Development of Electromagnetic Acoustic Transducers for Degradation Evaluation of FRP

研制了用于玻璃钢(GFRP)的电磁声换能器。分层是玻璃钢层合板的缺陷之一，它降低了层合板的抗压强度。因此，为了保证结构的安全性，有必要对其进行无损评估。对于金属，EMAT已经应用于无损检测。电磁超声换能器由线圈和磁体组成，利用洛伦兹力或磁致伸缩效应，可以在换能器和金属之间不存在耦合作用的情况下产生和检测超声波。然而，由于玻璃钢是不导电的，所以不能用于金属的EMAT。在这项研究中，我们建议在预浸料堆叠中嵌入一个线圈，并在线圈上放置一个磁铁来组成EMAT。首先，我们使用传统的压电式换能器来确保EMAT能够发射和接收体横波。我们还发现，由于感应电流的存在，EMATs在俯仰捕捉模式下不可能测量体波。然后我们改用两个线圈排成一条线来测量SH波的导波速度。通过取反向磁场下的两个信号的差值，从励磁信号的泄漏中提取出引导SH波信号。用方板试件检验了速度测量的准确性，提出了利用兰姆波速度检测玻璃钢层合板脱层的方法。采用曲线型线圈，其节距调整为最低对称模兰姆波的波长。研究发现，为了获得良好的探测性能，发射机线圈应嵌入中间层。我们发现，如果准确地检测到路径长度与通过时间之间的斜率的变化，就可以评估分层的位置。

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.