Mechanical reliability of optical fibre for strain sensors

The reliability of optical fibre for sensor applications is an important issue since they are embedded in structures. The production of these sensors sometimes requires that the coating is stripped from the optical fibres with hot sulphuric acid. Tests conducted on the static fatigue of optical fibres under water, where the time to failure is measured at different stresses showed a deviation from the power-law equation. This indicates an increase in the stress corrosion parameter, n, at lower stress. Tests done using a two-point bend machine to measure the fracture strength of fibre at different time intervals after previous loading to a fixed stress showed a slight increase with time before the strength starts decreasing. This can be explained by the suggestion that the silica bonds deform by twisting to decrease the stretch in the bonds themselves before cracking. However, this might be a problem for optical fibres strain sensors used at high stress.