Development of Electromagnetic Acoustic Transducers for Degradation Evaluation of FRP

针对玻璃纤维增强塑料（GFRP）材料，研制了电磁超声换能器（EMAT）.分层是FRP层合板的主要缺陷之一，它降低了FRP层合板的抗压强度。因此，为了保证结构的安全性，对结构进行无损评价是十分必要的.对于金属，电磁超声已经应用于无损检测。电磁超声换能器由线圈和磁铁组成，利用洛伦兹力或磁致伸缩效应，在换能器和金属之间不需要任何耦合剂的情况下产生和检测超声波。然而，由于GFRP是不导电的，因此不能应用于金属的EMAT。在本研究中，我们提出了在预浸料层中嵌入线圈，并在线圈上加磁铁的方法。首先，我们使用传统的压电换能器，确保EMAT能够发射和接收体剪切波。我们还发现，由于感应电流的存在，在一发一收模式下用EMAT测量体波是不可能的。然后，我们切换到测量引导SH波速度使用两个线圈对齐在一条线上。通过取反向磁场获得的两个信号的差，从激励信号的泄漏中提取引导SH波信号。利用方板试件验证了兰姆波速度测量的准确性，并提出了利用兰姆波速度检测玻璃钢层合板分层的方法。使用弯曲线线圈，其节距被调整为最低对称模式兰姆波的波长。发现发射线圈应埋入中间夹层中，以获得良好的探测能力。我们发现，分层的位置可以进行评估，如果路径长度和渡越时间之间的斜率的变化被准确地检测到。

Electromagnetic acoustic transducer (EMAT) was developed for glass fiber reinforced plastics (GFRP). Delamination is one of the defects in FRP laminates, which lowers the compressive strength. Thus the nondestructive evaluation is necessary in order to assure the structural safety. For metals, the EMAT is already applied in the nondestructive inspection. The EMAT consists of a coil and magnets, and it can generate and detect the ultrasonic wave without any couplant between transducer and the metal using the Lorentz force or magnetostriction. However, since the GFRP is nonconductive, the EMAT for metals cannot be applied. In this study, we suggested to embed a coil in the prepreg stacking and to compose the EMAT with a magnet on the coil.First, we ensured that the EMAT can transmit and receive the bulk shear wave, using the conventional piezoelectric transducer. We also found that the measurement of bulk wave by EMATs in pitch-catch mode is impossible because of the induction current. Then we switched to measure the guided SH wave velocity using two coils aligned in a line. The guided SH wave signal was extracted from leakage of the excitation signal by taking difference of two signals obtained for reversed magnetic fields. Accuracy of the velocity measurement was examined using the square plate specimen.We also proposed the detection of delamination in GFRP laminates using the velocity of Lamb wave. Meander line coil was used, pitch of which is adjusted to the wavelength of the lowest symmetric mode Lamb wave. It was found that the transmitter coil should be embedded in the medial interlayer to obtain the good detectability. We revealed that the position of the delamination can be evaluated if the change in the slope between path length and transit time is detected accurately.