Constitutive models of a time dependent material at finite strain levels

In this work a constitutive model relating the time dependent material response in a polymeric material has been studied experimentally and analytically. Uniaxial tensile loading is assumed. The load levels involved result in finite strain levels and stress levels above the yield point. The constitutive model was selected and developed in order to predict nonlinear time dependent behavior of the material.; Since finite strain levels were involved, true stress and true strain were used to describe the stress-strain response of the material. A combination of Schapery's nonlinear viscoelastic model and Lai's nonlinear viscoplastic model was used to characterize time dependent behavior at finite strain levels. Since the applied stress was allowed to exceed the yield point, plastic behavior was added to the constitutive model after separating the irrecoverable strain into instantaneous plastic strain and viscoplastic strain. The material properties involved in Schapery's viscoelastic model and Lai's viscoplastic model were determined by conducting several creep/creep recovery tests at several stress levels including the stress levels above the yield point.; The validity of the constitutive model was evaluated by comparing predicted and measured response in the creep/creep recovery tests. Reasonable agreement between predicted and measured strains at stress levels as high as 95% of the ultimate strength was achieved. In addition, two-step loading tests were conducted. In some cases the second step in stress was lower than the first step in stress, whereas in other cases the second step in stress was higher than the first step in stress. As before, these tests involved finite strain levels, and reasonable agreement between predicted and measured response was achieved.