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.     

A Molecular Mechanism for Stress Corrosion in Vitreous Silica

The mechanical strength of most glasses and ceramics decreases with time under static loading in an ambient environment. This strength loss is associated with slow growth of preexisting surface flaws due to stress corrosion by water from the surrounding environment. We studied stress corrosion in vitreous silica exposed to water and several nonaqueous environments; environments which enhance stress-corrosion crack growth in silica contain active groups with electron donor sites on one end and proton donor sites at the other. These results suggest a detailed chemical model for the interaction of the environment with mechanically strained bonds in the solid at the tip of a crack. The proposed model for stress-corrosion crack growth also has implications for the long-term strength behavior of a wide variety of brittle materials.     

Crack-Tip Structure in Soda–Lime–Silicate Glass

The topology of crack tips in soda–lime–silicate glass was investigated using atomic force microscopy (AFM). Studies were conducted on cracks that were first propagated in water and then subjected to stress intensity factors either at or below the crack growth threshold. Exposure to loads at the crack growth threshold resulted in long delays to restart crack growth after increasing the stress intensity factor to higher values. After breaking the fracture specimen in two, the "upper" and "lower" fracture surfaces were mapped and compared using AFM. Fracture surfaces matched to an accuracy of better than 0.5 nm normal to the fracture plane and 5 nm within the fracture plane. Displacements between the upper and lower fracture surfaces that developed after a critical holding time were independent of distance from the crack tip, and increased with holding time. Despite the surface displacement, crack tips appeared to be sharp. Results are discussed in terms of a hydronium ion–alkali ion exchange along the crack surfaces and corrosion of the glass surface near the crack tip by hydroxyl ions.     

Creep-Crack Growth by Damage Accumulation in a Glass-Ceramic

The growth rate, near-tip creep response, and damage processes of creep cracks in a pyroceram glass-ceramic were studied under tensile loading at elevated temperatures. The rates of crack extension were characterized as a function of the applied stress intensity factor. The damage processes which occurred near the crack tip and led to creep crack extension were identified using a replica technique and by direct observations in a scanning electron microscope equipped with a high-temperature loading stage. The accumulated creep strains near the crack tip were measured via the stereoimaging technique. The results indicate that creep-crack growth in the pyroceram glass-ceramic occurs in both continuous and discontinuous manners, with the damage processes manifested as the nucleation, growth, and coalescence of inhomogeneously distributed cavities and microcracks. Measurements of the total accumulated creep strain near the crack tip suggest that crack extension follows a critical strain criterion. Both the microcrack density and the total accumulated creep strain show similar dependence with distance from the crack tip. These observations suggest that damage accumulation and crack extension in the glass-ceramic are controlled by the near-tip creep rates.     

High-Temperature Stress Corrosion Cracking in Ceramics

Single-phase ceramics are shown to be susceptible to stress corrosion cracking at elevated temperatures in the presence of a wetting amorphous deposit. Crack arrest occurs when the crack is depleted of amorphous material, provided that the stress intensity is below a "blunting" threshold. A preliminary model of such cracking has been developed. The model considers stress corrosion as a process wherein the corrosive medium, by virtue of its wetting characteristics, allows the crack to propagate along grain boundaries. A crack velocity that depends sensitively on the dihedral angle is predicted. Furthermore, since the corrosive medium acts as a conduit for rapid atom transport to the crack tip, the crack velocity also exhibits a strong dependence on the viscosity of the fluid medium. Implications for such stress corrosion processes on premature failure are discussed.     

Real-Time Observation of a Non-Equilibrium Liquid Condensate Confined at Tensile Crack Tips in Oxide Glasses

As crack propagation in oxide materials at low crack velocities is partly determined by chemical corrosion, proper knowledge of the crack tip chemistry is crucial for understanding fracture in these materials. Such knowledge can be obtained only from in situ studies because the processes that occur in the highly confined environment of the crack tip are very different from those that take place at free surfaces, or that can be traced post mortem . We report on the occurrence of a hydrous liquid condensate between the two fracture surfaces in the vicinity of the tip of tensile cracks in silica. Observations are performed in real time by means of atomic force microscopy (AFM) at continuously controlled crack velocities in the regime of stress corrosion. Condensate formation and changes in the extent and the shape are demonstrated for a wide range of macroscopic humidities at different crack speeds. Its liquid character is confirmed by the study of AFM phase-contrast data. It is believed that this evidence of a nanoscale liquid hydrous phase at the crack tip will provide novel insights into the chemistry of failure of oxide materials.     

Phase angle analysis for stress corrosion cracking of carbon steel in fuel-grade ethanol: Experiments and simulation

The stress corrosion cracking (SCC) of carbon steel in simulated fuel-grade ethanol (SFGE) was investigated using electrochemical impedance spectroscopy (EIS) and slow strain rate test (SSRT). Phase angle at low frequency range (<1 Hz) is sensitive to SCC process of carbon steel in SFGE. Phase angle decreases during an active crack growth. Frequency at maximum phase angle also increases towards an active cracking region at around 1 Hz. A transmission line model (TLM) is used to simulate the EIS response for SCC based on real geometrical parameters. By systematically changing the values of the circuit elements, the activities of the sample surface, crack tip and crack wall were studied in detail. By comparing the Bode plots from both experiment and simulation during SCC, a detailed mechanistic picture is derived to describe the behavior of the stress corrosion crack on carbon steel in the fuel-grade ethanolic environment.     

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.     

Temperature/rate dependence of yield and fracture behaviour of phenolphthalein poly(ether ketone)

The rate/temperature dependence of yield stress, tensile modulus and crack opening displacement of phenolphthalein poly(ether ketone) (PEK-C) has been investigated. The rate/temperature dependence of crack opening displacement and the correlation established between stress intensity factor, K_IC, yield stress, and type of crack growth suggest that the extent of crack tip blunting largely governs the relative toughness of PEK-C and induces transitions in the types of crack growth observed. A quantitative expression is then presented which successfully describes the fracture toughness values over a wide range of temperatures and rates.     

氧化锆增韧陶瓷的室温动态疲劳

用动态疲劳试验法研究了３Ｙ－ＴＺＰ和３Ｙ－ＴＺＰ／Ａｌ_２Ｏ_３（２０ｗｔ％）陶瓷在空气中的室温动态疲劳，并讨论了疲劳慢裂纹扩展特性。另外利用动态疲劳数据对两种陶瓷的平均寿命进行了预测。两种陶瓷材料在室温下均存在着慢裂纹扩展，主要是由空气中水蒸汽的应力腐蚀所造成的，且裂纹是沿晶界玻璃相扩展的。相变诱发的表面压应力和裂纹尖端的正应变可提高疲劳抗力。在８００ＭＰａ应力作用下，３Ｙ－ＴＺＰ和３Ｙ－ＴＺＰ／Ａｌ_２Ｏ_３（２０ｗｔ％）的平均寿命分别为２４ｍｉｎ和７２ｈ，平均寿命随应力的增大而缩短．     

Static fatigue and compressive stress generation in an aged crack

In classic experiments by Michalske and others, it was found that cracks aged statically below the fatigue limit acquired a temporary strength increase compared to the non‐aged crack. In our previous publication we observed that cracks growing near the fatigue limit exhibited a time dependent slowing down of crack growth. Both of these phenomena are related to a toughening of the crack tip that we attribute to a water‐assisted surface stress relaxation mechanism. To test this hypothesis, the K‐v crack growth curves have been measured using the double cantilever beam (DCB) experimental technique for two commercial glasses, a sodium aluminosilicate, and a potassium aluminosilicate, both of which exhibit clear fatigue limits in air. Using polarimetry, it is shown that the stress state near an unloaded but previously aged crack tip is opposite in sign to the stress state near the tip of a crack held in Mode I loading. These results clearly indicate that a stress relaxation mechanism is occurring at the crack tip.     

Static and cyclic crack propagation in Ce-TZP ceramics with different amounts of transformation toughening

Ceria partially stabilized zirconia ceramics (Ce-TZP) with identical grain size and different amounts of transformation toughening were processed to investigate the influence of phase transformation on static and cyclic fatigue crack growth.Static crack growth is governed by environmentally stress induced corrosion at the crack tip and it is highly influenced by the crack shielding due to the phase transformation. Three fatigue mechanisms are expected to be operative at different proportions depending on the amount of transformation: wedge effect due to debris, degradation of bridging and modification of the shielding effect of the transformation zone. However, it is difficult to separate the contribution of the different mechanisms as grain bridging is induced by crack arrest due to phase transformation.     

Dynamic Fatigue of Treated High-Silica Glass: Explanation by Crack Tip Blunting

In previous papers, it was shown that when abraded high-silica glass is soaked in hot water or annealed in air, its crack tip becomes blunt. The dynamic fatigue characteristics in various liquids of high-silica glass with different treatments were examined. The stress rate dependence of fracture strength was determined for freshly abraded samples and for abraded and soaked (in hot water) or heat-treated samples, in water, hydrazine, formamide, and acetonitrile. The stress rate susceptibility of the fracture strength was represented by the commonly used n value. Freshly abraded samples exhibited approximately the same low n values for all the liquids, which is consistent with the slope of the slow crack growth data. Treated samples, on the other hand, showed a slightly larger n value in water and extremely large n values in nonaqueous liquids. When the crack tip is sharp, only slow crack growth is involved in the fatigue. When the crack tip is blunt, crack initiation is involved in addition to the crack growth. The equation of dynamic fatigue based upon slow crack growth was modified to incorporate this effect of crack initiation and was applied to the present experimental results. The analysis showed that the crack initiation stress is stress rate dependent in water and is stress rate independent in nonaqueous liquids. This conclusion was supported by the crack initiation behavior observed using the microhardness indenter.     

Deformation and fracture behaviour of a rubber-toughened epoxy: 2. Failure criteria

In part 1 the microstructure and fracture characteristics of a rubber-modified epoxy, and for comparison that of the unmodified epoxy, were examined in detail. Based on this analysis a qualitative mechanism involving cavitation, shear yielding and plastic flow was proposed. As an extension of this work, the present paper considers the yield behaviour of the epoxy material and uses the data determined, together with the previously reported fracture results, to calculate values of the crack opening displacement. The rate/temperature dependence of the crack opening displacement and the correlations established between stress intensity factor, K_Ic, yield stress and type of crack growth suggest that the extent of crack tip blunting largely governs the relative toughness of the epoxy materials and induces transitions in the types of crack growth observed. A quantitative expression is then presented which successfully describes the fracture toughness values over a wide range of temperatures and rates. The two parameters in this expression are shown to be material constants and therefore provide a unique failure criterion. They can be viewed simply as curve-fitting parameters but they may also have some significance in terms of a critical stress that must act over a critical distance ahead of the crack tip to produce crack growth.     

Scanning Electron Microscopy of Fracture Surfaces

Scanning electron microscopy has supplied information concerning the mechanism of failure of aluminium/epoxide joints: (1) evidence for a critical concentration of A-1100 silane that inhibits stress corrosion cracking; (2) evidence of plastic deformation at a crack tip; (3) observation of aluminum corrosion products.     

Stress and displacement fields at the tip of a craze containing a crack

Plane elasticity theory is utilized to obtain expressions for the stress and displacement fields at the tip of a craze containing a crack. The craze is modeled as a very thin elliptical inclusion with different elastic properties from hat of the surrounding bulk polymer. Problem is solved by superimposing the solution of a crack problem onto the solution for a uniformly loaded homogeneous craze. Invoking stress free boundary conditions on the crack surface provides a singular integral equation of Hilbert type with a unique solution. Contour lines of constant hydrostatic stress and constant maximum shear stress around the craze tip are shown graphically. These two stress combinations have played prominent roles in a number of proposed craze growth criteria. Results show that even for relatively long cracks within the craze, very little stress enhancement at the craze tip occurs. Only as the crack tip approaches the craze tip does the enhancement become significant, tending to drive the craze region ahead of the crack.     

Fatigue crack growth and craze-induced crack tip shielding in polycarbonate

This paper expounds a revised characterisation of the elastic stresses ahead of a crack tip in polycarbonate which takes account of the elastic–plastic boundary stresses induced by the presence of the crazed region that surrounds a crack. The advanced experimental techniques used in this work have provided insights into fractography, identification of the crazed region and location of the crack tip position (using confocal laser scanning microscopy and scanning electron microscopy). In addition, the four-parameter model of crack tip stresses has led to modified definitions for crack tip stress intensity factors which explicitly account for craze-induced shielding effects on the fatigue crack growth rate in polycarbonate. The model is generic and offers the potential for increased understanding of fatigue crack growth in polycarbonate.     

Fatigue fracture of reaction injection molded (RIM) nylon composites

This study was undertaken to determine how milled glass fibers affect the fatigue resistance of reaction injection molded (RIM) nylon 6. Specifically the effects of glass content, fiber length, orientation, and surface treatment were investigated. The fatigue crack growth rates for unfilled and glass-filled samples were observed to follow the well-known Paris equation in terms of dependence on cyclic stress intensity factor. For the unfilled nylon a line shaped zone was observed in advance of the crack tip. Fractography results suggest that the zone was the projection of the actual crack tip profile through the thickness of the sample rather than a distinct plastic or deformation zone. The fatigue fracture surface exhibited a patchy type structure with features 50–150 μm in size, suggesting a void coalescence type of mechanism as has been reported for injection molded nylons. A diffuse damage zone, several millimeters in size, was observed at the crack tip for the glass-filled RIM nylon 6. The zone was observed to pulsate with the applied oscillating load. The growth of the damage zone volume with increasing crack length (and thus increasing stress intensity factor range) followed the Paris law, as did the crack growth rate data. The damage mechanism is attributed to void formation and microcracking at the fiber–matrix interface. The results of this study show that, for milled glass-reinforced RIM nylon 6, the crack growth rates were much more rapid than observed for injection-molded nylon 6 containing chopped glass fibers. This difference is attributed to the greatly reduced glass fiber lengths for the milled glasses.     

Active centres of magnesium oxide surface and calculations of dissociative chemisorption of methane on modified MgO

In the framework of supermolecular approach using MINDO/3 method the various channels of dissociative chemisorption of methane molecule on pure and lithium doped magnesium oxide surfaces are considered. On the basis of the obtained calculation results the possible ways of methane activation on MgO and Li/MgO are discussed.     

Fluoride Ions Behavior in the Presence of Corrosion Products of Iron: Effects of Other Anions

The behavior of fluoride ions in the presence of corrosion products of iron in drinking and water solutions was analyzed and the adsorption capacities of the iron oxides (corrosion products of iron) for fluoride ions were determined. Drinking water containing naturally 2.45 mg of fluoride ions per liter was characterized and the concentrations of other anions were determined. The effect of contact time, the initial concentration of fluoride ions, and the effect of other anions naturally present in the drinking water were considered. The kinetic results could be adjusted to the pseudo-second order model, which indicated that the sorption mechanism was chemisorption and the equilibrium was reached in 24 hours. The presence of bicarbonate and chloride ions diminishes the removal efficiency of the fluoride ions, whereas other anions (sulfate, phosphate, and nitrate) did not show any significant effect. The results reflect that the iron oxides products from the corrosion of a hydraulic infrastructure allow the removal of fluoride ions from water.