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Nanoindentation in Elastoplastic and Viscoelastic Materials: Insights From Numerical Simulation Open Access

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This thesis examined finite element (FE) analysis to simulate nanoindentation to gain insights into the behavior of elastoplastic and viscoelastic materials. The study examined the behavior of an elastoplastic material model during indentations using different indenter tip geometries. Additionally, the study examined the calibration of a viscoelastic material model from reported data and its ability to predict indentation response. Comparisons were also made to experimental results. Extensive FE simulations were conducted to investigate the effects of indenter geometry on the load-displacement response of an elastoplastic material. A 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 2D axisymmetric analysis. 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 FE simulations to investigate the response of an elastoplastic materials using the two indenter geometries. Simulations showed a difference in the load-displacement response between the two indenters. The Berkovich geometry produced more localized contact stresses and plastic strains, leading to a smaller mobilized force for the same magnitude of displacement. Experimental results of nanoindentation into an aluminum specimen were compared to elastoplastic FE results. Comparisons suggest that machining-induced residual stresses have likely affected the experimental results. FE 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 of spherical indentation displayed good agreement with a previous study. Comparisons to experimental nanoindentation results showed good prediction of loading behavior; however, unloading behavior was not well captured. A parametric study was performed to determine if parameter adjustments could achieve a simulation that better predicted 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.

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