Nanoindentation in elastoplastic and viscoelastic materials: Insights from numerical simulation

This thesis examined the use of finite element analysis to simulate the process of nanoindentation to gain insights into the behavior of elastoplastic and viscoelastic materials. The study examined the behavior of a calibrated elastoplastic material model during indentations using different indenter tip geometries. The study also compared simulations and experimental results for the elastoplastic material. Additionally, the study examined the calibration of a viscoelastic material model from reported data and its ability to predict indentation response.;Nanoindentation is an important experimental technique for evaluation of mechanical properties (particularly hardness) of very small volume of materials. Despite the emergence of significant publications on interpretation of nanoindentation test results for material characterization at nanoscale, a number of key issues including the effect of tip geometry on the resulting load-displacement curves are yet to be fully investigated.;In this work, extensive finite element simulations were conducted to investigate the effects of indenter geometry on the load-displacement response of an elastoplastic material subjected to indentation by using Berkovich and conical indenters. The Berkovich indenter, widely used in nanoindentation experiments, is typically simplified to a theoretically equivalent 70.3° conical indenter in numerical simulations which allows for a less computationally intensive 2D axisymmetric analysis. Previous studies into the validity of this equivalence assumption for indentation in elastoplastic materials have led to varying conclusions.;This study employed two and three dimensional finite element simulations to investigate the response of elastoplastic materials, obeying a combined isotropic and kinematic hardening, during indentation with conical and Berkovich indenters. Simulations showed that there is a clear difference in the load-displacement response of the selected material to the two indenters. The Berkovich geometry was 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 aluminum specimen were compared to elastoplastic finite element simulation results. Comparisons suggest that machining-induced residual stresses have likely affected the experimental results.;Finite element simulations were also used to gain insights into the behavior of viscoelastic materials. Calibrations were performed using relaxation data for a polymer reported in literature. Numerical simulations displayed good agreement with spherical indentations reported in a previous study. Comparisons to reported Berkovich nanoindentation results showed good prediction of loading behavior; however, unloading behavior was not well captured. A study was performed to determine if adjusting Prony parameters could achieve a simulation that provides a better prediction of the complete indentation response. The adjusted model provided better prediction but still failed to fully capture the unloading behavior of the experimental data. However, the adjusted model compared well to other relaxation data for the polymer. The viscoelastic study suggests that plastic deformation plays a role in the response of the material during nanoindentation with a Berkovich indenter.