Existence of Negative Group Velocities in Lamb Waves

Abstract

It is known only fragmentarily that negative group velocities exist in some lower modes of Lamb waves whose phase velocities are very high. This paper reveals numerically that the existence of negative group velocities depends not only on the frequency-thickness product but also on Poisson's ratio σ of the plate material. Negative group velocities exist in S1 mode for σ<0.45, and in A2 mode for σ<0.31. In addition, S3, S4, S6 and A5 modes exhibit negative group velocities in respective narrow ranges of σ.