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

Ghassan Nihad Jawad,Muhammad Firdaus Akbar

doi:10.1109/access.2021.3095105

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 metal-backed 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.

Highlights

Polymer composites are widely used in many industries such as construction, power plants, and aerospace
The samples were pressed against a metallic surface such that the machined recesses acted as differently-sized delamination regions, while scanning was performed on the upper surface of the samples
In this paper, a novel microwave Non-Destructive Testing (NDT) technique is proposed for detection and thickness estimation of delaminated regions in dielectric coatings

Summary

INTRODUCTION

Polymer composites are widely used in many industries such as construction, power plants, and aerospace. Despite the ability of this method to detect various defects, estimating the delamination thickness proved to be challenging due to the nonlinear relation between the reflection peak amplitude and the actual time shift Another method proposed in [23] used zero-padding on the measured frequency domain reflection coefficient in order to artificially increase the bandwidth of the signal. Relying on the amplitude variations of IFFT results to obtain the delamination depth information represents a significant advantage over tracking the shift of the reflected peaks, as in the THz technology [16], [17] and previously-reported microwave NDT techniques [23]. Drawn from the analysis and results outlined in the previous sections

THEORETICAL BACKGROUND

RESULTS AND DISCUSSION

CONCLUSION