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Please use this identifier to cite or link to this item: http://hdl.handle.net/1860/2904

Title: On spherical nanoindentation stress-strain curves, creep and kinking nonlinear elasticity in brittle hexagonal single crystals
Authors: Basu, Sandip
Keywords: Materials science;Materials -- Creep;Engineering design
Issue Date: 23-Oct-2008
Abstract: Despite the fact that they can accurately delineate the onset of the elasto-plastic transition of solids, spherical nanoindentation, NI, experiments are less common than sharp indenters. Herein a novel, robust technique to convert NI load-displacement to stress-strain curves was developed and applied to fused silica, aluminum, iron and the hexagonal single crystals: ZnO, LiNbO3, GaN, sapphire and mica. In all cases, the NI stress-strain curves clearly showed the onset of yield and subsequent strain hardening. We also show, by Weibull analysis, that the pop-in stresses are stochastic in nature and depend on the presence of dislocation-nucleating defects. The ability to calculate the NI or mean stress in real time allowed us to run, for the first time, constant NI stress experiments, which, in turn, was exploited to quantify the room temperature creep of A-oriented ZnO single crystals. Analyses of the results clearly showed that the creep is a power law creep, with an exponent of ≈ 3 and a threshold stress that was a function of time. Repeated cyclic NI experiments, in the same location, were used to successfully demonstrate the fully-reversible hysteretic nature of kinking nonlinear elastic, KNE, solids, indirectly confirming the presence of incipient kink bands, IKBs, in these materials. Our recently developed micro-scale model, based on the reversible dislocation motion in the form of IKBs, shows excellent agreement with the nonlinear hysteretic behavior of C-oriented LiNbO3 single crystals. Combining the NI stress-strain results, we show that the energy dissipation per unit volume per cycle in KNE solids increases with increasing c/a ratios, as well as increasing domain sizes. In summary this work shows that spherical NI stress/strain curves are a powerful tool for quantifying and understanding the elastic-plastic transition in materials, especially brittle solids, where bulk uniaxial compression/tension experiments are not possible.
URI: http://hdl.handle.net/1860/2904
Appears in Collections:Drexel Theses and Dissertations

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