Viscoelastic response of hydrogel materials at finite strains
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Hydrogel materials are very soft materials consisting of polymer networks and solvent molecules. The materials may exhibit large volume changes depending on its external chemical and mechanical environment and have viscoelastic properties which is common for many polymeric materials. In order to model the material response with the finite element method, a hydrogel constitutive model have been combined with finite viscoelastic theory and the resulting viscoelastic hydrogel constitutive model have been coded in a UMAT-subroutine for analysis with the ABAQUS/Standard finite element modeling software. Material parameters have been extracted from a hydrogel relaxation experiment, and while the experimental data is variable, the constitutive model have successfully been able to mimic the viscoelastic material response shown in the experimental data. The Neo-Hookean and Yeoh hyperelastic models have also been combined with finite viscoelastic theory in order to model compression experiments performed on acrylic Ugelstad particles. The material models where not able to model the complex force-deflection curve shown in the experimental data and some different hyperelastic models should be considered in order to properly model the material. The constitutive models have been numerically tested, with finite element creep and relaxation tests, which show that the models are numerically stable at large deformations.