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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Viscoelastic Properties of Elastomer under Complex Loading

Author: SUPHADON, Nutthanun

Year: 2010

Supervisor(s): James Busfield

This thesis aims to explore the effect of a static 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 by using Bergstrom-Boyce model. 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 Kunzle1 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 as the loss modulus for an unfilled rubber is constant with strain up to an extension ratio of 2 and it is also isotropic in behaviour for filled rubber compounds such as carbon black and fumed silica 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 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 around the filler, and shape factor effects. These can help to further understand the mechanism of filler reinforcement in rubber materials. Another complex loading was also used to validate these results with a static pure shear superimposed with simple shear oscillation. The results confirm the loss modulus was 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 model2. The results show that this model can predict behaviour for uniaxial but in a complex loading the model was inappropriate.

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