No description available.

Thesis (Ph. D.)--University of Washington, 1983. / Vita. Includes bibliographical references.

No description available.

No description available.

The dependence of fracture strength upon the time of loading is commonly termed static fatigue or delayed failure. This has been attributed to the growth of subcritical flaws under stress. Hence the study of subcritical crack growth is important in predicting the life expectancy of a material when it is subjected to a stress. Subcritical crack growth of glass and glass-ceramics, at room temperature and in two different environments (toluene and water) was studied. Glass containing 17.8 wt% Li₂O - 82.2 wt% SiO₂ and crystallized glasses (glass-ceramics) were chosen. The double torsion technique was used to determine crack velocity at various stress intensity factors. It was shown that the slopes of the velocity-stress intensity factor diagrams for glass and glass-ceramics (having different volume fractions of crystalline phase) tested in water, remained constant. However, these plots shifted to the higher stress-intensity region, as the degree of crystallinity in the glass increased. The crack velocity-stress intensity factor plots of glass and glass-ceramics tested in toluene have shown a similar behaviour but the slope of these plots increased as the degree of crystallinity in the glass increased. A modification of the stress- corrosion model of Hillig and Charles²³ is proposed. Crack velocity data of glass and glass-ceramics tested in water agreed well with the proposed model. Crack velocity data of glass and glass-ceramics tested in toluene are discussed.in terms of the "lattice trapping theory". An equation is presented to predict.the life expectancy under stress of these materials from crack growth data. The transverse rupture test was used to determine the fracture strength of glass and glass-ceramics. These results have shown that the fracture strength of glass-ceramics is increased mainly due to the increase in the fracture surface energy. The critical stress intensity factor of glass-ceramics increases as the degree of crystallinity increases. The fracture surface. energy of these materials was calculated from the knowledge of the critical, stress intensity factor and it was shown that the fracture surface energy of glass-ceramics containing up to 0.5 volume fraction of. crystalline phase is related to the inter-particle spacing. This observation is further substantiated by fracto-graphic examination. The kinetics of crystallization of lithium disilicate from 17.8wt% Li₂0 - 82.2 wt % Si0₂ glass was studied by crystallizing this glass at 530°C for various lengths of time. It was shown that the crystallization of lithium disilicate is a diffusion controlled reaction. It was found that the diffusivity for this process is much lower than the diffusion coefficient of lithium ion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This experimental investigation examines the influence of surface treatment on the strength of low-fired tempered ceramic bodies. Specimens prepared with commercial raw materials are employed, either finger-smoothed, burnished, textured, slipped/burnished, or resin coated; two building techniques, coiling and paddle-and-anvil, are also included since these forming processes affect the vessel surface. The modulus of rupture or flexural strength is determined by means of a four-point bending test, which is sensitive to critical surface flaws. By means of a univariate analysis of variance, small differences in strength are found between ceramic briquettes with different surface treatments. Although the differences in means for the moduli of rupture values are statistically significant, the differences are of such a low magnitude that they cannot be considered behaviorally relevant. These results are only valid for the materials, mode of preparation, and test procedures employed.

No description available.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 204-211. / by Thomas Dale Ketcham. / Sc.D.

Thesis (Ph. D.)--University of Washington, 1988. / Vita. Includes bibliographical references.

Crack-microstructure interactions occurring during the flaw introduction process were studied in a model brittle composite, a cordierite glass-ceramic. Microstructural effects associated with the repropagation of the introduced flaws under the imposition of a mechanical load were also examined. Two general types of crystallized microstructures were investigated for samples heat-treated from the original glass: a fine structure composed of a uniform precipitation of very small (< 0.1 µm) crystallites, and a coarser structure characterized by crystallites, and a coarser structure characterized by crystallites ~ 1-2 µm in diameter dispersed within a much finer-grained (< 0.1 µm) crystalline matrix. Surface damage was simulated by the Vicker's microhardness technique, with indentations being made over a wide load range to duplicate varying degrees of severity in the contact events. Direct measurement of the indentation flaws was made by calibrated scanning electron microscopy. Fracture toughness values were determined by direct calculation from the indentation parameters. The repropagation of the indentation flaws was investigated by strength tests performed in biaxial flexure. The results indicated that flaw introduction, as well as strength, fracture toughness, and the magnitude of strength loss sustained from surface damage, were all significantly affected by crack interactions with the crystallites in the glass-ceramic samples. The crack-crystallite interactions were extensive in the coarse microstructure samples. Crack pinning by the dispersed phase crystallites occurred at flaw sizes approximately equal to the mean free path distance between the dispersions, while at larger flaw sizes, crack deflection around the dispersed crystallites took place. Crack-microstructure interactions were absent in these same samples at flaw sizes less than the mean free path distance, and were not observed at all in the original glass or in samples heat-treated to yield only the fine microstructure. In the coarse microstructure samples, the size of flaws introduced by surface contact was found to be limited by the crack pinning interaction, thus confirming the basic concept of the dispersion-strengthening model for brittle composites. A substantial toughening effect in these same samples was realized from the crack deflection. Fracture toughness for the coarse microstructure samples exhibited a crack size-dependency, with toughness values corresponding to that of the matrix measured at small flaw sizes, and to that of the composite, at larger flaw sizes. The phenomenon was not present in either the original glass or in the fine microstructure samples. The crack-crystallite interactions occurring in the coarse microstructure samples greatly improved mechanical performance. The combination of decreased flaw size from crack pinning and increased fracture toughness from crack deflection resulted in strength values which were superior to those of the original glass. The crack size dependent fracture toughness enhanced the ability of the coarse microstructure samples to avoid potential strength losses following surface contact. / Ph. D.