The inert failure strains of glass fibers with the molar compositions xNa2O · (1 − x)SiO2, where 0 less-than-or-equals, slant x less-than-or-equals, slant 0.35, were measured under liquid nitrogen (77 K) using a two-point bending technique. Failure strain increases with increasing Na2O content, from 17.7% for x = 0.00-23.5% for x = 0.35. The inert failure strain depends on the face-plate velocity (Vfp) of the two-point bending test, increasing with increasing Vfp for silica and decreasing with increasing Vfp for the sodium silicate glasses. The magnitude of the latter 'inert delayed failure effect' increases with increasing x. The addition of Na2O to a silicate glass weakens the glass network through the formation of non-bridging oxygens, and the weaker network distorts more at high strains and slower Vfp.