Non-destructive testing for carbon-fiber-reinforced plastic (CFRP) using a novel eddy current probe

Abstract Carbon-fiber-reinforced plastic (CFRP) is of low conductivity and has a layered structure. High-frequency transmitter-receiver (T-R) probes are widely chosen to inspect CFRPs using eddy current testing (ECT). However, in these works, the variation in the distance between the probe and test sample can cause a larger signal than that caused by defects and may cover up the defect. The detection sensitivity was also reduced by random noise resulting from lift-off change. To address these issues, it is meaningful to design a probe which can overcome the effect of lift-off variation and meanwhile offer high sensitivity to defects in CFRPs. In this study, a T-R probe with a special structure for detection of CFRPs was developed. The probe contains an 8-shaped transmitter coil (TX coil) and a circular receiver coil (RX coil), which is placed on a line equidistant from the two parts of the transmitter coil. Theoretically, regardless of how the lift-off changes, the output signal is always 0 if the azimuth of the probe agrees with one of the fiber orientations of an intact CFRP. Experimental studies demonstrate that the proposed probe is insensitive to lift-off compared with a traditional T-R probe and offers high sensitivity to defects. For defect detection, in-plane waviness can be detected with the proposed probe. Quantitative experiments for crack detection were performed. The cracks were clearly visualized in the scanning images. The length and location of the cracks can also be estimated from the scanning images.