The sign of the reaction volume for the water/silica reaction

In the present report, we deal with the question whether the equilibrium of the silica/water reaction shifts under tensile stresses to an increased or reduced hydroxyl concentration or equivalently, which sign the reaction volume shows. From our analysis on the basis of the Le Chatelier-Braun Theorem, it can be concluded that the equilibrium constant of the watersilica reaction is enhanced due to tensile stresses and that the reaction volume change is positive. In additon it will be shown that the apparently found enlarged water solubility under compressive stresses by Nogami and Tomozawa [2] is an artifact of the stress-dependent diffusivity

Contact Strength and Fracture Toughness from Opposite Cylinder Loading Tests

Bars loaded by oppositely concentrated forces via rollers are appropriate test specimens for the determination of strength and fracture toughness under contact loading. Test devices are described and solutions for the stress, the stress intensity factor, and the T-stress term are reported. Experimental results are compiled for the contact strength. For most investigated materials, measured contact strengths showed strongly reduced Weibull exponents compared with those from 4-point bending tests. This important effect is attributed to the strong stress gradients near the contact zone

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

Stress-enhanced silica/water reaction under torsion loading

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 the analysis in [1,2] 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. On the basis of the results in [1,2] it will be shown that under torsion loading the strongest effects on stress-enhanced hydroxyl generation and swelling strains should occur, although this stress state shows a disappearing hydrostatic stress term