Saint-Venant’s principle and the minimum length of a dual-coated optical fiber specimen in reliability (proof) testing

• The effect of the St.-Venant’s principle on the length of a coated optical fiber specimen in proof testing is addressed.
• It is shown that it is the longitudinal compliance of the coating system that is primarily responsible for this effect.
• It is shown also how to select the adequate length of the specimen to make the test results meaningful.

The tensile force experienced by a dual-coated optical fiber specimen during its reliability (proof) testing is applied to the fiber’s secondary coating and is transmitted to the glass fiber at a certain distance from the specimen’s ends. Although it is true that, in accordance with the Saint-Venant’s principle, the glass fiber will be subjected, at a certain distance from the specimen ends, to the same stress that it would experience if the external force were applied to both the fiber and the coating, it is also true that, because of the buffering effect of the coating, the effective length of the fiber under testing, when the testing force is applied to the coating only, might be reduced appreciably in comparison with the fiber actual length. The objective of the analysis is to develop a simple analytical (mathematical) stress model for the evaluation of this effect and to use this model to establish the appropriate minimum length of the test specimen, so that the experimental data are consistent and physically meaningful. It is shown that it is the axial compliance of the secondary coating experiencing the direct action of the external loading and the interfacial compliance of the coating system that determine the buffering effect of the coating. It is concluded that for any finite compliance of the coating, even a very low one, one can always employ a long enough specimen, in which the major mid-portion of the glass fiber will be loaded to practically the same level as in an infinitely long specimen, when the external force is distributed between the glass fiber and its coating proportionally to the axial rigidities of these structural elements. The developed model can be used for selecting the appropriate length of coated optical fiber specimens in reliability (proof) testing. It can be used also beyond the fiber optics technologies area, when composite structures of the type in question are employed and tested.