Stress limits for aramid fibres

Abstract

Aramid fibres should have many structural applications: they should be used as tendons in prestressed concrete, as stay cables in bridges and as ropes in marine industry owing to their good tensile properties, low weight and lack of corrosion. However, uncertainty about their ability to carry significant loads for a long period of time has meant that engineers have been reluctant to adopt them. Two new techniques (stepped isothermal method and stepped isostress method) are now available that allow accelerated testing to be carried out at low stress levels, in such a way that the long-term creep and creep rupture properties can be determined without having to extrapolate more than one decade on a log-time scale. Such tests have now been carried out on two slightly different aramid fibres: Kevlar 49 and Technora. The paper shows how this information can be used to predict the safe operating stresses for these fibres when under sustained load, which is precisely the sort of application for which they are most economically suited. The effects of yarn variability are considered, as are the effects of different temperatures and varying loads. The aim is to be able to predict the behaviour of aramid fibres when subject to load durations of 100 years or more, without having to provide excess material because of a lack of applicable test data.