Strength of Lead Silicate Glass After H2 Reduction

Hydrogen-reduced lead silicate glass canes showed nearly a 70% increase in four-point bending strength over like specimens of the unreduced parent glass. No difference was observed in the strength of the reduced and the unreduced glasses after abrasion. Fractographs showed the presence of a surface layer (∼5 μm) in the reduced glass where the fracture traveled slowly. Secondary ion mass spectroscopic analysis showed this surface layer had decreased oxygen but increased Si⁺ and SiOH⁺ species. Although the possibility of surface compression could not be ruled out because of a possibly anomalous stress optical coefficient, it was concluded that the reduction altered the glass network to retard the subcritical crack velocity intrinsic to the system.     

Development and mechanical characterization of Al2O3 platelet-reinforced glass matrix composites obtained from glasses coming from dismantled cathode ray tubes

A cold-pressing and pressure-less viscous flow sintering treatment for the manufacturing of dense alumina platelet-reinforced glass matrix composites was proposed for the recycling of glasses coming from dismantled cathode ray tubes (CRTs). Mixtures of three different glasses from CRTs and Al₂O₃ platelets were investigated regarding the introduction in glass of rigid, non-sintering, inclusions and the nature of the matrix glasses. An innovative short-time sintering procedure was found to be advantageous, leading to significant increases in bending strength, microhardness and fracture toughness, despite the relatively low Al₂O₃ platelet addition. Both the morphology of the residual porosity, due to the sintering process and particular chemical and physical interactions within the matrix, and the crack deflection effect, due to the specific matrix-reinforcement combination, were found to be the determinant of the observed mechanical properties. The obtained bending strength, Vickers’ microhardness and fracture energy are comparable to the values reported for glass-ceramics for technical applications in the building industry.     

Mechanism of Mechanical Strength Increase of Soda–Lime Glass by Aging

Two models have been proposed to explain the mechanical strength increase of abraded or indented soda–lime glasses upon aging, namely, crack tip blunting and the release of residual tensile stress near the crack tip. To clarify the mechanism, the time dependence of the strengthening of an abraded soda–lime glass was investigated. Effects of aging media, such as moist air, distilled water, 1 N HCI and 1 N NaOH solutions, as well as the abrasion flaw depth, were determined. The strength increase rate in water of abraded soda–lime glass was compared with those of borosilicate and high-silica glasses. The effect of stressing during aging was also investigated. It was found that the rate of strength increase was faster with decreasing abrasion flaw depth and with decreasing chemical durability. For a given flaw depth, an acidic solution produced the fastest strengthening. The strengthening rate was found to accelerate because of the "coaxing'effect of stressing during aging. From these observations, it was concluded that the strengthening rates relate to the diffusion process and chemical reactions, especially the alkali–hydrogen (or hydronium) ion-exchange reaction, near the crack tip. The role of the residual tensile stress appears to be similar to that of the applied tensile stress, helping the diffusion process near the crack tip. The observed strength increase of soda–lime glass by aging was thus attributed to the effective blunting of the crack tip geometry by the glass–water reaction.     

Modelling the strengthening of glass using epoxy based coatings

Glass strength can be increased by applying epoxy based surface coatings. A number of models have been presented in the literature to explain the strengthening afforded by these coatings but until now there has been no clear evidence to definitively support one model over another. In this work, finite element models (FEM) of four-point bending test specimens have been developed. These models have been used to study the strength of cracked uncoated and surface coated specimens in order to identify the strengthening mechanism. The FEM results showed that full filling of the crack using epoxy coating is sufficient to heal the crack if the coating inside the crack is ideally glued to the crack surfaces. It is also shown that under these circumstances the coating modulus is relatively unimportant parameter. FEA results for partially filled cracks show that increasing the filled percentage increases the strengthening. Fractographic analysis of the 10 kg indented and coated samples showed that the fracture surfaces do not follow the median crack symmetric plane and that fracture started from another plane when coated properly, however the fracture surface of these samples still starts from the indentation site. On the other hand, fractographic analysis of the 1 kg indented and properly coated samples showed that the samples failed from their edges which indicate that the crack was overcome. The finite element results show that the diamond imprint resulting from the Vickers indentation play an important role in this type of fracture.     

Fracture Surface Energy of Glass

Fracture surface energies of six glasses were measured using the double-cantilever cleavage technique. Values ranged from 3.5 to 5.3 J/m² depending on the chemical composition of the glass and the temperature of the test. The fracture surface energy increased with decreasing temperature and increasing Young's modulus; however, exceptions to this behavior were noted. The magnitude of the values obtained is discussed with respect to the theoretical strength of glass and possible irreversible effects at the crack tip such as stress corrosion and plastic deformation are considered.     

The fracture toughness of inorganic glasses

Measuring the fracture toughness (K_Ic) of glasses still remains a difficult task, raising experimental and theoretical problems as well. The available methods to estimate K_Ic are reviewed, with emphasis on their respective advantages and drawbacks. In view of our current understanding, this analysis gives precedence to the SEPB method. The ultimate glass strength, the critical flaw size, and the indentation load for the onset of crack initiation are discussed, in the light of the fundamentals of fracture mechanics and classical background regarding the mechanics of brittle materials. Analytical expressions were further proposed to predict the fracture energy and fracture toughness of glasses from different chemical systems from their nominal compositions. The theoretical values were compared with the experimental ones, as obtained by self‐consistent methods when available. The agreement observed in most cases suggests that measured K_Ic values correspond to the crack propagation regime (as opposed to the crack initiation threshold), and supports previous investigations in glasses and ceramics, which showed that a crack tip is nearly atomically sharp in these materials (but for metallic glasses). Some ideas to design tougher glasses are finally presented.     

Effects of acid leaching treatment of soda-lime silicate glass on crack initiation and fracture

Water or acid soaking surface treatments have been shown to increase the mechanical strength of soda-lime silicate (SLS) glasses. This increase in strength has traditionally been attributed to effects related to residual stress or changes in fracture resistance. In this work, we report experimental data that cannot be explained based on the existing knowledge of glass surface mechanics. In dry environments, annealed and acid-leached SLS surfaces have comparable crack initiation stress and fracture stress as measured by Hertzian indentation and biaxial bending tests, respectively. Yet, in the presence of humidity, acid-leached surfaces have higher failure stress than the annealed surfaces. This apparent enhancement in the crack resistance of the acid-leached surface of SLS glass in humid environments supports the hypothesis that acid-leached surface chemistry can lower the transport kinetics of molecular water to critical flaws.     

加固混凝土梁的GFRP板抗拉强度与界面切应力

 为了研究玻璃纤维布的拉伸强度,通过5个涂浸渍胶和未涂胶的试件进行了轴向拉伸实验。结果表明,环氧 树脂不仅有粘贴和保护玻璃纤维布的作用,而且能够提高其拉伸强度。为进一步分析玻璃纤维强化塑料(GFRP)板对 加固结构的影响,特制备9个单层GFRP板加固混凝土梁试件,对其进行四点弯曲加载实验,并在其中一个试件的 GFRP表面粘贴了应变片,进行电测跟踪测试,从而得到结构损伤破坏过程曲线和应变片粘贴处的应变值。对比两种 实验得知,GFRP板轴向拉伸强度比四点弯曲实验得到的强度大。又由测试数据和力学模型,得到FRP板轴力以及与 混凝土间界面上的切应力分布曲线。     

Correlation of Strength of Glass with Fracture Flaws of Measured Size

The purpose of this study was to develop a method of producing a single small crack on a glass surface which would become the initiating flaw in the fracture process. Techniques for measuring dimensions of this flaw also formed an essential part of the study. The cracks were formed by pressing a wedge-shaped tungsten carbide point against the glass. Measurements were made from visible patterns found on the fracture surface after the glass was broken. Strict adherence to carefully worked out procedures was found to be essential for obtaining satisfactory cleaved cracks. Based on fracture caused by a semicircular crack 0.002 in. deep, the 1-second breaking stresses of five different glasses were as follows: lead-alkali glass 6800, soda-lime glass 9000, 96% silica glass 11,500, low-expansion borosilicate glass 12,000, and aluminosilicate glass 13,800 lb. per sq. in. These values are believed to correlate with the intrinsic strengths of the glasses. This method should have definite advantages in the investigation of fracture phenomena in glasses and in other brittle materials, specifically the effects of composition on strength and the causes of stress fatigue     

Effect of Loading Rate and Surface Conditions on the Flexural Strength of Borosilicate Glass

This study evaluates the loading rate and surface condition dependence of the flexural strength of a borosilicate glass. The glass specimens are subjected to three different surface treatments before four-point bending tests to study the effect of surface flaws. Quasistatic (Material Test System 810) and dynamic (Kolsky bar) experiments are performed at loading rates ranging from 0.7 to 4 × 10⁶ MPa/s. The results show that the flexural strength of the borosilicate glass has a strong dependence on the loading rate. A chemically etched surface produces an enhanced flexural strength by about an order of magnitude. Scanning electron microscopy images on fracture surfaces indicate that the failure is governed by different types of flaws under different surface treatment conditions. Edge failure is also identified for samples possessing high flexural strength.     

Modeling Slow Crack Growth Behavior of Glass Strengthened by a Subcritical Tensile Stress Using Surface Stress Relaxation

Glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. It has been observed by several authors that when an oxide glass with a large crack is held under a subcritical stress intensity (where no slow crack growth occurs) in room‐temperature water vapor or liquid water, upon reloading to a higher stress intensity, a finite restart time is observed prior to measurable crack extension. This phenomenon of apparent strengthening, or crack arrest, has been attributed to concepts such as corrosive dissolution of the crack tip, crack tip blunting, or water diffusion, and subsequent swelling of the material around the crack tip. Recently, a newly observed surface stress relaxation process that is aided by molecular water diffusion was used to improve the mechanical strength of glass fibers and to explain the subsurface compressive stress peak observed in ion‐exchange strengthened glasses. The same process is employed here to explain these delayed slow crack growth data. A simple mathematical model has been developed utilizing water‐assisted surface stress relaxation and fracture mechanics. Predictions of restart times using the model agreed well with published experimental data, indicating that surface stress relaxation is responsible for the anomalous delayed slow crack growth behavior.     

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.     

Application of Fracture Mechanics to Space-Shuttle Windows

The fracture properties of an ultralow-expansion glass intended for use in windows for the Space Shuttle were characterized by strength and fracture-mechanics techniques to provide reliable design data. Proof-test diagrams for predicting minimum times-to-failure under specified service loads were developed from measurements of subcritical crack growth in water and air. Failure predictions were confirmed from strength measurements in water. In vacuum (<10⁻⁴ torr), the fracture behavior was similar to that of other high-SiO₂ glasses, as evidenced by the absence of subcritical crack growth and by insensitivity of the critical stress intensity factor to temperature.     

Quantitative Characterization of Mechanical Stress Field and Fracture Strength in Isotropic Brittle Materials During Crack Tip Propagation

A generalized method for characterizing mechanical stresses in brittle materials during crack tip propagation is presented. This approach was derived from classical fracture mechanics and is therefore mechanism independent and applies to any isotropic brittle material, including glasses, fine grained ceramics or metals, and high stiffness polymers. A practical implementation demonstrates the merits of this technique: the fracture strength can be determined by characterizing the angle between the free surface of a flexural overload fracture and stress intensity factor loci. The accuracy of this method was phenomenologically validated using flexural strength tests on glass as a model material system. In addition, such fractographic measurements can also be used to characterize an inhomogeneous internal stress field, and thereby, for example, help discriminate whether the sample failed due to pure bending loads alone, or whether membrane stresses were also present.     

Molecular dynamics study on nano‐particles reinforced oxide glass

Energy release rate and fracture toughness of amorphous aluminum nanoparticles reinforced soda‐lime silica glass (SLSG) were measured by performing fracture simulations of a single‐notched specimen via molecular dynamics simulations. The simulation procedure was first applied to conventional oxide glasses and the accuracy was verified with comparing to experimental data. According to the fracture simulations on three models of SLSG/‐Al₂O₃ composite, it was found that the crack propagation in the composites is prevented through following remarkable phenomena; one is that a‐Al₂O₃ nanoparticles increase fracture surface area by disturbing crack propagation. The other is that the deformation of a‐Al₂O₃ nanoparticle dissipates energy through cracking. Moreover, one of the models shows us that the crack cannot propagate if the initial notch is generated inside a‐Al₂O₃ nanoparticle. Such strengthening is partly due to the fact that the strength of the interface between nanoparticle and SLSG matrix is comparable to that of SLSG matrix, implying that their interface does not reduce crack resistance of the oxide glass.     

Rotational flexural strength of cylindrical brittle specimens

Conventional static flexural strength testing of brittle cylindrical rods only subjects a small fraction of the entire specimen's area or volume to the maximum tensile stress. Thus, a nonconservative measured strength likely results since most flaws on the surface or in the bulk are not subjected to a sufficiently high tensile stress that can cause fracture. To mitigate this, a rotational flexural tester and corresponding test method were developed whereby rotation and monotonically increasing three-point flexure were superimposed to investigate fracture response of solid glass cylinders. This combination of rotation and flexure subjects more area and volume of a cylindrical test specimen to tensile stress than a standard static (nonrotating) flexural test. As anticipated, failure stresses were lower for the rotational flexural test. Expressions for effective area and volume are provided for rotating solid rods and tubes subjected to three-point, four-point, uniform, and uniformly distributed load bending configurations.     

On the kinetics of crack growth in glass

This paper reports on the results of investigations of the crack growth kinetics in glasses under different test conditions (in media containing moisture and under vacuum with the complete removal of water from the glass surface) in the test temperature range from room temperature to 430°C. The specific features of the glass fracture at the subcritical stage of crack growth and the critical fracture stage have been discussed. The results obtained have been analyzed with invoking the data available in the literature.     

Surface damage resistance and yielding of chemically strengthened silicate glasses: From normal indentation to scratch loading

We report on surface elasticity, plastic deformation and crack initiation of chemically strengthened soda-lime silicate and sodium aluminosilicate glasses during lateral indentation and scratch testing. Instrumented indentation using a normal indenter set-up corroborated previous findings on the effects of chemical strengthening on surface Young's modulus, hardness, and indentation cracking. Using lateral indentation in the elastic-plastic regime, we find a pronounced increase in the scratch hardness as a result of chemical strengthening, manifest in higher work of deformation required for creating the scratch groove. Thereby, the glass composition is found to play a stronger role than the absolute magnitude of surface compressive stress. Using a blunt conical stylus for instrumented scratch testing reveals three distinct modes of scratch-induced surface fracture, which occur during scratching or after unloading. Occasional micro-cracking caused by pre-existing surface flaws at low scratching load can be completely suppressed through chemical strengthening. The intrinsic defect resistance to microcracking is reduced as a result of ion stuffing, depending on the initial glass composition, whereas the resistance to abrasive yielding is enhanced by several hundred MPa.     

Fracture Strength of α- and β-SiAION Measured by Biaxial and Four-Point Bending

The fracture strength of an α- and β-SiAION was measured by biaxial and four-point bending. By utilizing Weibull statistics a close prediction could be made of the four-point values from the biaxial values. At a fracture strength of 460 MPa, fracture was initiated by linking of surface and subsurface flaws.