A novel four-point bend test for strength measurement of opticalfibers and thin beams. II. Statistical analysis

In the first part of this work, a novel implementation of the well-known four-point bend test is described that determines the strength of thin beams and optical fibers by measuring the loading pin displacement, rather than the applied load. This paper extends the analysis of the nonlinear bending behavior to account for the stochastic nature of strength. A statistical analysis is presented that determines the effective tested length in bending and the tension to bending strength ratio. Results are given for both surface and volume flaws as well as for specimens of both circular and rectangular section. Strength measurements on a deliberately weakened silica optical fiber are consistent with the predictions of the analysis     

Strength and fatigue of silica optical fibers

After a brief review of early lightguide work, the authors assess advances since the mid-1970s. They note important work on strength distributions and procedures for improving strength. This is followed by a discussion of long-length strength, prooftesting and splicing. Flaw character is studied along with strength degradation, fatigue limits, aging and fatigue, and hermetic coatings     

Diffusion of moisture through optical fiber coatings

The strength of polymer-coated fused silica optical fiber is dependent on the ambient moisture because of stress corrosion. The diffusion rate of water vapor through the polymer coating can be estimated by monitoring the strength as a function of time after suddenly changing the ambient humidity. Not only is this technique a new way of sensing humidity, but it also allows estimation of the time needed for the coated fiber to equilibrate with its environment. A comparison has been made between the diffusion coefficients measured in this way and by monitoring the weight of free-standing polymer films as a function of time. Fickian diffusion has been assumed and is justified by the good agreement between the experiment and theory. The diffusion of water vapor both into and out of various polymers was investigated     

Mechanical durability of ZBLAN and aluminum fluoride-based opticalfiber

Zero stress aging and static fatigue experiments were performed on fluoride glass fibers of ZBLAN and aluminum fluoride-based compositions to examine the mechanical durability in aqueous solutions. ZBLAN fiber, which has the higher initial strength, becomes weaker than initially lower strength aluminum fluoride-based fiber in less than one hour in pH 7 buffer solution at 30°C. This is shown to be due to dissolution of the glass and precipitation of crystals at the glass/coating interface. In 1N NaOH the solubility is higher, resulting in less precipitation and hence less strength degradation. Although ZBLAN fiber degrades more quickly when a high strain is applied, at low applied strain the residual strength is higher than is observed when do strain is applied. These results indicate that future work on fluoride glass fibers for use in aqueous environments should focus on optimization of the as-drawn strength of durable glass, such as aluminum fluoride-based material, rather than on making small strength improvements to already strong but less durable glass, since such improvements will be quickly lost in aggressive environments     

Structural changes upon reduction of dipyrido[2,3-a:3',2'-c]phenazine probed by vibrational spectroscopy, ab initio calculations, and deuteration studies

A series of bridging ligands, dipyrido[2,3-a:3',2'-c]phenazine (ppb), dipyrido[2,3-a:3',2'-c]-6,7-dichlorophenazine (ppbCl2), and dipyrido[2,3-a:3',2'-c]-6,7-dimethylphenazine (ppbMe2), and their binuclear copper(I) complexes have been synthesized, and their spectral properties were measured. The single-crystal structure of the complex, [(PPh3)2Cu(mu-ppbCl2)Cu(PPh3)2](BF4)2 in the monoclinic space group P21/c, 18.2590(1), 21.1833(3), 23.2960(3) A with Z = 4 is reported. The copper(I) complexes are deeply colored through MLCT transitions in the visible region. The vibrational spectra of the ligands have been modeled using ab initio hybrid density functional theory (DFT) methods (B3LYP/6-31G(d)) and compared to experimental FT-Raman and IR data. The DFT calculations are used to interpret the resonance Raman spectra, and thus the electronic spectra, of the complexes. The preferential enhancement of modes associated with the phenanthroline section of the ligands with blue excitation (lambda(exc) = 457.9 nm) over phenazine-based modes with redder excitation (lambda(exc) = 514.5 and 632.8 nm) suggests the 2 MLCT transitions terminated on different unoccupied MOs are present under the visible absorption envelope. The radical anion species of the ligands are prepared by the electrochemical reduction of the binuclear copper(I) complexes; no evidence of dechelation prevalent in other copper(I) complexes is observed. The resonance Raman spectra of the reduced complexes are dramatically different from those of the parent species. Across the series common bands are observed at about 1590 and 1570 cm(-1) which do not shift with reduction but are altered in intensity. The normal-mode analysis of the radical anion species suggests that these normal modes primarily involve bond length distortions that are unaffected by reduction.     

Diffusion of moisture through fatigue- and aging-resistant polymer coatings on lightguide fibers

In previous work the diffusion rate of water vapor through the polymer coating on optical fiber was estimated by monitoring the strength as a function of time after suddenly changing the ambient humidity. This technique is used here to measure the diffusion of moisture both into and out of two novel fiber coatings. The first specimen is a dual-coated fiber with silica particles added to its secondary coating. It is shown that the improvement in this fiber's reliability is not due to the silica particles adsorbing/absorbing the moisture. The second fiber, coated with a fluorinated polymer, was designed to have higher fatigue resistance as a result of having a lower permeability to moisture. It is found that even though this fiber had higher than normal resistance to fatigue, the diffusion of moisture through this coating was not substantially different than through typical coatings used on fibers for telecommunications applications.     

Axi-symmetric contact on thin compliant coatings

This paper presents a theory for the indentation of a soft thin coating by a rigid body. The coating is assumed to be bonded to a rigid substrate and to behave linearly elastically. A simplifying approximation enables the stresses within the coating, averaged through its thickness, to be determined for particular indenter profiles. The results are shown to be sensitive to the thickness and compressibility of the coating material. Unlike much previous work, the results can be expressed analytically for certain indenter profiles and have been substantiated by experiment. The theory has many useful applications, in particular for situations where the layer acts as a protective coating and for the accurate, in situ and non-destructive measurement of the elastic modulus of the coating material.     

Acid stripping of fused silica optical fibers without strengthdegradation

Glass optical fibers are almost always coated with a polymer immediately after drawing to protect them from subsequent handling damage. When studying the strength and fatigue properties of the fibers, it is useful to be able to remove this coating in order to directly observe the fatigue properties of the glass in immediate contact with the environment. Fused silica optical fibers are frequently stripped by immersion in hot (~200°C) concentrated sulfuric acid. Two recent papers have claimed that hot acid stripping significantly degrades the strength and increases the width of the strength distribution. However, there is a large literature that implies that, at least for most coating systems, acid stripping does not degrade the strength provided sufficient care is taken to protect the bare fiber surface during stripping and subsequent testing. This paper explicitly proves this result, showing that careful complete stripping has little or no effect on the strength of fiber tested in both tension and bending. It is also shown that the immersion time in the hot acid has no noticeable effect on the strength. Experimental protocols are described that minimize the likelihood of accidental damage to the fiber during stripping     

A novel four-point bend test for strength measurement of opticalfibers and thin beams. I. Bending analysis

A novel four-point bend apparatus is described for strength measurement of thin compliant beams that avoids the loading and gripping problems associated with other techniques. The apparatus has proved particularly useful for strength measurement of relatively weak optical fibers. In this four-point bend system, loading pin displacement rather than applied load is the measured quantity from which failure stress is calculated, avoiding the load based instability at high deflection. A single-ended support design for the loading pins permits the specimens to be conveniently immersed in the test environment and enables several specimens to be tested simultaneously. Nonlinearities in the deflection/stress relationship are analyzed and a correction factor to the linear bending theory is presented. Friction between the specimen and support pins is found to increase local stresses at the pins. In the second part of this work, a statistical analysis is presented that determines the effective tested length in bending and the tension to bending strength ratio. The predictions of the analysis are confirmed by strength measurements on a weak silica fiber