THE NECESSITY OF BOTH PLASTICITY AND BRITTLENESS IN THE FRACTURE THRESHOLDS OF IRON

In order to explain discontinuous brittle cracking in materials subjected to hydrogen, single-crystal and crack/dislocation simulation studies have been made. The computational scheme allows the local stress tensor near a crack in an anisotropic elastic solid with shielding dislocations to be determined. The model system is iron with a load axis perpendicular to the {100} cleavage plane, growth in the macroscopic <010> direction, and an applied stress intensity of 16 MPa m1/2. The result is that the elastic stress distribution is translated out from the crack tip by about 23 nm and that large stresses, although not of a singular nature, still exist. These stresses near 20 000 MPa are sufficient to concentrate dilute solutions of hydrogen to nearly one hydrogen for every iron atom. Such stresses, combined with these extremely high hydrogen concentrations are proposed for the initiation of decohesion which grows locally over 1-mu-m size areas, as observed experimentally.