The dynamics of tensile crack fronts restricted to advance in a plane are studied. In an ideal linear elastic medium, a propagating mode along the crack front with a velocity slightly less than the Rayleigh wave velocity, is found to exist. But the dependence of the effective fracture toughness $\gamma \left(v\right)$ on the crack velocity is shown to destabilize the crack front if $\frac{d\gamma }{\mathrm{dv}}<0\mathrm{}$. Short wavelength radiation due to weak random heterogeneities tends to lead to this instability at low velocities. The implications of these results for the crack dynamics are discussed.

• Received 23 January 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.877