Reliability of Dynamic Fatigue Data for Plastic Coated Fused Silica Optical Waveguide Fibers

The reduction in strength of glass fibers under stress and in the presence of moisture or water follows a log stress vs. log time to failure dependence theoretically and experimentally. Errors in the acquisition of fatigue data for plastic coated fibers are minimized when the time dependent parameter in the dynamic case is actual time to failure and not strain rate. Also for those fibers tested in humid or water environments, the transport time of water through the fiber coating must be considered for those systems demonstrating a time and/or temperature dependence of transport. The time dependent reduction in strength by slow crack growth is fatigue and its rate is given by the slope of the log stress vs. log time to failure curve. The intercept of this curve is the "instantaneous" strength. For plastic coated fibers this intercept shows marked reduction in a water environment as compared to drier ambient conditions. This reduction is attributed to surface energy decrease, but this concept has not been rigorously researched. Slope and intercept data given for a variety of plastic coatings on fused silica fiber and compared statistically in ambient and water environments, enable engineering worst case predictions of lifetime to be made. These predictions, however, are for fibers having high instantaneous strengths in > 20 Km lengths and are therefore not representative of fibers exhibiting low proof-test strength levels. Also, the possibility of interfacial coating-glass interactions occuring and having adverse effects on these predicted fiber lifetimes is not considered.