Nanoindentation in elastoplastic materials: insights from numerical simulations

Finite element simulations of nanoindentation were performed on an elastoplastic material using Berkovich and conical indenters to investigate the effects of geometry on the load-displacement response of the material. The Berkovich indenter, widely used in nanoindentation experiments, is typically simplified to a theoretically equivalent 70.3° conical indenter for numerical simulations, which allows for a less computationally intensive two-dimensional (2D) axisymmetric analysis. Previous studies into the validity of this equivalence assumption for indentations in elastoplastic materials have varying conclusions. Using 2D and 3D finite element simulations, the present study investigates the response of elastoplastic materials, obeying a combined isotropic and kinematic hardening, to indentation with conical and Berkovich indenters. Simulations show that there is a clear difference in the load-displacement response of the selected material to the two indenters. The Berkovich geometry is found to produce a more localized pattern of contact stresses and plastic strains, leading to a smaller mobilized force for the same magnitude of displacement. To further validate the numerical simulations, experimental results of nanoindentation into an aluminium specimen were compared to elastoplastic finite element simulation results. Comparisons suggest that machining-induced residual stresses have likely affected the experimental results.