Chemically Corroded Pristine Silica Fibers: Blunt or Sharp Flaws?

Bare silica fiber is aged in the vapor of a hydrofluoric acid solution, the inert strength is determined, and the resulting surface topography is characterized with an atomic force microscope. This chemical treatment results in well-defined, spatially resolved etched holes whose dimensions can be readily characterized. The aspect ratios range from 10−3 to 1, with c≤a, where c is the flaw length and a is its half width, and the inert strength ranges from 5.4 to 12.5 GPa. Better agreement is observed for the measured and calculated strength when the flaws are defined as blunt rather than sharp (i.e., c/a rather than c1/2). This model agrees with results in the literature for chemically corroded pristine fiber. The geometry of the maximum stress concentrator is characterized as a partially embedded hemispherical pit and, therefore, suggests a lower limit for the inert strength of a chemically corroded pristine fiber when c/a= 1. In addition, these results suggest reevaluation of the long-term mechanical reliability assessment of optical fibers, because a sharp flaw model (i.e., c≫a) is currently used.

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