Effect of Humidity on Small‐Crack Growth in Silica Optical Fibers

Subcritical small‐crack growth in silica optical fibers was measured directly in air at 25°C for several humidity conditions from 30% to 90% relative humidity. The crack velocity data were shown to be closely fitted by a power function of the stress intensity factor. It was found that the power n is independent of humidity and takes a value of 21.7. It was also revealed that crack velocity varies with relative humidity to a power of 2.5, which agrees with estimates from the published dynamic fatigue data concerning the humidity dependence of the strength of pristine silica fibers.