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13 May 2000 Nondestructive characterization of corrosion-protective coatings on aluminum alloy substrates
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This paper describes the initial phase of the development of a nondestructive, multi-sensor approach for detecting, quantifying and monitoring degradation of organic coatings applied to aircraft aluminum frame structures. Two ultrasonic techniques are discussed: the well-established pulse/echo scanning acoustic microscopy, employing a 200 MHz transducer with a focusing lens and, as a proposed alternative, continuous acoustic wave measurements with a probe in contact to the sample. The High spatial and depth resolution of scanning acoustic microscopy provides the possibility to obtain information about coating inhomogeneities, e.g. density variations due to non uniform curing of the polymeric coating or interface voids, e.g. sites of weak adhesion. This is achieved by altering the probe/sample-distance, i.e. changing the focus point of the lens. Since the echoes from the topsurface and the interface can be separated, thickness measurements are possible, too. However, only down to a thickness of 10 - 15 micrometer. Here, continuous acoustic wave measurements can be considered to be a good alternative for acoustic measurements in the pulsed regime. The method enables very accurate thickness evaluation, but can not reach the excellent lateral resolution of scanning acoustic microscopy.
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Jochen Hoffmann, Shamachary Sathish, and Mohammad Khobaib "Nondestructive characterization of corrosion-protective coatings on aluminum alloy substrates", Proc. SPIE 3993, Nondestructive Evaluation of Aging Materials and Composites IV, (13 May 2000);

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