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. 2018 Feb 27;9(3):98. doi: 10.3390/mi9030098

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© 2018 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 (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Appearance contrast before and after ultrasonic surface strengthening. (a) Appearance contrast before and after strengthening (20×); (b) Before strengthing (200×); (c) After strengthing (200×).