Kilometer lengths of optical fiber have a much lower strength than short lengths due to occasional defects of an extrinsic nature. The fatigue properties of these defects are hard to study due to their rarity. Subthreshold indentation flaws in silica optical fibers (i. e. Vickers indentations produced under sufficiently low load to avoid radial crack formation) have been shown to exhibit environmental fatigue similar to "pristine" silica fiber. Thus the indentation technique may be used to introduce controlled flaws into the fiber that model the strength limiting defects found in long length specimens. This paper presents the results of fatigue studies on subthreshold indentation flaws that have strengths of up to 1 GPa (typical of proof stress levels).
Ab,stract-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 systeim, 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 singleended 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 dleflectiodstress relationship are analyzed and a correction factor to the linear bending theory is presented. Friction between the slpecimen 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.
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