The latter profile was found to be 10 times less extended than the peak position profile ( Lezzi and Tomozawa, 2015). This conclusion was derived from the surface profiles of the IR reflection peak position at ≈1120 cm –1, which indicates structural relaxation, and that of the Si-OH concentration. These experiments on bend fused silica fibers, held under subcritical tensile stress at low water pressure below T g, indicate that molecular water can cause stress relaxation as a very mobile species. (2013a) explaining why the strength of fused silica fibers is increased by soaking in water as reported in Ito and Tomozawa (1982a).Īlternatively, water-induced toughening can be explained by water induced stress relaxation as found for fused silica, E-glass and soda-lime silicate glass fibers ( Lezzi and Tomozawa, 2015 Seaman et al., 2015). Water penetration and swelling were later described in Wiederhorn et al. In fused silica, swelling at the crack tip was also found and attributed to stress-induced water diffusion into the glass surface ( Tomozawa, 1996 Fett et al., 2005 Wiederhorn et al., 2013a) as it was previously stated ( Nogami and Tomozawa, 1984 Han and Tomozawa, 1991). Sodium can be enriched at the fractured glass surface ( Langford et al., 1991 Celarie et al., 2007), dissolve into the narrow water film at the crack tip ( Wiederhorn, 1967) and cause ion exchange related swelling and compressive stress ( Lanford et al., 1979). Further, glass dissolution and reprecipitation at different curvature can cause crack tip blunting ( Ito and Tomozawa, 1982a). In contrast to Michalske and Freiman (1982) it was found that compressive instead of tensile stress promote glass dissolution at the crack tip ( Ito and Tomozawa, 1981). However, more recent studies indicate that crack propagation is affected by other phenomena and that a simple ≡ Si-O-Si ≡ + H 2O reaction ( Michalske and Freiman, 1982) is unlikely to be the stress corrosion reaction ( Ito and Tomozawa, 1981 Tomozawa, 2007 Ciccotti, 2009 Wiederhorn et al., 2013c). Stress corrosion is therefore widely accepted as the underlying mechanism, assuming that water molecules break strained Si-O-Si network bonds at the crack tip into silanol groups ( Charles, 1958 Hilling and Charles, 1965 Michalske and Freiman, 1982). Focusing mainly on commercial glasses, crack velocity was studied in liquids ( Wiederhorn and Bolz, 1970 Simmons and Freiman, 1981 Gehrke et al., 1987a Gehrke et al., 1990 Gehrke et al., 1991 Dunning et al., 1994), humid air ( Wiederhorn, 1967 Richter, 1983 Evans and Johnson, 1975 Gehrke et al., 1987b Muraoka and Abe, 1996 Freiman et al., 2009), and vacuum ( Pukh et al., 2009 Wiederhorn et al., 1974), where ambient water proved to be the key accelerant for slow crack propagation ( Wiederhorn, 1967). Glass strength and fatigue are controlled by the presence and propagation of surface microcracks ( Ciccotti, 2009 Wiederhorn et al., 2013a). In ambient air, a largely extended region II is observed for the hydrous glass, which indicates that crack growth is more sensitive to ambient water. Further, inert crack growth in hydrous glass is found to be divided into sections of different slope, which indicates different water related crack propagation mechanism. In vacuum, a decreased slope of logarithmic crack growth velocity versus stress intensity factor is evident for the hydrous glass in line with an increase of β-relaxation intensity indicating more energy dissipation during fracture. Stable crack growth was measured for nominal dry and water-bearing (6 wt%) soda-lime silicate glasses in double cantilever beam geometry and combined with DMA studies on the effects of dissolved water on internal friction and glass transition, respectively. 3Leibniz University Hanover, Institute of Mineralogy, Hanover, Germany.
2Clausthal University of Technology, Institute for Non-Metallic Materials, Clausthal-Zellerfeld, Germany.