| Rope, as the earliest tools used by human beings, play a key role in the development of human civilization. During a very long time after the rope has been invited, the materials of rope are natural cellulose fiber, especially the jute fiber. As the coming of industrial time, the steel wires replaced the natural fiber as the material of rope and used in hoisting and pulling industries. Until recent decades, as the creation of high performance fibers, such as HMPE and Aramid fibers, the fiber rope with high strength to weight ratio are concerned by scientists and engineers.Ropes wood be objected two kinds of force. One is the force in axis, which is the main style of towing process; another is unsymmetrical force, which is the main force type in winding process and are the composition of twisting, pressing, bending, torsion and friction. The classical situations are ropes bent over sheaves and slipping over a beam or hook with big curvature. This work focused on the geometrical modeling of tubular braided structures and the bending fatigue of fiber rope bent over sheaves. In geometrical modeling, the Solid Works software was used as the tool to realize the modeling process. In the fatigue research, the effects were discussed systematically, including the structural parameters, the experimental parameters and environmental elements. Based on series of experiments, this work discussed the failure mechanism of rope bent over sheaves which would be helpful for the design and usage of ropes.(1) Firstly, the structures of braided rope were analyzed and characterized. By generalizing the rose curve, we obtained the generalized rope curve which would be used to characterize the strands in braids. By the comparisons between simulated models and real ropes, it could be observed that models based on the generalized rose curve could be good at reflecting the morphology of strands in braids.(2) Based on the decomposition and re-composition of strand motions in braiding, two kinds of surfaces are abstracted, which are braiding surface and helical surface. the braided strand could be obtained by the intersection of these two surfaces. Based on this method, series of braided models are realized including common braided structures, triaxial braids, loose braided structures and tight braids structures, multistrand braid and noncylindrical braids.(3) Furtherly, based on realized models, this work developed the automatic modeling software of braided structures using the Solid Works API. This system could realize the adjustment of parameters including diameter, pitch, braiding angle, the number of strands and the interlacing pattern etc. and could realize the calculation of curve length etc. the software would fasten the geometrical modeling and parameter adjustment of braided structures and improve the design efficiency.(4) Based on the relevant theories of differential geometry,this work deduced the spatial equations of bent strand in braided rope and discussed the relative slip among strands. Based on the mathematical analysis and finite analysis, this work also analyzed the distribution of stress and strain in the strands and ropes. The relative motions and distribution of stress and strain provided the base for the analysis of bending fatigue. Relative calculation and simulation illustrated that the slip among the adjacent strands would decline with the increase of braiding angle; the slip would be lines with the diameter ratio of sheaves to rope.(5) By experiments, this study systematically analysis the parameters that affect the bending fatigue of rope over sheaves, which include the structural parameters, the experimental parameters and the environmental elements.(a) For the aspects of rope structures: compared with the braided rope, the fatigue life of the twisted rope would has a 30% discount; the exist of sheath would increase the friction between sheath and the rope core and would increase the hairiness of core after bending fatigue.; for braided rope with same diameter and different braiding patterns, the diamond braids have more interlacing points which would increase the abrasion among strands, the Hercules braids would have serious linear friction for their long contact. so the regular braids would be the alternative choice.(b) For experimental parameters: the cross-section of sheaves should be in coincident with the shape of ropes which would ensure that there are the least slip when bending over sheaves. For sheaves with circle, trapezoid and rectangular cross-section, the sheaves with circle would be better for rope bent over sheaves compared with the other two sheaves. For the diameter ratio, the experiments illustrated that it would be better if the ratio is bigger than 20. The bending frequency reflects the speed of bending process. The experiments show that the high frequency would obviously decrease the fatigue life. When the frequency is more than 0.5Hz, this kind of effectiveness would be little. So in practice, the frequency should be no more than 0.5Hz.(c) For experimental environment: under high stress, water would play lubricant and heat diffusive effects which would obviously improve the fatigue life. Under lower stress, the lubricant oil would decrease the abrasion among strands and improve the fatigue life.(6) By the experimental observation and tests, the fatigue mechanism would be concluded into two aspects: the abrasion and the heat damage. The abrasion is the dominating aspects for the bending fatigue over sheaves and its happen and evolution would include the extension and pulling of fiber, the break of fiber, the extension of abrasion area and the final break of rope. According to the contacting situation, the abrasion would be divided in to point friction, line friction and surface friction. Under a certain stress, the strands in braids would experience four steps in these three kinds of friction styles and at last lead to the fatigue of ropes. Another aspect of bending fatigue would be heat damage, the reflections of which is the adhesion of fibers among strands. The temperature of rope core and sheath would be increased obviously tested by heatsensitive sensor. The source of heat mainly include two parts. One is heat generated by the friction among strands, the other would due to the viscoelasticity of the synthetic which would transform certain energy to heat and lead to the increase of temperature, especially when the frequency is big. Based on the fatigue mechanism, in the last part of this paper, certain methods are given for rope usage bending over sheaves.The above work woud deepen the acknowledge of rope structures,improve the rope design and clear the relationship among the relevant parameters including rope structures, experimental and environmental parameters on the bending properties. At the same time, the work would be meaningful for the rope desin for bending over sheaves and rope selections. |
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