Crack growth in the high crack velocity region for binary sodium silicate glasses

The crack growth behavior in the high crack velocity region (region III) for binary sodium silicate glasses was investigated by using a DCDC (double cleavage drilled compression) specimen under compressive stress. For 15 mol%Na2O glass, despite the anomalous characteristic of the bulk modulus, subcritical crack growth in region III was detected. The slope of stress intensity factor (K-I)-crack velocity (nu) curve for silicate glass depended on the number of non-bridging oxygen per SiO4 tetrahedron rather than on the alkali content in the glass. This compositional dependence of the slope of K-I-nu curve for silicate glasses could not be explained in terms of thermally activated crack growth process where the crack-tip shape would vary with the pressure derivative of bulk modulus. The smaller K-I-nu slope and the higher K-IC value of 35 mol%, Na2O glass map be related to the structural change from a three-dimensional network to a two-dimensional network because of the introduction of many non-bridging oxygens into the silica network.