Density Studies on the Function of Rare-Earth tons in Glass Matrices

The effect on density of the inclusion of low concentrations of rare-earth ions in lead silicate, soda-lime, and phosphate glass matrices has been studied. Significant changes are noted in these density values that are particularly noticeable in phosphate glasses and least noticeable in lead glasses. It is considered that the variable function of the Pb²⁺ ion obscures the general effect of rare earths in lead silicate glasses; but in soda-lime, and more specifically in phosphate glasses, evidence is highly satisfactory that the lanthanons (La through Lu) function only as network modifiers, and that scandium and yttrium in concentrations of >0.2 and 1.0 atomic % function strongly as network formers. This is shown to be in accord with concepts associating glass function with cation electronegativity.     

Promotion of Attack on Glass and Silica by Citrate and Other Anions in Neutral Solution

The observation that essentially neutral solutions of sodium citrate attacked sulfur-treated soda-lime glass bottles of generally high chemical resistance much more than was anticipated led to a general investigation. It has been found that neutral solutions of sodium citrate attack silicate glasses and silica with a severity similar to that of substantially alkaline solutions. This effect was observed for both soda-lime and borosilicate glasses and also for vitreous, crystalline, and amorphous forms of silica. The effect of citrate becomes perceptible at a pH of about 5 and increases rapidly with pH up to 7.6. Solutions of higher pH were not studied. Solutions of certain other complex-forming anions show the effect on sulfur-treated soda-lime glass containers in varying lesser degrees. Although the active anions studied are complexing agents for alkaline earths and aluminum, it was observed that citrate had more action on a fused silica container than on containers made of certain glasses containing calcium and aluminum. Therefore a digestive action on silica and the Si–O–Si structure of glasses is to be recognized. Fluoride, the only anion tested that forms previously known complexes with silicon, had slightly less action on sulfur-treated bottles than citrate. The results suggest that soluble silicon complexes are formed at pH values in the region of 7 by citrate and some of the other organic anions studied.     

Effect of Test Environment on Stress-Corrosion Susceptibility of Glass

The stress-corrosion susceptibility of abraded and acid-polished soda-lime and borosilicate glasses in test environments of 6N NaOH, distilled water, and 6N HCI was measured by dynamic fatigue techniques. Dynamic-fatigue data for these glasses agree well with crack-velocity data for the water and 6N NaOH environments. The lack of agreement in the 6N HCI environment suggests that the failure mechanism for glass in HCI is not simply crack propagation by stress corrosion. The agreement in failure predictions based on strength and crack-velocity data in 6N NaOH and water suggests that either set of data may be used for effective design calculations; however, caution should be used when basing strength calculations on crack velocity data in 6N HCI.     

Dependency of Fatigue Predictions on the Form of the Crack Velocity Equation

A computer search technique was developed to analyze fatigue strength data using both exponential and power law forms of the subcritical crack velocity equation. All crack velocity equations would fit a given set of fatigue data equally well in the data range but failure predictions based on the different crack velocity equations diverge from each other outside the data range. The exponential form of the crack velocity equation best fit both the static and dynamic fatigue data of hot-pressed Si₃N₄ and optical glass fibers, whereas the power law form best fit the static and dynamic fatigue data of soda-lime glass and A1₂O₃. To determine the most appropriate crack velocity equation for a given material/environment system, it is recommended that fatigue data be obtained under different loading conditions and the data numerically regressed using the computer search technique with each of the possible crack velocity equations to find which best fits the data.     

Effect of Fining Agents on the Solarization of a Borosilicate Glass

The effects of both fining agents and TiO₂ concentration on the solarizability of a borosilicate glass proposed for solar energy applications have been determined. Optical and ESR measurements indicate that chlorine-fined glasses are much less susceptible to solarization than are arsenic-fined glasses. Titanium dioxide acts as a solarization suppressant with either fining agent. The observed increase in optical absorption in the visible region is in marked contrast with the recent report of a solar-induced decrease in optical absorption in the infrared range for soda-lime silicate glasses.     

Infrared Study of Soda-Lime Glass During Hydration and Dehydration

Infrared spectra of two soda-lime silicate glasses were measured after different times of hydration and dehydration. The height of OH and molecular water bands increased proportionally to the square root of time during hydration. The amount of dehydration with time was fit with a diffusion model; the diffusing species was probably molecular water. The rate of dehydration of the two laboratory glasses was many orders of magnitude faster than the rate for a commercial soda-lime glass, consistent with the presence of a transformed layer on the surface of the hydrated laboratory glasses.     

Origin of the Static Fatigue Limit in Oxide Glasses

Oxide glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. The log of crack velocity decreases linearly with decreasing stress intensity factor in Region I. For some glasses, at a lower stress intensity, K_o, log v asymptotically diminishes where there is no measurable crack growth. The same glasses exhibit static fatigue, or a decreasing strength for increasing static loading times, as cracks grow and stress intensity eventually reaches the fracture toughness. In this case, some glasses exhibit a low stress below which no fatigue/failure is observed. The absence of slow crack growth under a low stress intensity factor is called the fatigue limit. Currently, no satisfactory explanation exists for the origin of the fatigue limit. We show that the surface stress relaxation mechanism, which is promoted by molecular water diffusion near the glass surface, may be the origin of the fatigue limit. First, we hypothesize that the slowing down of slow crack growth takes place due to surface stress relaxation during slow crack growth near the static fatigue limit. The applied stress intensity becomes diminished by a shielding stress intensity due to relaxation of crack tip stresses, thus resulting in a reduced crack velocity. This diminishing stress intensity factor should result in a crack growth rate near the static fatigue limit that decreases in time. By performing Double Cantilever Beam crack growth measurements of a soda‐lime silicate glass, a decreasing crack growth rate was measured. These experimental observations indicate that surface stress relaxation is causing crack velocities to asymptotically become immeasurably small at the static fatigue limit. Since the surface stress relaxation was shown to take place for various oxide glasses, the mechanism for fatigue limit explained here should be applicable to various oxide glasses.     

Determination of Fictive Temperature of Soda-Lime Silicate Glass

Peak positions of silica structural bands, both in infrared absorption and reflection modes, were used earlier to measure the fictive temperature of silica glass. In the present study, the method was applied to determine the fictive temperatures of a soda-lime silicate glass. For the silicate glass, the IR absorption spectra produced a broad structural band which made the precise determination of peak position difficult, and only the IR reflection band was used. Equilibrium peak positions of ∼1056 cm⁻¹ IR band, due to Si-O stretching, were found to be directly correlated with the fictive temperature of the soda-lime silicate glass. The soda-lime glass exhibited an opposite dependence of the IR band position on the glass fictive temperature as compared to silica glass.     

Computerized Prediction of Glass Compositions from Properties

A computer technique is described which allows prediction of glass compositions which have arbitrary sets of properties. The technique involves equations which can be used to calculate the properties of a glass when only its composition is known. The application of the method to soda-lime silicate glasses is discussed.     

Stress Corrosion and Static Fatigue of Glass

Stress corrosion cracking of six glasses was studied using fracture mechanics techniques. Crack velocities in water were measured as a function of applied stress intensity factor and temperature, and apparent activation energies for crack motion were obtained. Data were consistent with the universal fatigue curve for static fatigue of glass, which depended on glass composition. Of the glasses tested, silica glass was most resistant to static fatigue, followed by the low-alkali aluminosilicate and borosilicate glasses. Sodium was detrimental to stress corrosion resistance. The crack velocity data could be explained by the Charles and Hillig theory of stress corrosion. It is probable that stress corrosion of glass is normally caused and controlled by a chemical reaction between the glass and water.     

Crack Initiation Tendency of Chemically Strengthened Glasses

The relationship between crack initiation tendency before chemical strengthening and fracture strength of chemically strengthened glass was investigated. Four kinds of glass compositions were used in this study: soda-lime silicate (SLS), aluminosilicate (ALS-1, ALS-2), and aluminoborosilicate (ALBS) glasses. The fracture strength of sheet glass was measured by the four-point bending test, the ring-on-ring test, the ball drop test, and the predamaged four-point bending test. ALBS glass, which has the lowest crack formation probability before chemical strengthening, is more damage tolerant than other glasses, and one of the best candidates for chemical strengthening.     

Effect of Electrolyte pH on Crack Propagation in Glass

Crack velocities in glass in various acids, bases, and neutral solutions were studied using the double-cantilever-beam technique. Results are explained in terms of crack-tip pH, which is controlled by the electrolyte at low crack velocities and by the glass composition at high velocities. The crack-velocity data are consistent with the known dependence of strength on pH for soda-lime silicate glass. Results also suggest that the slope of the universal fatigue curve should depend on surface pH.     

Statistical investigation of the influence of maximum shear stresses induced by thermal treatments on fracture toughness of soda-lime silica glasses: Comparison between Hertzian and Vickers indentations

Hertzian and Vickers indentation tests have been performed to estimate the hardness and the fracture toughness of a soda-lime silica glass fabricated by the float process. A comparison between as-prepared glass, annealed glass (90 min at 680°C), and tempered glass (quenched from 660°C to 25°C) has been conducted to investigate the influence of thermal treatments on fracture toughness. In this study, a new method based on acoustic emission, recorded during Hertzian indentation tests, has been used in order to determine precisely the minimum load for fracture of these glasses having various thermal histories. Experimental results have shown the existence of a threshold load below which no crack can be propagated in glass. These critical loads have been used to determine Weibull’s fracture laws as a function of surface quality and maximum shear stresses. It has been also shown that the presence of residual stresses induced by quenching leads to a shift of this threshold load and modifies Weibull’s laws. Therefore, this method, which requires no measurement of any crack length, can be used to accurately estimate residual stresses induced by quenching in soda-lime silicate glasses.     

Effect of Fictive Temperature on Dynamic Fatigue Behavior of Silica and Soda-Lime Glasses

The dynamic fatigue characteristics of silica glasses with fictive temperatures of 1000°, 1100°, and 1300°C and soda-lime glasses with fictive temperatures of 470° and 530°C were measured in air. For both glasses, samples with higher fictive temperatures had a greater fatigue resistance. Inert strength of silica glasses with flctive temperatures of 1000° and 1300°C was also measured at liquid nitrogen temperature. Glass with higher flctive temperature had a greater inert strength.     

Environment-dependent indentation recovery of select soda-lime silicate glasses

Indentations made on silicate glasses can easily be affected by the environment. In the present work, indentations were made on select commercial float glasses as well as on experimental soda-lime silicate glasses using a 1 mm diameter spherical tungsten carbide ball-mounted Brinell indenter. Recovery of indentations made on the glass samples was measured in different environments, namely, 100 °C, room temperature/room humidity and 100% relative humidity, as a function of time by using a Zygo laser non-contact profiliometer. Elastic (Young's modulus, bulk modulus, shear modulus and Poisson's ratio) and indentation (Vickers hardness, fracture toughness, brittleness and fracture surface energy) properties of the glasses were also determined by a pulse-echo and Vickers indentation methods, respectively, to correlate with the recovery of indentations. The elastic properties and Vickers hardness are directly proportional to the packing ions present in the glass structure and the strength of an individual bond, whereas the brittleness and fracture toughness more likely depend on molar volume of the glasses. According to the applied environment, a recovery rate of indentations follows the order: room temperature/room humidity <100% relative humidity <100 °C, regardless of glass composition. The reason for higher recovery rate of indentations is attributed to the structural relaxation, which is promoted by a thermodynamic driving force at 100 °C, and stored strain energy in deformation zone, allowing the indentations to regain their original configurations at certain points.     

Modeling the nanoindentation response of silicate glasses by peridynamic simulations

Nanoindentation is a widely used method to probe the mechanical properties of glasses. However, interpreting glasses’ response to nanoindentation can be challenging due to the complex nature of the stress field under the indenter tip and the lack of in situ characterization techniques. Here, we present a numerical model describing the nanoindentation of an archetypical soda-lime silicate window glass by means of peridynamic simulations. We show that, although it does not capture shear flow and permanent densification, peridynamics exhibits a good agreement with experimental nanoindentation data and offers a direct access to the stress field forming under the indenter tip.     

Measurement of Crack Tip Displacement Fields in Mixed Mode I–Mode III Fracture

A new method has been devised to propagate stable subcritical mixed mode I–mode III cracks in soda–lime–silicate and borosilicate glasses. The mode I content was measured using an original optical tunneling technique and mode III content via multiple-beam interferometry. Measurement of the mode III displacement field allows values of mode III fracture toughness, K_III, to be derived. K_III vs crack velocity (v) diagrams have been constructed for both glasses, and, in the case of the soda–lime–silicate glass, v vs K_III relationships have been determined for a variety of humidities. Comparison has been made with published K_I vs v data for this glass.     

Borofloat and Starphire Float Glasses: A Comparison,

Borofloat^® borosilicate float glass and Starphire^® soda-lime silicate float glass are used in transparent protective systems. They are known to respond differently in some ballistic and triaxial loading conditions, and efforts are underway to understand the causes of those differences. Toward that, a suite of test and material characterizations were completed in this study on both glasses so to identify what differences exist among them. Compositional, physical properties, elastic properties, flaw size distributions and concentrations, tensile/flexure strength, fracture toughness, spherical indentation and hardness, transmission electron microscopy, striae, high-pressure responses via diamond anvil cell testing, laser shock differences, and internal porosity were examined. Differences between these two float glasses were identified for many of these properties and characteristics, and the role of three (striae, high pressures where permanent densification can initiate, and submicrometer-sized porosity) lack understanding and deserve further attention. The contributing roles of any of those properties or characteristics to triaxial or ballistic loading responses are not definitive; however, they provide potential correlations that may lead to improved understanding and management of loading responses in glasses used in transparent protective systems.     

Dynamic Fatigue Analysis of Indentation Flaws Using an Exponential-Law Crack Velocity Function

The effect of residual contact stresses on the dynamic fatigue behavior of indentation flaws was analyzed using an exponential-law crack velocity function based on stress corrosion theory. Analysis of strength data from Vickers-indented soda-lime glass specimens in water showed that the contact residual stress can be properly accounted for and that the exponential-law fatigue parameters can be straightforwardly obtained.