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.     

Observations on Finger-like Crack Growth at a Urethane Acrylate/Glass Interface

Interfacial crack growth behavior along a urethane acrylate/glass interface is characterized by the development of finger-like perturbations along the advancing crack front. The finger-like perturbations grow from a slightly irregular crack front until they reach a steady-state where the velocity of the finger tips equals the velocity of the finger valleys. Once the fingers reached steady-state, the crack velocity was dependent on the applied strain energy release rate via a power law relationship where the exponent was independent of test humidity; however, the multiplicative constant A decreased by an order of magnitude from 80 to 15% RH. The spacing of the fingers was found to be independent of the crack's velocity and the relative humidity of the environment.     

Observations on Finger-like Crack Growth at a Urethane Acrylate/Glass Interface

Interfacial crack growth behavior along a urethane acrylate/glass interface is characterized by the development of finger-like perturbations along the advancing crack front. The finger-like perturbations grow from a slightly irregular crack front until they reach a steady-state where the velocity of the finger tips equals the velocity of the finger valleys. Once the fingers reached steady-state, the crack velocity was dependent on the applied strain energy release rate via a power law relationship where the exponent was independent of test humidity; however, the multiplicative constant A decreased by an order of magnitude from 80 to 15% RH. The spacing of the fingers was found to be independent of the crack′s velocity and the relative humidity of the environment.     

Subcritical Crack Growth in Silica Optical Fibers in Wide Range of Crack Velocities

Crack growth in silica optical fibers was measured directly in a wide range of crack velocities d a/ d t (10⁻¹³ < d a /d t < 10⁻⁵ m/s) by improving our previous method. The slope in log-log plots of the crack velocity versus the stress intensity factor data was revealed to increase with the crack velocity, e.g., slope, 23.3 for d a /d t < 10⁻¹⁰ m/s, and 39.7 for d a /d t > 1(10⁻⁸ m/s. The data were also found to be fitted well by the exponential law in the whole range of d a /d t , rather than the power law. Additionally, the fiber data were shown to be close to those reported for bulk silica specimens having large cracks for high crack velocities where the bulk specimens were tested.     

Rapid synthesis of polymer-silica hybrid nanofibers by biomimetic mineralization

Biomimetic formation of silica from polyamines such as poly(ethylene imine) (PEI), inspired by the proteins found in diatoms and sponges, has been actively investigated recently as a potential route to silica formation compared to the conventional sol-gel process. We report silica formation onto nanofibers of PEI blended with poly(vinyl pyrrolidone) (PVP) obtained via electrospinning of their 50:50(w/w) blend. The active component, PEI, catalyzes rapid silica formation, within minutes, upon immersion of the PEI/PVP nanofibers in silica precursor tetramethylorthosilicate (TMOS). The silica formation in nanofibers was then investigated by scanning electron microscopy (SEM), Energy Dispersive X-ray analysis (EDX), differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and Fourier-transform infra-red (FTIR) spectroscopy. The silica content in the PEI/PVP nanofibers could be controlled by pre-treatment of the fibers at different conditions of relative humidity prior to the silicification. Fibers exposed at higher (80%) relative humidity led to higher inorganic (silica) content compared to those exposed to relative dry conditions (<20% relative humidity). Calcination of the fibers indicated that silicification proceeded across the whole fiber cross-section that consisted of nano-structured silica. Such a simple route to rapid formation of organic-inorganic hybrid nanofibers could have applications ranging from catalysis to tissue engineering, and nanocomposites in general.     

Dependency of Fatigue Predictions on the Form of the Crack Velocity Equation

A computer search technique was developed to analyze fatigue strength data using both exponential and power law forms of the subcritical crack velocity equation. All crack velocity equations would fit a given set of fatigue data equally well in the data range but failure predictions based on the different crack velocity equations diverge from each other outside the data range. The exponential form of the crack velocity equation best fit both the static and dynamic fatigue data of hot-pressed Si₃N₄ and optical glass fibers, whereas the power law form best fit the static and dynamic fatigue data of soda-lime glass and A1₂O₃. To determine the most appropriate crack velocity equation for a given material/environment system, it is recommended that fatigue data be obtained under different loading conditions and the data numerically regressed using the computer search technique with each of the possible crack velocity equations to find which best fits the data.     

Fracture resistance of mineral reinforced polyamide 6

Tensile tests have been carried out over a wide range of test speeds on compact tension specimens of polyamide 6 containing spherical silica particles, whose size and content had been adjusted to give optimum impact performance in conventional impact tests after conditioning at 50% relative humidity. The tensile test results confirmed there to be a significant improvement in the high speed crack initiation resistance at room temperature and at high moisture contents on addition of the silica particles. However, at low moisture contents and/or temperatures well below the glass transition temperature, the crack initiation resistance was reduced. It is hence inferred that for the chosen silica particle distribution, toughening requires a certain minimum level of matrix ductility in order to be effective.     

Developing porous fibers by electrocentrifuge spinning system

In this study, polysulfone fibers with various surface morphologies were developed using electrocentrifuge spinning system. The effects of a number of parameters, including the solvent system, spinning system angular velocity, and relative humidity, on the fiber morphology were studied using field emission scanning electron microscopy, porosimetry [Brunauer–Emmett–Teller (BET)], and contact angle test. The results showed that the fibers prepared from the acetone/dimethylformamide (DMF) solvent system had higher micro/nano roughness than those fabricated from the tetrahydrofuran/DMF solvent system, that is to say, the higher the vapor pressure of the solvent, the higher the surface roughness. The acetone/DMF system created fibers that had internal porosity. Also, the relative humidity had a significant effect on providing micro/nano roughness, so that increasing the relative humidity led to an increase in the surface roughness. The increase in the angular velocity caused to stretch the micro/nano patterns and increase the fiber diameter. The results of the BET confirmed the microscopic observations. With the increase in the relative humidity and the use of the acetone/DMF system, porosity, and specific surface area of the fiber increased. X-ray diffraction analysis was also performed and it was found that the presence of moisture did not affect the crystallinity of the fibers. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47513.     

Static Fatigue of Silica in Hermetic Environments

Static fatigue tests on metal-coated silica fibers have shown subcritical crack growth with crack velocities ranging from ∼10⁻¹⁴ to 10⁻¹⁷ m.s⁻¹. To induce subcritical crack growth, however, it is necessary that the test be performed at stress intensities, K_I, of >0.8 K_IC. Metal-coated fibers provide a unique medium for studying static fatigue at these high stress levels because the metal coating protects the fiber from the external environment. Crack growth,for these tests demonstrates the sensitivity of crack velocity to OH⁻ concentrations at the crack site.     

Elastic Properties of Silica Xerogels

To calculate elastic constants, longitudinal and tranverse acoustic wave velocities were measured for silica xerogels as a function of relative humidity (rh). The silica xerogels studied are microporous with open porosity of 53 vol%. The longitudinal wave velocity exhibits a minimum at about 35% rh. The transverse wave velocity decreases to a constant value for 35% rh. Consequently, Young's modulus is a minimum at about 35% rh, whereas the shear modulus decreases to a constant value at 35% rh. The bulk modulus and Poisson's ratio exhibit minimum values at about 15% rh. Young's modulus decreases from 4.91 to 3.42 GPa at 35% rh and then increases to 3.60 GPa at 55% rh. Poisson's ratio decreases from 0.184 to 0.164 at 15% rh and then increases to 0.272 at 55% rh. Below 35% rh, silica xerogels adsorb a monolayer of hydroxyls, whereas above 35% rh silica xerogels show pore filling.     

Fracture mechanics properties of fused silicas used for international space station windowpanes

The fracture toughness and slow crack growth (SCG) parameters of a quartz-based silica and a high-purity fused silica were measured as part of a program to review the reliability of the International Space Station windows. The materials exhibit the same fracture toughness (.75 MPa m^1/2 in N₂) and very similar SCG parameters. The literature on fused silica indicates excellent agreement of fracture toughness, but a very wide range of SCG parameters, even from the same institution, with strength-based methods usually yielding a lower power law exponent than direct crack velocity measurements. Use of the exponential function is shown to provide better agreement between test methods, with velocity curves derived from strength tests of bare fiber and polished or ground test specimens paralleling those from wide-range, direct crack velocity observations, implying that constant stress rate tests can predict long lifetime via the exponential function. However, much variation still exists. SCG parameters for soda–lime silicate are much less sensitive to the test method than fused silica. Static load tests and stress intensity measurements resulted in a fatigue threshold of .3 MPa m^1/2 for fused silica.     

Melt spinning of nylon 6: Structure development and mechanical properties of as-spun filaments

The structure of melt-spun nylon 6 filaments was studied using on-line x-ray diffraction and birefringence measurements. Measurements were also made on as-spun and treated filaments. On-line wide-angle x-ray scattering measurements indicated that crystallization did not occur on the nylon 6 spinline at spinning rates up to 1000 m/min when spinning was done into either ambient air of 60% relative humidity or into wet saturated air. The filaments did crystalline gradually on the bobbin to a paracrystalline pseudohexagonal (γ) form. The rate of crystallization was dependent on the molecular orientation developed in the spun filaments. Crystalline orientation factors based on hexagonal symmetry were computed as a function of take-up velocity for fibers which were conditioned 24 hr in air at 65% relative humidity. Annealing in air or treatment in water or 20% formic acid solution causes a transformation from the pseudohexagonal form to the α monoclinic form. The tangent modulus of elasticity and tensile strength of spun and conditioned filaments increase with increasing take-up velocity and spinline stress, while elongation to break decreases with these variables.     

Silica fiber–polybenzoxazine–syntactic foams; Processing and properties

Polybenzoxazine filled with chopped silica fibers and their syntactic foams of varying composition and densities were processed. The composition and density variations were achieved by regulating the relative concentrations of silica fiber and glass microballoons (MB). The variation of tensile, compressive, and flexural properties with change in composition was investigated. For the silica‐fiber filled materials, the property attained a maximum at about 40% volume content of fiber, and thereafter, the properties showed a diminishing trend. The incorporation of microspheres significantly lowered the strength of silica filled materials. However, the decrease in the specific flexural strength was less pronounced and the strength was unaffected beyond a certain microsphere content. During processing and mechanical testing, a large quantity of fibers was fractured, which reduced the strength of silica filled systems. The diminution in material strength on embedding microspheres is attributed to the presence of stress concentrating loci as evidenced from SEM analysis. The various factors leading to the property variation with composition are discussed with microscopic analyses, like clustering of fibers, crack propagation, fiber pull out, and debonding of fibers from resin phase. Dynamic mechanical analysis revealed an improved damping property for the filled materials in contrast to the unfilled polymer. The T_g (deduced from tan δ maximum from DMTA) decreased in silica fiber containing materials and on incorporating the MB, the values reverted back to that of the neat polymer. Both silica and MB conferred better thermal and thermooxidative stabilities to the polybenzoxazine. However, the degradation mechanism is nonoxidative in nature. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Environmental Fatigue of Silicate Glasses in Humid Conditions

Failure strains of commercial silica, soda-lime-silicate, and E-glass fibers were measured using two-point bending in room temperature humid air. Humidity dependence and dynamic fatigue behavior were studied, and the fatigue reaction orders in terms of humidity were determined. In the humidity range tested (~0.1% to ~100%), the dynamic fatigue parameters for silica and E-glass are found to be greater in lower humidity (~0.1% to ~10%), whereas the fatigue parameter for soda-lime-silicate is independent of humidity. The humidity dependence of failure strains for all three glasses was more pronounced in high humidity (~10% to ~100%) than in low humidity (~0.1% to ~10%), indicating that the reaction order decreases with decreasing humidity. These observations were correlated with the different structures of the glasses and their corresponding fatigue mechanisms.     

Flue gas desulfurization in an internally circulating fluidized bed reactor

An internally circulating fluidized bed reactor (ICFBR) was used as a desulfurization apparatus in this study. The height of the bed was 2.5 m, and the inner diameter was 9 cm. The bed materials were calcium sorbent and silica sand. The effects of the operating parameters of the flue gas desulfurization including relative humidity, particle size of the calcium sorbent, inlet concentration of SO₂, difference between the superficial gas velocities in the draft tube and the annulus, and superficial gas velocity in the draft tube on SO₂ removal efficiency (RE) were investigated. It was found that when the relative humidity (RH) was varied from 40% to 80%, the steady state RE had a largest value of approximately 15% when the relative humidity was 60%. When RH = 50%, 60% and 70%, RE decreased initially and then increased. After that RE decreased again until a steady state was reached. In addition, RE decreased with increasing calcium particle size or inlet SO₂ concentration. A larger difference between the superficial gas velocities in the draft tube and the annulus had a higher RE resulting from increasing reactivity of the calcium sorbent caused by a higher attrition rate. Moreover, a higher attrition rate had a higher total volume of the flue gas treated. Finally, a model to predict the steady state RE in ICFBR was proposed. It assumed that the draft tube section was a bubbling fluidized bed while the annulus section was a moving bed. In addition, the effects of the calcium sorbent conversion, attrition rate and gas-bypassing fractions on RE were also taken into account in this model. It was found that the values of RE predicted by this model agreed with the experimental results.     

MIL-101(Cr)开式吸附性能实验

采用水热合成法制备水热稳定金属有机骨架MIL-101(Cr),基于太阳能吸附式空气取水选取不同的实验工况,将MIL-101(Cr)、细孔硅胶作为研究对象,相对湿度控制在50%、温度范围5～45℃条件下,测试并对比了MIL-101(Cr)与细孔硅胶的吸附性能。实验表明,35℃、50%RH条件下,吸附过程进行1000min,MIL-101(Cr)水吸附量为22.05g/100g,其吸附量相比细孔硅胶提高93%左右;当系统平衡时,MIL-101(Cr)有效平均吸附速率相比细孔硅胶提高120%左右。此外,在相对湿度（RH）50%、温度范围5～45℃条件下,MIL-101(Cr)的平衡吸附量在11.40～23.47g/100g之间。在所控温度下,MIL-101(Cr)在25℃时平衡吸附量最大,在5℃时平衡吸附量最小,25℃时MIL-101(Cr)的平衡吸附量相比5℃时提高106%左右。该实验可以为四季工况不同温度下MIL-101(Cr)用于太阳能吸附式空气取水提供基础数据。     

Influence of Temperature and Humidity on Dynamic Fatigue of Optical Fibers

The influence of temperature and humidity on dynamic fatigue characteristics of silicone resin-coated optical glass fibers is examined. Dynamic fatigue is shown to result from subcritical crack growth. Test results verify that subcritical crack growth during dynamic loading is a thermally activated process and that the effect of humidity on crack growth is a function of water vapor pressure.     

Subcritical Crack Growth in Lead Zirconate Titanate

Subcritical crack growth in terms of velocity–stress intensity factor ( v – K ) curves in lead zirconate titanate (PZT) were experimentally characterized on poled and unpoled compact tension specimens. The poled specimens were tested under open- and short-circuit electrical boundary conditions, which resulted in an increase in fracture toughness by 0.2 MPa·m^1/2 for the accessible velocity range ( v = 10⁻⁹ to 10⁻⁴ m/s) in the open-circuit case. Subcritical crack growth of unpoled specimens was obtained under ambient (relative humidity = 35%) and dry (relative humidity ∼ 0.02%) conditions over a regime in stress intensity factor of 0.5 MPa·m^1/2.     

Oxidation of Silicon Carbide Fibers During Static Fatigue in Air at Intermediate Temperatures

The static fatigue of SiC-based fiber bundles and single fibers has been examined in previous papers, with emphasis placed on the analysis of the stress–rupture time data, and on the modelling of delayed failure from slow crack growth. The present paper investigates the oxidation of the fibers during static fatigue, at temperatures in the intermediate temperature range (500°–800°C). Two oxidation-induced phenomena have been evidenced: the formation of a thin silica film at the surface of fibers and the delayed failure of fiber bundles and single filaments. The stress–rupture time data are interpreted with respect to the chemical and structural characteristics of fibers, and to the oxide film growth rate. The structural analysis of the fibers was carried out using scanning electron microscopy and Auger electron spectroscopy. Delayed failure was found to result from slow crack propagation from surface defects, as a result of the consumption of the free carbon at grain boundaries and the local stresses induced by the SiC→SiO₂ transformation at the crack tip. The respective contributions of these phenomena to static fatigue are discussed.     

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.