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Viscoelasticity of Aramid Fibres

Yarn jaws

The stress-rupture performance of aramid fibres governs the allowable stresses that can be applied to them when used as prestressing tendons for concrete. Such tendons would have design lives of 120 years, so we clearly cannot wait that long to obtain the relevant test data. This project, undertaken by by Nadun Alwis, Ioannis Giannopoulos and Matt Causier has been evaluating different strategies for using accelerated testing. It has been concluded that the Stepped Isothermal Method can be used to produce test results in reasonable time scales. The method can also produce master curves for the creep response of the fibres.

Various aspects have been studied and have resulted in publications that are listed below.

Statistical Predictions of Stress Rupture

It is necessary to use statistical methods to predict the long term stress-rupture lifetime from short-term data.  This paper looked at how these predictions should be made.

 

Do aramids behave linearly or non-linearly?

There has been uncertainty about whether the viscoelasticity of aramid fibres is linear or nonlinear; creep tests tended to show it was linear while stress-relaxation tests tended to show it was nonlinear.  This was because creep tests tended to be done as part of investigations of stress-rupture, which were carried out at high load.  Stress-relaxation tests were carried out at the sort of loads used in practice, which were lower.  So a programme of both creep and stress-relaxation tests were undertaken at stresses ranging from very low to very high.  This concluded that the behaviour changes at about 40% of the short-term breaking strength.  Above this load the behaviour is linear, below that it is nonlinear.

 

Development of Stepped Isothermal Testing (SIM) for testing aramid fibres

It is impractical to conduct creep tests for a very long period, so some sort of accelerated testing is required.  Thornton developed the Stepped Isothermal Method for testing polyester fibres for use in soil reinforcement.  A single yarn is put into tension inside an oven.  The temperature is then increased in steps - typically by a few degrees centigrade each time - and then held constant for 4 or 5 hours.  Various adjustments then have to be applied to allow for the different rates of creep, and the past history at each stage.  The result is a master curve for creep that ends with a stress-rupture failure of the specimen.  We have adapted that method for use with the much higher modulus aramid fibres.  We showed that the Activation Energy is constant, which implies that the some process is underway at all stresses and temperatures.

 

Application of SIM testing to polyester fibres

This project related to a SIM study of polyester fibres for use in offshore mooring lines.

 

Development of Stepped Isostress Method (SSM)

An alternative to accelerating creep by means of raising the temperature is to supply energy to accelerate creep by increasing the stress.  Similar adjustments have to be made as for SIM testing.  The factor controlling the acceleration is the Activation Volume.  More papers are in preparation.


Application of SIM and SSM testing to Kevlar 49 and Technora fibres

Having shown that SIM and SSM testing work for aramid fibres, the project then applied the techniques to Kevlar 49 and Technora aramid fibres.  It was shown that the SIM testing gave predictions that matched extrapolations from conventional creep tests, but that SSM predicted slightly different behaviour.  It was shown that Technora offers longer predicted lifetimes than Kevlar, but there was a bigger difference between SIM and SSM.


Practical Implications for Designers

The work so far has enabled predictions to be made for allowable stresses using aramid fibres for structural applications.

 

Work is continuing, applying SIM and SSM testing to a different fibre, PBO, and using molecular modelling to relate measured properties to the physical and chemical structure of the fibres.  We are also studying the Retained Strength after Creep for all thee fibres.

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