Validated tensile characterization of the strain rate dependence in soft materials

Abstract

A major obstacle in the accurate measurement of the dynamic tensile response of soft materials is the issue of obtaining material response under simultaneous stress equilibrium, stress uniaxiality and constant strain rate. To resolve this issue, a split-Hopkinson pressure bar (SHPB)-based dynamic tension experiment is developed in this study, which has sufficient fidelity to ensure temporal and spatial accuracy of the stress state, to investigate soft materials under large deformations and intermediate dynamic strain rates (100-102 s−1). Cross-linked polydimethylsiloxane (PDMS) elastomer is chosen as a model material. By examining the evolution of full-field strain and strain rate measured using digital image correlation (DIC) and employing analytical criteria for quantifying stress equilibrium and deformation uniaxiality, it is demonstrated that the proposed test method yields a true steady-state and inertia-free stress-strain material response when all three conditions of equilibrium, uniaxiality and strain rate constancy are satisfied. Quasi-static experiments at 10−3-10−1 s−1 strain rate are also conducted for comparison. By analyzing the quasi-static and dynamic stress-strain plots and the variation of uniaxial moduli in the investigated range of strain rates, it is shown that the tensile response of PDMS points to limiting chain extensibility and exhibits a weak nonlinear strain rate sensitivity. Finally, a visco-hyperelastic model based on the Gent strain energy density function and a recently proposed viscous dissipation potential is employed to capture the experimentally obtained response. It is shown that all the features of the rate-dependent stress-strain response are modeled with good fitting accuracy.