Eddy Current Evaluation of Two-Layer Aircraft Structures with Carbon Fiber Reinforced Polymer Layers on Aluminum Alloy Substrates

Carbon fiber reinforced polymers (CFRPs) combine high load-carrying capacity with low specific weight, making them increasingly important for lightweight design in modern aerospace structures. Their use is expanding not only in primary components but also in repair patches for reinforcing aluminum alloy (AA) elements. However, suitable nondestructive evaluation (NDE) techniques for CFRP–AA assemblies remain limited. CFRPs are heterogeneous materials composed of carbon fibers and a polymer matrix, characterized by pronounced interfaces and anisotropic electrical conductivity due to fiber orientation. For evaluation purposes, CFRPs can be approximated as a homogeneous medium with an effective specific conductivity determined by composition and design. This study investigates the feasibility of measuring CFRP layer thickness on AA substrates using eddy current (EC) techniques. Conventional EC instruments designed for dielectric coatings are unsuitable due to the relatively high conductivity of CFRP. To address this, single-coil ferrite-core EC probes (8 mm diameter) were developed and operated at low frequencies in resonant mode. Experiments on specimens provided by the State Enterprise “ANTONOV” demonstrated that CFRP thicknesses up to 12 mm can be measured reliably. The results will contribute to the development of a dedicated EC instrument for CFRP thickness measurement. Moreover, the proposed technique is sensitive to delamination between CFRP and AA substrates or between CFRP layers, enabling potential application in in-service inspection and structural health monitoring. Such use requires initial baseline measurements at reference points for subsequent comparison during the service life of the structure.