Creep experiments and examples for viscoelastic material under combined stress with abrupt changes in state of stress

Creep experiments under combined tension and torsion loading are reported for tubular specimens of polyvinyl chloride copolymer. Experiments include a sequence of several combinations of tension and torsion on each specimen with abrupt changes between combinations, including reversal of sense of torsion, recovery and reloading after recovery. Results show a strong path and time dependence for the strain and for the direction of the strain‐rate vector at constant state of stress following a change in state of biaxial stress. The character of the results is described reasonably well by the Boltzmann superposition principle of linear viscoelasticity using a constitutive equation of the form:Examples are given of the use of this equation for common states of stress. Better agreement with the data is obtained from a similar but non‐linear equation employing hyperbolic sine functions of stress.