DISLOCATION-CONTROLLED STABLE CRACK-GROWTH IN MO AND MGO

Four-point bend tests were performed on precracked single-crystal MgO and Mo specimens over a range of temperatures and strain rates. Etching was used to reveal dislocations around the crack tip and pre-existing dislocations in the bulk material. In both materials slow, stable crack growth was found to occur before fast fracture, the degree of which increased with temperature. A high dislocation density was revealed in the crack advance region. Different dislocation geometries were observed to be associated with the precrack, the crack advance region and the fast fracture zone. We propose that dislocation sources at or near the crack tip emit dislocation loops with both shielding and antishielding dislocations. The action of the antishielding dislocations promotes crack advance, while the shielding dislocations can retard fast fracture until substantial crack advance has occurred. The model is described and initial simulations to test its validity are reported.