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.
[Cambridge University | CUED | Structures Group | Geotechnical Group]