The factors that influence the tensile fatigue strength of large-diameter wire ropes and the effects of socket materials on wire breakage in sockets were investigated with use of the newly developed wire breakage detecting system. The fatigue strength of wire ropes was defined at five percent wire breakage rate and had a tendency to increase with an increase in wire tensile strength and diameter and a decrease in self-rotativity. The effect of these factors increases in that order. The size effect was recognized, the larger the rope diameter, the greater the fatigue strength, in case the similar rope construction and the same wire tensile strength were used. These phenomena was explained qualitatively by the fact that wire breakages were observed mostly at points where abrasion had occurred due to contact between wires. The breaking strength of fatigued rope dropped only by three percent when wire breakage rate was eight percent. The epoxy resin proved satisfactory as a socket material.
More and more offshore oil wells are being drilled in deeper waters. For the purpose of economy, offshore structures for deep-sea use, such as the tension leg platform (abbreviated TLP) and Guyed Tower, are moored by wire ropes (abbreviated ropes). The ropes used in such applications directly receive the loads repeatedly applied by waves and tides. Therefore, the fatigue behaviour of rope constitutes an important factor in the design of such offshore structures. Ropes are used also for long-span suspension bridges such as those being constructed to connect Honshu and Shikoku in Japan. In such bridges, the fatigue strength is important for hanger ropes on which bridge traffic imposes much live load.
Most of the conventional rope fatigue data have been obtained by bending tests. Only a few tensile fatigue test data are available, and those being mainly for such small-diameter ropes as 1/2 and 1/4 inch. 1,2 Few reports have been made for larger diameter wire ropes. 3,4,5 Besides the fatigue tests so far conducted have given no specific definition on fatigue life. The best attempt made in this connection has been visual examination of broken wire in the outermost layer of the rope. In the fatigue test of large-diameter ropes comprising a large number of wires, however, wire breakage does not always occur in the outermost layer, but can take place in the inner layers, and even in sockets under certain conditions. To exactly determine the fatigue strength of large-diameter ropes, therefore, it is necessary to detect wire breakage during the fatigue test and thereby determine their fatigue life. Using the developed device for detecting wire breakage, on occasion, tensile fatigue tests have been conducted on various kinds and types of 50 mm diameter ropes that were made to the similar rope construction and the same wire strength as those for practical applications. By comparing the test results obtained, a study was made on the factors affecting the fatigue strength of large-diameter ropes, along with an investigation on socketing materials having high fatigue performance, as reported hereunder.
