Fracture of Glass in Vacuum

TLDR

It is believed that subcritical crack growth in glass is a thermally activated process dependent on crack‑tip structure rather than alkali‑ion diffusion or viscous flow. The fracture of six glasses was examined in vacuum (<10⁻⁴ torr) over temperatures from 25 °C to 775 °C. Four of the six glasses showed subcritical crack growth with activation energies between 60 and 176 kcal mol⁻¹; the two anomalous glasses lacking slow growth had abnormal thermal and elastic properties and their critical stress‑intensity factors increased ~10 % up to ~600 °C, while a narrow cohesive region at the crack tip favors subcritical growth and a wide region favors abrupt fracture.

Abstract

The fracture of 6 glasses was studied in vacuum, &lt;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.