Growth and Coalescence of Indentation Cracks in Glass

Indentation cracks show the initial decrease and subsequent increase in crack velocity with the stress intensity factor whether the cracks are single or coalescent. The decrease in crack velocity is mainly due to the residual stress effects. For two collinear cracks, the individual cracks before coalescence show crack velocity much higher than would be expected on the basis of the stress intensity factor for single or coalescent cracks.     

Fatigue Crack Growth from Small Surface Cracks in Transformation-Toughened Ceramics

A theoretical model based on the theory of complex potentials and dislocation formalism is used to simulate the fatigue crack growth of small cracks in a transformation-toughened ceramic. Assuming power-law crack growth in which the growth rate depends on the effective stress intensity at the crack tip instead of the applied stress intensity, it is shown that the crack growth rate decreases with the applied stress intensity in the initial stage of fatigue crack growth. This is in agreement with existing experimental evidence for the growth of small cracks in Mg-PSZ. New experimental results obtained by in situ observation in a scanning electron microscope of a similar material confirm this behavior. The numerical results also confirm the plausibility of using the steady-state toughness value obtained from quasi-static crack growth as a normalizing parameter in the power-law for fatigue crack growth.     

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.     

Cyclic Fatigue Life and Crack-Growth Behavior of Microstructurally Small Cracks in Magnesia-Partially-Stabilized Zirconia Ceramics

Cyclic fatigue stress/life ( S / N ) and crack-growth properties are investigated in magnesia-partially-stabilized zirconia (Mg-PSZ), with particular reference to the role of crack size. The material studied is subeutectoid aged to vary the steady-state fracture toughness, K_c , from ∼3 to 16 MPa · m^1/2· S / N data from unnotched specimens show markedly lower lives under tension—compression compared with tension—tension loading; "fatigue limits"(at 10⁸ cycles) for the former case approach 50% of the tensile strength. Under tension—tension loading, cyclic crack-growth rates of "long"(> 3 mm) cracks are found to be power-law dependent on the stress-intensity range, Δ K , with a fatigue threshold, Δ K _TH, of order 50% of K_c . Conversely, naturally occurring "small"(1 to 100 μm) surface cracks are observed to grow at Δ K levels 2 to 3 times smaller than Δ K _TH, similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the restricted role of crack-tip shielding (in PSZ from transformation toughening) with cracks of limited wake, analogous to the reduced role of crack closure with small fatigue cracks in metals. The implications of such data for structural design with ceramics are briefly discussed.     

Arrest of Fatigue Cracks in Transformation-Toughened Ceramics

A theoretical model based on the theory of complex potentials and dislocation formalism is used to simulate fatigue crack growth in a transformation-toughened ceramic. The effective stress-intensity factor is calculated during crack growth, because it is believed to determine the crack-growth rate similar to the Paris-type growth law. For certain combinations of transformation strength and load, the effective stress-intensity factor decreases to zero, indicating crack arrest. A detailed parametric study of this phenomenon reveals that the applied load and minimum transformation strength parameter necessary to cause crack arrest are linearly related, independent of initial crack length. This suggests that a threshold stress similar to the endurance limit in the conventional stress/life (S/N) approach should be used instead of the threshold stress-intensity factor in the design of transformation-toughened ceramics against fatigue.     

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.     

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.     

Healing of Lithographically Introduced Cracks in Glass and Glass-Containing Ceramics

The morphological evolution of lithographically defined cracklike flaws in glass and glass-containing ceramics was studied at elevated temperatures. The systems studied have glass contents from 100 to approximately 0.5 vol%, providing insight to the contribution of viscous flow of the glass to crack healing over a range of glass contents spanning many industrial ceramics. Healing behavior is found to be controlled by viscous flow of glass in all cases except the lowest glass content, for which significant mass transport is only accomplished by diffusional mechanisms. This implies a change of mechanism below some critical glass content.     

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 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.     

Effect of Cracks on Water Sorption of Glass

Water sorption on glass surfaces was measured by recording the weight gain of specimens exposed to a controlled-humidity environment. The difference between water sorption on glass surfaces with and without cracks introduced by controlled loading of a disk cutter is described. The increased sorption is explained in terms of condensation within the cracks.     

切口及纤维方向对短玻璃纤维增强尼龙疲劳特性的综合影响分析

在注塑成型形式下,通过设计两种不同方式及位置浇注口进浇,研究在不同进浇情况下,试样切口及纤维方向对短玻璃纤维增强聚酰胺6(尼龙6)拉伸及疲劳特性的综合影响。采用电子拉力机对试样进行拉伸及疲劳测试;采用计算机断层成像技术(CT)获得试样内部微观结构图像,采用平均截距长度对其试样的形态参数实施具体分析并讨论了相关结构;采用数字图像相关技术(DIC)对切口周围数字散斑动态应变实施测量分析。结果表明,不同方式与位置进浇可对试样特定几何形状切口的拉伸和疲劳强度产生很大的影响;拉伸和疲劳特性产生差异不是由纤维方向引起的,切口应变分布是相关特性产生差异的原因。     

Fatigue Behavior of Alumina–Zirconia Multilayered Ceramics

The influence of sustained and cyclic loading on the crack growth behavior of a multilayered alumina–zirconia composite exhibiting high internal compressive stresses is investigated. The study was conducted on precracked notched samples and focused on evaluating the static and cyclic fatigue resistance to crack extension beyond the first arresting interface (threshold) as well as the mechanisms involved during stable crack growth through the layered structure for each loading condition studied. Although it is found that the layered composite is prone to subcritical crack growth, the effectiveness of operative toughening mechanisms, i.e., compressive residual stresses as well as crack bifurcation and delamination at interfaces, is observed to be independent of the loading conditions. As a consequence, fatigue degradation of the multilayered ceramics studied is restricted to the intrinsic environmental-assisted cracking of the individual layers, pointing them out as toughened composites practically immune to variable stresses and much less static and cyclic fatigue sensitive than other structural ceramics.     

Behavior of Cyclic Fatigue Cracks in Monolithic Silicon Nitride

Cyclic fatigue-crack propagation behavior in monolithic silicon nitride is characterized in light of current fatiguecrack growth models for ceramics toughened by grainbridging mechanisms, with specific emphasis on the role of load ratio. Such models are based on diminished cracktip shielding in the crack wake under cyclic loads due to frictional-wear degradation of the grain-bridging zone. The notion of cyclic crack growth promoted by diminished shielding is seen to be consistent with measured (long-crack) growth rates, fractography, in situ crack-profile analyses, and measurements of back-face strain compliance. Growth rates are found to display a much larger dependence on the maximum applied stress intensity, K _max than on the applied stress-intensity range, Δ K , with behavior described by the relationship da/dN ∞ K²⁹_maxΔK^1.3. Fatigue thresholds similarly exhibit a marked dependence on the load ratio, R = K _min/ K _max; such effects are shown to be inconsistent with traditional models of fatigue-crack closure. In particular, when characterized in terms of K _max growth rates below ∼10⁻⁹ m/cycle exhibit an inverse dependence on load ratio, an observation which is consistent with the grain-bridging phenomenon; specifically, with increasing R, the sliding disance between the grain bridges is decreased, leading to less frictional wear, and hence less degradation in shielding, per loading cycle. The microstructural origins of such behavior are discussed.     

Static and Cyclic Fatigue Behavior of a Polycrystalline Alumina

The fatigue behavior of a polycrystalline alumina was investigated. Stress conditions consisted of a static tensile stress and a static tensile stress with superposed sinusoidal cyclic stress. The alumina exhibited the expected static fatigue behavior; a cyclic fatigue effect characterized by a frequency and amplitude dependence was also observed. Possible mechanisms of cyclic fatigue in brittle materials are discussed.