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) Bueckn er-Rice weight function theory for the crack-dislocation interaction. Dislo cation image analyses showed that dislocations ahead of the crack tip obser ved in the present study were almost right-handed (R-H) screw dislocations lying on the (0 (1) over bar 1) plane, while in the crack wake many of the dislocations were left-handed (L-H) on the (0 (1) over bar 1) plane. Format ion of such dislocation configuration can be understood by the emission of dislocation loops from sources at a crack tip, where crack jogs, crack kink s and intersections of crack planes with free surfaces may act as significa nt sources for dislocation emission. 3D stress analysis exhibits that the l argest component of crack tip stress intensities induced is mode I shieldin g type, and that mode II component is not negligible in the present case, s uggesting the induction of the crack tip shielding for the mixed stress mod es of I and II through the dislocation emission from the sources indicated above. (C) 2000 Elsevier Science S.A. All rights reserved.