Lamb waves in stratified plates: appearance of “forbidden” phase velocities

It is known that Lamb waves in homogeneous traction-free plates can propagate with arbitrary phase velocity, spanning the admissible speed interval (0;\(+\infty )\). However, as the current research shows, Lamb waves propagating in two-layered traction-free plates may have ‘forbidden’ phase velocities, at which no Lamb waves can propagate. The analysis is based on the approach comprising Cauchy complex formalism and the exponential fundamental matrix method.