Materials 2023, 16, 506. https://doi.org/10.3390/ma16020506 www.mdpi.com/journal/materials Article Nondestructive Examination of Carbon Fiber-Reinforced Composites Using the Eddy Current Method Ryszard Łukaszuk 1, * and Tomasz Chady 2 1 Doctoral School, West Pomeranian University of Technology, 70-313 Szczecin, Poland 2 Faculty of Electrical Engineering, West Pomeranian University of Technology, Sikorsky 37 St., 70-313 Szczecin, Poland * Correspondence: ryszard.lukaszuk@zut.edu.pl Abstract: This paper presents the results of experiments using the eddy current system designated for nondestructive inspection of carbon fiber-reinforced composites. For this purpose, the eddy cur- rent testing system with a differential transducer with two pairs of excitation coils oriented perpen- dicularly and a central pick-up coil was utilized. The transducer measures the magnetic flux differ- ence flowing through the pick-up coil. The transducer of this design has already been successfully utilized to inspect isotropic metal structures. However, the anisotropy of the composites and their lower conductivity compared to metal components made the transducer parameters adjustment es- sential. Thus, various excitation frequencies were considered and investigated. The system was evaluated using a sample made of orthogonally woven carbon fiber-reinforced composites with two artificial flaws (the notches with a maximum relative depth of 30% and 70%, respectively, thickness of 0.4 mm, and a length of 5 mm). The main goal was to find a configuration suitable for detecting hidden flaws in such materials. Keywords: nondestructive testing (NDT); nondestructive evaluation (NDE); eddy current testing (ECT); carbon fiber-reinforced composites; differential eddy current probe; hidden flaws detection 1. Introduction Composites are manufactured by joining together two materials that differ signifi- cantly in their chemical and physical properties. The undoubted advantages of compo- sites are simple modifiability of the structure for the target use, low production cost, good corrosion resistance, and high strength–thickness ratio. Due to these facts, composites found extensive use in various branches of modern industry, such as shipbuilding (ma- chine enclosures) [1], offshore (pipelines) [2], civil engineering (reinforced foundations, sewage pipes) [3], power engineering (rotor blades in wind turbines) [4], aerospace (crew capsules, satellites) [5], and even in biomedicine (implants) [5]. Composites are the first choice in case of weight reduction, fire and thermal protection, and invariable dimensions or stiffness. Unfortunately, the strength of composites, like the steel commonly used in industry for years, is limited. Moreover, the composite structure damage may already oc- cur at the manufacturing stage and substantially affect the structure’s performance and lifetime. For example, in the power energy industry, the production of larger wind turbine blades causes an increased failure rate due to the weight reduction of the glass composite [6,7]. Carrol et al. indicate that wind turbine blade malfunctions account for 6.2% of failure cases [8]. Sometimes, a cost-intensive structural repair is necessary to resume the turbine operation. Health monitoring and frequent inspections, especially nondestructive ones, are necessary to enhance the productiveness of the turbines [9]. In the automotive indus- try, the production of hydrogen fuel cell vehicles is gaining popularity, given the need to reduce fossil fuel consumption and air pollution [10]. Hydrogen storage vessels are man- ufactured from carbon fiber-reinforced composites, which are lightweight and corrosion- Citation: Łukaszuk, R.; Chady, T. Nondestructive Examination of Carbon Fiber-Reinforced Composites Using the Eddy Current Method. Materials 2023, 16, 506. https://doi.org/10.3390/ma16020506 Academic Editor: Jianbo Wu Received: 24 November 2022 Revised: 23 December 2022 Accepted: 31 December 2022 Published: 4 January 2023 Copyright: © 2023 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).