Abstract
Factors influencing tensile fatigue strength of 50-mm wire ropes were investigated with a wire-breakage detecting system. The fatigue strength increased with an increase in wire strength and diameter and a decrease in self-rotativity of ropes. The epoxy resin was satisfactory as a socketing material.
Introduction
More and more offshore oil wells are being drilled in deeper waters. For the purpose of economy, offshore structures for deepsea use, such as the tension leg platform (TLP) and guyed tower, are moored by wire ropes. The wire ropes used in such applications directly receive the loads repeatedly applied by waves and tides. Therefore, the fatigue behavior of wire rope is an important factor in the design of such offshore structures. Wire 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 does not cause problems for main cables receiving little live load, but it is important for hanger ropes on which bridge traffic imposes much live load. Thus, there is a strong demand to determine fatigue characteristics and clarify fatigue behavior in relation to tensile breaking load of such large-diameter wire ropes-i.e., size range of 85, 130, and 180 mm.Most conventional wire rope fatigue data have been obtained by bending tests. Few tensile fatigue test data are available, and those are mainly for small-diameter wireropes (6.4 and 12.7 mm).Few reports have been made for larger-diameter wire ropes and the fatigue tests conducted have given no specific definition of fatigue life. The best attempt has been visual examination of broken wire in the outermost layer of the wire rope. In the fatigue test of large-diameter wire 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 determine the fatigue strength of large-diameter wire ropes exactly, therefore, it is necessary to detect wire breakage during the fatigue test and thereby determine fatigue life.It was against such a background that a device for detecting wire breakage in wire rope being fatigue tested was developed by making use of acoustic emission (AE) and an accelerometer. With this device, tensile fatigue tests have been conducted on various kinds of 50-mm-diameter wire ropes that were similar to practical wire rope in construction and strength. A study comparing test results was conducted on factors affecting the fatigue strength of large-diameter wire ropes, along with an investigation on socketing materials having high fatigue performance.
Experimental Procedure
Commercial large-diameter wire ropes are manufactured with many diameters - e.g., hanger ropes for long-span suspension bridges are 85 mm, those for guyed towers are 130 mm, and those for TLP's are 180 mm. To test full-size wire ropes, a fatigue-testing machine should have a capacity of more than 4 MN.Only a 2-MN fatigue-testing machine was available, so the maximum testable nominal diameter was 50 mm. Therefore, wire ropes having a 50-mm nominal diameter were prepared with wires having the same strengths and similar constructions as those of full-size commercial wire ropes. Table 1 lists the specifications of the fatigue-tested wire ropes. Ropes 9S6, 9SL6, 6S7, and 6S8 are of the center-fit-rope-core (CFRC) type for hanger-rope use.
SPEJ
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