This paper analyzes the appearance of proper and degenerate Dirac cones arising during Lamb wave propagation in an isotropic functionally graded layer satisfying the Wiechert condition. It is found that Dirac cones appear in a layer with a distribution of physical properties asymmetric with respect to the midsurface and for any Poisson’s ratio. The study is based on the Cauchy formalism and the exponential fundamental matrix method.
S.G. Saiyan (Wed,) studied this question.