Computational and experimental study of instrumented indentation

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references (p. 167-175).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.The effect of characteristic length scales, through dimensional and microstructural miniaturizations, on mechanical properties is systematically investigated by recourse to instrumented micro- and/or nanoindentation. This technique is capable of extracting mechanical properties accurately down to nanometers, via rigorous interpretation of indentation response. Such interpretation requires fundamental understandings of contact mechanics and underlying deformation mechanisms. Analytical, computational and experimental approaches are utilized to elucidate specifically how empirical constitutive relation can be estimated from the complex multiaxial stress state induced by indentation. Analytical formulations form a framework for parametric finite element analysis. The algorithms are established to predict indentation response from a constitutive relation (hereafter referred to as "forward algorithms") and to extract mechanical properties from indentation curve (hereafter referred to as "reverse algorithms"). Experimental verifications and comprehensive sensitivity analysis are conducted. Similar approaches are undertaken to extend the forward/reverse algorithms to indentations using two ore more tip geometries. Microstructural miniaturization leads to novel class of materials with a grain size smaller than 100 nm, hereafter referred to as "nanocrystalline" material. Its mechanical properties are observed to deviate greatly from the microcrystalline counterparts.(cont.) In this thesis, experimental, analytical and computational approaches are utilized to elucidate the rate and size dependent mechanical properties observed in nanocrystalline materials. Indentations, as well as micro-tensile tests, are employed to attain various controllable deformation rates. A simple analytical model, hereafter referred to as Grain-Boundary-Affected-Zone (GBAZ) model, is proposed to rationalize possible rate-sensitivity mechanism. Systematic finite element analysis integrating GBAZ model is conducted with calibration against the experiments. The same GBAZ model, further utilized in the parametric finite element study, is capable of predicting the inverse Hall-Petch-type phenomenon (weakening with decreasing grain size) at the range consistent with the literature.by Nuwong Chollacoop.Ph.D

Relationship between hydrothermal temperatures and structural properties of CeO2 and enhanced catalytic activity of propene/toluene/CO oxidation by Au/CeO2 catalysts

A simple hydrothermal synthesis of CeO2 was implemented to obtain a series of CeO2-supported gold (Au) catalysts, used for the total oxidation of propene/toluene/CO gas mixtures and the oxidation of CO. CeO2 preparation started from a cerium hydrogen carbonate precursor using a range of different hydrothermal temperatures (HT) from 120 to 180°C. High-resolution transmission electron microscopy, X-ray diffraction, and H2-temperature-programmed reduction data indicated that CeO2 morphology varied with the HT, and was composed of the more active (200) surface. Following Au deposition onto the CeO2 support, this active crystal plane resulted in the most widely dispersed Au nanoparticles on the CeO2 support. The catalytic performance of the CeO2-supported Au catalysts for both oxidation reactions improved as the reducibility increased to generate lattice oxygen vacancies and the number of adsorbed peroxide species on the CeO2 support increased due to addition of Au. The Au catalyst on the CeO2 support prepared at 120°C was the most active in both propene/toluene/CO oxidation and independent CO oxidation

OPTIMIZATION OF MACHINING PARAMETERS ON THE SURFACE ROUGHNESS OF ALUMINUM IN CNC TURNING PROCESS USING TAGUCHI METHOD

In this research, Taguchi method is employed by focusing on spindle speed, feed rate, and depth of cut to optimize the CNC turning parameters for aluminum alloy 6063. The main goal of this study is to improve the surface roughness of the material. A L9 orthogonal array is used for experimentation, and the results are subsequently analyzed using ANOVA (Analysis of Variance). A spindle speed of 1300 rpm, a feed rate of 0.5 m/min, and a depth of cut of 1.5 mm are the optimal conditions to achieve the minimum average surface roughness (Ra). The main effect plot of the signal-to-noise (S/N) ratio provides significant evidence supporting the primary research goal. Furthermore, the ANOVA table reveals that spindle speed contributes 59.71%, feed rate contributes 29.80%, while depth of cut only contributes minimally at 0.72%. Based on the research findings, spindle speed and feed rate can be adjusted to control surface roughness. Both factors are highly significant in influencing the surface roughness of the material. The prediction equation from the linear regression analysis is Ra = 1.745 – 0.001024 spindle speed + 0.3000 feed rate – 0.0233 depth of cut. A coefficient of determination or R-squared value of 0.9115 indicates that the independent variables can explain 91.15% of the variation in the dependent variable. The experimental and predicted surface roughness (Ra) values have a predicted error percentage of 2.26%

The Influence of Direct Non-Thermal Plasma Treatment on Soot Characteristics under Low Exhaust Gas Temperature

This study aimed to assess the effectiveness of nonthermal plasma (NTP) technology utilizing a dielectric barrier discharge (DBD) reactor, both with and without exhaust gas recirculation (EGR), in reducing soot particles and their impact on nitrogen oxides (NOx). The experiment involved maintaining a constant flue gas flow rate of 10 l/min, employing high voltage values of 0, 6, and 10 kV, fixed frequency of 500 Hz and setting the various IMEP of 5, 6, and 7 bar and the engine speed at 2,000 rpm. The findings demonstrated that NTP was successful in removing NOx by approximately 16.84% and 17.01%, achieving particle matter (PM) removal efficiencies of around 60.79% and 81.13%, and effectively reducing activation energy by approximately 18.34% and 31.5% (with and without EGR, respectively) at a high voltage of 10 kV. These results highlight the potential of NTP technology in mitigating emissions and reducing the environmental impact associated with diesel engines

Activity for Diesel Particulate Matter Oxidation of Silver Supported on Al2O3, TiO2, ZnO, and CeO2: The Effect of Oxygen Concentration

Particulate matter (PM) is a problem for human health the major producer of PM are diesel engines. The diesel particulate filters (DPFs) are used for the limitation of the PM. The DPF operation consists of two sequential functions: PM filtering and regeneration. One of the main contributing factors affecting the regeneration of DPF is the oxygen concentration in the exhaust gas. This study investigates the impact of different oxygen concentrations (99.99%, 10%, and 5%) on (PM) oxidation when using silver catalysts supported on CeO2, ZnO, TiO2, and Al2O3. The synthesized catalysts were characterized using XRD, SEM, SEMEDX, and H2-TPR techniques, and the PM oxidation activity was evaluated using TGA. The results demonstrated that different oxygen concentrations had little effect on light VOCs oxidation compared to no catalyst or the same catalyst. However, heavy VOCs and soot combustion, which require a higher oxygen concentration, significantly reduce combustion performance when the oxygen concentration decreases

Microstructural simulation of three-point bending test with pre-crack: Effect of sample size and loading configuration

Deformation behavior in three-point bending test of Mo-Si-B alloy specimens with pre-crack was investigated byrecourse to finite element analysis (FEA) with heterogeneous microstructures incorporated. The loading configuration wascarried out by moving the top pin down, while the bottom pin remained fixed. The Mo-Si-B alloy consists of hard, brittle T2(Mo5SiB2) phase embedded in a soft matrix of Mo solid solution. The sample contains pre-crack configuration at the middlein order to study the effect of the second phase (T2 particles) onto a crack tip during the bending test. Real optical micrographswere scanned, digitized, and meshed into a two dimensional model with constitutive relations of both phases. It was found that the plastic strain localization was not only caused by the hard T2 particles ahead of the crack tip, but also caused by the top pushing pin. This interaction between two strain fields was also observed if the microstructure were homogeneous. Two methodologies were explored in order to minimize this interaction in order to solely separate the effect of hard T2 particles ahead of the crack tip. The first one was to increase the distance between the top pin and the crack tip, while the second one was to approximate the pushing pin loading by end moment application. It was found that the strain interactionstill appeared in both cases

Three-dimensional simulation of deformation fields underneath vickers indenter: effects of power-law plasticity

The effects of power-law plasticity (yield strength and strain hardening exponent) on the plastic strain distribution underneath a Vickers indenter was systematically investigated by recourse to three-dimensional finite element analysis, motivated by the experimental macro- and micro-indentation on heat-treated Al-Zn-Mg alloy. For meaningful comparison between simulated and experimental results, the experimental heat treatment was carefully designed such that Al alloy achieve similar yield strength with different strain hardening exponent, and vice versa. On the other hand, full 3D simulation of Vickers indentation was conducted to capture subsurface strain distribution. Subtle differences and similarities were discussed based on the strain field shape, size and magnitude for the isolated effect of yield strength and strain hardening exponent

Biodiesel as a lubricity additive for ultra low sulfur diesel

With the worldwide trend to reduce emission from diesel engines, ultra low sulfur diesel has been introduced with thesulfur concentration of less than 10 ppm. Unfortunately, the desulfurization process inevitably reduces the lubricity of dieselfuel significantly. Alternatively, biodiesel, with almost zero sulfur content, has been added to enhance lubricity in an ultralow sulfur diesel. This work has evaluated the effectiveness of the biodiesel amount, sourced from palm and jatropha oil,and origin in ultra low sulfur diesel locally available in the market. Wear scar from a high-frequency reciprocating rig isbenchmarked to the standard value (460 m) of diesel fuel lubricity. It was found that very small amount (less than 1%) ofbiodiesel from either source significantly improves the lubricity in ultra low sulfur diesel, and the biodiesel from jatropha oilis a superior lubricity enhancer

Points of Consideration on Climate Adaptation of Solar Power Plants in Thailand: How Climate Change Affects Site Selection, Construction and Operation

Solar energy is planned to undergo large-scale deployment along with Thailand’s transformation to a carbon neutral society in 2050. In the course of energy transformation planning, the issue of energy infrastructure adaptation to climate change has often been left out. This study aims to identify climate-related risks and countermeasures taken in solar power plants in Thailand using thematic analysis with self-administered observations and structured interviews in order to propose points of consideration during long-term energy planning to ensure climate adaptation capacity. The analysis pointed out that floods and storms were perceived as major climate events affecting solar power plants in Thailand, followed by lightning and fires. Several countermeasures were taken, including hard countermeasures that require extensive investment. Following policy recommendations were derived from the climate-proofing investment scenario study. Policy support in terms of enabling regulations or financial incentives is needed for implementation of climate-proofing countermeasures. Public and private sectors need to secure sufficient budget for fast recovery after severe climate incidents. Measures must be taken to facilitate selection of climate-resilient sites by improving conditions of power purchase agreement or assisting winning bidders in enhancing climate adaptability of their sites. These issues should be considered during Thailand’s long-term energy planning