FRACTURE MECHANISMS IN POLYCRYSTALLINE NONMETALLIC MATERIALS.
Final rept. 31 Mar 67-30 Mar 68,
IIT RESEARCH INST CHICAGO ILL
Pagination or Media Count:
Fracture phenomena and their relation to dislocations and dislocation motion were studied during the course of the investigation. High-purity single crystals were stressed up to 60,000 psi. Although cracks formed in the crystals, no etch pits or associated dislocation motion was observed. X-ray techniques and transmission electron microscopy of thin films prepared by ion bombardment are proposed for determining dislocation velocity stress relationships. Present results indicate that the impurities play a major role in dislocation behavior in alumina and their presence is essential for formation of etch pits. Polycrystalline Lucalox of 5 micron, 20 micron, and 30 micron grain size was fractured at room temperature, 400, 700, and 1000C. Extensive cleavage and intercrystalline fracture were observed using the Scanning Electron Microscope. The extent of cleavage decreased with increase in temperature and grain size. These results are explained in terms of temperature dependence of fracture stress for cleavage and grain boundary fracture, stress concentration at sharp-edged pores, and favorable orientations of some grains relative to the loading axis for cleavage. Author
- Ceramics, Refractories and Glass