It was recalled that the primary mechanism of fatigue crack growth was crack-tip dislocation emission, followed by glide of the emitted dislocations. Both of these processes were controlled by the crack-tip resolved shear stress field, as characterized by the resolved shear stress intensity factor. A dislocation barrier model for the fatigue crack growth threshold was constructed. The model assumed that a fatigue crack stopped growing when the crack-tip slip bands were incapable of penetrating the primary dislocation barrier. The derived threshold behaviors agreed with the observed constant threshold which was observed in the low-R region, and the constant threshold observed in the high-R region. The low-R threshold stress intensity factor value was related to the resistance of the primary dislocation barrier which, in most cases, was a grain boundary. The high-R constant was related to the resistance of secondary barriers. The analysis showed that the former was proportional to the square root of the grain size. Such a relationship was observed in steels. The model helped to explain the characteristics of the transition from microstructure-sensitive to microstructure-insensitive growth.

A Dislocation Barrier Model for Fatigue Crack Growth Threshold. H.W.Liu: International Journal of Fracture, 1998, 93, 261-80