Contact-Induced Static Fatigue of Annealed and Tempered Glass

The role of environmentally assisted crack growth in the contact-induced strength degradation of brittle surfaces was studied. Indentation fracture mechanics, incorporating a standard crack-velocity function, are used to predict remaining strength as a function of contact load and duration. Strength tests on annealed and tempered glass disks, indented with a diamond pyramid or tungsten carbide sphere in a water environment, are in accord with the predicted degradation characteristics. The results indicate that fatigue effects are likely to be of only secondary importance in designing for maximum resistance to in-service contact damage.     

Stress Corrosion of Silica Glass

Dissolution rate of silica glass in water was measured as a function of hydrostatic pressure at 285°C and was found to increase with increasing pressure. This observation appears to be contrary to the stress-corrosion hypothesis of the static fatigue of glass .     

Static Fatigue Limit with Particular Reference to Glass

After a review of the literature on the static fatigue limit, it was concluded that the normal method for its determination, using time-to-rupture data, is inadequate. There are very few experimental demonstrations of the existence of a true fatigue limit, and it is difficult to measure its value with any statistical confidence. An alternative experimental approach is suggested which is based on measuring the change in strength distribution of a representative sample after a high-temperature anneal under a fixed tensile stress. This stress weakens weak specimens in the as-received distribution, whereas strong specimens gain in strength. Those specimens whose strength remains constant demonstrate the existence of a fatigue limit and can be used to calculate a numerical value. The results are analyzed on the basis of the Charles-Hillig stress corrosion theory, which has been slightly modified to facilitate the examination of the parameters associated with the fatigue limit. This analysis relates the static fatigue limit to the effective surface energy appropriate to the fatigue mechanism, i.e. corrosion caused by water vapor. Some implications of the modified model are discussed.     

石油化工静设备的应力腐蚀与防护

以石油为原料的制品已经走入各行各业,石油化工行业对于社会繁荣和经济发展起到了不可磨灭的作用,其良好的发展对于国家富强,人民幸福有着重要的意义。然而,在石油化工行业的生产中,静设备经常受到应力腐蚀而损坏,对行业的生产带来了很大不便,也是一种巨大的损失,主要对静设备应力腐蚀成因及防护进行探讨。     

Stress Corrosion Cracking of Bioactive Glass Composites

Bioactive glass was reinforced with 20 vol% Ag particles and 20 vol% SiC whiskers. Fatigue parameters and lifetime data for the two composites were determined via the stressing rate dependence of the bend strength in a bio-simulating buffered solution (pH of 7.4). A lifetime of 10 yr was estimated for the SiC-reinforced composites under bend stresses of 34 MPa and for Ag composites under 23 MPa. Stresses for similar survival of pure bioactive glass are 17 MPa.     

石油化工静设备的应力腐蚀开裂与防护

石油是当前应用的最普遍的化石能源,石油化工行业也是当今社会上最为重要的支柱产业之一,它的良好发展对于社会经济的发展起着相当重要的作用。但是在石油化工行业的生产中却经常发生静设备的应力腐蚀破坏问题,对行业的正常生产活动产生重大的不良影响,并且会造成经济损失。为了改善因设备腐蚀而造成的生产效率下降和经济损失的问题,应当对设备进行相关的防护。     

Predicted Static Fatigue Behavior of Specially Coated Optical Glass Fibers

Long-term static fatigue behavior of optical glass fibers is controlled by their strength, fatigue resistance, and zerostress aging behavior. The effectiveness of four special coatings in preventing the long-term static fatigue deterioration of optical glass fibers was evaluated by determining the dynamic fatigue behavior and the effects of zero-stress aging on strength of the four specially coated optical glass fibers in water from 25° to 85°C. The results clearly show that the strength, fatigue resistance, and aging behavior varied significantly between these specially coated fibers. By analysis of these experimental results in terms of fracture mechanics principles, the predicted static fatigue behaviors of the four fibers were compared. Ideally the optimum fiber is one that exhibits a high strength, low strength variability, high fatigue resistance, and high aging resistance. Each of these specially coated fibers had a deficiency in at least one of these properties.     

Steric Effects in Stress Corrosion Fracture of Glass

Previous studies show that stress corrosion crack growth in glass is controlled by chemically enhanced crack tip bond rupture reactions. The brittle nature of fracture in glass suggests that the region where bond rupture reactions occur must be on the order of the atomic spacings in the material. Crack growth kinetics and zeolite diffusion data were used to determine the relation between molecular size and reactivity at the crack tip. Crack growth rates in silica glass were measured in the presence of a series of chemical species that have comparable chemical features and systematically increasing molecular diameters. Results show that chemically active species with diameters greater than 0.5 nm are ineffective as stress corrosion agents. A comparison of crack growth results and zeolite diffusion measurements was used to conclude that the opening to the crack tip is less than or equal to 0.5 nm. This crack tip dimension is consistent with the concept of atomic scale brittle fracture in silica glass.     

Extended Charles–Hillig Theory for Stress Corrosion Cracking of Glass

The work originally performed by Charles and Hillig (C&H) on the chemical stress corrosion cracking of glass is based on the chemical reaction rate theory and restricts the analysis to only the kinetic change at the exact location of the crack tip. As a result, crack sharpening/blunting is predicted when the applied stress lies above/below the static fatigue limit. The present paper extends the investigation within the same physical framework to the geometric change of the entire cavity surface, particularly in the vicinity of the cavity apex region. It has been found that a physical-property-dependent parameter ( m ) exists which exerts a strong influence on the crack-tip morphology. In the case of m = m _th, where m _th is a threshold m which assumes a value of ∼ 45 for an elliptical cavity having a minor to major axes ratio of 0.01, the current predictions reduce to the C&H results. In general, however, m ≠ m _th, and the single-valued fatigue limit degenerates into a range of applied stresses under which either enhanced blunting ( m > m _th) or necking ( m < m _th) is predicted to take place. Evaluation of m for soda–lime glass reveals that m > 45, suggesting that enhanced blunting takes place at the crack tip when external loads are applied at a moderate level for a typical crack having an initial major to minor axes ratio > 100 in a soda–lime glass specimen.     

Effect of Growth and Coalescence of Multisurface Cracks on Static Fatigue Behavior of Glass Ceramics

In order to study the effect of the interaction and coalescence of multisurface cracks on the static fatigue behavior of brittle materials such as glass and ceramics, a single crack and three collinear cracks were introduced on the surfaces of plate specimens of glass ceramics by Vickers microhardness indentation and were subjected to a constant four-point bending load. It was found that time-to-failure for a certain applied stress significantly decreased with the decrease in the crack center distance of indentation cracks, and the effect of interaction and coalescence of multicracks was shown to be significant. On the basis of the crack growth parameters for a single crack, time-to-failure for multicracks was predicted by taking the interaction and coalescence into consideration. It was demonstrated that the proposed multicrack coalescence model gives better predictions.     

Crack Growth in Soda–Lime–Silicate Glass near the Static Fatigue Limit

The atomic force microscope (AFM) was used to explore the nature of features formed on the surfaces of cracks in soda–lime–silicate glass that were held at stress intensity factors below the crack growth threshold. All studies were conducted in water. Cracks were first propagated at a stress intensity factor above the crack growth threshold and then arrested for 16 h at a stress intensity factor below the threshold. The stress intensity factor was then raised to reinitiate crack growth. The cycle was repeated multiple times, varying the hold stress intensity factor, the hold time, and the propagation stress intensity factor. Examination of the fracture surface by optical microscopy showed surface features that marked the points of crack arrest during the hold time. These features were identical to those reported earlier by Michalske in a similar study of crack arrest. A study with the AFM showed these features to be a consequence of a bifurcation of the crack surface. During the hold period, waviness developed along the crack front so that parts of the front propagated out of the original fracture plane, while other parts propagated into the plane. Crack growth changed from the original flat plane to a bifurcated surface with directions of as much as 3° to 5° to the original plane. This modification of crack growth behavior cannot be explained by a variation in the far-field stresses applied to the crack. Nor can the crack growth features be explained by chemical fluctuations within the glass. We speculate that changes in crack growth direction are a consequence of an enhancement in the corrosion rate on the flank of the crack at stresses below the apparent crack growth threshold in a manner described recently by Chuang and Fuller.     

Static Fatigue of Graphite

The static fatigue behavior of an extruded graphite was determined. The relation between time-to-rupture and the homologous fatigue stress (i.e. the ratio of the applied stress to the instantaneous fracture stress) is shown at two levels of applied stress. The significance of this relation is discussed, and its use in predicting cumulative fatigue damage is indicated.     

Strength and Static Fatigue of Abraded Glass Under Controlled Ambient Conditions: II, Effect of Various Abrasions and the Universal Fatigue Curve

A controlled grit blast was found to be a reproducible method of producing standardized damage to a glass surface. The effects of grit size, blast pressure, and amount of grit on the strength of the resulting specimens are reported. Static fatigue curves (strength vs. load duration) were obtained for specimens immersed in room-temperature distilled water and in liquid nitrogen (77°K.) after the specimens had been subjected to various abrasion treatments. The low-temperature strength was independent of load duration, and for surface damage of simple geometry it was inversely proportional to the square root of the initial crack depth, consistent with the Griffith theory. Abrasions of different geometry produced differing static fatigue curves at room temperature, and in one case curves actually crossed. If, however, the strength values for each abrasion were divided by the low-temperature strength for that abrasion and plotted vs. a reduced time coordinate, all the data could be fitted to a single "universal fatigue curve." This analysis led to a clear distinction between "linear" and "point" flaws, the former being flaws (such as emery scratches) which have an extension in a direction perpendicular to the applied stress and the latter being of a more localized character. Linear flaws fatigue more rapidly than point flaws by a factor of fifty and for each type of damage the fatigue rate is inversely proportional to the exponential of the initial flaw depth. A detailed analysis of the data in terms of several static fatigue theories from the literature shows that none of them provides a complete and adequate explanation of these results.     

Mechanisms of Glass Corrosion

An approach is developed for understanding formation of gel films on glass surfaces during water exposure. Compositional and microstructural data from ir reflection spectroscopy, scanning electron microscopy, and electron microprobe analysis are used with parameters defined from solution-analysis data to produce an SiO₂ composition profile of the corroded surface. This model profile is compared with a measured profile and used to interpret differences in corrosion resistance of binary Li₂O- and Na₂O-SiO₂ glasses. Effects of corrosion temperature on surface gel structure are also discussed.     

Strength and Static Fatigue of Abraded Glass Under Controlled Ambient Conditions: I, General Concepts and Apparatus

An extensive experimental study of the factors affecting the tensile strength and static fatigue of bulk glass has been conducted. To minimize the effects of past history of the specimens, all specimens were subjected to a controlled, reproducible surface abrasion. Time, temperature, and chemical environment were subject to control and systematic variation during the period from abrasion to test and during the strength test itself. Specimens consisted of flat laths tested in cross bending with the abraded spot in the center of the tension face. An electronically controlled electromagnetic tester permitted applications of pulse loads or constantly increasing loads with controllable durations from about 0.0025 second up to any desired value. The apparatus and methods for producing the abrasions, controlling the environment, and performing the tests are described in this paper. A brief review of the experimental background on the strength of glass is also presented as an introduction to the aims and concepts of this study.     

Determination of Crack Velocity as a Function of Stress Intensity from Static Fatigue Data

The estimation of lifetime of high-strength (≥1 GPa) glass samples with small cracks at low stress levels (≤ 150 MPa) requires a knowledge of the crack velocity, V , as a function of the stress intensity factor, K , in the range of V values lower than the current limit of measurements (about 10 ^–11 m/s). It is shown that V(K) data in this range can be obtained directly from static fatigue measurements without assuming a priori any form for V(K) . The method has been applied to the static fatigue data of Proctor, Whitney, and Johnson on silica fibers, and values of V in the range of 10^–11 to 10^–16 m/s (corresponding to K values of 0.35 to 0.20 Mpa·m^0.5) are reported.