Crack tip dislocations in MgO thin crystals have been investigated using a high voltage electron microscope (HVEM). Particular emphasis is laid on determining the sign of the Burgers vectors of those dislocations, because it is essentially important for understanding the fracture toughness of materials. For this determination, image simulation for a dislocation inclined to the foil surface has been performed, and characteristics of image contrast are discussed. Crack tip shielding due to dislocations is also discussed briefly.
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