Fracture studies on optical fibres

The fracture behaviour of optical fibre was studied both in pure tension and combined tension and torsion. The results were analysed in terms of failure statistics and fracture mechanics. Electron microscopy techniques were employed to ascertain possible flaw size and their location which may contribute to the failure of the fibre. Three flaw mechanisms were shown to exist arising from large surface flaws, bulk flaws and inherent surface defects due to the pulling process. The long term mechanical reliability of optical fibre is of major concern to the manufacturers. The phenomenon of subcritical crack growth was investigated by dynamic fatigue tests both in air as well as in acidic and alkaline environments. The parameter 'stress corrosion susceptibilities' in the various environments were evaluated. Results indicated that the presence of water in the environment significantly contributes to the subcritical crack growth rate in the fibre. In solutions, the presence of OH- ions further enhances this effect. Removal of water and OH- ions after presoaking the fibre was shown to effect a complete recovery of the strength. Design diagrams for optical fibre were constructed. These diagrams incorporate the statistical nature of the strength as well as the time dependent behaviour of the fibre. The value of such a diagram is to estimate the proof stress required for a given working stress and a given length of time if no failure is to be assured