Inert failure strains of sodium aluminosilicate glass fibers

The inert failure strains ( ε _f ) of glass fibers from the compositional series 25Na ₂O · xAl ₂O ₃ · (75 − x)SiO ₂, where 0 ⩽ x ⩽ 32.5, were measured at 77 K using a two-point bending technique. Failure strain decreases when Al ₂O ₃ replaces SiO ₂, from 20.9% for x = 0 to 13.7% for x = 32.5. The failure strain depends on the testing speed, or faceplate velocity ( V _fp ) of the two-point bender. For glasses with relatively low alumina contents ( x < 10), ε _f decreases with increasing V _fp , a behavior noted in an earlier study of sodium silicate glasses [J. Non-Cryst. Solids, in press]. For glasses with greater alumina contents ( x ⩾ 15), ε _f shows little dependence on V _fp , as is found for silica fibers. The replacement of SiO ₂ with Al ₂O ₃ strengthens the glass network through the replacement of non-bridging oxygens with oxygens that bridge neighboring alumina and silica tetrahedra. This stronger (more cross-linked) network is less distorted at high stresses and so does not allow such large strains before failure. The dependence of ε _f on testing rate ( V _fp ) appears to be related to the presence of non-bridging oxygens in the glass network and the relaxation of these sites, as indicated by internal friction studies of similar glasses. This may explain the greater values of ε _f at the slower V _fp for the less cross-linked networks.