Fracture of Glass in Vacuum

The fracture of 6 glasses was studied in vacuum, <10 −4 torr (10 −2 N/m 2 ), as a function of temperature from 25° to 775°C. Subcritical crack growth was observed in 4 of the glasses. Activation energies for crack motion ranged from 60 to 176 kcal/mol. The glasses which did not exhibit slow crack growth were “anomalous” glasses with abnormal thermal and elastic properties. Critical stress intensity factors for these 2 glasses increased ∼10% as the temperature increased to ∼600°C. It is felt that subcritical crack growth is not the result of alkali‐ion diffusion or viscous flow but rather of a thermally activated growth process which depends on the crack‐tip structure in the glass. A narrow cohesive region at the crack tip favors subcritical crack growth, whereas a wide region favors abrupt fracture.