Strobo‐photoelastic visualization of Lamb‐type waves with negative group velocity in a solid/fluid/solid trilayer

Abstract

An elastic wave with negative group velocity transports energy in the direction opposite to the phase velocity. This phenomenon exists in certain modes of Lamb waves propagating in a solid plate, but only over a very narrow range of frequency‐thickness products and Poisson’s ratios of a plate material. A minimum value of the negative group velocity of a mode is dominated by the Poisson’s ratio of a material. A similar phenomenon also appears in Lamb‐type waves that are guided in a symmetric solid/fluid/solid trilayer. Derivation of the dispersion equation of Lamb‐type waves emphasizes the existence of the negative group velocity in certain modes. The existence of negative group velocity depends not only on the modes, frequency‐thickness product, and Poisson’s ratio, but also on the fluid layer thickness. The magnitude of negative group velocity is controlled by changing the fluid layer thickness. Direct evidence is presented of the negative group velocity that is obtained by means of stroboscopic photoelastic visualization of pulsed Lamb‐type waves propagating in a glass/water/glass trilayer. A time‐sequence photoelastic image shows the wave‐phase behavior and the pulse envelope propagating in the opposite direction.