Elastodynamic degenerate states for a crack in an orthotropic medium

Motivated by recent theoretical studies concerning degenerate states of crack propagation in an orthotropic medium, the analysis of the corresponding elastodynamic Mode-I is resumed in order to point out its main features compared with those of an ordinary state of crack propagation as well as of Mode-II degenerate states. It will be shown that a Mode-I degenerate state of crack propagation is very sensitive to a small perturbation of the instantaneous crack velocity which may be responsible for crack instability.