## 1 Introduction  
Carbon fiber-reinforced plastic (CFRP) is widely used in aerospace, automobile, shipping, sporting goods, and other industries due to its excellent strength to weight ratio, wear, and corrosion-resistant properties [1]. However, a defect in the manufacturing stage of CFRP composites can lead to premature failure during its service time [2]. In-plane waviness is one such defect that can significantly reduce the mechanical properties such as tensile and compressive strength and stiffness of CFRP composites [3]. In-plane waviness is difficult to observe visually during manufacturing and will be impossible to see once a surface coating is present or if the defect occurs below the surface lamina. Thus, rigorous detection and evaluation of in-plane waviness requires the use of non-destructive testing (NDT) and non-destructive evaluation (NDE) techniques. Many NDT methods for inspecting CFRPs exist, and each has unique strengths and weaknesses for certain applications and types of defects. NDT methods that have shown success at detecting fiber orientation in CFRPs include ultrasonic testing (UT) [4], eddy current testing (ECT) [5], optical microscopy, X-ray computed tomography (CT), and microwave synthetic aperture radar (SAR) polarimetry [6]. Except for microwave polarimetry, these methods typically use high-resolution imaging and post-processing based on transforms such as the Fast Fourier Transform (FFT) or the Discrete Radon Transform (DRT) to determine the dominate fiber orientation in one or more laminate. Previously, researchers used high resolution pulse/echo UT and the 2D FFT to detect the ply orientation within \(\pm 3^{\circ}\) for each lamina in a 20-ply cross-weave laminate, with the exception of the top three laminae [4]. ECT imaging has also been developed by [PERSON] et al. to determine the fiber orientations and stacking sequence of unidirectional laminates using the FFT and DRT [7]. Because of its high precision and accuracy, CT scanning has also been applied to CFRPs as a comparison for error analyses and verification of other methods. In-plane waviness refers to a defect within woven or unidirectional CFRPs where the fiber orientation has unwanted variations throughout the part. A common variation of this defect is an in-plane wrinkle, which can be characterized by the parameters width, amplitude, and maximum angle. The literature contains many successful approaches to detecting fiber orientation and ply layup order, whereas fewer methods have been developed for fiber waviness detection and quantification. The existing CFRP NDT methods struggle with scanning this type of defect for a variety of reasons. To detect in-plane waviness, a scanning method must identify changes in the fiber direction at multiple points in the scanning area. Generally, the existing imaging methods have been optimized to detect a single dominant fiber orientation over the entire scan area and thus do not have the resolution to accurately determine the changing fiber's orientation throughout an image. One counterexample is in the work of [PERSON] et al., where edge detection techniques were used to obtain angle changes at the edges of the waviness section [8]; however, this was not applied to quantify the waviness, as datapoints were only extracted at the strongest identifiable edges, not the entire part or the waviness region. Non-imaging methods have shown promise in efficiently and accurately determining varying fiber orientation in CFRP. These methods rely on the directionality of ECT probes and include detection [9; 10], estimation [11], and visualization [12] of in-plane waviness. The consistent and reliable quantification of fiber waviness, however, is still a sought-after goal for the NDT toolbox, as most of the existing methods focus on detection or parameter estimation. In this work, we apply rotational eddy current testing to measure the fiber angle throughout a unidirectional CFRP laminate. This approach differs from those found in the literature in several ways.