Nondestructive Evaluation of Delamination in CFRP Laminates Using Electromagnetic Acoustic Transducers

将电磁声换能器（EMATs）应用于CFRP复合材料分层的无损检测。由于兰姆波速度既取决于频率又取决于板厚，因此由于分层导致的厚度减少导致速度变化。在逐步引入分层的情况下，利用emat测量了CFRP层合材料中Lamb波的群速度。EMAT由线圈和永磁体组成，利用在电路板表面附近产生的涡流和静磁场发射兰姆波。为了产生涡流，我们使用了普通编织的碳预浸料片，将其堆叠并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板下放置了钢块。尽管有这些改进，兰姆波信号仍然被激励信号的泄漏所掩盖。通过对接收机施加两个相反的磁场，取信号之差，提取兰姆波信号。还考察了电磁铁代替永磁体的可行性。在分层的每个阶段接收和采样不同路径长度的Lamb波信号。计算了各频率分量在时域内的幅值分布。振幅峰值时间与路径长度成反比，其斜率与群速度成反比。对比声区和分层区群速度，发现分层区群速度增大，与理论预测一致。然而，声段的速度与引入分层之前的速度不同。结果表明，在声部速度已知的情况下，分层部分可以被检测为速度升高区域。

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.