The fundamental question as to whether the tip structure of brittle cracks is atomically sharp or has a rounded contour is examined in relation to current descriptions of strength‐controlling flaws. The distinction between the two opposing viewpoints lies in the controlling flaw dimensions in the strength formulation; crack length in the first and tip radius in the second. Definitive evidence on the issue is obtained from aging tests on soda‐lime glass, using indentations as controlled flaws. An increase in the inert strength is observed with increased exposure of the newly created flaws to moist environments prior to stressing to failure. This strength increase saturates after approximately 1 day, depending on the environmental species. The trend mirrors that reported by Mould in an earlier aging study on abrasion flaws. However, whereas Mould concluded that the strengthening must be due to tip rounding, the present tests reveal that the indentation‐induced cracks actually extend during the aging period. A fracture mechanics analysis shows that such extension relaxes residual crack‐opening stresses associated with the central contact deformation zone. It is accordingly concluded that the cracks remain sharp throughout their postindentation evolution; the influence of extraneous conditions on the strength is manifested only through the driving forces on these cracks. Flaws which have been annealed (i.e., which have had their residual driving forces removed) show no such aging effects. The fracture mechanics analysis also shows that in the region of saturated aging, where the indentation cracks appear to stop growing, the fracture driving force is in the region of the zero‐velocity threshold described in the macroscopic crack growth studies by Michalske. Contrary to previous interpretation, the indication is that the cracks do not blunt out in this region, for otherwise the strengthening would steepen rather than level out. Implications of this result concerning the inviolate nature of basic crack growth laws are discussed.

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The effect of room-temperature erosion on material removal from and strength properties of soda-lime glass and sintered alumina was determined. The results were compared to the elastidplastic indentation fracture model. The dependence of erosion rate and strength of soda-lime glass on the kinetic energy of the impacting particles was in good agreement with predictions. The lack of agreement between theory and experiment for sintered alumina was attributed to microstructural aspects of the erosion damage that are not modeled by indentation fracture.