Double ZGV implies an Airy phase

The Airy phase is the frequency at which group velocity of a surface acoustic wave (SAW) attains a stationary value. In turn, a stationary value of the group velocity causes SAW to propagate with a lesser attenuation than at other frequencies. At present, no conditions for the appearance of stationary values of the group velocity are known for dispersive modes of Lamb waves propagating in stratified plates. Herein, the closed form solutions are derived for double zero group velocity (ZGV) belonging to the same Lamb mode. It is also found that the double ZGV leads to the appearance of the Airy phase located between the twofold ZGV. The analysis relies on the Cauchy formalism and the exponent fundamental matrix method.