Understanding the scope and limitations of non-destructive testing procedure is essential for selecting the appropriate test parameters for material inspection. This paper presents the scope of material (δs\documentclass[12pt]{minimal}
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\begin{document}$$ \delta_{s} $$\end{document}) and probe dependent (δt\documentclass[12pt]{minimal}
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\begin{document}$$ \delta_{t} $$\end{document}) penetration depths for determining the optimal test frequency (fopt)\documentclass[12pt]{minimal}
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\begin{document}$$ f_{opt} ) $$\end{document} for detection of sub surface defects in electrically thick conducting specimens. Numerical modelling is carried out for a pancake coil above an electrically thick aluminium plate, t/δt\documentclass[12pt]{minimal}
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\begin{document}$$ t/\delta_{t} $$\end{document} > 1, to study the influence of the EC probe and defect location (tdf\documentclass[12pt]{minimal}
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\begin{document}$$ t_{df} $$\end{document}) on the test frequency for near and deep sub surface defects. The study concludes that the optimal test frequency, fopt\documentclass[12pt]{minimal}
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\begin{document}$$ f_{opt} $$\end{document} for detection of deep sub surface defects (tdf/t≈1\documentclass[12pt]{minimal}
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\begin{document}$$ t_{df} /t \approx 1 $$\end{document}) is determined by the probe dependent skin depth, δt\documentclass[12pt]{minimal}
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\begin{document}$$ \delta_{t} $$\end{document}, and the plate thickness is related to fopt\documentclass[12pt]{minimal}
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\begin{document}$$ f_{opt} $$\end{document} by, t∝1/fopt\documentclass[12pt]{minimal}
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\begin{document}$$ t \propto 1/\sqrt {f_{opt} } $$\end{document}. The numerical observations were experimentally validated for machined sub surface notches on a 10 mm thick (t\documentclass[12pt]{minimal}
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\begin{document}$$ t $$\end{document}) aluminium plate.