Application of ECT technology with a miniature PCB-based spiral coil in the visualization of CFRP laminates

An eddy-current testing (ECT) method with miniature-printed circuit board (PCB)-based planar spiral coils for carbon fibre-reinforced plastics (CFRP) is proposed in this paper. First, the electromagnetic model of the rectangular spiral coil is discussed, thus the analytical expressions of the impedance in free-space, and impedance change due to the eddy-current are provided. Next, the intensity areas of impedance change of a certain PCB-based planar spiral coil for metal and CFRP are theoretically given with different frequencies. Then, an ECT system for CFRP is established, and a resonant probe with a PCB-coil is designed by coupling to the parasitic capacitance of the cables. Finally, a series of ECT experiments for unidirectional, four-directional angle-ply, and orthogonal woven CFRP laminates are conducted. The results show that the minimum defect depth that can be detected on the unidirectional CFRP laminate reaches 0.1 mm. This study is an effective attempt for CFRP ECT based on planar spiral coils fabricated by PCB technology and lays the foundation for further ECT large-scale array scanners manufactured by PCB technology.