PHY23 Dispersion of ultrasonic waves (Lamb waves)

The phenomenon of Lamb waves causes by superimposing of ultrasonic transverse and shear waves in thin plates, whose thickness is smaller than the ultrasonic wave length. What is interesting about lamb waves is that they, on the one hand, show a frequency-dependent change of their propagation velocity (dispersion). On the other hand, lamb waves are present in the form of symmetrical and antisymmetrical modes which propagate within the material independent of each other.In the experiment, different Lamb wave modes are stimulated in thin glass plates using specific angle beam wedges and their frequency and group velocity are determined. The formation of different oscillation modes of a plate, the influence of plate thickness and the connection of Lamb waves with transverse and shear waves in conjunction with the elastic constants of the material is discussed.

Velocity frequency diagram

S0, A1 and S2 modes were stimulated in the glass plates by combination of different incidence angles and sound frequencies. In the dispersion chart below, the determined group velocities are entered depending on the product of frequency and thickness of the respective glass plate. Furthermore, the chart shows the theoretical curse of curve (numerical solution) of the dispersion properties of the stimulated Lamb wave modes.