Supersonic Crack Research Articles

On the possibility of a supersonic crack in a crystal lattice

A simple model of a crystal containing a crack is analyzed to show that the crack velocity, in this model, is not limited to the maximum velocity of waves in the perfect crystal. Supersonic motion in the lattice is possible because of the existence of localized modes that do not exist in a continuum model. The model used here is such that subsonic motion is not possible, but the model is well behaved in the supersonic region and a closed form expression for the velocity-applied stress relation can be derived. The necessary applied stress increases rapidly with velocity, but remains below the perfect lattice breaking stress for finite speeds. The supersonic crack opens into the shape of a wedge whose included angle decreases as the velocity increases. Some arguments are given for believing that the same qualitative results, for the supersonic region, also hold for real crystals.

Crack propagation at supersonic velocities

A theoretical model for laser-induced supersonic cracks in crystals with weak cleavage planes is developed. The crack is pictured as being driven by a laser-induced expanding plasma. The theory predicts the crack length-time history to be expected for a given energy-time input curve. The conditions for laser-induced supersonic crack production are seen to be a large input energy relative to fracture energy, a short loading time, and a weak fracture plane. The theoretical calculations are in rough agreement with experimental observations.