The dynamic tensile stress-strain behaviour of an EPDM rubber was characterized at quasi-static (<0.01 s−1), medium and high strain rates (100–600 s−1). The quasi-static experiments were conducted by a simple uniaxial tensile test; the medium and high strain-rate tests were performed using drop-weight and gas-gun apparatuses. In these dynamic tests, high speed imaging and digital image correlation were used to measure dynamic displacement fields in the specimen. The dynamic stress state is not in equilibrium, which is a usual requirement for a conventional dynamic experimental analysis. Instead, the non-equilibrium deformation was analysed by the Virtual Fields Method (VFM) using inertial forces, clearly generated due to the non-equilibrium state, as a virtual load cell. The linear VFM associated with a linear isotropic model was applied to the drop-weight test data, in these experiments specimens were subjected to various static pre-stretches before dynamic loading was applied. For the gas gun experiments, in which the dynamic strain and experimental durations were larger, the nonlinear VFM was developed to include the one-term Ogden hyperelastic model so that the long deformation history could be analysed. The material parameters identified by these two techniques were used to reconstruct uniaxial true stress-strain curves which showed a clear and consistent rate dependency.
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Effort sponsored by the Air Force Office of Scientific Research, Air Force Material Command, USAF, under grant number FA8655-12-1-2015. The U.S Government is authorized to reproduce and distribute reprints for Governmental purpose notwithstanding any copyright notation thereon. The authors thank S Fuller and JL Jordan of AFOSR and M Snyder and R Pollak of EOARD for their support. The authors would like to thank R Froud and R Duffin for the construction of the experimental apparatus used in this research, and their helpful advice when designing this apparatus. Finally we thank Professor F Pierron for his invaluable help with the Virtual Fields Method.
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