On the lower limiting velocity of a dynamic crack in brittle solids

The existence of forbidden velocity gap in dynamic crack propagation in brittle crystals has been proposed previously, based on analytical calculations and numerical simulations. These suggested that the minimal velocity of a dynamically propagating crack is a significant portion of the Rayleigh wave speed. On the other hand, theoretical analysis based on continuum mechanics does not identify any lower limit to the crack velocity. In this work, we studied experimentally the crack velocity in glass and single-crystal silicon, in a geometry that constrains the crack profile to a nearly quarter elliptical shape, such that at a certain part of the crack it is forced to move appreciably slowly. Direct measurements show that the crack velocity along this profile decreases to less than 1% of the Rayleigh wave speed, both at room temperature and at 77 K, which is notably below the expected velocity gap. (C) 2003 American Institute of Physics.