Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Charles R. Kurkjian, Rolf A. Frantz, and Hakan H. Yuce, "LIGHT GUIDE FIBERS: PROBLEMS AND PROSPECTS," Optics & Photonics News 8(7), 34-39 (1997) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

The intrinsic strength ((sigma) i) is defined as the strength in the absence of slow crack growth or fatigue. It is of general interest in the study of brittle fracture, but it is of particular importance in the evaluation of lightguide fiber lifetimes since it is required for the calculation of the constant B. In this work we review the existing literature relating to the inert strength, indicate the limits that can be proposed for and suggest possible approaches to more satisfactory estimates of value(s) of (sigma) i. It is suggested that reasonable values of the inert strength may be obtained by taking 85 - 90% of the liquid nitrogen strength.

An ultrasonic surface acoustic wave technique for studying the growth behaviour of small fatigue cracks is described. The technique allows crack depth and opening stress to be monitored continuously during the course of a fatigue test. Results are given for a 1740 MPa yield strength, silicon-modified, AISI 4340 steel tested under zero-to-tension cyclic loading. Good agreement is shown between acoustically determined crack depth and that measured by post-fracture optical microscopy. The monitoring of changing crack depth-to-surface length ratios during tests is also demonstrated. Acoustically determined crack opening stresses were found to be about 10% higher than values determined by measurements of crack tip opening displacements by scanning electron microscopy. Effects on crack growth of two different specimen surface preparations, electropolishing and diamond paste polishing, are also reported. Growth rates in electropolished specimens were as much as an order of magnitude higher than in diamond paste polished specimens which had a shallow but significant layer of compressive residual stress.

Fiber optic reliability is increasingly important because of increased telecommunication traffic density and migration of optical fiber closer to the subscriber. An overview is given of what fiber optic standards are, their benefits, where they are generated, and what has been published (or soon will be) on fiber reliability standards. These concern test procedures for measuring fiber reliability parameters, and also concern theory for using these parameters for predicting fiber reliability. We update a previous paper on standards work, concentrating on recent achievements.

It is now established that the long term mechanical reliability of fused silica optical fiber can be determined by surface etching of the fiber that results in roughness that acts as a source of strength degrading defects.1,2 This suggests that suppression of surface dissolution should have a beneficial effect on fiber reliability. In previous work3,4 this was achieved by incorporating in the coating colloidal silica particles which then, presumably, dissolve preferentially due to their higher solubility and reactivity and so protect the fiber sacrificially. It was found that the static fatigue lifetime could be increased by up to a factor of 30 in liquid water by just 0.75 wt% of colloidal silica. However, this first such fiber exhibited two undesirable properties due to the crudity of the preparation techniques; there was large scatter in the fatigue data and the tensile strength showed a low strength mode. This paper shows that, by careful production of the coating, the disadvantages of the earlier fiber can be avoided, and that even more dramatic improvements in lifetime can be achieved. It is also shown that the coating additive is effective in vapor as well as liquid aqueous environments.

Polymer coated, silica based optical fibers are being rapidly deployed for telecommunications with applications moving rapidly from trunk routes into the distribution plant. The sensitivity of glass strength to water is well recognized as a major reliability concern for optical fibers.1 However, as fibers are used in the loop plant and on customer premises, they are likely to be exposed to a wide variety of chemical environments. These can range from gasoline leaking into underground ducts and manholes to chemicals such as acetone used to clean fibers in some splicing procedures to household cleaners containing ammonia inadvertently splashed on fibers.

In Part I of this study, an ultrasonic surface acoustic wave (SAW) technique was used to monitor the depth of surface fatigue microcracks in 300M steel as they grew. This technique also provided information on the stress which must be applied to cause the cracks to fully open. Values of crack opening stress obtained by scanning electron microscope (SEM) measurements of crack tip opening displacements were compared with values determined acoustically. Good agreement was obtained. 300M steel, heat treated to yield strength of 1737 MPa, was tested at two different zero-to-maximum tension stress levels. Effects of residual stresses produced by two different surface preparations on the growth of small cracks were studied. One preparation was electropolishings while the other was stress-relieving followed by diamond paste polishing. Fatigue microcrack growth rate agreed well with that of large cracks in electropolished specimens but, the presence of shallow compressive residual stresses in other specimens caused growth rates as much as an order of magnitude lower than in electropolished specimens. In Part II, four different microstructures of 4140 steel were investigated. Small fatigue crack behavior was monitored using a surface acoustic wave ultrasonic technique to accurately measure crack depth and crack closure stress. Cracksmore » from 50 to 200 ..mu..m in depth in specially designed catilevered bending samples were investigated. 143 refs., 74 figs., 5 tabs.« less

The theory of Kino and Auld which relates the reflection coefficient of acoustic waves from a crack to its size is summarized. A scattering model is evaluated from this theory concerning the reflection of surface acoustic waves (SAW) from a small surface fatigue crack at a frequency such that the crack depth is much smaller than the acoustic wavelength. Acoustic predictions of crack depth are compared to postfracture measurements of depth for small surface cracks in Pyrex glass, 7075-T651 aluminum, and 4340 steel. Additionally, the minimum detectable crack depth as limited by the acoustic noise level is determined for several typical aluminum and steel alloys. The utility of SAW reflection coefficient measurements for inferring crack depth, crack growth, and crack opening behaviorin situ during fatigue cycling is discussed.