Hydroxyl Damage in Silica: Full-range description including large damages

When water diffuses into silica glass it reacts chemically with the glass forming nanometre sized pores that change the physical properties of the glass. In earlier papers and reports, we discussed the effect of water/silica reaction on the strength via volume swelling, and showed by use of damage mechanics that the water reaction reduces Young’s modulus E and intrinsic strength in thin surface layers. In this paper, the dependency between hydroxyl concentration and damage will be derived for the full damage range by using experimental results from literature. For small water concentrations, we used sound velocity measurements from literature. The suggested relations describe the dependence between the hydroxyl concentration and the Young’s modulus for the damaged glass and allow the strength decrease due to hydroxyl generation to be computed. From an example of application, it can be concluded that damage by hydroxyl generation has little effect on strength even in the case of completely damaged surface region so far the water-affected surface zone is much thinner than the bulk material

Effect of torsion loading on swelling in silica

Water reacting with silica causes the generation of hydroxyl SiOH accompanied by a volume or swelling expansion. The principle of LeChatelier ensures that the hydroxyl concentration increases with increasing externally applied stresses. From literature it becomes obvious that (a) the hydroxyl concentration must depend on the multi-axiality of the applied stresses, and (b) that the swelling effect is anisotropic. Based on Finite Element results it can be shown that for instance under torsion loading a strong stress-enhancement of the water/silica reaction with increased hydroxyl content must occur although this stress state shows a disappearing hydrostatic stress term. Due to the disappearing hydrostatic stress term under torsion loading, no mechanical effect would have been expected in torsion tests. Seen from this point of view, the torsion test seems ideal to us to provide evidence of anisotropic swelling. Further effects that can also influence the torsion test are addressed in the Appendix

Crack-tip models by Irwin and Dugdale for hydroxyl-damaged crack-tip zones

Due to the hydroxyl generation, the silica network ahead of crack tips is damaged. The consequence is a damaged crack-tip zone showing a reduced Young’s modulus. The linear-elastic fracture mechanical treatment by application of stress intensity factors becomes doubtful especially for large zones. In this report, we use a description via well-known models of Elastic-Plastic Fracture Mechanics, namely, the models by Irwin and Dugdale. As an application we compared the results with experimental observations on crack profiles from literature. The computed and observed Crack-Tip Opening Displacements (CTOD) were found to be in good agreement

Validation of the swelling formalism via evaluation of bending moments from literature: Comparison of theoretical predictions and measurements

The volume of silica expands by the hydroxyl generation when silica surfaces react with water. These volume strains are proportional to the mass concentration of the hydroxyl. They were measured by Wiederhorn et al. by evaluating the curvature of disks undergoing this reaction on only one side. The obtained bending moments were found to be proportional to square-root of heat-treatment time. In the present considerations we pay particular attention to global bending moments caused by swelling, which, in contrast to local swelling stresses, do not require assumptions about the type of stress distribution. It can be stated that • The bending moments from disk curvature and hydroxyl measurements via the IR-evaluation procedure by Libowitzky and Rossman are in excellent agreement. • Predictions of moments M based on diffusivities and surface water concentrations by Helmich and Rauch also show good agreement with correlation coefficients R$^2$>0.95. Consequently, we can sufficiently write: M$_{predict}$ = M$_{measured}$

Mass Transfer of Water at Silica Surfaces - Extension of the data base to lower temperatures

In two previous reports we have dealt with the description of the time dependence of the water content at the surface of silicate glass by mass transfer (SWP 73 and SWP 94). Literature measurements in the range of 200°C100°C. Measurements by Helmich&Rauch serve as a basis. A step change in the mass transfer coefficient is found at 200°

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

Analysis of corrosion damage by Proctor et al

In a study by Proctor et al. a strength reduction was observed due to surface damaging in humid air at high temperatures. The strength tests on these damaged specimens were carried out in normal lab atmosphere. Unfortunately, these tests are affected by subcritical crack growth. In the present report we show on the strength results obtained in humid air, in which way these data can be used to determine the inert strength of the damaged surface. Proctor et al. provided images of the surface defects and identified, localized corrosion centers. In our report we show how the effects of these local corrosion defects can be handled fracture mechanically

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$_{tip}$

Heart-shaped crack-tip zones - Effect of the zone length

In this report the general influence of different elastic modules ahead of a crack tip and in the bulk on the stress intensity factors will be discussed again. From the FE results obtained in Report [1], and the theoretical solution by Merkle [3] K = K$_{appl}$$\sqrt{\frac{E}{E_{0}}}$ it becomes clear that the FE-results for relative zone sizes $\omega$/a >0 must deviate from the exact solution but are also in rather good agreement with the theoretic results for finite $\omega$/a. In this report also results are compiled for cracks fully embedded in zones with reduced module. Such cracks show stress intensity factors completely different from a $\sqrt{ E/E_{0}}$ -dependency. Nevertheless, it can be concluded that the stress intensity factors on the basis of the J-integral agree with those obtained by COD evaluation. As an approximate description of the observed E/E$_{0}$-dependency we suggest $\frac{K}{K_{appl}}$ $\cong$$\frac{2E}{E+E_{0}}