Development and validation of a novel data analysis procedure for spherical nanoindentation

Siddhartha Pathak

doi:10.17918/etd-3056

This dissertation presents a novel approach for converting the raw load-displacement data measured in spherical nanoindentation, from indentation depths as small as a few nanometers,into much more meaningful indentation stress-strain curves. This new method entails a new definition of the indentation strain, a new procedure for establishing the effective zero-load and zero-displacement point in the raw dataset, and the use of the continuous stiffness measurement (CSM) data. The concepts presented here have been validated by finite element models as well as by the analyses of experimental measurements obtained on isotropic metallic samples of aluminum and tungsten. It is demonstrated that these new methods produce indentation stressstrain curves that accurately capture the loading and unloading elastic moduli, the indentation yield points, as well as the post-yield characteristics in the tested samples. A further development of this approach, without the need for the CSM - an option available only on a limited number of machines - is also outlined. Subsequent validation of this approach on a wide range of material samples including metals, carbon nanotubes (CNTs), ceramics and bone - confirms the ongoing success andversatility of this technique. In particular, the success of these data analysis techniques has been demonstrated in correlating the elastic moduli measured in loading and unloading segments, and explaining several of the surface preparation artifacts typically encountered in nanoindentation measurements in metals. In an extension of this technique to anisotropic polycrystalline samples, a judicious combination of the results from Orientation Imaging Microscopy (OIM) and nanoindentation were used to estimate, for the first time, the changes in slip resistance in deformed grains of anisotropic metallic samples of Fe-3%Si steel at a micron length scale. These results also represent the experimental validation of Vlassak and Nix's theory1 for nanoindentation in anisotropic solids and the first report of experimentally measured nanoindentation yield strengths in anisotropic crystallographic solids as a function of crystal lattice orientation. Similar studies on dense CNT brushes, with ~10 times higher density than CNT brushes produced by other methods, demonstrate the higher modulus (~17-20 GPa) and orders ofmagnitude higher resistance to buckling in these dense CNT brushes than vapor phase deposited CNT brushes or carbon walls. This work also demonstrates the viscoelastic behavior, caused by the increased influence of the van der Waals' forces in these highly dense CNT brushes, showing their promise for energy-absorbing coatings. Even in a complex hierarchical materials system like bone, this indentation analyses technique has been able to elucidate trends in elastic, yield and post-yield indentation behavior at the lamellar level in the femora (thigh bone) of two different inbred mouse strains, A/J and B6, to the corresponding structural information measured using Raman Spectroscopy at similar micron (lamellar) length scales. Thus bone with a higher mineral-to-matrix ratio is shown to demonstrate a trend towards a higher local modulus and yield strength and the B6 mouse strain exhibits a trend towards lower modulus and yield values than the more mineralized A/J strain. An extension of the above study to indentation testing of bone in the 'wet' or hydrated conditions (which represents more closely bone's naturally hydrated invivo environment), demonstrates a novel approach to characterize bone's dynamic mechanical behavior under contact loading. Bone having a higher collagen content or a lower mineral-tomatrix ratio was found to demonstrate a trend towards a higher viscoelastic response - confirming the trends shown in the dry bone results. In summary, the success of the analyses techniques demonstrated in this dissertation constitute a crucial first step in the formulation of a rigorous framework for establishing structure-property linkages in various materials models at the submicron length scale.

Development and validation of a novel data analysis procedure for spherical nanoindentation

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Development and validation of a novel data analysis procedure for spherical nanoindentation

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