DISLOCATION ACTIVITY, STABLE CRACK MOTION, AND THE WARM-PRESTRESSING EFFECT IN MAGNESIUM-OXIDE

Four-point bend tests were performed on precracked single-crystal MgO specimens at different temperatures and strain rates. A large amount of stable crack growth before fracture occurred; etching revealed dislocations in the crack advance region. We believe that dislocation sources near the crack tip emit loops, producing shielding and anti-shielding dislocations; the latter promote crack advance by a repeated microcleavage mechanism. ''Warm-prestressing'' experiments, which improve the room-temperature fracture toughness by dislocation shielding, were performed. There was a prestress below which no ''warm-prestressing effect'' (WPSE) was exhibited and the magnitude of the WPSE increased with increasing prestressing temperature.