Study on the dislocation emission around a crack tip by using high-voltage electron microscopy

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Dislocation emission from a crack tip is most fundamental process to understand the toughening mechanism of crystalline materials. In the present study, dislocation emission from the tip of a crack in MgO thin crystals has been investigated based on the geometry of crack tip dislocations observed using high voltage electron microscope(HVEM). In-situ experiments in the HVEM using tensile holder were also made. The effect of crack tip shielding due to the dislocations was also calculated using 3-D Buekner-Rice weight function theory for 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(011)plane, while in the crack wake many of the dislocations were left-handed(L-H)on the (011)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. 3-D 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.