Combined Stress-Creep Experiments on a Nonlinear Viscoelastic Material to Determine the Kernel Functions for a Multiple Integral Representation of Creep

Abstract

A multiple integral functional relationship has been employed as a constitutive equation for nonlinear creep of viscoelastic material under combined stress. The kernel functions of this representation for first, second and third order stress terms have been determined from tests on a single tubular specimen of polyvinyl chloride copolymer. Tests needed to determine kernel functions adequate to describe multiaxial creep under constant stress were found to be three pure tension tests at different stress levels, three pure torsion tests at different stress levels, and two combined tension‐torsion tests. Experiments include twenty tests under various combinations of tension and torsion of the same specimen. The use of only one specimen was made possible by the fact that following 2 hr of creep the recovery in 4 days was almost complete. Agreement between the multiple integral representation and experimental results was very satisfactory. Agreement with a hyperbolic sine representation of stress dependence was less satisfactory owing in part to the fact that the synergistic effect of tension plus torsion in the nonlinear range was not accounted for.