FRACTURE MECHANISMS IN POLYCRYSTALLINE NONMETALLIC MATERIALS.
Final rept., 1 Oct 65-1 Oct 66,
IIT RESEARCH INST CHICAGO ILL
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The objective of this program was to study the mechanism of fracture in Al2O3 and to investigate the role of dislocations in crack nucleation. The grain size dependence of strength was found to have a slope of 13, and the results were analyzed in terms of the plastic work term associated with a moving crack. Metallographic work confirms the motion of dislocation within the grains. It also suggests twinning as a mode of deformation. In single crystal alumina, the surface condition and heat treatment have a pronounced influence on strength. Heat treatment of sapphire develops low angle grain boundaries which serve as a source of mobile dislocation. Dislocation velocity-stress measurements were made in the 1120 plane. The stress exponent was found to be 32, which is surprisingly low. The actual stress necessary for moving dislocations is of the order of 90 of the fracture stress. Author
- Ceramics, Refractories and Glass