The Tensile Fatigue of Wire Rope: A New Approach
Authors: Thorpe T.W. , Rance Andrew 
Published: October 2008

The fatigue behaviour in air and seawater of zinc coated steel wire taken from a 40 mm diameter wire rope has been studied. Seawater had little effect on short term tensile strength but it reduced fatigue life by an amount which increased with increasing mean stress and decreasing test frequency. The application of fretting during fatigue testing resulted in very low endurances, which were similar to those measured in fatigue tests on wire ropes.

INTRODUCTION

Wire ropes are used for mooring certain types of offshore platforms, (semi-sumbersibles, guyed towers etc.), and are proposed as elements of the mooring systems of wave energy devices. In these types of application, wave loadings on the structures can subject the ropes to fluctuating stresses, which can give rise to fatigue failure. In practice, two types of fatigue failure occur: that due to bending over fairleads etc. and that due to tensile stresses in the main length of the rope. In this paper only the latter failure mode is considered, though several of the observations and proposals made can be applied, with modification, to fatigue failure in bending.

Corrosion in the presence of a seawater environment can accelerate failure by a process known as corrosion-fatigue. Wire ropes are manufactured from individually galvanised wires and usually incorporate a blocking compound, which is intended both to lubricate and to prevent ingress of seawater. Whilst these protective measures appear to be effective in delaying the reduction of tensile strength by corrosion1, few data exist on the influence of corrosion on fatigue life2,3,4. Since corrosion is a time-dependent process5, 6 corrosion-fatigue testing should be carried out at realistic, in-service cyclic loading frequencies. For large diameter ropes this is a difficult, expensive and time-consuming task. In order to attempt to overcome this restriction the Department of Energy supplimented their programme of high frequency, (4 Hz), corrosion fatigue tests on 40 mm diameter wire rope, (carried out at the National Engineering Laboratory, NEL), with tests on single wires at both 4 Hz and 0.1 Hz (carried out at AERE, Harwell). It was intended that the latter programme should be used to gain a better understanding of the fatigue failure processes operating in wire ropes. Successful achievement of this would provide the basis for assessing the relevance of the NEL tests to in service loading frequencies and, perhaps, for interpreting the deleterious effect of bending on fatigue life.

This paper describes the work done in the single wire programme and interprets the data in the light of its potential application to the fatigue of wire ropes.

EXPERIMENTAL

1.8 to 2.3 mm diameter single wires were provided by British Ropes. Some of the wires, (typical of those used in manufacturing the NEL ropes), had been galvanised using a 'hot-dip' method, whilst other wires, (more representative of large diameter ropes), had been galvanised by electro deposition. These two processes produced a 0.05 rom thick layer of zinc on the underlying high carbon steel wire.