Quasi-static and Dynamic Nanoindentation of Soft and Spatially Distinct Materials and Structures
Quasi-static nanoindentation has been used to assess the mechanical properties of soft and spatially distinct materials for several years. Most of the soft materials exhibit time-dependent (viscoelastic) behavior; thereby dynamic nanoindentation analysis increased the possibility of obtaining an accurate mechanical response from the materials. Normally, the heterogonous microstructure of specimens can result in experimental error when analyzing nanoindentation results. The accurate assessment of nanoindentation on soft materials with spatially distinct structures is not fully understood in previous studies. Some existing features in specimens, such as the residual stresses generated during polymer parts processing, also significantly influences nanoindentation data analysis. The objective of this study is to systematically consider some of the uncertainties when it comes to characterize soft materials by nanoindentation and thus develop several improved characterization methods, and provide guidance for future measurement. The study sought to clear out four main uncertainties within nanoindentation analysis for viscoelastic and heterogeneous materials: 1. Does the free edge close to the indents affect the dynamic nanoindentation results? How can we improve the analysis method? 2. How should indentation results be utilized to estimate the potential residual stresses? 3. Could we perform the statistical nanoindnentation to obtain the comparable results of volume fraction of individual phases in heterogeneous materials? 4. During nanoindentation, what is the appropriate combination of the loading rate, unloading rate and the holding time setup in terms of viscoelastic materials? In this dissertation, the correlation of quasi-static nanoindentation analysis methods with the structural compliance, residual stress, sampling volume and various relaxation processes will be covered in the following chapters. Dynamic nanoindentation will be used to access the time-dependent behavior of soft materials in different phenomenon, and used to compare the data reliability from dynamic measurement with that from quasi-static measurement.
Bahr, Purdue University.
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