍。这表明，在高频区域中，信噪比将得到显著改善。

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.