PSI - Issue 57

Francis Blanc et al. / Procedia Structural Integrity 57 (2024) 343–354 Francis BLANC / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Synthetic fiber ropes offer many advantages over wire ropes, such as light weight, easy handling, low stiffness drive, low maintenance, high resistance to water (absence of corrosion) and many chemical agents [1] [2]. The maritime sector has used synthetic fibers since they exist, but other sectors are also taking a close interest in them. The literature is very rich on studies concerning moorings [3] [4] [5] [6] and there are also several specific applications such as robotics where the cables imitate tendons [7]. Crane manufacturers have significant requirements on knowledge of the fatigue behavior of ropes for their lifting applications so that maneuvers take place in absolute safety. Lifting applications have many pulleys which prove to be the most severe elements for the tensile strength of a rope. As [8] [9] schematically shows, bending a rope around a curved surface can cause damage within the rope. The strands abrade as they move relative to each other. If the rope then unbends, additional abrasion takes place. Strands on the outside of the bend stretch, a re more highly loaded, and may break. Strands on the inside compress and may experience axial compression fatigue. Like maritime sector [10] [11] [12], industrial lifting companies are therefore interested in high-strength fibers ropes such as high modulus polyethylene (HMPE) and aramid fibers ropes. A well-known method is based on cyclic bend-over-sheave (CBOS) fatigue testing [13] [14] [15] [16] [17] [18] [19] [20]. The developed test bench and the realized tests that are presented in this paper are based on this method. In this paper, presented experimental results concern aramid fiber ropes. Tests on HMPE fibers ropes are still in progress. Aramid fibers are also studied closely, both in terms of strength resistance [21] [22] or environmental resistance [23] [24]. 2. Principle and methods

2.1. Principle

The principle of the test bench is to reproduce the stresses to which a rope is subjected in service on a lifting application. The stresses are therefore traction to take up the load and cyclic bending to simulate the passage of the rope in the pulleys and the winches. This principle is called cyclic bend-over-sheave (CBOS).

2.2. Methods

The method is broken down into 4 steps, synthetized in Figure 1.

The first step is to check the tensile strength of the of the rope, by performing a monotonic tensile test. Although this data is given by the manufacturerof the rope, as it is used as a reference, it is preferable to check it upon receipt of a new rope but also from time to time to ensure that there are no manufacturing errors. The second step consists in estimate the relationship between the number of cycles to failure and the bending load level . To identify this fatigue law, at least 3 load levels, between 10% and 33% of the static tensile strength, are carried out. It is interesting to identify the failure mode of the rope for each load level by fracture analysis. The number of achievable cycles being known, the third step consists in performing cycles up to a certain percentage of this lifetime . We considered 3 to 4 levels in duration generally between 15% and 60% of number of cycles until failure. It is also interesting to know the mode of degradation of the rope after a certain number of cycles carried out. The last step has for objective to determine the residual static tensile strength of the rope after fatigue tests of the third step, by applying pure tensile test up to fracture of the rope.