Dislocation emission from a crack tip in MgO thin crystals

Dislocation emission from the tip of a crack in MgO thin crystals has been studied based on the geometry of crack tip dislocations observed using high voltage electron microscopy (HVEM). The effect of crack tip shielding due to the dislocations was also calculated using three-dimensional (3D) Bueckner–Rice weight function theory for the crack–dislocation interaction. Dislocation image analyses showed that dislocations ahead of the crack tip observed in the present study were almost right-handed (R-H) screw dislocations lying on the (01̄1) plane, while in the crack wake many of the dislocations were left-handed (L-H) on the (01̄1) plane. Formation of such dislocation configuration can be understood by the emission of dislocation loops from sources at a crack tip, where crack jogs, crack kinks and intersections of crack planes with free surfaces may act as significant sources for dislocation emission. 3D stress analysis exhibits that the largest component of crack tip stress intensities induced is mode I shielding type, and that mode II component is not negligible in the present case, suggesting the induction of the crack tip shielding for the mixed stress modes of I and II through the dislocation emission from the sources indicated above.