Anomalous Lamb waves in cubic crystals at a non-semisimple degeneracy of the fundamental matrix

An explicit equation is derived for the condition of non-semisimple degeneracy of the fundamental matrix and, hence, the occurrence of anomalous guided waves propagating along the crystallographic axes of a cubic crystal. It is revealed that the observed non-semisimple degeneracy appears at a certain phase velocity in any cubic crystal of type I, but this kind of degeneracy is not possible in cubic crystals of type II. The analysis is based on the Cauchy sextic formalism combined with the spectral analysis of the fundamental matrix.