Is a crack an effective stress riser for triggering of plasticity?

From the stress intensity factors induced by a circular loop attached to a crack front, the resulting image force acting on the dislocation was calculated. The relaxation of the elastic energy stored due to neighbouring surfaces is obtained through integration of the variation of the image force while shifting the loop towards the inside of the material. The amount of relaxation, as numerically determined in this paper under a given loading, is too small to sufficiently decrease the energy barrier for triggering of dislocation emission. Homogeneous nucleation on a perfect crack front is impossible. Since dislocation emission has been experimentally observed under the loading considered, it must come from defects along the crack front.