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. 2015 Jun 25;15(7):14932–14945. doi: 10.3390/s150714932

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© 2015 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).

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Raw data of a single impact (Left Column) and Fourier spectra (Right Column) of the Impact-Echo test measurement. The arrows indicate the frequency of the Impact-Echo resonance.