Elimination of swelling stresses from measurements of the equilibrium constant in silica

The reaction of water with silica is described by the equilibrium constant k. This property depends on temperature and is affected by stresses. The experimental determination of k is in most cases carried out in presence of swelling stresses. Knowledge of the reaction volume $\Delta$$\overline{V}$ allows an elimination of the swelling stresses and the determination of an equilibrium constant for the case of a stress-free material. An analytical expression of k$_{0}$ as a function of temperature, k$_{0}$=f(T), will be suggested

Evaluation of crack profiles by Bando et al. Analysis of crack opening displacements

The behavior of cracks in glass is discussed contrary for many years. Both purely elastic material behavior as well as plasticity and viscosity were discussed. One of the key results were the crack opening measurements of Bando which were considered for an indication of non-elastic behavior. There are two effects of water, damage by breaking the ring structure due to chemical water reaction and swelling. In this short note, the effect of damage by the water-glass reaction is addressed. In this report we deal with a purely linear attempt by the J-integral

Psychosis and urbanicity – a review of the recent literature from epidemiology to neurourbanism

Purpose of review: Epidemiological studies associate city living with an elevated psychosis risk. Urban (social/economic) stress and exposure to environmental toxins, pollution or disease agents have been proposed to underlie this association. This review provides an update on the recent evidence (May 2017 - November 2018). Recent findings: Of 645-screened studies, 17 on: (1) urbanicity-psychosis associations in worldwide high, middle and low-income countries, (2) explanatory mechanisms, including nature exposure, social and economic stressors and genetic risk; (3) urbanicity effects on the brain and coping; and (4) urbanicity and resources, were included. The reviewed evidence revealed complex patterns of urbanicity-psychosis associations with considerable international variation within Europe and between low, middle and high-income countries worldwide. Social and economic stressors (e.g. migration, ethnic density, economic deprivation), nature exposure and access to resources could only explain part of the urbanicity effects. Risk factors differed between countries and between affective and non-affective psychosis. Summary: Urbanicity-psychosis associations are heterogeneous and driven by multiple risk and protective factors that seem to act differently in different ethnic groups and countries. Interdisciplinary research combining approaches, e.g. from experimental neuroscience and epidemiology, is needed to unravel specific urban mechanisms that in- or decrease psychosis risk

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

Social neuroscience in psychiatry: unravelling the neural mechanisms of social dysfunction

Social neuroscience is a flourishing, interdisciplinary field that investigates the underlying biological processes of social cognition and behaviour. The recent application of social neuroscience to psychiatric research advances our understanding of various psychiatric illnesses that are characterized by impairments in social cognition and social functioning. In addition, the upcoming line of social neuroscience research provides new techniques to design and evaluate treatment interventions that are aimed at improving patients’ social lives. This review provides a contemporary overview of social neuroscience in psychiatry. We draw together the major findings about the neural mechanisms of social cognitive processes directed at understanding others and social interactions in psychiatric illnesses and discuss their implications for future research and clinical practice

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$_{tip}$

Swelling strains from gamma-irradiated silica - Evaluation of results by Shelby

Water introduced in silica reacts with the silica network under hydroxyl generation that causes a volume swelling. This effect has been discussed quantitatively in much detail in [1] for the case of water entrance by a heat treatment procedure. In the present Report we include also data by Shelby [8] obtained by in-situ generation of hydroxyl in gamma-irradiated silica. This procedure resulted in stronger density changes compared with the heat-treated material. Independent of this fact, we can show that the same volume swelling results for both methods. As an application of the results an estimation of the molar volume of hydroxyl is made