Computational modeling of indentation of materials

This thesis focusses on computational modeling of several different indentation problems involving the microindentation, nanoindentation and reference point indentation (RPI). The goal of this thesis is to develop better understanding of indentation processes in different materials utilizing numerical methods. Indentation has been used for a long time for characterizing hardness of materials. Nanoindentation is a technique where a micron scale indenter is forced into a material of interest while monitoring load and displacement. Indentation techniques can be used to measure elastic modulus, hardness and viscoelastic properties of materials. The RPI technique uses a reference probe which sits on the surface of substrate and defines datum for test probe. The test probe indents the substrate multiple times using a cyclic load and computes nine different RPI outputs. The RPI outputs are not completely understood and there is a need for more research. This thesis presents four different studies. The first study deals with experimental and numerical investigation of indentation on Cr(Mo)-NiAl eutectic alloy. More specifically, the depth dependent strain partitioning between the layers of NiAl layered structure was measured using X-Ray microdiffraction. Finite element simulations were conducted to evaluate thermal strains due to cooling. Results from this simulation were further utilized to model microindentation of the eutectic composite. Strain data from simulations were compared with experimental results confirming experimentally observed trends. The second study deals with simulation of RPI on bone. Bone was modeled as a material that exhibits elastic, plastic, viscous and continuum damage behaviors. The RPI process was simulated as a cyclic indentation process using a finite element package Abaqus®. Effects of changing material properties, indentation force and number of cycles on the RPI outputs were evaluated computationally and compared with experimental works of similar nature and good coherence was demonstrated. Third study presents a statistical approach to study the RPI outputs. Various mechanical tests (compression tests, stress relaxation test, tensile tests) were conducted on multiple 3D printed polymers and properties were obtained. These properties were analyzed against the RPI data of polymers in order to find correlations. The study is currently ongoing and data collected till date is presented in this thesis. Fourth study presents a fast and efficient way of characterizing viscoelastic properties of soft materials based on experimental nanoindentation data and viscoelastic indentation tool on Nanohub. PMMA (polymethyl methacrylate) was subjected to nanoindentation to obtain experimentally time-displacement data. This data along with experimental parameters (loading time, force and indenter radius) serve as inputs to the Nanohub viscoelastic simulation tool. This tool can provide 3 or 5 viscoelastic constants for materials, assuming a Standard Linear Solid model for solids. Numerical data from simulations was matched against experimental data for two different loading rates and good match was observed