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Crack growth velocity in alkali silicate glasses was measured in vacuum across 10 orders of magnitude with double cantilever beam technique. Measured and literature crack growth data were compared with calculated intrinsic fracture toughness data obtained from Young´s moduli and the theoretical fracture surface energy estimated from chemical bond energies. Data analysis reveals significant deviations from this intrinsic brittle fracture behavior. These deviations do not follow simple compositional trends. Two opposing processes may explain this finding: a decrease in the apparent fracture surface energy due to stress-induced chemical changes at the crack tip and its increase due to energy dissipation during fracture.
Hydrous silicate and borate glasses of up to 8 wt% water were prepared using wet condition of melting under external pressure. Rheological experiments were conducted to determine the effect of the total water content on viscous flow and kinetic fragility. The findings indicate that dissolved water acts as a strong fluxing agent in both glass types. Water decreases particularly viscosity at the glass transition range, makes the viscosity-temperature curve "longer" and thus decreases kinetic fragility. Recent IR and NMR data are presented to link the dynamics of hydrous silicate and borate glasses with their structure particularly with regard to changes in the water-, silicon- and boron-speciation.