Crack-tip shielding in silica at room temperature

When water penetrates into silica surfaces near a crack tip, it reacts with the SiO${}_2$ network and generates hydroxyl $\equiv$SiOH. Due to the hydroxyl generation, the glass must expand. Since a free expansion is not possible for the thin layers on the undeformed bulk material, compressive stresses occur which shield the crack-tip region from externally applied tensile load. The consequence is a fracture mechanics shielding stress intensity factor K${}_{sh}$<0. So far we only determined the shielding stress intensity factor from theoretical considerations on water diffusion und the high tensile stresses at crack tips. Since water concentration measurements on crack surfaces of uncritically driven cracks are available in literature, we determine the shielding term Ksh from experimental data. This evaluation is done with and without consideration of damaging the initial ring network by hydroxyl generation. It can be concluded that the shielding stress intensity factor is clearly overestimated, when crack-tip damage is ignored. Finally, it is illustrated in which way the shielding stress intensity factor influences the v-K-curve for subcritical crack growth.