Abstract
The model that describes the wedging open of a cleavage crack in double cantilevered specimens constrains the tip ends of the crystal to move with a constant velocity. In rapidly loaded specimens, cracks have been observed to propagate as rapidly as bending moment pulses produced by flexure waves. These bending moment pulses are generated when the ends of the crystal are deflected at a constant rate. A maximum velocity of the tip end of the specimen to ensure stable crack propagation is derived by comparing the propagation and magnitude of the dynamical bending moment pulses to the propagation of stable cracks. Fracture surface energies calculated from observations of the motion of a crack are therefore not reliable under fast loading conditions.