*3.4. Crack Initiation Ahead of the Main Crack*

Finally, we treat the growth of a short crack initiated ahead of the main crack, similar to that shown in Figure 3b. The equilibrium configuration of both the glide and cleavage dislocations are shown for selected fractions of hydrogen coverage. With an increase in hydrogen coverage, the material becomes more brittle, or from the dislocation point, the cleavage crack becomes narrower, while the glide part of the crack becomes wider, reflecting crack opening displacements. Figure 17 shows (a) the short crack grows more towards the main crack, (b) the crack tip plasticity or glide component is more for the segment facing the main crack, and (c) with increasing hydrogen coverage, the glide components at both ends decrease and approach the cleavage mode of crack growth. In these calculations, the blunting radius of the main crack is held constant. Future calculations will involve consideration of the plasticity at both the main crack and the initiated short crack, although such calculations will be time-consuming since the number of dislocations involved becomes large.

**Figure 17.** Discrete dislocation models of growth of an incipient crack from ahead of the main crack for various hydrogen coverages.
