The creep behaviour of oriented and isotropic polyethylene

The long term plastic deformation, (creep), behaviour in tension of both
oriented and isotropic polyethylene has been studied using a variety of relatively simple
experimental techniques, primarily dead loading creep tests.
In an attempt to gain an understanding of the mechanisms controlling creep two
different viscoelastic models have been applied to the data. The first of these was a two
process model, involving two activated Eyring processes in parallel. The second was a
Fotheringham and Cherry type model, with a co-operative jump based on a single
activated process.
In the oriented state the two process model has been successful in describing the
behaviour of the material. The Fotheringham and Cherry type model was not successful
in describing the behaviour of this material. A complete expression of the two process
model has been shown to fully describe the behaviour of one grade at one draw ratio,
and an approximation of the model for the other draw ratios and grades has indicated
that creep of oriented polyethylene is controlled by a c-shear mechanism.
Because of time constraints it was only possible to analyse the behaviour of one
grade of the isotropic material with the Fotheringham and Cherry model. Whilst this
was successful the activated parameters obtained from it seem very low and would
seem to indicate a chain rotation mechanism. Whilst it appears that the two process
model should also describe this data, it has not been possible to produce a conclusive
fit. It is considered that this is due to limitations in the computing software used for
fitting. Hence an approximation of the model was used to produce approximate
activation parameters.
In addition to the investigation into the rate controlling mechanism of creep the
data obtained for the oriented state has been shown to be consistent with the existence
of a unique relationship between three important mechanical properties, (strain, strain
rate and stress). This relationship holds provided that the initial morphology of the
material is equivalent prior to orientation taking place. It has also been shown that the
creep data obtained for the isotropic state are consistent with the existence of double
yield points in polyethylene.
Finally the creep data have been used in a computer model of craze deformation
in order to investigate the rate controlling mechanism of slow crack growth. This has
shown that craze failure, and hence crack lifetime is controlled by tensile creep to
failure of fibrils at the base of the craze.