The effect of water penetration on crack growth in silica glass

Due to the high stresses at crack tips, stress-enhanced water diffusion into silica occurs and gives rise to a volume expansion zone around the tip. This zone leads to a fracture mechanics shielding effect. In the present paper, the diffusion equation for water into glass is solved in cylindrical coordinates including the effect of the mechanical swelling stresses on the diffusivity. As an application of the volume swelling in the crack-tip region, the shielding stress intensity factor is calculated and the relation between subcritical crack growth rate and the crack tip stress intensity factor is derived from an existing crack growth rate versus applied stress intensity factor curve. The general mathematical procedure is outlined in detail and experimental results are discussed, giving evidence for crack-tip diffusion of water and a consequent zone of swelling around the crack tip.