Dispersion properties of Lamb and leaky Lamb modes in elastic and viscoelastic plates

The understanding of the complex dispersion curves for guided ultrasonic waves in solid plates is essential in applications and research involving such waves. In the present work, complex three-dimensional (3D) dispersion curves for Lamb, leaky Lamb, viscoelastic Lamb, and viscoelastic leaky Lamb modes in a homogeneous and isotropic solid plate of uniform thickness are discussed in relation to influences of fluid loading and viscoelastic absorption losses in the plate. This includes influences on the real frequency - complex wavenumber dispersion diagram, and the corresponding group velocities. An example is given for a vacuum- and water-embedded steel plate with small viscoelastic losses. Significant influences are noted in the frequency region of backward waves. That is, in the frequency band near mode cutoff and the zero group velocity (ZGV) point of certain Lamb modes.