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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: Stress-strain behaviour of rubber

Author: GOUGH, Julia

Year: 2000

Supervisor(s): Craig Davies

Measurements of the deformation behaviour of an elastomer containing a compressible filler are used to assess theoretical equations for the compression modulus of rubber pads bonded to rigid endplates. The volume fraction of filler is estimated from a simple model.
The first cycle stress-strain behaviour of filled and unfilled rubbers is characterised from uniaxial tests and by measuring both non-zero principle stresses with a novel pure shear technique. Various theoretical forms for the strain energy density function are assessed. The results support the assumption that the strain energy of filled natural rubber is a function only of the first strain invariant.

Finite element modelling of the behaviour of a hyperelastic material in simple shear reveals that the proximity of the free edges in conventional simple shear testpieces strongly influences the stresses and deflections in the thickness direction. These finding are qualitatively supported by experiment. The effect of free edges on the shear modulus is also assessed.

Deviations from hyperelastic behaviour are investigated through experimental studies of stress relaxation, cyclic stresses softening and the superposition of a torsion on a uniaxial extension. Anisotropic deformations can result in corresponding differences in the amounts of stress relaxation or stress softening in different directions. Isotropic models cannot model these features but may be adequate for most practical applications.

The relationship between the modulus and crystallinity of partially crystalline rubber is determined experimentally. The reinforcing effect of the crystals is found to be approximately independent of their morphology and of the modulus of the amorphous rubber. Studies of yielding of partially crystalline rubber show that the yield stress increases with increasing amounts of crystallization whereas the yield strain remains roughly constant.