Dynamical Stability of the Crack Front Line

Dynamical stability of the crack front line that propagates betweentwo plates is studied numerically using the simple two-dimensionalmass-spring model. It is demonstrated that the straightfront line is unstable for low speed while it becomes stable for high speed.For the uniform model, the roughness exponent in the slower speedregion is fairly constant around 0.4 and there seems to be arough-smooth transition at a certain speed.For the inhomogeneous case with quenched randomness, the transition is gradual.