Abstract
Based upon the results of previous work [K. M. Lin et al., Maler. Sci. Engng 95, 167 (1987)], the system of a screw dislocation dipole interacting with an internal crack is studied. The analytic formulae of the physical variables, such as stress field, strain energy and flipping energy, image force and image torque, and stress intensity factor, are obtained. Both dipole emission from the crack and the effect of the dipole on crack shielding are discussed in detail. If a dipole near an internal crack is originated from the crack, it must be at an orientation angle α max corresponding to a state of maximum energy when emission occurs, and then rotates to lower its strain energy to an orientation angle α min in the range of (90° + λ/2) and (270° + λ/2) due to the image torque, where λ denotes the angle between emission and crack planes. A dipole orienting at an angle α min shields both crack tips simultaneously. In addition, the behavior of a dipole coplanar with the crack is also investigated. It is found that (1) under the action of a mode III applied stress, a zero-force point exists in front of the crack tip when the dipole orients at α = 180°; and (2) a maximum shielding (for α = 180°) or antishielding (for α = 0°) on the left-hand crack tip occurs when the ratio of half crack length to the distance from the right-hand tip to the dipole center (l/p) is equal to 0.25. © 1993.