Uncovering the hidden structure: A study on the feasibility of induction thermography for fiber orientation analysis in CFRP composites using 2D-FFT

The induction heating process elicits a heating response in a carbon fiber reinforced polymer (CFRP) composite distinctly different from those in metal. Prior work in our lab and elsewhere has established that the intensity and spatial distribution of the heating patterns are governed by the fiber orientations in each layer and the degree of electrical contact between layers. Based on more extensive work on this non-conventional heating behavior, we show that the analysis of induction heating patterns enables the characterization of the fiber orientations and stacking order within the material. In the first step, 2D Fast Fourier Transform (2D-FFT) is used to extract the fiber orientations from the spatial characteristics of the heating pattern recorded with an infrared camera. In the next step, the extracted fiber orientation is used to design bandpass filters to carry out the inverse 2D-FFT and obtain the layer stacking order. Our findings demonstrate that this approach can accurately identify the layer orientations (maximum error of 6o) and the stacking sequence in quasi-isotropic CFRP laminates with up to 12 layers. We believe that the proposed approach has the potential as a valuable nondestructive, non-contact tool for large-area inspection and quality control in manufacturing fiber-reinforced composites.