Experimental and numerical investigation of fibre orientation characterization in CFRP laminae using a racetrack eddy-current probe

The fibre orientation within the plies of a planar carbon fibre reinforced plastic (CFRP) laminate is characterised using eddy-current testing with a racetrack coil probe. The measured quantity is the variation in the probe impedance relative to a reference orientation during a rotational sweep. The procedure is simpler and faster than the surface scanning methods commonly used for fibre orientation detection. Experimental results are presented for 12 CFRP specimens comprising between 2 and 24 plies. The fibre orientations in the uppermost three plies are accurately reconstructed using a simple signal-processing algorithm applied to the measured impedance variation. Numerical studies are carried out using an electromagnetic simulation based on the plane wave decomposition (PWD) method, and the simulated results show good agreement with the experimental measurements. The operating principle and the limitations of the reconstruction method are explained through the electromagnetic field distributions calculated using the PWD method. The influence of the low-conductivity transition layers between adjacent CFRP plies is also investigated.