Negative group velocities of Lamb waves

Abstract

Negative group velocities, which mean that the direction of propagation of wave energy and that of wave phase are opposite, were found experimentally in S2 mode of Lamb waves excited on an A 1 plate of 1 mm thick at frequencies near 3 MHz. A stroboscopic schlieren technique was used to observe an ultrasonic pulse reradiated in water from the Lamb mode, together with specular-reflected and transmitted pulses. The phase velocity of the Lamb mode was derived from the angle of incidence, and the direction of group velocity was determined from the displacement of reradiated pulse relative to the specular reflection. Phase velocities observed at various frequencies were in good agreement with theoretical predictions, which also described negative group velocities appearing at phase velocities above 10 km/s. Such a high phase velocity is negligibly affected by water loading. Existence of negative group velocity seems inherent in S2 mode and independent of plate materials. Also in S4 mode, negative group velocities are predicted in a very narrow frequency range.