Accelerated Testing for Long-term Stress-rupture Behaviour of Aramid Fibres

Summary

Aramids have been proposed as an alternative material to steel for many civil engineering applications.  However, their usage is limited due to a lack of understanding of the long-term stress-rupture behaviour.  Many stress-rupture models have been developed to predict the rupture times, based on molecular and statistical theories.  Stress-rupture data obtained at high stress levels has been used to predict the behaviour at low stress levels using the extrapolation techniques.  The reliability of these life models is questionable.

In this thesis, a different statistical approach has been presented to predict the long-term stress-rupture behaviour of Kevlar (one type of aramid) rope data reported in the past.  Unlike the other methods, various life models have been investigated and the best model that ‘optimally’ fits the data has been chosen based on the Kullback-Leibler model selection criterion.  

To improve the reliability of the life models it is necessary to include the stress-rupture data obtained at low stress levels.  However, stress-rupture data at low stress levels are expensive to obtain under conventional creep testing as a long time span is needed to reach the failure of a specimen.  To overcome this problem, and to obtain stress-rupture data at low stress levels within a reasonably short time scale (hours), two accelerated testing methods have been investigated.  In these methods, creep rate is accelerated by elevated temperatures.

In the Time Temperature Superposition Principle (TTSP) several yarns are tested at different temperatures and a single curve, known as the master curve, is obtained to predict the long-term behaviour.  In the Stepped Isothermal Method (SIM) a single yarn specimen is tested at a specific stress level at a series of increasing temperature steps. Some manipulation of the data is required in order to compensate for the temperature steps.  This technique has many advantages over the TTSP and conventional creep testing and can be considered as an automated testing method to obtain the long-term stress-rupture data points. 

In this thesis, both these techniques have been applied to Kevlar yarns and their applicability has been checked.  The SIM method seems promising as it shows repeatability when a number of tests are carried out.  Therefore, the SIM can be introduced as a sound method to generate more rupture data at low stress levels. 

Key words:  Stress-rupture, Life models, Accelerated testing, Time Temperature Superposition Principle, Stepped Isothermal Method, Kullback-Leibler model selection criterion.