SLSR-Based Microwave NDT Array Sensor for Surface Cracks Inspection on Coated CFRP Structures

A stub-loaded spiral resonator (SLSR) array sensor is presented for the detection of surface cracks on coated or painted carbon fiber-reinforced polymer (CFRP) structures based on nondestructive testing (NDT). Any alteration in the applied electric field toward the cracks is considered a perturbation in permittivity. This results in the sensor functioning as a bandpass filter in the presence of cracks and a bandstop filter when cracks are absent within the sensing band for the sample under test (SUT). The unit sensor based on SLSR utilizes a fringing field sensing capacitor (FFSC) serving as a “sensing pad,” and it direct interacts with SUT for surface crack detection. The minimum crack size detection capability of sensor can be adjusted by using sensing pad parameters. The sensor operates within the frequency range of 1–2 GHz and has the capability to detect cracks from minimum dimensions as small as 0.5 mm in terms of width, length, or height having any orientation. In this work, two different array sensor configurations are investigated: uniform array for multiple cracks and nonuniform array for multiple cracks with location. The coated CFRP sheet with cracks embedded in the surface is used to demonstrate the detection of cracks with location. The sensor’s effectiveness is confirmed through a comparison of simulated and experimental results. These proposed sensors have potential applications in the detection of surface cracks in high-value infrastructure, such as aircraft, bridges, pipelines, nuclear power plants, and wind turbines, contributing to their safety and integrity.