Silicon nitride powder containing yttria was densified using hot isostatic pressing. Cylindrical compression specimens were creep tested in air in the temperature range 1350 to 1500 C. Four-point bend tests were conducted at a crosshead speed of 0.127 mm/min. on rectangular bars. Strength degraded by subcritical crack growth.
this report discusses experiments and theory concerning the effects of biaxial loads on ceramic strengths. a ball-on-ring apparatus has been built to produce balanced biaxial tension. the measured stress distribution agrees with finite element analysis and preliminary results on a glass-ceramic suggest that biaxial strengths may be greater than uniaxial for this material/surface- finish combination. biaxial strengthening and weakening were uncovered during an extensive literature survey. a new fracture mechanics ...
Fracture stresses in specimens of four commercial polycrystalline ceramics differing in each linear dimension by a factor of four or five were measured at room temperature under controlled conditions. Data obtained were analyzed with the aid of fractographic examinations for applicability of Weibull statistics. A central problem in attempting to use ceramic materials in demanding structural applications is uncertainty about the stresses to which they can be safely subjected. A ...
Fourteen C/C composites and three reentry-grade bulk graphites were evaluated experimentally to determine their applicability for impact member use in radioisotope heat sources. The various materials were evaluated as energy absorbing materials using two fundamental types of tests. The first determined the energy associated with the formation of a projected area of new surface by a single fracture, e.g., work of fracture. In the other test, a relative measure of ...
strength measurements of ceramics coupled with fractographic examinations indicate two specimens of the same material and containing flaws of the same depth can have different strengths due to variable flaw shape. a mathematical analysis is presented showing that flaw area can account for both flaw depth and shape. (author)
griffith equation is used to explain fracture of brittle materials from stress-intensifying flaws. strength measurements coupled with fractographic examinations for three commercial ceramics are presented along with relations found between fracture stresses and characteristics of the fracture-initiating flaws. (author)
Bend strengths of a commercial sintered alumina, 3M Company's Alsimag 614, are reported. Fracture stresses in specimens differing in each linear dimension by a factor of five were measured at room temperature under environmental conditions which either minimized or enhanced subcritical crack growth prior to catastrophic fracture. Strength was found to be dependent on specimen size under both test conditions. Fractographic identification of strength-controlling flaws, coupled with the analysis of ...
A central problem in structural designing with brittle materials results from an observed size dependence of fracture stress and a related or accompanying stress-distribution dependency. The ultimate goal of this project is to define and explain size-strength relationships exhibited by ceramics that are of interest for Navy structural applications. In the interests of obtaining strength reproducibility in specimens machined from billets of a dry-pressed high-alumina ceramic, physical characteristics of spray-dried ...
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