An electron microscope study of crack tip deformation and its impact on the dislocation theory of fracture

Abstract

Dislocation behavior has been observed near the crack tip of metals during in situ deformation in an electron microscope. All three modes of crack propagation and deformation were observed depending on the specimen geometry, although shear cracks of mode III type with screw dislocations in the plastic zone were most frequently observed. Dislocations were emitted from the crack tip during early stages of crack propagation and were driven out of the crack tip area, leaving behind a dislocation-free zone. The dislocations were piled up in the form of a linear array on a slip plane which was coplanar with the crack in metals of low stacking fault energy. In metals of high stacking fault energy the dislocations cross slipped out of the original slip plane and formed a broad plastic zone. The cracks propagated by a combination of plastic and elastic processes in which the plastic portion of the crack opening was created by the dislocations that were emitted from the crack tip. The elastic process occurred as a result of brittle fracture of the dislocation-free zone. As the cracks moved into the thicker part of the specimen, they often propagated in a zigzag manner by emitting dislocations on two alternating slip planes. In order to understand the observed behavior of dislocations near the crack tip, the elastic interaction between a crack and a dislocation must be examined and the force on the dislocation close to the crack tip estimated. It was shown that a dislocation was generated if the applied stress was sufficiently large that the force on the dislocation was repulsive down to the core distance from the crack tip. The emission condition was expressed in terms of a critical stress intensity factor for dislocation generation. The magnitude of the critical stress intensity factor for dislocation generation relative to the critical stress intensity factor for brittle fracture could be used as a criterion for determining the ductile versus brittle behavior of a metal. Once generated, the dislocations moved out of the crack tip area because of a repulsive force. The dislocations came to rest at a point where the repulsive force was balanced by the lattice friction. The dislocation-free zone thus created would restore the elasticity to the crack tip region and it was possible to define a local stress intensity factor. Without the dislocation-free zone, the local stress intensity factor approached zero and the crack did not propagate because there was no elastic energy release associated with the propagation. The dislocation-free zone model was also applied to describe the mechanisms of ductile fracture as well as the ductile-to-brittle transition. A mechanism of brittle fracture in the presence of crack tip deformation was also proposed from the model. The shielding of a crack tip by the dislocations in the plastic zone was discussed in terms of the local stress intensity factor. Finally, the local stress intensity factors were determined experimentally from the observed crack tip geometry and were compared with the values predicted by theory.