The decrease in stress intensity factor, expected from dislocation emission at a crack tip. can be achieved either through a change in crack tip radius (blunting) or through a crack closure induced by the dislocation (shielding) opposite to the opening imposed by the external stress field. So an apparent contradiction arises between the shielding concept and the crack opening displacement (COD) concept. In this paper a relation linking the COD to the stress intensity factor K is demonstrated. The explicit crack opening displacements induced by a dislocation are derived from this equation. Opposite to modes II and IlI for which either positive or negative displacement (or K) are allowed, physical limitations in mode I lead to changes in crack length: full or partial closure can be noticed. Special attention is paid to the latter case arising from dislocations whose Burgers' vectors are parallel to the crack plane, dislocations which have been observed experimentally under mode I loading: it is shown that these dislocations can account for both the crack shielding and the crack opening. The evaluation of the closed crack length with the applied K is discussed and the minimum stress intensity factor for crack opening as well.