Identification of finite viscoelasticity and adhesion effects in nanoindentation of a soft polymer by inverse method

In the present study a procedure to characterize the finite viscoelasticity and to simultaneously identify the influence of adhesion in nanoindentation experiments of soft polymers is developed. Silicone rubber, which is assumed to be an isotropic elastomer, is chosen to be examined. Different nanoindentation testing protocols are used to visualize and proof the viscoelastic properties and the adhesion behavior of a soft silicone rubber in contact with a Berkovich tip. It could be shown that the analytical solution of linear viscoelastic indentation has some limitations in order to predict the experimental data that contains finite viscoelasticity as well as adhesion effects. The inverse method is applied by using the finite element computation combined with a numerical optimization subroutine. A viscoelastic model at finite strain is chosen to represent the silicone rubber’s behavior. The default contact pressure-clearance relationship used in ABAQUS®is modified; a surface-based adhesive behavior in traction-separation law is incorporated into the contact pairs. The real geometry of the Berkovich tip is considered in order to minimize the systematic errors between the numerical model and the experiments. Finally, the parameters of the chosen viscoelastic constitutive model and the adhesive contact model are identified by matching the response of the numerical model with the experimental force-displacement curves. The present model contains the surface adhesion and is verified to show better reproducibility regarding the experiments than the analytical solution. There are several drawbacks of the analytical solution that are also presented in this work.