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UBC Theses and Dissertations

The fracture mechanics of lithium disilicate glass and glass-ceramics Rao, Avaral S.


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.

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