Off-axis estimation of lay-up orientation in CFRP laminated composites using eddy current testing

This paper presents a traditional eddy current testing (ET) method to estimate the off-axis of lay-up in the cross-ply and multidirectional carbon fiber reinforced polymer (CFRP) laminates. The transmitter receiver (TR) probe equipped with two identical circular coils was utilized to scan surface of the CFRPs with artificially induced off-axis layers in different depths. A 2D fast Fourier transform (2D-FFT) was applied to the C-scan experimental images for determining the layer orientation. It was observed that several highlighted lines are located at different directions in the Fourier transform images, which correspond to the higher energy and indicate a periodicity in the corresponding direction. From these images, the layer orientation with a minimum -5 degrees off-axis was measured, and the actual fibers direction 40 degrees can be obtained by subtracted 90 degrees from the frequency angle in the Fourier transform image. Additionally, mechanism of the output signal change caused by the off-axis lamina was analyzed using the finite element (FE) analysis. Numerical results showed that deformation of the space magnetic field occurs because of the redistribution eddy current induced in the off-axis lamina, which implies the possibility to measure and estimate the deviated fibers orientation of the CFRPs using the traditional ET method.