| An in-situ electron microscope study has been conducted to characterize the fracture and crack tip deformation behavior of metals under tensile and cyclic loading in a transmission electron microscope (TEM). The results show a number of new findings regarding the dislocation behavior near the tip and its relationship to the mode of crack propagation. Dislocation emission at the crack tip was found to depend strongly on the geometry of plastic zone, which contains a dislocation-free-zone(DFZ) just ahead of the crack tip. The difficulty of dislocation emission was, in turn, manifested by the fracture toughness. These experimental results were then compared to the theoretical analysis based on the DFZ model of fracture. Furthermore, this in-situ method has been extended to study the cyclic loading on the behavior of crack-tip-generated dislocations. It is found that dislocations emitted from a crack tip during tensile loading can be retracted and annihilated partially upon unloading. The change of plastic zone, dislocation distribution, and the DFZ during the retraction process were carefully examined using a TV/video camera. Their consequent impact on the crack propagation were then discussed in connection with the previously proposed dislocation theory of fracture and dislocation shielding for a moving crack. |
|
