IFFT-Based Microwave Non-Destructive Testing for Delamination Detection and Thickness Estimation

Abstract

Microwave Non-Destructive Testing (NDT) techniques for dielectric coatings are vital processes in many industrial applications due to their superior defect detection capabilities over traditional NDT methods. However, the limited bandwidth and post-processing complexities of these techniques cause them to fall short of accurately estimating the delamination thickness in such structures. In this paper, a novel low-complexity signal processing method is proposed to estimate the delamination thickness in metalbacked dielectric coatings using bandwidth-limited Open-Ended Rectangular Waveguide (OERW) probes. Here, Inverse Fast Fourier Transform (IFFT) process is used to convert the measured complex reflection coefficients at the surface of the coating to the time domain. Next, the amplitude of a specific time step is used to accurately estimate the delamination thickness without extra time-consuming processing. Using a 3 mm-thick macor samples with machined defects, the proposed technique is validated by correctly estimating delamination thickness down to 0.4 mm using an OERW probe with 13.5 GHz bandwidth. Hence, this technique overcomes the challenge of the OERW's limited bandwidth, which highly affects the possibility of tracking shifting in the peak reflection in the time domain. Moreover, the proposed method does not add to the complexity of the NDT process, which makes it suitable for in-situ real time applications. Hence, following this approach would be of great importance to numerous industries, where accurate thickness estimation of minute delamination in coatings is essential to avoid system failure.

Assessment of Delamination Under Insulation Using Ridge Waveguide

“…Instead of time arrival, many TDR techniques are proposed for microwave near field inspection. In [2] and [39], reflection coefficients obtained using OERW are converted into time-domain using inverse fast Fourier transform (IFFT). After that, the magnitude variation in the second peak corresponding 𝑡 is monitored to illustrate the variation between the defect and defect-free locations.…”

Prospect of Using Machine Learning-Based Microwave Nondestructive Testing Technique for Corrosion Under Insulation: A Review

“…Compared to the conventional NDT, microwave NDT is a powerful technique for inspecting the under-coating defect such as CUI. Electromagnetic waves at electromagnetic frequencies (300 MHz-300 GHz) can deeply penetrate the composite insulation and are sensitive to the thickness changes in the surface of the metal substrate [27, 116,117]. The microwave NDT technique provides several advantages, such as non-contact inspection, and does not need a couplant for the signal transmission into the sample under test [118][119][120][121].…”

Assessment of Defects under Insulation Using K-Medoids Clustering Algorithm-Based Microwave Nondestructive Testing

“…As a result, any decrease in the magnitude of the second peak can infer that delamination occurs in the ceramic coating. The defect and defect-free zones can be simply grouped utilizing the magnitude of the reflected microwave signals in the time domain using this deduction [33].…”

“…Notably, determining the IFFT with a greater number of points offers a better representation of the reflecting pulse, since it performs oversampling in the time domain. There is a downside, however, as oversampling significantly increases the detection time and processing complexity [33]. Therefore, PCA is required to reduce dimensionality.…”