The viscoelastic properties of rubber under a complex loading
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This thesis aims to explore the effect of pre-strain on the viscoelastic behaviour of
rubber materials. There are various conflicting theories in the literature regarding the
strain dependence and resulting anisotropy of the viscoelastic behaviour. This thesis
seeks to measure the behaviour and to study the possibility of using Finite Element
Analysis (FEA) to predict the static behaviour of a rubber cylinder in combined
torsion-tension and also the viscoelastic behaviour of rubber under various complex
loadings using a Bergstrom-Boyce model1-4.
To measure the induced anisotropy, a rubber test piece is subjected to a simple
extension l and then it is subjected to small strain oscillations in the direction of the
pre-extension or in shear. These two different deformations will allow the extent of
the anisotropy in the viscoelastic behaviour induced by the pre-extension to be
measured. Kuhn and Kunzle5 found that the loss factor resulting from a small
oscillation decreased as a function of the static pre-strain. They and many others have
interpreted this as a lowering of internal viscosity due to chain orientation. However,
a simple analysis shows that this effect is due to geometric changes alone and that the
essential viscoelastic behaviour expressed in terms of the deformed dimension after
the application of the pre-strain as the loss modulus for an unfilled rubber is constant
with strain up to an extension ratio of 2. It is also isotropic in behaviour for filled
rubber compounds such as carbon black. For fumed silica filled rubber, the picture is
more complex. For a moderately carbon black (25 phr) filled rubber, the loss
modulus is still independent of the pre-strain for normal working strains but at highly
filler contents (above 50 phr), the loss modulus increases with pre-strain at extension
ratios somewhat less than 2. With silica, the coupling agent dominates the
viscoelastic behaviour. For filled rubber, the change in loss modulus with strain can
in part be explained by strain amplification, slippage of rubber around the filler, and
shape factor effects. This approach can help to further understand the mechanism of
filler reinforcement in rubber materials.
Another complex loading is also used to validate these results with a static pure shear
superimposed with simple shear oscillation. The results confirm the loss modulus is independent of the pre-strain for unfilled rubber and lightly filled rubber but for the
most highly filled rubber, the test is unsuitable as the smallest oscillating strains were
too great for linear viscoelastic behaviour.
The Finite Element Analysis (FEA) shows that a rubber cylinder in combined
torsion-tension test can be modelled accurately as an elastic component provided that
the appropriate strain energy function (SEF) and geometry are used in the model. The
correct torque and the second order effect whereby a reduction in the axial force
resulting from the torsion of a pre-strained rod can both be accurately represented.
The viscoelastic behaviour under various complex loadings was modelled using the
Bergstrom and Boyce model1. The results show that this model can predict behaviour
for uniaxial but in a complex loading the model was inappropriate.
Authors
Suphadon, NutthanunCollections
- Theses [3706]