Image forces and shielding effects of a screw dislocation near a finite-length crack

Some confusion in the literature about the interaction between a screw dislocation and a finite-length crack is clarified. It makes a difference whether the dislocation is emitted from the crack or comes from elsewhere. For example, both the image force and the shielding effect are always larger for a dislocation emitted from the crack than for the same dislocation originating elsewhere. They become equal only if the crack is very long (semi-infinite). Furthermore, the shielding effect is finite for the dislocation which is emitted from the crack and moved far away. It would be zero if the dislocation is not emitted from the crack. These and other details are presented.