Origin of region II slow crack growth of glasses : internal friction of crack tip

Abstract

The slow crack growth behavior was observed for oxide glasses under stress in the presence of water vapor. In general, the slow crack growth is divided into three regions: region I, region II and region III slow crack growth. With decreasing applied stress intensity, the slow crack growth transitions from region III to region II and finally to region I. In slow crack growth region II, the crack growth rate is nearly independent of the applied stress intensity, while in region I or region III, the logarithmic crack growth rate linearly depends on the applied stress intensity. The traditional explanation for the region II slow crack growth is the limited rate of the transport of water vapor molecules towards the crack tip. Based on the transport limited theory, the region II slow crack growth rate was predicted to be nearly temperature independent. However, the region II slow crack growth was found to increase with temperature for some glasses under a constant water vapor pressure. It is known that water diffuses into glass during the slow crack growth and that water in glass produces a large internal friction. In this thesis, the region II slow crack growth of oxide glasses is attributed to the transition between region I, where there is a lower relaxed modulus near crack tip, and region III, where the crack tip has a higher unrelaxed modulus. The proposed mechanism is supported by the observation that there is a residual stress pattern near crack tip after region I slow crack growth while no residual pattern is observed near crack tip after region III slow crack growth. The residual stress pattern exhibits a match, ignoring the magnitude variation, to the theoretical stress field near the crack tip predicted by the linear elastic fracture mechanics model for both soda-lime silicate glass and sodium trisilicate glass. Furthermore, the residual stress pattern is found to exhibit a time-dependent behavior: as the time increases, the residual stress pattern decays. The decay process can be modeled using empirical equation: stretched exponential decay function. The time dependent behavior indicates that oxide glasses exhibit the viscoelastic behavior even at room temperature. Due to water diffusion into glass, internal friction plays a role and reduces the strength. These observations support the proposed model of region II slow crack growth.