On the crossing points of Lamb wave velocity dispersion curves

Standard dispersion curves relating the phase velocity of Lamb waves to the frequency-plate thickness parameter fd indicate that in some cases symmetrical and antisymmetrical mode velocity curves cross each other. Viktorov’s equations are used to show that the crossing points are points where the dispersion curves are discontinuous so that no distinct Lamb mode exists for these particular velocity-fd combinations. Procedures are given to predict how many such points exist and where they are located in a given range of fd.