Characterization Of Torsion Fatigue Of High-Performance Fibers

As one of high intensity, high modulus fibers, high performance fibers (HPF) are widely used in industries, such as composites materials, ropes, conveyer belts and so on. These fields require the fiber has good torsion fatigue properties. The fatigue of fibers is of importance because, during production, processing, and end use, they are more likely to undergo repeated than steady loadings. Hence fatigue is more likely to break the fibers than the application of a constant tension.Taking the PBO fiber, Nomex? fiber and high modulus polyethylene as the samples, the torsion fatigue of the three fibers had been characterized. At the same time, the mechanisms of torsion failure were also discussed. The details were as follows:(1) During the study of three fiber's microstructure, it could be found that: PBO fiber is made up of micro fibrils with the diameter of 10nm-50nm. The fiber is a skin-core structure, the skin without void is very thin and the core consists of micro fibrils, which are all extendedchain structure having axial shift. The a-axis of the crystals shows a radial orientation in cross-section of filament. The extended chains consisting of covalent bonds produce extremely high fiber strength as well as high modulus and tenacity, which highly depend on molecular orientation. Nomex fiber is also consisted of micro fibrils. HMPE fiber is with high crystal;the crystal region is extended along the fiber axial. The fibers microstructures are characterized by the tension morphological observation.(2) During the direct torsion to fracture experiment, we could find that as HPF with the flexible molecular chains, HMPE and Dyneema SK65 had a bigger twist angle than the other fibers with rigidity chains. So Dyneema SK65 and HMPE had better torsion resistance properties than the other high performance fibers. Three kinds of Para-amid had the similar torsion angle. In this experiment, several high performance fibers' twist angle was compared.(3) The torsion fatigue experiment showed that the torsion fatigue life was dispersed, from less than ten cycles to several hundreds. In the experiment, the conclusion of the relations among pre-tension, torsion angle and fatigue cycling numbers were obtained. At same condition, with the increase of pre-tension or torsion angle, the torsion fatigue lifetime was decreased, but the torsion angle had the stronger affection than the pre-tension. In these parts, a data calculating method andprogram were used after dividing the data into groups by utilizing logarithm.(4) During the experiment of direct torsion failure and torsion fatigue test, it was found that most of the fibers were failure at the end closed to the torsion motor, because at this area, the twist distribution was relatively centralized which led to strain concentration. As the result, fibers were failed because of the exquisite shearing strength.(5) Through microscope and SEM, the typical pictures of torsion fatigue breakage appearances were obtained. The fracture or damages on PBO fibers, Nomex? fibers and high performance fibers were discussed in details. The SEM observations show that the PBO fiber was usually laminated and spitted into fibrils, while the Nomex? fiber's torsion fracture section was shrinkage. Which didn't have the obviously fibrils split. High modules polythene fiber also showed fibrils splitting. Its main morphology was divaricator and spiting, moreover, it could be found that there was obvious twist at the end closed to the driving motor, but less twist at the other end.