Reflection Coefficient of a Fluid-Coupled Elastic Layer

Abstract

An elastic layer, or plate, immersed in a fluid possesses plane-wave reflection and transmission properties which are related to the propagation characteristics of guided waves in the layer. If the fluid density is much less than that of the plate, this relationship amounts to a correspondence and has been used to deduce the velocity dispersion of Lamb waves in numerous studies of ultrasonic reflection [1–3]. Under conditions of heavy fluid loading (ie, when the ratio of fluid to solid density approaches or exceeds unity), the fluid begins to play a decisive role in controlling both the propagation and reflection characteristics of waves in the immersed plate. A vivid illustration of this behavior is seen in the locus of curves determined by the transmission maxima (reflection coefficient zeroes) for a graphite-epoxy composite plate immersed in water. It has been shown through both measurement and calculation [4,5] that for certain values of incident angle and frequency, the total transmission loci in this case correspond neither to the Lamb waves in vacuum nor to the leaky guided waves in the fluid-coupled plate.