Abstract
Crack tip dislocations in a (001) MgO thin plate introduced by (010) cleavage were investigated using high voltage transmission electron microscopy. The stress concentration at a crack tip was modified by the multiplication of (a) (011)[011] and (011)[011] screw dislocations or (b) (110)[110] edge dislocations. The former case was dominantly observed in a thin plate sample as predicted by Hahn's model. Burgers vector was determined by the image absence technique and weak-beam method, indicating that these dislocations contribute to a reduction of the stress concentration in mode I fracture by producing a back stress toward the crack tip. The possibility was pointed out that the former type of dislocations is also related to mode III.