The viscoelastic response of highly-stretches rubber specimens to superposed torsional deformation

Abstract

This paper describes and discusses the anomalous mechanical response of tubular specimens of natural rubber which have been highly stretched in their axial direction and to which a small torsional twist is applied while stress relaxation is taking place. It has been found that whereas the response conforms to the theory of large elastic deformations for stretch ratios up to 3.5, i.e. the torsional modulus is directly proportional to the axial force, for stretch ratios greater than this the response becomes anomalous, and at stretch ratios in excess of about 4.0 the torsional modulus increases as the stress relaxes. This is attributed to the crystallization of the rubber which is known to take place in this range of stretch ratio. The results of recent experiments both of torsional oscillations where the rubber specimen provides the restoring couple and of the response of stretched rubber tubes to small step twists are described, and the theoretical representation of this type of response is outlined. The effects of different ambient temperatures on the response are also described.