Zero stress aging in notched multi-component glass fibers

Degradation of glass under zero applied load in the presence of humidity at ambient temperature is of great interest to the container and fiber glass industries. The phenomenon is well documented for fused silica used in optical fibers, but has not been studied in detail for multi-component glasses. In this work notches of varying length (500-1500 nm) were placed with a focused ion beam into two types of multi-component glass fibers, E-glass (48 μm diameter) and soda-lime-silicate (35 μm diameter). Notched specimens were exposed to dry and humid conditions for up to 32 days. Transmission electron microscopy revealed the presence of extensive reaction products within the root of the notch, even after only 1 day of aging in the nominally dry environment for the soda-lime-silicate glass. Surprisingly, the extensive reactions have no measurable effect on the fiber strength. The uniaxial tensile strength of the notched glass fibers, measured with the fracture surface mirror radius method, does not follow a classic fracture mechanics prediction, implying that the notches are not classic Griffith flaws. Fracture mechanics is applied to show that sharpness at the notch base may be important, especially when subcritical crack growth is present during the strength measurement.     

Dynamic and Static Fatigue of Silicate Glasses

Fatigue of silicate glasses was studied using both static and dynamic tests. Static fatigue data for acid-etched soda-lime silicate glass determined at 74°F in 50 and 100% rh can be represented by a single universal fatigue curve (UFC). The UFC for acid-etched glass does not lie on the UFC of Mould and Southwick, which was determined for abraded soda-lime silicate glass; the acid-etched glass was less susceptible to static fatigue. The susceptibility of acid-etched soda-lime silicate glass to static fatigue differed little from that of pristine E-glass and fused SiO₂ fibers. Dynamic fatigue data for soda-lime, E, borosilicate, and fused quartz glasses agreed well qualitatively and quantitatively with fundamental crack velocity data for these glasses; the dynamic fatigue theory of Charles was used in the comparison. The theoretical implications of these results are discussed.     

Effect of Humidity on Small-Crack Growth in Silica Optical Fibers

Subcritical small-crack growth in silica optical fibers was measured directly in air at 25°C for several humidity conditions from 30% to 90% relative humidity. The crack velocity data were shown to be closely fitted by a power function of the stress intensity factor. It was found that the power n is independent of humidity and takes a value of 21.7. It was also revealed that crack velocity varies with relative humidity to a power of 2.5, which agrees with estimates from the published dynamic fatigue data concerning the humidity dependence of the strength of pristine silica fibers.     

Mechanochemical Wear of Soda Lime Silica Glass in Humid Environments

The mechanochemical wear of multicomponent glasses was studied under controlled humidity conditions using a reciprocating ball‐on‐flat tribometer. For dry conditions, the surfaces were extensively damaged by scratching for all of the glasses, while for humid conditions the wear behavior varied with the glass composition suggesting a chemical effect on scratch behaviors of glass surfaces. The wear of soda lime silica (also called sodium calcium silicate) glass was suppressed with increasing humidity, while the borosilicate and barium boroaluminosilicate glasses showed an increase in wear volume with increasing humidity. The unique humidity dependence of the observed mechanochemical wear of soda lime silica glass supports the hypothesis that hydronium ion formation in the sodium‐leached sites of the soda lime glass enhances its wear resistance.     

Recycled content in polymer matrix composites through the use of A-glass fibers

The utility of recycled A-glass (primarily composed of soda-lime-silicate) fibers as reinforcement for structural composites has been studied. A series of plaques of unsaturated polyster composite were resin transfer molded with an A-glass continuous strand mat (CSM) and a control with E-glass CSM. The influence of fiber volume fraction on the physical and thermo-mechanical properties of the resultant composites were investigated, both before and after environmental exposure. At the maximum fiber fraction considered (nominally 29 vol%), the use of A-glass reinforcement lowered the warp direction tensile modulus from 8.6 to 7.6 GPa and strength from 139 to 100 MPa, relative to the control. Similar results were observed for both the flexural and the tensile properties, irrespective of fiber fraction and test direction (warp vs. weft), for the A-glass reinforcement. Environmental exposure was found to affect equally the properties of A-glass and E-glass fiber reinforced composites. Based upon microscopic analyses and constituent properties, the lower mechanical properties of the A-glass fibers composites have been linked to the lower properties of A-glass fibers relative to E-glass fibers. The experimental results were also used to test a micro-mechanics models for random fiber reinforced composites. Reasonable correlation was found between the experimental results and the theoretical predictions. To offset their lower mechanical properties, A-glass fibers could be used as a reinforcement in composite applications by simply increasing the fiber fraction relative to their E-glass counterpart.     

Effect of Fictive Temperature on Dynamic Fatigue Behavior of Silica and Soda-Lime Glasses

The dynamic fatigue characteristics of silica glasses with fictive temperatures of 1000°, 1100°, and 1300°C and soda-lime glasses with fictive temperatures of 470° and 530°C were measured in air. For both glasses, samples with higher fictive temperatures had a greater fatigue resistance. Inert strength of silica glasses with flctive temperatures of 1000° and 1300°C was also measured at liquid nitrogen temperature. Glass with higher flctive temperature had a greater inert strength.     

Humidity Dependence of the Fatigue of High-Strength Fused Silica Optical Fibers

Strength and dynamic fatigue behavior of silica fibers has been measured as a function of ambient humidity. Bare and polymer-coated fibers were compared to determine the influence of the coating. The results verify earlier work that suggests the degradation reaction is approximately second order with respect to humidity. However, we verify this result using rigorous data analysis techniques and, unlike the earlier work, the result is shown to be independent of the form of the kinetic model for crack growth. Trends in the calculated fatigue parameters illustrate that a simple exponential crack growth law best describes the humidity data. No significant differences were found between coated and bare fibers, provided the coated fibers were properly equilibrated. A data analysis methodology is given for obtaining valid reaction orders independently of the crack growth law form.     

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.     

Reaction of Ammonia with Fumed Silica

The reaction between fumed silica and ammonia has been studied between 600° and 1200°C. Up to 24 wt% nitrogen can be introduced into vitreous silica by adjusting the partial pressure of ammonia, temperature, and reaction time. The experiments show that the amount of nitrogen incorporated into silica has a linear dependence on the partial pressure of ammonia and a square root dependence on the reaction time. The activation energy for the reaction was found to be 76.0 kJ/mol. The nitrided powders as well as several hotpressed samples were crystallized at 1550° to 1650°C. Silica glasses containing 0 to 1 wt% nitrogen crystallize to cristobalite. Glasses containing 2 to 10 wt% nitrogen are stable against devitrification in nitrogen, even at 1650°C. Glasses with 12 to 28 wt% nitrogen crystallize to fine-grained Si₂N₂O with residual glass.     

Creep Behavior of Soda-Lime Glass in the 100–500 K Temperature Range by Indentation Creep Test

The loading time and temperature dependences viscoelastic behavior of a soda-lime-silicate glass (SLS) were studied by indentation creep experiments. Experiments were conducted in air, water and silicone oil with 15–10 000 s loading times, and temperatures ranging between 100 and 500 K. The indentation size was found to depend much on the loading time and temperature. Hardness was found to decrease significantly with increasing loading time, even at 173 K, and to decrease rapidly with rising temperature, even well below the glass transition temperature ( T _g). Water on the surface of glasses appeared to reduce the hardness and indentation viscosity of the glass.     

Effect of Aluminum and Neodymium Admixtures on Devitrification of Silica Glasses

This investigation concerned the devitrification kinetics (in the temperature range 1100° to 1500°C) of silica glasses doped with aluminum oxide (0.1 and 1.0 mol%), neodymium oxide (0.08 and 0.25 mol%), and with aluminum and neodymium oxides simultaneously, at their mutual molar ratio close to 4. The glasses were obtained by electric melting of quartz under vacuum and by the sol/gel method. On the basis of the obtained results, conclusions have been drawn concerning the mode in which the admixtures become incorporated into the silica glasses as well as the structural differences between the melted and sol/gel glasses.     

Effect of silica reinforcement on natural rubber and butadiene rubber vulcanizates by a sol–gel reaction with tetraethoxysilane

The effect of silica reinforcement was studied for natural rubber (NR) and butadiene rubber (BR) vulcanizates by a sol–gel reaction with tetraethoxysilane at different temperatures. The formation of silica in the rubber vulcanizates was investigated analytically with Fourier transform infrared spectroscopy and energy-dispersive X-ray analysis. The variations of the mechanical and dynamic properties were measured in the NR and BR vulcanizates with silica filling. The hardness of the rubber vulcanizates increased with silica filling in the rubber matrix. The tensile strength and elongation at break decreased with silica filling in the NR vulcanizates. The moduli at 50, 100, and 300% elongation increased with silica filling in the rubber matrix. The storage modulus of silica-filled rubber vulcanizates became higher than that of pure rubber vulcanizates. The temperature dependence of the loss modulus also increased with silica filling. The temperature dependence of the loss tangent was maintained, regardless of silica filling in the BR vulcanizates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

The nanomechanical deformations on glass surfaces near the elastic–plastic load boundary have been measured on various glasses by nanoscratching using an atomic force microscope (AFM) to mimic the mechanical interactions of polishing particles during optical polishing. Nanoscratches were created in air and aqueous environments using a 150‐nm radius diamond‐coated tip on polished fused silica, borosilicate, and phosphate glass surfaces; the topology of the nanoscratches were then characterized by AFM. Using load ranges expected on slurry particles during glass polishing (0.05–200 μN), plastic‐type scratches were observed with depths in the nm range. Nanoscratching in air generally showed deeper & narrower scratches with more pileup compared to nanoscratching in water, especially on fused silica glass. The critical load needed to observe plastic deformation was determined to range from 0.2–1.2 μN for the three glasses. For phosphate glass, the load dependence of the removal depth was consistent with that expected from Hertzian mechanics. However, for fused silica and borosilicate glass in this load range, the deformation depth showed a weak dependence with load. Using a sub‐T_g annealing technique, material relaxation was observed on the nanoscratches, suggesting that a significant fraction of the deformation was due to densification on fused silica and borosilicate glass. Repeated nanoscratching at the same location was utilized for determining the effective incremental plastic removal depth. The incremental removal depth decreased with increase in number of passes, stabilizing after ~10 passes. In water, the removal depths were determined as 0.3–0.55 nm/pass for fused silica, 0.85 nm/pass for borosilicate glass, and 2.4 nm/pass for phosphate glass. The combined nanoscratching results were utilized to define the composite removal function (i.e., removal depth) for a single polishing particle as a function of load, spanning the chemical to the plastic removal regimes. This removal function serves as an important set of parameters in understanding material removal during polishing and the resulting workpiece surface roughness.     

The effect of glass composition on the reactivity of synthetic glasses

The reactivity of synthetic glasses depends on their chemical compositions. In far from equilibrium dissolution experiments, the reactivity of Ca‐rich glasses with compositions similar to blast‐furnace slag is found to be much higher (up to ~60 wt.% after 7 days) compared to Si‐rich glasses with compositions similar to type F fly ash (up to ~20 wt.% after 7 days). Isothermal calorimetry and TGA experiments conducted on model systems containing portlandite and calcite and on glass‐blended Portland cement confirmed the higher reactivity of the Ca‐rich glasses. The degree of glass reaction after 91 days ranged from 7 to 20 wt.%. The results showed also a higher reactivity of the glasses containing more aluminum (both for Ca‐rich and Si‐rich glasses) indicating that not only calcium but also aluminum acted rather as network modifier than as network former. The results confirm a strong dependence of the glass reactivity on the degree of polymerization of the glass network.     

Phosphate-based glasses and glass ceramics for immobilization of lanthanides and actinides

Borosilicate glasses used to be a choice to vitrify nuclear wastes in most of the countries. However, Russian HLW vitrification plant at the Mayak Production Association (Chelyabinsk Region, Urals) uses phosphate-based glasses. Initially, Russian HLW glasses were based on sodium-aluminophosphate with the following approximate composition (mol.%): 40 Na₂O_, 20 Al₂O₃, 40 P₂O₅. Recently we have modified this composition by replacing of up to 50% Al₂O₃ with Fe₂O₃. Such replacement increases chemical durability and resistance to devitrification and radiation of the glasses. The phase composition and structure of these glasses containing ~ 10?wt% RE oxides and ~ 10, ~ 50, and ~ 100?wt% UO₃ (over 100?wt%) were studied in details using XRD and FTIR. Glasses and glass-ceramics have high chemical durability. Thus, the glasses and glass ceramics on sodium-aluminum-iron-phosphate basis are good waste forms for lanthanide fraction of HLW generated at spent nuclear fuel pyroprocessing and minor actinides.     

Flexural fatigue of marine laminates reinforced with woven roving of E-glass and of Kevlar® 49 aramid

A comparison of the flexural fatigue characteristics of E-glass and Kevlar® 49 aramid in polyester and vinyl ester resin hand lay-ups typical of boat hull laminates is presented. Data on unidirectional E-glass and aramid composites from epoxy preimpregnated tapes are reported for comparison. The data indicate that while the initial flexural strength of E-glass woven roving laminates is greater than that of aramid laminates, the runout stress of aramid laminates at 10⁶ fatigue cycles is similar or superior to glass. S-N curves for aramid laminates are flatter indicating better flexural fatigue resistance.     

An Overview of the Strengthening of Glass Fibers by Surface Stress Relaxation

Pristine glass fiber is well known to become mechanically weaker when heat-treated in the presence of water vapor. However, recently, the same fiber was found to become stronger if heat-treated while held under a subcritical tensile stress at a temperature below the glass transition temperature. The added strength was attributed to the formation of a surface compressive layer on the glass created by a surface stress relaxation process that occurred while being held under the tensile stress in air. Silica glass fibers with strengths estimated to be ~7–8 GPa were produced, exceeding the ~5.5 GPa strength of fresh optical fiber commonly reported at room temperature in air. Similar degrees of strengthening, a 20–30% improvement, have been observed previously for E-glass and is reported here for the first time for soda-lime silicate glasses. This process is a new glass strenthening method that may be applied to all oxide glasses and is not subject to the same constraints as currently available methods.     

Effect of Forming and Aging Atmospheres on E-Glass Strength

E-glass fibers were drawn in various atmospheres. The specific surface area measured by water vapor adsorption was larger than the geometric area. Specific surface increases with the square root of drawing gas pressure. Exposure of glass fibers to ambient atmosphere decreases the strength and increases the area of macropores (r_p> 100 Å). The reciprocal strength of E-glass filaments aged at ambient humidity for various times is proportional to the square root of macro pore area. This relation is independent of the drawing atmosphere, at least for water and tetrafluoromethane.     

Degradation of Fatigue Properties of Weld-Bonded Aluminum Exposed to Moisture and Elevated Temperature

Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-O/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38°C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 × 10⁶ cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.     

Degradation of Fatigue Properties of Weld-Bonded Aluminum Exposed to Moisture and Elevated Temperature

Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-O/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38°C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 × 10⁶ cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.