THE BRITTLE FRACTURE OF ALUMINA.
Final technical rept. Feb 66-Jan 67,
NEWCASTLE-UPON-TYNE UNIV (UNITED KINGDOM)
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This work aims at increasing understanding of the mechanisms of brittle fracture in alumina and at application of this to its strengthening. In particular, the role of dislocation motion is investigated. Direct evidence of dislocation motion accompanying fracture has been obtained for alumina and magnesia and deformation twinning has been shown to occur during the fracture of alumina even at -196 C. Effective surface energy measurements up to 500 C have been obtained for alumina and a detailed study made of the fracture surfaces. The bend strength of pure and chromium doped single crystals of alumina has been determined from -268 to 800 C. The strength of alumina increases markedly on decreasing the temperature from -196 C to -268 C. Also, small additions of chromium give appreciable strengthening. Both these findings support the thesis that plastic deformation is involved in the brittle fracture of alumina. Fracture velocities have been measured from Wallner lines on the surfaces of alumina and glass. Crack branching occurs not at a critical velocity, but when a critical stress intensity condition is fulfilled. A theory accounting for the detailed mechanism of crack branching is given. Author
- Ceramics, Refractories and Glass