The torsional behaviour of 1, 3 and 5 phr peroxide crosslinked natural rubber has been characterized over a range of strains from near the undistorted state (γ ≈ 0.017) to γ ≈ 1.0. Isochronal measurements of both torque and normal force were used to calculate values of the derivatives of the strain energy function W with respect to the first and second stretch invariants I1 and I2. In the course of our work we found that, contrary to many reports in the literature, ∂W ∂I1 was affected significantly by the amount of crosslinking. Finally for the 1 phr peroxide crosslinked rubber it was found that, while ageing for 14 months at ambient conditions did not significantly affect the small-strain torsional modulus, G = 2( ∂W ∂I1 + ∂W ∂I2), it did significantly affect the individual derivatives ∂W ∂I1 and ∂W ∂I2.
- normal force
- strain energy function