Threshold crack speed controls dynamical fracture of silicon single crystals

Fracture experiments of single silicon crystals reveal that after the critical fracture load is reached, the crack speed jumps from zero to 2km/sec, indicating that crack motion at lower speeds is forbidden. This contradicts classical continuum fracture theories predicting a continuously increasing crack speed with increasing load. Here we show that this threshold crack speed may be due to a localized phase transformation of the silicon lattice from 6-membered rings to a 5-7 double ring at the crack tip.