Nondestructive evaluation of aircraft stealth coating by Terahertz-time domain spectroscopy: experimental and numerical investigation

ABSTRACTParticle-reinforced polymer matrix composites (PMCs) are often used as radar-absorbing materials (RAM) in aircraft stealth applications. The thickness evaluation of such multilayered coatings with micrometre-level thickness is highly challenging from single-side access. In this study, we used Terahertz Time-domain Spectroscopy (THz-TDS) in the transmission mode to extract the refractive index of the dielectric coatings for measuring the thickness using the time-of-flight method. A numerical study based on the Finite Element method (FEM) has also been developed to validate the transmission experiments. A frequency-dependent complex dielectric parameters must be considered for coatings with high absorption in the THz regime. This was addressed by performing the finite element simulations in a frequency domain. The thickness of each layer of a multi-layered coating is estimated by carrying out the experiments in reflection mode. Since the interface echoes were overlapping, a deconvolution algorithm and frequency thresholding were employed to reconstruct the signals reflected from the different interfaces. Using this technique, the thickness of each layer of coating is estimated accurately in a single measurement, which was challenging to measure using other conventional non-destructive testing (NDT) methods.KEYWORDS: Terahertz-time domain spectroscopyfinite element methodmulti-layer coatingthickness measurementterahertz pulsed imagingnon-destructive evaluation AcknowledgmentsThis study was supported by the Defense Laboratory Jodhpur, DRDO. We thank Dr Abhinandan Jain and Dr M. K. Pathra from Defense Laboratory Jodhpur, DRDO, for their continuous support throughout this project. We also want to thank Dr Sreedhar Unnikrishnakurup from the Institute of Material Research and Engineering (IMRE), A*STAR, Singapore, for providing us with the CT images and his technical inputs in the THz domain.Disclosure statementNo potential conflict of interest was reported by the author(s).Data availability statementThe data that supports the findings of this study are available from the corresponding authors upon reasonable request.Additional informationFundingThis work has been funded by the “Samsung-IITM Pravartak Fellowship” by Samsung India Electronics Pvt. Ltd as part of their CSR initiative.