The behavior of dislocations in the vicinity of a moving crack tip was investigated. It was found that dislocations which resided within a well-defined strip along the crack path, and which had Burgers vectors of a suitable orientation, were strongly attracted to the crack tip. The motion of such dislocations with respect to the moving crack tip was determined. When they were drawn into the immediate vicinity of the crack tip, these dislocations then caused atomic-scale blunting of the tip. The resultant crack trapping mechanism was modelled phenomenologically, and yielded a quantitative estimate for the actual crack tip fracture toughness. This estimate depended upon the temperature, crack speed and density of pre-existing dislocations. This fracture toughness was suggested to represent an measure, of the resistance to fracture, that was more realistic than the ideal Griffith surface-energy estimate. Possible scenarios for gradual or abrupt transitions from brittle to ductile behavior, as a function of temperature, were considered within a framework which was based upon the present model and a continuum plasticity model for the functional relationship between the measurable far-field toughness and the crack-tip fracture toughness. The nature of the transition depended upon the relationship between these two functions. The anomalous low-temperature toughness of Fe was explained in terms of the typically high dislocation-density of annealed Fe crystals.

The Influence of Pre-Existing Dislocations on Cleavage Crack Propagation Behavior in Crystals. S.D.Mesarovic: Journal of the Mechanics and Physics of Solids, 1997, 45[2], 211-38