A combined dislocation-cohesive zone model for fracture in nanocrystalline materials

A combined dislocation-cohesive zone model was proposed to describe the fracture toughness of nanocrystalline (nc) materials. In the framework of the model, cohesive stress near crack tip initiates edge dislocations, which move to the opposite grain boundaries. The emitted dislocations provide a shielding effect of the crack. The dependence of both the maximum number of dislocations, emitted by a crack, and the critical stress intensity factor on grain size d (ranging from 20 to100 nm) for Cu was calculated. The calculated results show that (i) nc materials have low fracture toughness, (ii) the critical stress intensity factor decreases with decreased grain size, and (iii) the grain size effect is not high; for instance, increasing the grain size from 20 to 100 nm increases the value of critical stress intensity factor only by 0.035 MPa/m ^1/2.