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，将缺陷定位在发射器和接收器之间，相距200mm。为了使超声波在整个检测区域内均匀传播，通过改变传播频率来改变超声波的传播角度。接下来，将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：前表面缺陷的差值信号在500KHz和700KHz处最大，下表面缺陷的差值信号在560KHz处最大。我们发现产生最大单幅值的传输频率等于超声波的传播方向。在样品厚度与波长接近的情况下，当电磁声换能器（EMATs）在保持发射端和接收端距离不变的情况下扫过缺陷时，其振幅随缺陷位置的变化而变化。结果表明，在厚度减小3mm和5mm的情况下，由拟合线梯度计算的厚度减小值在最大误差10%以内。为了获得较高的传输幅值，提出了一种采用磁线圈的新型电磁调制器设计方案。对于所有类型的磁线圈，电流驱动EMAT具有高幅值性能。特别是当电流驱动EMAT采用两个宽度为1.0 mm的堆叠磁线圈时，得到的幅值是磁体型EMAT的1.8倍。这表明在高频区域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.