Visco-Elastic Behavior of Rubber in a Combined Torsion-Tension Test

If a rubber rod is subjected to a simple extension of extension ratio and then either twisted or excited by small strain oscillations in the direction of the pre-extension, it is possible to measure the anisotropy in the visco-elastic behaviour induced by the pre-extension. Previous work has found that tan dropped with increasing extension which was interpreted as lowering of internal viscosity due to chain orientation. These experiments present further work on rubber materials under dynamic loading and confirm the general finding of decreasing tan with increasing extension. However, we believe that the results can be understood in terms of changes to the test piece geometry alone. The assumption we propose is that under conditions of a small torsional or tension oscillation superimposed on a large static tensile deformation, the fundamental dissipative process is itself little affected by moderate static strains that are insufficient to cause substantial orientation of the monomer units in a chain. This seem reasonable, as up to 100% strain the statistical theory of rubber like elasticity suggests that the amount of orientation should not be substantial for moderately crosslinked rubber. In torsion therefore, the energy dissipation will depend on the strained dimensions of the rubber cylinder and the square of amplitude of the small strained produced by torsion alone. This theory is supported by the experimental observations.