Bending Performance of Fiber-Optic Cable Sheaths: Application of New Apparatus and Techniques

Apparatus and techniques were developed by which moment-curvature curves can be generated over a wide range of curvatures, cable diameters, and environmental conditions. The resulting curves can be used to analyze bending performance in the same way as stress-strain curves are used to analyze tensile performance. In the first series of tests using this apparatus, five wire-reinforced fiber-optic cable sheaths were tested at room temperature. The wire reinforcement had a dominant, theoretically predictable effect on the bending stiffness of certain designs, while it was relatively unimportant in others. The key difference is the degree of encapsulation of the reinforcement, which can be controlled by simple changes in design. In addition, buckling performance can be improved by increasing the cross-sectional rigidity. In both cases, tensile performance is relatively unaffected. These findings are exemplified by the performances of a single-ply and a crossply design. Compared to the single ply, the crossply has the more rigid cross section and the lower degree of encapsulation. Both sheaths have nearly equal bending stiffness throughout the entire range of curvature, and the buckling radius of the crossply design (7.4 cm) is better than that of the single-ply design (8.4 cm) despite a 2 to 1 advantage in tensile stiffness. An effective design sequence has evolved from the results of this study: (a) select reinforcement for tensile performance, (b) adjust cross-sectional rigidity for buckling performance, and (c) adjust reinforcement encapsulation for bending stiffness performance. In this sequence, each step is nearly independent of the others.