Loading rates and the dynamic initiation toughness in brittle solids

The experimentally determined marked rise of the stress intensity factor re quired to initiate crack propagation in brittle solids under variably high loading rates, is analyzed. This problem of fracture initiation at the tip of a crack is considered in terms of activating a flaw at some distance awa y from the tip. By using a semi-infinite crack in an unbounded two-dimensio nal solid subjected to spatially uniform but temporally varying crack-face pressure, we consider the evolution of stress at the failure initiation sit e. Fracture initiation is assumed synonymous with attaining a critical stre ss at the fracture site. The results conform to typical experimental data o f dynamic crack initiation in brittle solids.