Dynamic responses of Ca-alginate/polyacrylamide hydrogels at high strain rates

Abstract

Many biological hydrogels possess remarkable mechanical performances under both quasi-static and impact loadings, yet such mechanical robustness over a wide range of strain rates has not been achieved in synthetic hydrogels due to the lack of understanding of dynamic responses at high strain rates. Here we study the dynamic responses of tough hydrogels at high strain rates by using the Ca-alginate/polyacrylamide hydrogel as a model material and a modified split Hopkinson press bar (SHPB) system. We show that the Ca-alginate/polyacrylamide hydrogel is also much tougher than the two constituents at high strain rates. The strength of Ca-alginate/polyacrylamide hydrogel at 1.2 × 104 s−1 is two orders of magnitude higher than the stress at the same strain at 0.25 s−1. We identify the fracture points by using snap rings to control the maximum compressive strains during the loading process and validate them by subsequent cyclic loading and unloading at 0.25 s−1. We formulate a five-parameter constitutive model to describe the visco-hyperelastic behaviors and achieve satisfactory agreements between theory and experiment. We further interpret the underlying mechanisms of the strain rate-dependent deformation and fracture behaviors by comparing them with the responses of the two constituents. The importance and opportunities of studying the dynamic responses of tough hydrogels at high strain rates are discussed.