TS3a1 - IN SITU TEM OBSERVATIONS OF CRACK TIP DISLOCATION BEHAVIORS UNDER A MIXED MODE LOADING

Abstract

Direct observations of crack tip dislocation behaviors have been made in bcc metals during in situ tensile and cyclic deformation in a transmission electron microscope (TEM). During the crack propagation, dislocations are usually emitted from the crack tip to be rapidly driven out of the area, and consequently piled-up in a plastic zone, leaving a dislocation-free zone (DFZ) between the crack tip and the plastic zone. The behaviors of crack tip dislocations depends on the loading condition and microstructure of the specimen. The present experimental results were focused on the dislocation emission at a crack tip under a mixed mode loading. The observed geometry of the crack tip, the DFZ and the plastic zone represented the difficulty associated with dislocation emission from the crack tip. The results were discussed and compared with the theoretical DFZ model of fracture.