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

Sachinlal Aroliveetil (Corresponding Author), Nithin Puthiyaveetil, Krishnan Balasubramaniam

Research output: Contribution to journalArticleScientificpeer-review

2 Citations (Scopus)


Particle-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.

Original languageEnglish
JournalNondestructive Testing and Evaluation
Publication statusE-pub ahead of print - 2023
MoE publication typeA1 Journal article-refereed


  • finite element method
  • multi-layer coating
  • non-destructive evaluation
  • terahertz pulsed imaging
  • Terahertz-time domain spectroscopy
  • thickness measurement


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