Fracture Wake Patterns in Brittle Solids

The interactions of a moving crack with self-emitted acoustic waves are studied in ion-implanted circular plates of crystalline silicon, where complex reproducible surface patterns made of local roughness variations are observed. A simple geometrical model, considering the sole propagation of A₀ Lamb waves inside the assembly, allows full prediction of all of these pattern shapes and their dependence on system parameters (crack velocity, elastic properties). Acoustic waves propagating along and behind the crack front are shown to play a central role in fracture-pattern formation. When the crack front is curved, surface patterns originate independently of any edge reflection or frequency preselection from acoustic waves propagating along and behind the crack. As in case of Kelvin wake patterns, fracture patterns emerge from geometrically induced coherence.