Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

The deformation and crystallization of Li2O·2SiO2 and Li2O·1.6SiO2 glass fibers subjected to a bending stress were measured as a function of time over the temperature range ∼50 to ∼150°C below the glass transition temperature (Tg). The glass fibers can be permanently deformed at temperatures about 100°C below Tg, and they crystallize significantly at temperatures close to, but below Tg, about 150°C lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams- Watt (KWW) approximation), rather than a single exponential model.