Crack length and velocity
Boudet and Ciliberto plotted the product of crack length, l, and velocity, V(l), against l. This yielded a straight line at velocities lower than the critical velocity (a), but two distinct slopes are seen for cracks travelling at higher velocities (b). One is observed at low crack velocities, while the other indicates a higher terminal velocity due to interaction between the sound emitted by the crack and the crack tip.