Complex poles of Lamb waves propagating along anisotropic and absorbing fluid-loaded plates

Abstract

This work presents theoretical and experimental investigations on Lamb wave generation along anisotropic immersed plates. The structure of these guided waves is strongly dependent on the nature of the source (transient source, bounded beam...). An incident plane wave for which the signal is time-limited is considered, and leads to transient Lamb waves mathematically defined by complex frequencies. These frequencies are the poles of the reflection/transmission coefficients of the fluid-loaded plates. This point of view, recently developed for isotropic plates, is completely different from the classic approach in terms of leaky Lamb waves, which are harmonic waves. New interesting results are then obtained for the NDT of thin anisotropic plates: the dispersion curves calculated for complex frequencies are very different from those calculated for real frequencies (harmonic waves). Moreover, some modes have a negative imaginary part of the frequency that implies transient phenomena increasing with time. The waveforms of the reflected signals are obtained and explained by the calculation of a contour integral in the complex frequency plane. The roles of anisotropy and absorbing effects are discussed.