Effect of sea-water environment on the tensile and fatigue properties of synthetic yarns

Synthetic fibers are used for critical performance applications of marine rope and cable industries, apart from military applications. The high strength-to-weight ratio and corrosion resistance offered by polymeric synthetic fibers makes them superior alternatives to steel wire ropes particularly in marine applications. These marine ropes and cables are subjected to a complex history of static and cyclic mechanical loading during service, leading to sudden and unexpected failure. The presence of corrosive sea water medium during service adds to the complexity of the problem. Thus, it is essential to study the tensile and fatigue performance of these synthetic fibers in the presence of sea water. In this study, experiments were conducted to examine the effect of marine environment on Vectran (Liquid Crystal Polymer) yarns. The first set of experiments analyzed the tensile strength degradation of Vectran yarns when exposed to simulated sea water for two months. The experiments were performed on Parallel continuous filament yarns and Twisted continuous filament yarns (0.5 twists per centimetre) and the results compared. In the second set of experiments, Vectran yarns were subjected to load controlled fatigue in dry state and continuously wetted state under tension-tension loading at a nominal frequency of 0.1-0.5 Hz. Stress vs. Number of cycles graphs were plotted to compare the fatigue performance in dry and wet conditions. Fatigue experiments were performed on Parallel and Twisted yarns to study the combined effect of twisting and wetting. Scanning Electron Microscopy was used to observe filament surface and failure mechanism. The results indicate that, twisting the continuous filament yarns improves both tensile strength and fatigue performance. Sea water exposure degrades the tensile strength of Parallel yarns. Twisted yarns show no such degradation. The fatigue performance of Parallel yarns appears to be higher in dry state compared to wet state. The fatigue performance of Twisted yarns seems to increase with wetting. SEM images show that failure of filaments is by severe fibrillation.