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.
- Alwis K.G.N.C. and Burgoyne C.J., Statistical lifetime predictions for aramid fibres, Journal of Composites in Construction, 9/2, 106-116, March/April 2005.
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.
- Burgoyne C.J. and Alwis K.G.N.C., Viscoelasticity of aramid fibres, J. Mat Sci, 43, 7091-7101, Nov 2008. doi:10.1007/s10853-008-3032-0
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.
- Alwis K.G.N.C. and Burgoyne C.J., Accelerated techniques to predict the stress-rupture behaviour of aramid fibres, 6th Int. Conf. on Fibre reinforced Polymers for Reinforced Concrete Structures, (FRPRCS-6), 111-120, Singapore, July 2003 (Received Best Paper (Research) Award).
- Alwis K.G.N.C. and Burgoyne C.J., Stepped isothermal method to predict the stress-rupture behaviour of aramid fibres, 4th Conf on Advanced Composite Materials in Bridges and Structures, Paper 046, Calgary, Canada, July 2004.
- Alwis K.G.N.C. and Burgoyne C.J., Stepped isothermal method to predict the stress-rupture behaviour of aramid fibres, 7th Int Conf on Fibre reinforced polymers for Reinforced Concrete Structures (FRPRCS7), ACI SP-230, 181-193, New Orleans, November 2005.
- Alwis K.G.N.C. and Burgoyne C.J., Time-Temperature Superposition to determine the Stress-Rupture of Aramid Fibres, Applied Composite Materials, 13/4, 249-264, July 2006
- Alwis K.G.N.C and Burgoyne C.J., Accelerated creep testing for aramid fibres using the Stepped Isothermal Method, J. Mat Sci, 43/14, 4789-4800, doi: 10.1007/s10853-008-2676-0. Jul 2008
Application of SIM testing to polyester fibres
This project related to a SIM study of polyester fibres for use in offshore mooring lines.
- Lechat C., Bunsell A.R., Davies P. and Burgoyne C.J., Characterisation of long term behaviour of polyester fibres and fibre assemblies for offshore mooring lines, Oilfield Engineering with Polymers 2008, 7-8 October 2008, Cavendish Conference Centre, London, UK.
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.
- Giannopoulos I.P. and Burgoyne C.J., Stepped Isostress Method for Aramid Fibres. FRPRCS9, Sydney, 2009
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.
- Giannopoulos I.P. and Burgoyne C. J., Accelerated technique to predict stress-rupture behaviour of Aramid Fibres, 1st Conference on Structural Members and Elements, Athens, Greece, 21-23 May 2008.
- Giannopoulos I.P. and Burgoyne C.J., SIM Test Results for Aramid Fibres, paper 79, ACMBS-V, Winnipeg, September 2008.
- Giannopoulos I.P. and Burgoyne C.J., Design Criteria for Aramid Fibres, ACIC Conference, Edinburgh, 2009.
- Giannopoulos I.P. and Burgoyne C.J., Jaw Effect in Yarn Testing, 16th Greek Concrete Conference, Paphos, Cyprus, Oct. 2009.
- Giannopoulos I.P. and Burgoyne C.J., Viscoelasticity of Kevlar 49 fibres, 16th Greek Concrete Conference, Paphos, Cyprus, Oct. 2009.
Practical Implications for Designers
The work so far has enabled predictions to be made for allowable stresses using aramid fibres for structural applications.
- Giannopoulos I.P. and Burgoyne C.J., Stress limits for aramid fibres, Proc. Inst. Civ. Engrs, Structures and Buildings, 162/SB4, 221-232, Aug 2009, doi 10.1680/stbu.2009.162.4.221.
- Burgoyne C.J., Allowable Long Term Stresses in Aramid Tendons. Keynote paper at 1st Middle East Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures, Dubai UAE, Februray 2011.
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.