Fracture strengths (u), fracture-initiating flaw sizes, and mirror radii ( r ) , as outlined by either the mist or the hackle boundary, were measured for silicate and nonsilicate glasses (e.g. AsZ& and glassy carbon). For all glasses, ur112= constant. The average ratios of inner and outer mirror radii to flaw size were -1O:l and -13:1, respectively, for most of the glasses. Critical fracture energies calculated from either flaw or mirror size agreed very well with those obtained by double-cantilever-beam measurements.

Variations in the thermal expansion coefficients, elastic moduli, and fracture strengths of LiaO-SiOz glass-ceramics were determined as a function of nucleation treatment and volume fraction of crystals present. Strength enhancement was attributed to a decrease in the mean free path between crystals as crystallization proceeds. It is postulated that the eventual reduction in strength in some glass-ceramics is caused by the development of localized cracks at the crystal-glass interface as a result of the volumetric changes which occur during crystallization.

Fracture mechanics is combined with fracture surface analysis to analyze brittle failure of glass bars which were tested relative to the direction of grinding. Grinding essentially produces two sets of flaws from which failure occurs. In the most severe set, formed basically parallel to the grinding direction, the ratio of the average depth (a) to the half‐width (b) is 0.5. In the less severe set, formed perpendicular to the grinding direction, the average a/b ratio is 1.6. In both sets the most severe flaws are generally associated with a particularly deep grinding groove or gouge. The strength reduction resulting from testing perpendicular to the grinding direction results from the larger flaw size and slightly higher stress‐intensity factor resulting from the greater ellipticity of the flaws formed parallel to the grinding grooves and perpendicular to the tensile axis. Detailed analysis of these 2 sets of flaws causing failure of appropriately oriented specimens shows that (1) the fracture mirror radius, r, occurs at a constant stress‐intensity level independent of flaw geometry; (2) unsymmetric fracture mirrors result from unsymmetric, irregular flaws leading to unsymmetric stress‐intensity distributions; (3) is constant for semielliptical flaws; and (4) fracture energy calculated from an expression including mirror constants, the flaw‐to‐mirror size ratio, and the flaw geometry agrees with measured values over a wide range of a/b values.