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，缺陷位于发射器和接收器之间，间隔200 mm。为了使超声波在整个检测区域内传播均匀，通过改变超声波的传播频率来改变超声波的传播角度。接下来，将来自缺陷的波形与无缺陷情况下的参考波形数据进行比较。结果表明，在前表面有缺陷的情况下，微分信号在500 KHz和700 KHz处有最大幅度，在下表面有缺陷的情况下在560 KHz处有最大幅度。研究发现，产生最大单幅值的传输频率与超声波的传播方向相同，在样品厚度与波长基本相同的情况下，当电磁声换能器(EMAT)扫过缺陷时，当发射器与接收器之间的距离保持不变时，波的幅度随缺陷位置的变化而变化。结果表明，对于3 mm和5 mm的厚度减薄，根据拟合线的斜率得到的减厚值的最大误差在10%以内。为了获得较高的传输幅度，提出了一种新的磁线圈EMAT的设计方案。对于所有类型的磁线圈，电流驱动的电磁兼容具有很高的幅值性能.特别是，对于电流驱动的EMAT，在宽度为1.0 mm的两个堆叠的磁线圈的情况下，获得的幅度是磁性型EMAT的1.8倍。这表明，在高频区，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.