Remarks on symmetric Lamb waves with dominant longitudinal displacements

Proof of the vanishing of the normal component of the particle displacement vector on the free surfaces of an isotropic, homogeneous plate for nonzero-order symmetric Lamb waves is given. Such a conclusion was stated without proof in the book by Viktorov [A. I. Viktorov, Rayleigh and Lamb Waves: Physical Theory and Applications (Plenum, New York, 1967), 1st ed., Chap. II, p. 121]. An expression for the frequency-thickness products where this occurs is also given. This enables practical (e.g., experimental) selection of such modes. Other interesting features of such modes, such as the independence of their group velocity on the mode's order or its nondispersitivity, are also addressed. The features discussed here are of great practical importance, e.g., in the nondestructive testing (NDT) of plates loaded by liquids and/or in the ultrasonic tensometry of a thin plate.