The effects of grain size and of crack-tip blunting radius on the fracture toughness of tungsten polycrystals were studied by using a combined dislocation dynamics/cohesive zone model. Two-dimensional dislocation dynamics were employed to analyze crack-tip plasticity and crack propagation was characterized by a cohesive zone model. The geometry of the crack and the corresponding boundary conditions were described by means of a boundary element method with dislocation dipoles as fundamental solution. Grain boundaries were introduced as obstacles for dislocation motion. Numerical experiments reveal that the fracture toughness decreases with grain size, because grain boundaries confine the plastic zone. This effect was particularly pronounced at small loading rates, where the unconfined plastic zone was large. The present results also showed that fracture toughness scaled with the tip radius as the stress concentration at the crack tip was reduced and the plastic zone was enlarged.

Modeling Size Effects on Fracture Toughness by Dislocation Dynamics. X.H.Zeng, A.Hartmaier: Acta Materialia, 2010, 58[1], 301-10