[Univ of Cambridge] [Dept of Engineering]

Variability and viscoelasticity of parallel-lay ropes

by G. Amaniampong


The advent of high strength synthetic materials has led to the production of synthetic ropes by parallel construction (parallel-lay ropes). These ropes have been identified for use in many offshore and bridge structures, particularly where corrosion resistance and low weight are of paramount concern, and may soon be preferred to traditional materials because of their desirable properties.

The ropes are made from fibres and yarns which exhibit large scatter in short and long-term strengths and this profoundly affects the behaviour of the ropes. This behaviour is very different from that of conventional materials and has contributed to the hesitation of engineers in exploiting the advantages of the ropes. The present study aims at modelling the tensile strength and the time-dependent behaviour of parallel-lay ropes from a knowledge of the properties of the constituent elements by means of probability theory and to study the viscoelastic behaviour of the ropes. This would help to boost the confidence of engineers in exploiting the advantages of parallel-lay ropes.

A model is presented which is used to predict the bundle strengths of short ropes from their constituent elements. In this model, the slack and the cross-sectional area of the elements can be varied, as can the stiffness to model ropes with linear or non-linear stress-strain behaviour. Crude models developed in the 1940s were restricted to elements which exhibited a linear relationship between the force (or stress) and elongation (or strain) under load. These models in which the fibre breaking stress was allowed to vary were later modified in terms of strain to allow for slack.

Yarns were tested to study the scatter in stiffness, area, failure stress and failure strain. Predictions of the tensile strengths of parallel-lay ropes were obtained by putting this yarn data into the model and the results compared with rope tests. The results were then extended to predict the tensile strengths of long ropes. This model allows a better and more reliable prediction of the short-term strengths of parallel-lay ropes.

Monte-Carlo simulations were used to study the time-dependent failure of bundles to parallel elements. This casts light on the failure process. A comparison of creep-rupture lifetimes of Parafil (parallel-lay) ropes from the simulations with limited empirical results confirms the creep-rupture behaviour can be successfully predicted using these simulations.

The stress-relaxation properties of Parafil ropes were studied. An empirical relationship was developed to predict the long-term relaxation results. The effect of mechanical pre-conditioning on the stress-relaxation behaviour was investigated and quantified. The linearity of the viscoelasticity of Parafil ropes was not clarified because a non-linear behaviour is deduced from the relaxation experiments as opposed to a linear behaviour suggested by creep results.

Keywords: Bundle theory, parallel-lay ropes, viscoelasticity, Parafil ropes, Monte-Carlo simulations, creep-rupture, stress-relaxation, Kevlar, aramid, polyester.

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