The appearance of proper and degenerate Dirac cones during the propagation of Lamb waves in an isotropic functionally graded layer satisfying the Wiechert condition is analyzed. It has been revealed that Dirac cones appear in a layer, in which the distribution of physical properties is asymmetric with respect to the midsurface and in any Poisson’s ratio. The study is based on the Cauchy formalism and the exponential fundamental matrix method.
Saiyan et al. (Mon,) studied this question.