Anomalous temperature behaviour of subcritical crack growth in silica

Silica shows the effect of subcritical crack growth in humid environments. Measurements in liquid water show increasing subcritical crack growth velocities when the temperature is increased as was shown by Wiederhorn and Bolz. Since this has been generally found for glasses, this effect is called normal subcritical crack growth. For measurements on silica in water vapour environment the astonishing effect of decreasing crack-growth rate v at an increased temperature was observed for constant partial water pressure in the humid environment. This surprising result observed in v-K experiments by Suratwala and Steele is called anomalous subcritical crack growth behavior. In the present report we consider the effects of reduced water concentration at silica surfaces and volume swelling by hydroxyl generation as the reasons for anomalous subcritical crack growth. From our computation, we can conclude that silica shows normal subcritical crack growth, when it is taken into account that the real physical stress intensity factor K$_{I}$ is used that describes the stresses in the singular crack-tip field, i.e. when v-is plotted vs. K$_{tip}$.