  Maintaining the correct fibre orientations and stacking sequence in
carbon-fibre reinforced polymers (CFRP) during manufacture is essential for
achieving the required mechanical properties of a component. This paper
presents and evaluates a method for the rapid characterisation of the fibre
orientations present in CFRP structures, and the differentiation of different
stacking sequences, through the Radon-transform analysis of complex-valued
eddy-current testing (ECT) inspection data. A high-frequency (20 MHz)
eddy-current inspection system was used to obtain 2D scans of a range of CFRP
samples of differing ply stacking sequences. The complex electrical impedance
scan data was analysed using Radon-transform techniques to quickly and simply
determine the dominant fibre orientations present in the structure. This method
is compared to 2D-fast Fourier transform (2D-FFT) analysis of the same data and
shown to give superior quantitative results with comparatively fewer
computational steps and corrections. Further analysis is presented
demonstrating and examining a method for preserving the complex information
inherent within the eddy-current scan data during Radon-transform analysis.
This investigation shows that the real and imaginary components of the ECT data
encode information about the sacking sequence allowing the distinction between
composites with different stacking structures. This new analysis technique
could be used for in-process analysis of CFRP structures as a more accurate
characterisation method, reducing the chance of costly manufacturing errors.
