Terahertz wave propagation in marine protective coatings and its non-destructive testing (NDT) capability were studied by the finite difference time domain (FDTD) method. The FDTD model was used to calculate the propagation and reflection of THz radiation from marine protective coatings. The reflected terahertz waves could be employed in coating thickness analysis of the paint layers. In order to clearly identify the interface between antifouling and anticorrosive coatings, stationary wavelet transform (SWT) approach was applied to decompose the obtained terahertz impulse functions into approximation and detail coefficients; SWT detail coefficients were used for the feature extraction of the coating thickness. SWT provides a more accurate identification of salient features in a signal, such as the weak feature between antifouling and anticorrosive coatings. We found that the developed model and SWT-based algorithms could be used to evaluate the occurrence of defects beneath the coatings (e.g., paint-off and corrosion defects). The proposed method provides the solution for coating thickness of marine protective coatings and it would benefit the effective maintenance to avoid coating failure and facilitate marine protective coating design. Therefore, non-destructive testing and evaluation of marine protective coating system by terahertz waves with SWT could be recommended for engineering applications

Terahertz time-domain spectroscopy is a contactless and nondestructive testing technique that is often used to measure the thickness of layered materials. However, the technique presents limited thickness detection resolution, especially in the thin thermally grown oxide (TGO) of thermal barrier coatings whose thickness is below 30 µm. In this study, an SWT-BP algorithm combining a stationary wavelet transform (SWT) and a backpropagation (BP) neural network was proposed, and the regression coefficient of SWT-detailed results was 0.92. The prediction results were in good agreement with the real-time results; it demonstrated that the proposed algorithm was able to achieve a thickness prediction of up to 1–29 µm of the TGO. The proposed algorithm is suitable for thin thickness detection of the TGO.

We investigated the spoof surface plasmon polaritons (SSPPs) on 1D grooved metal surface for terahertz sensing of refractive index of the filling analyte through a prism-coupling attenuated total reflection setup. From the dispersion relation analysis and the finite element method-based simulation, we revealed that the dispersion curve of SSPP got suppressed as the filling refractive index increased, which cause the coupling resonance frequency redshifting in the reflection spectrum. The simulated results for testing various refractive indexes demonstrated that the incident angle of terahertz radiation has a great effect on the performance of sensing. Smaller incident angle will result in a higher sensitive sensing with a narrower detection range. In the meanwhile, the higher order mode SSPP-based sensing has a higher sensitivity with a narrower detection range. The maximum sensitivity is 2.57 THz/RIU for the second-order mode sensing at 45° internal incident angle. The proposed SSPP-based method has great potential for high sensitive terahertz sensing

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