Multi-parameter characterization of thin specimens using ultrasonic NDE
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The problem of Ultrasonic NDE is defined as determining one or more of the following acoustical parameters: thickness, wavespeed, density and attenuation. In this paper, we describe exact theory-based experimental techniques for simultaneous measurement of the acoustical parameters of thin specimens; the qualifier thin is used to describe a situation when the time-duration of the ultrasonic wave exceeds the time-duration between two successive echoes from the front and the back surfaces of the specimen. A plane longitudinal wave incident normally upon the plate is considered. Transfer functions have been derived for the specularly reflected and the transmitted waves. An inversion scheme, based upon the method of least squares in conjunction with the Gauss-Newton Algorithm, has been developed to determine two or more of the acoustical parameters. Results are presented for two distinct classes of materials: elastic plates of aluminum, brass, steel, and glass; viscoelastic plates of plexiglas, graphite/epoxy, E-glass/epoxy and bulk aerospace adhesive. Finally, techniques are described to determine the acoustical properties of a thin adhesive layer bonding a pair of thick aluminum adherends; its properties are then compared with that of the bulk adhesive.
