LARGE-STRAIN VISCOELASTIC CONSTITUTIVE MODELS FOR RUBBER .2. DETERMINATION OF MATERIAL CONSTANTS

A method for determining material constants in large strain viscoelast ic materials was demonstrated for a highly saturated nitrile rubber. M aterial constant selection was based an viscoelastic stress relaxation data at small and large strains, under both tension and compression, and was constrained to assure Drucker stability. Assuming that the vis coelastic strain energy function was both time and strain separable, a Prony series was constructed for the time dependent material constant s. For comparison, four different Prony series were developed from col location methods and a nonlinear regression analysis, each separately based on either large or small tensile strain relaxation data. III add ition, a final Prony series was constructed from dynamic data. These P rony series were included in this comparison to judge their ability to predict both large and small strain material behavior. Finite element analyses of large and small step-strain relaxation tests and a single cycle hysteresis loop at large deformations were performed for each s et of Prony series. The results were then compared to experimental beh avior. The Prony series based on the constrained method accurately pre dicted step-strain relaxation behavior at all strain levels, for both tension and compression. The finite element results for the other Pron y series show that large strain material behavior was best predicted b y those Prony series based on large strain material behavior. Similar findings were found for small strain material behavior. The constraine d Prony series and the two large strain based Prony series best modele d the experimental hysteresis loop.