Mechanism of mechanical fatigue of silica glass. Final technical report, July 1985--June 1995

Abstract

Objective is to study the static fatigue mechanism of silica glasses such as optical communication fibers. It is shown that the strength increase by thermal or hydrothermal treatment can be explained by formation of blunt crack tips. Specimens with blunt cracks exhibited fatigue only in water and NH3. While fatigue of glasses with sharp cracks involves only crack propagation, that of glasses with blunt cracks involves both crack initiation and propagation. Nonaqueous liquids can be adsorbed on the glass surface only, thus can aid crack propagation only. Water/NH3 can exhibit both adsorption and diffusion, and the ability of water to initiate a crack appears related to its diffusion into the glass. Mechanical fatigue of pristine silica fibers takes place in water but is not expected in nonaqueous liquids. Water entry into silica glass is accelerated by applied stress. Water entry (diffusion) into silica glasses at low temperatures was found closely coupled with structural relaxation of the glass, which lowers the fictive temperature. The relaxation can be monitored by simple IR spectroscopy; IR absorbance measures the fictive temperature over the entire thickness while IR reflection measures that of the surface. By combining IR reflection peak position measurement and successive etching, the depth profile of the fictive temperature can be determined. This was done in a communication fiber and in a specimen heat treated below the glass transition temperature. Glasses with higher fictive temperatures exhibit greater fatigue resistance.