Analysis of reversible and irreversible strains in the creep of a nonlinear viscoelastic polymer

The results of an experimental investigation and analytical description of creep of a nonlinear viscoelasticplastic material (a Upilex-S polyimide film) in tension are presented. The elastic modulus, yield stress, and ultimate strength were found from quasi-static tests with a constant deformation rate. The total strain of the material was assumed to be the sum of elastic, viscoelastic, and irreversible strains. The reversible elastic and viscoelastic and the irreversible deformation of the material were investigated in experiments on creep with the subsequent recovery of deformations (creep recovery). The nonlinearity of its viscoelastic behavior was taken into account using the principle of stress-time superposition. The results obtained allow one to describe the creep behavior of a material at various stages of loading and unloading.