A Dislocation barrier model for fatigue crack growth threshold

The primary mechanism of fatigue crack growth is crack-tip dislocation emission followed by the glide of the emitted dislocations. Both of these two processes are controlled by the crack-tip resolved shear stress field, which is characterized by the resolved shear stress intensity factor, . A dislocation barrier model for fatigue crack growth threshold is constructed. The model assumes that a fatigue crack stops growing when crack-tip slip bands are incapable of penetrating the primary dislocation barrier. The derived and deduced threshold behaviors agree with the observed constant threshold K_max,th in the low R region and constant threshold ΔK_th in the high R region. K_max,th is the K_max at the threshold. The constant K_max,th is related to the resistance of the primary dislocation barrier, which in most of cases is grain boundary; and the constant ΔK_th is related to the resistance of secondary barriers. Furthermore, the analysis shows that K_max,th is proportional to √d, where d is the grain size. The relation has been observed in steels. The model also helps to explain the characteristics of, and the transition from, microstructure-sensitive to microstructure-insensitive growth. This revised version was published online in July 2006 with corrections to the Cover Date.