Interaction of sound with fast crack propagation:: An equation of motion for the crack tip

The influence of ultrasound on crack dynamics in brittle materials is experimentally studied by using both the natural sound emitted by the propagating crack and an artificially generated ultrasound burst. We show that, although the acoustic energy is much weaker than the energy needed to propagate the crack, the presence of sound waves in the specimen cannot be neglected because sound interacts with the crack tip and may strongly modify the fracture dynamics. The characterization of this interaction allows us to find an equation of motion describing the crack propagation and the energy balance in mode I.