FRACTURE AND FRACTURE-TOUGHNESS OF STOICHIOMETRIC MGAL2O4 CRYSTALS ATROOM-TEMPERATURE

The fracture toughness and path of stoichiometric spinel (MgAl2O4) cry stals were determined at 22 degrees C for key low-index planes by doub le cantilever beam, as well as fractography of flexure specimens faili ng from either machining or indentation flaws. These results are compa red with other single and polycrystalline MgAl2O4 fracture toughness v alues measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental a nd theoretical results shows (1)the fracture toughness of the spinel { 110} plane is only a limited amount (e.g. 6%) higher than for the {100 } plane (similar to 1.2 MPa m(1/2)), (2) fractography of machining fla w fracture origins was the most effective source of K-IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of e lastic anisotropy (especial ly for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especi al ly for notch-beam tests), and the frequent lack of symmetric plasti c deformation and fracture (especially for indentation techniques).