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.
The dynamic fatigue test is often preferred to the static fatigue test to measure stress corrosion parameters due to the short time required to conduct tests. The results can be interpreted using the power law or the exponential law for crack growth. However, the results are usually interpreted using the power law which can be analytically integrated in the dynamic fatigue form in contrast to the exponential law which cannot be integrated analytically. Nowadays, the two-point bend machine is used widely to carry out dynamic fatigue tests. Two loading modes are available for this machine: constant plate speed and constant stress rate. In the constant plate-speed mode the mobile plate moves with constant speed towards the other plate thus increasing the stress at the apex of the bent fibre in a non-linear profile. The constant stress-rate mode, which is the more advanced, requires that the mobile plate moves at a variable speed to maintain a constant rate of stress increase at the apex of the optical fibre. In this paper we examine dynamic fatigue tests on optical fibre in water at different temperature using both the constant stress-rate and the constant plate-speed loading-profiles. Then we analyze the results using the power law and the exponential law for crack growth. The parameters for stress corrosion are then used to predict static fatigue behaviour of the optical fibre and are compared with experimental results of static fatigue carried out in two-point bend under water.
In addition to the numerous assessments presented in literature of the radiation-induced optical property changes in
optical fibres, we investigate the impact of MGy dose levels on the mechanical properties of different single and
multimode fibres. We compare the tensile test and two-point bending test results obtained by two different laboratories
with commercially available optical fibres before and after irradiation up to 15 MGy. Our results show a significant
strength reduction of about 50 % at high dose levels, relevant for particular applications in nuclear power facilities and in
large nuclear physics experiments. This strength reduction seems to depend on both the coating materials and the test
conditions, as suggested by the roughening of the outer glass surface of the optical fibre, observed with atomic force
microscope (AFM) images.
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.
Effect of the hot acid stripping of optical fiber polymeric coating on the dynamic and static fatigue is studied for assessing the impact of the stripping process on the life expectancy of embedded sensors.