Relationship between leaky Lamb modes and reflection coefficient zeroes for a fluid-coupled elastic layer

Abstract

It is shown in this article how the zeroes of the plane-wave reflection coefficient for a fluid-coupled elastic layer are modified under fluid loading, throughout the entire range of fluid density. Transformation and exchange behavior of branches of the reflection coefficient zero spectrum with increasing fluid density has been found and discussed. This study extends an earlier investigation of the complex mode spectrum (reflection coefficient poles). The behavior of the zeroes of the reflection coefficient is compared with that of the poles as a function of the fluid density. In addition, we derive an analytical expression for the intersection of the fluid-modified fundamental zero branches and the Rayleigh wave speed. This number can be useful as a measure of the sensitivity of a solid material to fluid loading. More significantly, the results of the current study have immediate relevance for an important class of advanced materials: polymer-matrix composites. It is demonstrated that the spectral modifications similar to those induced in the reflection coefficient of aluminum at high fluid density occur below the density of water, if the solid plate is a polymer-matrix composite.