Nondestructive Evaluation of Delamination in CFRP Laminates Using Electromagnetic Acoustic Transducers

将电磁声换能器(EMAT)应用于碳纤维复合材料层合板分层损伤的无损检测。由于Lamb波的速度依赖于频率和板的厚度，分层导致的厚度减小导致了速度的变化。在本研究中，利用电磁法测量层合板中兰姆波的群速度，利用层合板表面附近感应出的涡流和静磁场，由线圈和永磁体组成的电磁传动仪发射兰姆波。为了能够产生涡流，我们使用了平纹编织的碳素预浸料，这些预浸料堆叠在一起，在高压灭菌器中固化。为了增强磁场强度，在发射机相对位置的钢板下方放置钢块。尽管有了这些改进，兰姆波信号仍然被激励信号的泄漏所遮挡。将两个相反磁场作用下得到的信号进行差值处理，提取出兰姆波信号。研究了用电磁铁代替永磁体的可行性。在分层的每个阶段，接收并采样了不同路径长度的兰姆波信号。计算了每个频率分量在时间域中的幅度分布。振幅峰值时间与路径长度成正比，路径长度的斜率与群速度成反比。比较了声区和分层区域的群速度，发现分层区域的群速度如理论预测的那样增大。然而，声段中的速度与引入分层之前的测量结果不同。结果表明，在声速已知的情况下，分层部分可以被检测为声速升高的区域。

Electromagnetic acoustic transducers (EMATs) were applied to nondestructive evaluation of delamination in CFRP laminates. Since Lamb wave velocity depends on both frequency and plate thickness, the thickness reduction due to delamination results in the velocity change. In this study, the group velocity of Lamb wave in CFRP laminates was measured by the EMATs while introducing delamination stepwise.The EMAT, which consists of a coil and a permanent magnet, launches the Lamb wave using the eddy current, induced in the vicinity of the plate surface, and the static magnetic field. To enable the eddy current to be generated, we used plain-woven carbon prepreg sheets, which were stacked and were cured in an autoclave. To enhance the magnetic field strength, steel block was put below the plate at the opposed position of the transmitter. In spite of these improvements, Lamb wave signal was still obscured by the leakage of the excitation signal. By taking difference of signals obtained applying two opposite magnetic fields for receiver, Lamb wave signal was extracted. The availability of electromagnet in place of permanent magnet was also examined.Lamb wave signals were received and sampled for various path lengths at each stage of delamination. Distribution of amplitude, in the time domain was calculated for each frequency component. Time of amplitude peak was plotted against path length, the slope of which gives the inverse of group velocity. Comparing the group velocities in the sound and delaminated sections, we found that the group velocity increased in the delaminated area, as predicted theoretically. However, the velocity in the sound section differed from that measured before introduction of delamination. It was revealed that the delaminated part may be detected as the area of elevated velocity when the velocity in the sound part is known.