Tension Bending and Sheave Bending Fatigue of Parallel Lay Aramid Ropes

R. E. Hobbs, Reader and J. Strzemiecki, Research Assistant, Department of Civil Engineering, Imperial College, London, UK

C. J. Burgoyne

Lecturer, Department of Engineering, University of Cambridge, Cambridge, UK

As part of a wider project involving several different organisations (Baxter, 1988), tests have been carried out to investigate the bend fatigue performance of Parafil ropes. These ropes contain a core of parallel filaments; in the case of Type G ropes used in these tests, the core yarn is Kevlar 49, which gives a rope with strength of 1926 N/mm² at a breaking strain of about 1.6%. The yarn is contained within a black polyethylene sheath which maintains the shape of the rope, protects the core from superficial damage, and shields the core from ultra-violet light. All the ropes tested had a nominal breaking load of 600kN, equivalent to a cross sectional area of 305 mm². 

The advantages of these ropes for mooring large structures become apparent as the water depth increases. With a specific gravity close to unity, the ropes are effectively weightless in water, thus giving a significant consequential increase in the payload of the floating structure. (Kevlar has a specific gravity of about 1.44, but with the polyethylene sheath the ropes have a specific gravity of about 1.09 when saturated). In addition, the ropes are easier to handle than steel alternatives, and the resistance to corrosion is better.