Effect of loading mode on the measurement of fatigue parameters in optical fibres

摘要

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.