Identification of elasto-viscoplastic material parameters by indentation testing and combined finite element modelling and numerical optimization

Using nanoindentation experiments, the material parameters of an elasto-viscoplastic material model with non-linear isotropic and kinematic hardening are determined by minimizing the least-squares difference between experimental data and results from a finite element model of the indentation test. The objective function is minimized by a gradient-based numerical optimization algorithm. The gradient of the objective function with respect to the material parameters is calculated using the direct differentiation method. The parameter coupling occurring in the indentation test is systematically analyzed, and virtual indentation tests are used to assess the reliability of the material parameter identification method. Experimental nanoindentation curves obtained on an aluminium alloy and annealed copper are analyzed. Objective functions consisting of load versus displacement-into-surface curves are used. The efficiency of the use of residual imprint data resulting from nanoindentation testing in the objective function is investigated in order to assess their appropriateness at such small scales.