THE FORMATION OF DISLOCATION RINGS ON A CRACK FRONT

The formation of dislocation rings at crack tips is considered when th e glide plane is tilted against the crack plane and does not contain t he crack front. The emanating ring is described in the Peierls model b y a distribution of infinitesimal dislocations and the influence of th e interplanar potential in the glide plane on the emission process is studied. The magnitude of the displacement, the radius of the ring and the profile of the displacement are allowed to adjust during the emis sion process by minimizing the energy. The formation of rings in tilte d glide planes can avoid the formation of ledges, which must occur whe n the dislocation is emitted in a plane containing the crack front. Si nce ledge formation severely obstructs the emission of dislocations, i t is suggested that ring formation is the more favourable process.