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.     

Failure Mechanism Associated with the Thermally Grown Silica Scale in Environmental Barrier Coated C/SiC Composites

The barium strontium aluminosilicate and Y₂Si₂O₇‐BSAS‐coated C/SiC composites were corroded in 50%H₂O–50%O₂ environments at 1250°C, respectively. It was found that the coated composites suddenly lost their strength as the corrosion time was up to 250 and 750 h, respectively. During the water vapor corrosion, a continuous silica scale was formed between the SiC bond coat and environmental barrier coatings, leading to the growth stress. The thickness of silica scale grew with the prolonged corrosion time, accompanied with the accumulation of growth stress in the silica scale. When the growth stress was greater than the bond strength between silica scale and SiC bond coat, the cracks would form and propagate along their interface, resulting in the spallation of EBCs, followed by the failure of the composites.     

Post-Indentation Slow Growth of Radial Cracks in Glasses

An expression is developed for the slow growth of radial cracks after indentation using established notions of slow crack growth and residual stresses at the tip of radial cracks. This expression is substantiated by data gathered on indentation of microscope slides tested in air and water environments. The agreement is reasonable with the published values of the glass stress-corrosion susceptibility coefficients. It is suggested that the present approach represents a simple and straightforward method of measuring the stress-corrosion susceptibility of glasses.     

Stress Corrosion of Ionic and Mixed Ionic/Covalent Solids

Slow crack growth data, surface reaction studies, and solubility results for sapphire and magnesium fluoride crystals are used to identify stress corrosion mechanisms for each material. Results suggest that stress corrosion in sapphire proceeds by the dissociative chemisorption of environmental species on strained crack tip bonds. Crack growth rates measured in nonaqueous environments and isotopically labeled water suggest that ion solvation rather than dissociative chemisorption dominates the crack tip interaction in magnesium fluoride.     

Biaxial strength and slow crack growth in porous alumina with silica sintering aid

Biaxial strength, fracture toughness and subcritical crack growth are reported for coarse grained porous alumina ceramics. The materials were prepared with a varying amount of a silica sintering aid, which resulted in the formation of a glassy secondary phase at the grain boundaries. Crystalline mullite was additionally found in the material with the highest silica content. The biaxial strength, measured by Ball-on-Ring and Ball-on-3-Balls, was highest for the material without mullite at the grain boundaries, and the biaxial strength decreased with increasing porosity. The fracture toughness of the materials was in the range of 1.7–1.9 MPa m^0.5. Measurements of subcritical crack growth by a modified lifetime method in air and aqueous environments demonstrated a higher crack growth rate in water and acid relative to in air. The effect of porosity and grain boundary phase were discussed in relation to subcritical crack growth and fracture mode in the coarse grained alumina ceramics.     

Delayed Failure of Plasma-Sprayed Al2O3 Applied to Metallic Substrates

Aluminum oxide coatings were applied to the ends of cylindrical metal samples of both 316L stainless steel and TS–6Al‐4V ELI by a plasma spray process. These samples, fabricated into 4-point bend specimens, were used to determine both the strength and slow crack growth characteristics. The data obtained indicated that both systems were susceptible to stress corrosion. When the substrate was the titanium alloy, the delayed failure behavior was characterized by two parallel stress-corrosion reactions: one at the ceramic-metal interface and one in the coating itself. The latter became dominant at low stresses. Finally, the nature of the epoxy adhesive used to fabricate the 4-point bend samples was found to strongly influence the fatigue behavior at long failure times.     

Scanning Tunneling Microscopy of Optical Fiber Corrosion: Surface Roughness Contribution to Zero-Stress Aging

Strength, fatigue resistance, and zero-stress aging behavior control the long-term mechanical reliability of optical fibers. Zero-stress aging refers to the loss of strength of high-strength glass fibers after exposure to some corrosive environments in the absence of stress. Understanding the effect of the chemical environment under zero stress on the subsequent fracture strength of optical fibers is important because optical fibers in service will probably encounter water and other chemical species while exposed to zero- or low-stress conditions. In this work, the strength of fibers aged under zero-stress conditions at 80°C in deionized water has been measured. Scanning tunneling microscopy was also used to measure the roughening of the fibers from corrosion at intervals during the aging. The product of the median inert strength of fibers aged for various times and the square root of the roughness depth of fibers was constant within experimental error. The results show that surface roughening contributes to zero-stress aging in silica fibers.     

Failure of Glass with Subthreshold Flaws

Conventional postthreshold crack analysis cannot be used to predict the strength and fatigue behavior of glass with subthreshold flaws. Therefore, a fracture mechanics model for failure of glass with subthreshold indentation flaws was developed. This model accounts for both the near- and farfield residual stresses associated with the indentation impression. It is shown that these stresses play a major role in the initiation and subsequent propagation of cracks that eventually cause failure. The model predicts "pop-in" of a well-developed crack and failure under continuous and discontinuous crack growth in both inert and fatigue conditions. The results of experiments with bare fused silica fibers with indentation subthreshold flaws in inert and fatigue (water) environments were in good agreement with the predictions by the model.     

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.     

Evolution of Stress Failure Resulting from High-Temperature Stress-Corrosion Cracking in a Hot Isostatically Pressed Silicon Nitride

Stress-corrosion cracking in a commercially available, hot isostatically pressed (HIPed), yttria-fluxed, silicon nitride was the prevalent mode of failure in specimens creepruptured at 1370°C. High-temperature diffusional processes associated with oxygen were responsible for the creation of an advancing stress-corrosion front that had formed at the specimen surface and advanced radially inward. The volume of material in the wake of the stress-corrosion front possessed a high concentration of lenticular cavities at two-grain boundaries, a high concentration of multigrain junction cavities, and large amorphous "pockets" in other multigrain junctions that were abnormally rich in oxygen and yttrium. The combination of tensile stress and the high concentration of cavities in the near-surface volume of the material resulted in microcrack coalescence or the formation of a planar, stress-corrosion crack . The concurrent growth of the stress-corrosion front and crack during the tensile creep-rupture tests ultimately led to stress-induced failure.     

第1水加热器底封头与接管部位裂纹成因及改进

分析了第1水加热器底封头与接管区域裂纹部位的化学组分、裂纹走向、裂纹断口、裂纹剖面形态及泥状残留物等特征,判定裂纹属应力腐蚀裂纹。根据应力腐蚀断裂机理并针对腐蚀原因,提出相应的改进、防范措施。新设备投运后,经8年多的运行,效果良好。     

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.     

Subcritical Crack Propagation in 3Y-TZP Ceramics: Static and Cyclic Fatigue

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y₂O₃ (3Y-TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10^-12-10^-3 m/s in several environments, including air, water (in the temperature range of 3°-85°C), secondary vacuum (10^-5 mbar), and silicon oil. Analysis of the experimental results-three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs-shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static-fatigue/cyclic-fatigue sequences show that both stress corrosion by water and pure cyclic-fatigue effects are operative under alternative stresses.     

Corrosion and Crack Growth in 33% Na2O-67% SiO2 and 33% Li2O-67% SiO2 Glasses

Environmentally enhanced crack growth data for 33% Na₂O-67% SiO₂ and 33% Li₂O-67% SiO₂ (mol%) were obtained. Corrosion data for the same glasses as determined by infrared reflection spectroscopy and atomic absorption spectroscopy were also measured. It was determined that, of the tested environments, those and only those environments which caused surface corrosion also enhanced crack growth. Crack growth curves were more complicated for these materials than for fused silica. Water-dominated regions occurred even in environments which are known to enhance crack growth.     

Fatigue Mechanisms in High-Strength Silica-Glass Fibers

We use experimentally determined crack growth data for silica glass and a fracture mechanics model for delayed failure to predict the fatigue behavior for high-strength silica-glass fibers. The results of this model indicate that fracture mechanics methods can be used to adequately describe the fatigue behavior observed for high-strength silica-glass fibers at room temperature in humid conditions. The key feature to properly interpreting the fatigue of high-strength fibers is the use of a fracture-rate law in which the crack extension rate increases exponentially with applied stress. We show that a fracture mechanics approach to highstrength fiber fatigue can provide the basis for identifying additional fatigue mechanisms that may control failure in more aggressive fatigue environments.     

液化气球罐Zare合金防护层涂装工艺

液化气球罐在使用过程中因球罐内壁存在SSCC(硫化物应力腐蚀开裂)腐蚀环境,造成球罐内壁焊道热影响区出现裂纹,导致球罐内壁腐蚀.本文介绍了采用涂覆稀土合金(Zare)涂层和封闭剂的方法解决液化气球罐内壁SSCC腐蚀问题,着重讨论了球罐内壁防腐涂装工艺及其注意事项.     

Degradation of Stressed Optical Fibers in Water: New Worst-case Lifetime Estimation Model

Simultaneously determined results from zero-stress aging and static fatigue measurements in water at 65°, 80°, and 95°C have been combined into a worst-case fiber lifetime model which shows good agreement between predicted and observed failure times. The model shows that the lifetime in a wet environment of the largest crack at normal service strain (0.1%) is nearly identical to the lifetime of the pristine fiber and that the initial strength and the stress corrosion exponent are of minor importance for the lifetime. In a separate experiment, it is shown that degradation in a water-saturated, jelly-filled cable is significantly slower than when fibers are directly immersed in water. For a fast-degrading fiber, evidence of a strength-increasing mechanism is presented.     

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.     

Mechanical Properties of Densely Sintered High-Purity Titanium Diborides in Molten Aluminum Environments

Titanium diboride powder, produced in an arc-plasma reactor, was pressureless-sintered to near theoretical density to produce materials of different grain sizes. Mechanical properties in an aluminum environment at 960° to 1000°C were studied by strength vs stressing rate and KIc tests. All materials experienced liquid-metal embrittlement in aluminum containing Fe, Si, and P impurities. During exposure, these metals penetrate TiB₂ grain boundaries and the impurities preferentially segregate there. Fine- and coarse-grained materials exhibited stress-strengthening (negative N ) and stress-corrosion (positive N ) slow crack growth behavior, respectively. Results were interpreted in terms of opposing slow crack growth mechanisms.     

Mixed-mode fatigue behavior of degraded toughened epoxy adhesive joints

Open-faced asymmetric double cantilever beam (ADCB) specimens of toughened epoxy-aluminum adhesive joints were aged either in a constant humidity environment or a cyclically changing environment to study the mixed-mode fatigue behavior. Under constant humidity environments, the fatigue threshold strain energy release rate initially decreased with aging time until it reached a constant minimum value for long times. In contrast, the crack growth rates continued to increase with aging time. It is hypothesized that at crack growth rates close to threshold the fatigue behavior is governed by the epoxy matrix, whereas at relatively high crack growth rates the fatigue behavior is governed by the loss of the rubber toughening mechanism. Increasing the aging temperature accelerated the degradation of the joints leading to a reduction in the time to reach the constant minimum value and increased the crack growth rates.Under a cyclic aging environment with intermittent salt spray, neither the threshold strain energy release rate nor the crack growth rates degraded until four weeks of aging. The superior fatigue performance of these joints compared to joints aged in constant humidity environments was due to the lower water concentrations in the adhesive while aging. This conclusion was supported by moisture uptake measurements of the adhesive in deionised and salt water environments that showed simple Fickian behavior at room temperature and dual-Fickian behavior at higher temperature. The salt water environment produced osmotic pressure that decreased the moisture concentration in the second stage of diffusion.