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 topograp hy is characterized with an atomic force microscope. This chemical tre atment 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 less-than-or-equal-to a, where c is the flaw lengt h and a is its half width, and the inert strength ranges from 5.4 to 1 2.5 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 literatur e for chemically corroded pristine fiber. The geometry of the maximum stress concentrator is characterized as a partially embedded hemispher ical pit and, therefore, suggests a lower limit for the inert strength of a chemically corroded pristine fiber when c/a = 1. In addition, th ese results suggest reevaluation of the long-term mechanical reliabili ty assessment of optical fibers, because a sharp flaw model (i.e., c > > a) is currently used.