Reflection coefficient of a fluid‐loaded anisotropic plate

Abstract

The acoustic reflection and guided wave behavior of an orthotropic plate immersed in an ideal fluid is examined as a function of fluid loading. Coincidence of zeros and real parts of reflection coefficient poles occurs only in the limit of vanishing or infinite fluid density. The modification of the pole and zero plate spectra in the presence of a variable fluid density will be discussed in the context of Lamb wave measurements on low density solid plates, such as graphite‐reinforced plastics (GRP). Numerical results are presented showing functional dependence of pole and zero branches on fluid density and plate elastic properties at constant phase velocity or constant frequency‐thickness product. It will be shown that propagating and evanescent wave modes exchange portions of their branches, and that reflection coefficient zeros also undergo extensive, but differing, transformations. Even very low fluid density (< 0.1 g/cm3) values can cause significant changes in the wave behavior of the fluid‐loaded GRP plate. Fluid loading transforms the wave spectrum from Lamb waves in a traction‐free plate to “constrained‐slip” waves in a plate satisfying mixed boundary conditions.