The mechanical reliability of optical fibre for long-term applications

Optical fibre is often required for long-term applications in harsh environments, eg. underwater cables or aerial cables in snowy areas. However, stress corrosion or the growth of cracks present on the surface of the glass results in the failure of the fibres, especially if they are to be deployed in humid areas and bearing a static stress. The presence of a fatigue limit, a stress under which the crack does not grow has not been established in pure silica glass, in contrast to borosilicate and the soda-lime glasses. A method to check the presence of a fatigue limit was developed by Gupta1 by studying the relation between the power and exponential laws of crack growth. Here, dynamic fatigue tests were carried out to estimate the fatigue parameters. The limiting stress is found and is compared to static fatigue results from tests carried out under hot water to accelerate the corrosion process. The extrapolation of the static fatigue stress for a time of 25 years predicts a failure stress that is higher than the fatigue limit.