Browsing by Subject "Nanoindentation"

Now showing 1 - 3 of 3
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    Finite element modeling of articular cartilage at different length scales
    (2012-04) Chiravarambath, Sidharth Saktan
    The composition and structure of articular cartilage (AC) are inhomogeneous within the tissue and vary throughout its depth. Its extracellular matrix can be considered as a fiber-reinforced composite solid consisting of a dense stable network of collagen fibers embedded in a proteoglycan (PG) gel. Several studies have shown that this specialized structure plays a vital role in the mechanical function of AC. In pathological conditions, such as osteoarthritis (OA), degeneration of cartilage due to changes in mechanical properties is observed. Osteoarthritis is the most common cause of disability in the elderly and affects more than 20 million people in the USA alone. The focus of this work is to understand the mechanical response of AC using finite element models using ABAQUS, a commercial FEA package that is widely used in the field of cartilage mechanics. This is done at two different scales - the macroscale and the mesoscale. At the macroscale, AC is considered as a homogeneous isotropic poroviscoelastic (PVE) material saturated by the interstitial fluid (water). Indentation tests are performed on cartilage from the mouse tibia plateau using two different sized flat-ended conical indenters with flat-end diameters of 15 μm and 170 μm. A finite element (FE) model of the test is developed and the PVE parameters identified by using inverse methods to minimize the errors between FE simulated and test data. Data from the smaller indenter is first used to fit the viscoelastic (VE) parameters, on the basis that for this tip size the gel diffusion time (approximate time constant of the poroelastic (PE) response) is of the order of 0.1 s, so that the PE response is negligible. These parameters are then used to fit the data from the larger indenter for the PE parameters, using the VE parameters extracted from the data from the smaller indenter. At the mesoscale the inhomogeneities of AC need to be addressed to understand the microstructural behavior of AC. The problem of interest in this part of the work is to understand the mechanical role of interfibrillar cross-links (IFLs), if they exist, suspected in AC and most collagenous tissues. A 3D FE model of AC meso-structure motivated by the parallel fibril geometry of the mid and deep zones of the patella is developed consisting of a PE matrix, unidirectional, bilinear fibrils (different stiffness in tension and compression), and the IFLs. Parametric studies are then performed for the model in simulated compression tests along the fibril direction and the effect of the IFLs and matrix are predicted and compared. Results suggest presence of IFLs would increase the effective modulus in compression. This is due to maintaining organization of the fibrils into a network due to IFLs imparting stability to the network by preventing early bending of fibrils and effectively reducing the Poisson effect. Finally, with a set of literature based parameters, compression tests for AC using the mesomodel show that removing the cross-links results in a significant (43%) drop in the effective compressive modulus, suggesting resolution necessary to experimentally detect the IFLs. At the mesoscale, the IFLs would play the mechanical role of stabilizing the fibril network and enhancing its stiffness.
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    The mechanical response of common nanoscale contact geometries
    (2008-03) Mook, William Moyer
    Characterizing the mechanical response of common nanoscale contact geometries is vitally important to fields such as microelectromechanical systems (MEMS) where the behavior of nanoscale contacts can in large part determine system reliability and lifetime. Therefore a research program was undertaken that focused on the development of innovative nanoindentation-based techniques capable of quantifying the mechanical response of freestanding nanostructures. Nanoindentation was used since it is a non-destructive, high resolution technique that has been proven to be very useful in characterizing materials at the nanoscale. Examples of tested structures include single crystalline nanoparticles and polycrystalline nanoposts. From these experiments methods to characterize the structures' effective elastic modulus, flow stress, fracture toughness and activation volume required for plasticity have been developed. It was noted that both modulus and toughness in nanoparticles scale with average contact stress. This result has lead to the development of an experimental analysis technique that accounts for the hydrostatic component of pressure which develops in a material under contact. The effect of hydrostatic pressure on indentation modulus is currently not accounted for in nanoindentation even though it is shown to be important at length scales below 100 nm.
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    Temperature and Rate Effects on the Mechanical Behavior of Tungsten and Its Nanocomposites
    (2022-05) Schmalbach, Kevin
    Methods to predict material failure frequently rely on large experimental datasets tuned to the properties of one material or are based on computationally expensive modeling. Both approaches are slow and do not adapt well to changes in material chemistry or processing. Development of analytical models with easily measured, physically meaningful parameters are key to alleviating bottlenecks in new materials development. One such model is presented in this work, which relies on easily measured physical parameters, effective stress and activation volume, referred to as activation parameters. This approach requires knowledge of both the temperature and strain rate effects on mechanical properties, which is the primary focus of this dissertation.Nanoindentation is an ideal way to measure these physical parameters due to its ability to measure properties at a length scale appropriate for grain- or phase-dependent deformation mechanisms and the small quantities of material required for testing (~mm3). Newer nanoindentation tests, such as strain rate jump tests, can quickly determine activation parameters, but lack standardized protocols for experiments and analysis. For this work, tungsten (W) was chosen as a model system in which to understand temperature and strain rate effects. This is in part due to its elastic isotropy, which simplifies data analysis, and its relatively experimentally accessible brittle-ductile transition temperature. In the first half of this dissertation, I will describe the use of nanoindentation strain rate jump tests to predict the fracture behavior of macroscale tungsten single crystals. I begin by presenting necessary protocol development for strain rate jump testing and analysis using a new, freely available software package. Included in the software package are a Python-based load function generator and a series of Matlab functions for data analysis. These tools were validated on single crystal tungsten, yielding a strain rate sensitivity nearly identical to that reported in the literature. The techniques were then applied at low temperature (-100 °C) and high temperature (50-300 °C) to measure activation parameters, effective stress and activation volume, as a function of temperature. The activation parameters, in combination with an analytical model for the strain energy release rate, accurately predict the brittle-ductile transition temperature along particular fracture systems in single crystal tungsten. Activation parameters measured from indentation of the (100) surface of single crystal tungsten accurately predict the brittle-ductile transition and fracture toughness along the {100}<011> fracture system of macroscale tungsten single crystals. Use of data from bulk tension of single crystal tungsten from the literature accurately predicts the fracture toughness in the {110}<110> fracture system of macroscale tungsten single crystals. In the second half of this dissertation, I will describe the synthesis and mechanical properties of 3-dimensionally ordered macroporous (3DOM) tungsten and a nanocomposite based on the porous framework. 3DOM tungsten was made with 35-40 nm wide ligaments, which exploit material size effect to have a ligament yield strength of 6.1 GPa at room temperature, approaching the ideal strength of tungsten. Considerable plasticity was observed above 125 °C, implying a brittle-ductile transition around this temperature, consistent with the predictions based on the earlier-presented model. Filling of the 3DOM tungsten framework with a silicon oxycarbide (SiOC) reinforcing phase resulted in a heterogeneous structure containing tungsten, silicon oxycarbide glass, and small domains of free carbon. The failure strength of 3DOM W-SiOC was 1.1 GPa, a factor of 22 greater than the 3DOM W framework. Although an increase in deformability was observed at 225 °C, pillars still failed by abrupt crack propagation. Fracture was only prevented at the next testing temperature of 425 °C. This places the brittle-ductile transition temperature of the W-SiOC composite significantly higher than that of pure tungsten. Additionally, the composite retains high strength even to 425 °C, achieving a yield strength of 400 MPa.