Zero stress aging and the static fatigue transition in optical glass fibers

Optical glass fibers can exhibit, under severe environmental conditions, an accelerated static fatigue behavior at long times under moderate stresses. This behavior is manifested by a transition, or 'knee', in the plot of ln (time to failure) versus ln (applied stress). This accelerated fatigue effect depends on several variables, including temperature, relative humidity, and pH, as well as on the composition of the cladding and polymeric coating. Similarly, optical fibers can exhibit a pronounced strength degradation due to zero stress aging. It was found that the onset of significant strength loss during zero stress aging occurred at about the same time as the static fatigue transition. Also, the spread in the distributions of time to failure at a given applied stress narrows after the transition. These results suggest that zero stress aging represents a second mechanism of crack growth which takes place on a longer time scale and is superimposed on the crack growth of stress corrosion. Finally, a model was used to predict this accelerated fatigue behavior in optical glass fibers by assuming the combined influence of zero stress aging and stress corrosion on crack growth.