High-speed inspection of delamination defects in unidirectional CFRP by non-contact eddy current testing

The anisotropic bulk conductivity of carbon fibre reinforced polymers is a challenge for eddy current testing (ECT). The conductivity is markedly higher in longitudinal fibre direction compared to the transverse directions. This means that ECT has mainly been used for detecting imperfections that affect the direction and magnitude of the longitudinal conductivity. That is, fibre misalignment and broken fibres respectively. Damage in the matrix, such as delamination, affects the already low transverse bulk conductivity and is therefore very difficult to detect using ECT. A customized eddy current probe developed for detecting delamination damage in carbon fibre reinforced polymer (CFRP) composite at 4 m/s is presented in this paper. The design of the probe is based on numerical simulation of eddy current flow in the anisotropic CFRP composite. It was found that the optimal coil configuration has a current flow parallel to the conducting carbon fibres, but in two in-line coils with opposite current flow direction operating at 1 MHz frequency. This forces the eddy currents between the probes to flow perpendicular to the fibres, making the probe sensitive to changes in transverse conductivity caused by delamination. The simulated probe configuration was validated experimentally, and the constructed inspection device was able to detect real delamination damage in a CFRP component with demand for high-quality inspection.