Ductile materials are found to sustain brittle fracture when the crack moves at high speed. This fact poses a paradox under current theories of dislocation emission, because even at high velocities, these theories predict ductile behavior. A theoretical treatment of time-dependent emission and cleavage is given which predicts a critical velocity above which cleavage can occur without emission. Estimates suggest that this velocity is in the neighborhood of the sound velocity. The paper also discusses the cleavage condition under mixed mode loading, and concludes that the cleavage condition involves solely the mode I loading, with possible sonic emission under such loadings
Brittle fracture investigation from disc specimen weakened by U- notch in mixed mode I + II
