Browsing by Subject "Mechanical"
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Item Ab Initio Study of Nanostructures for Energy Storage(2014-05-07) Cristancho Albarracin, DahiyanaNanomaterials are expected to overcome the challenges imposed from bulk materials in the design of electronic devices. With the help of nanotechnology smaller, lighter, and more energy efficient materials can be used in the development of smart nanodevices. The goal of this research is to characterize the chemical, electrical, and mechanical properties of nanostructures for energy conversion and storage. In this dissertation, three materials are studied at nano level using theoretical calculations: carbon nanotubes (CNTs), lithium silicon (Li_(4n)Si_(n)), and polyvinyl alcohol (PVA). The coupling of mechanical and electronic properties of carbon nanotubes are studied, we estimate a modulus of elasticity of 1.3 TPa and find that the mechanism of CNT structure deformation is chirality dependent. Armchair and chiral nanotubes have ductile deformation fracture while zigzag have both ductile and brittle. Furthermore, the HOMO-LUMO gap of CNT increases under plastic deformation. We conclude that mechanical forces affect the electromagnetic absorption properties of CNTs. Silicon has been proposed as a promising material for anodes in Li ion batteries; a layer called: the solid electrolyte interphase (SEI) is formed on the electrodes during charging process that may restrict the ion mobility. Preliminary electrical characterization shows the external potential effects of SEI on electron transport as a function of SEI thickness. Furthermore, the rotation of the Li_(2)O molecules in SEI plays a big role in the electron transport in Li-Ion Batteries. Mechanical and thermal properties of polyvinyl alcohol (PVA) are characterized using in situ X-ray photoelectron spectroscopy (XPS) and theoretical calculations. It is found that the carbon peaks in PVA shifted under mechanical and thermal stretching. At different temperatures, the C-O bond was the most stable carbon group than others. We find that Hartree-Fock/10-31G (d) reproduces the binding energy of core carbon electrons, which is enough to characterize bonds and corroborate the spectroscopic analysis.Item Effect of electro-mechanical loading in metallic conductors(2010-12) Gallo, Federico Guido; Ravi-Chandar, K.; Mear, Mark E.; Satapathy, Sikhanda S.; Liechti, Kenneth M.; Landis, Chad M.The development of high powered electro-magnetic devices has generated interest in the effect of combined electromagnetic and mechanical loading of such structures. Materials used in high-current applications – aluminum alloys and copper – are subjected to heat pulses of short duration (in the range of a few hundred microseconds to a few milliseconds); immediately following or along with such heat pulses, these materials are also subjected to large mechanical forces. In previous work reported in the literature, ejection of material from the vicinity of preexisting defects such as cracks, notches or discontinuities have been observed resulting from short-duration high-intensity current pulses; after a series of pulses, permanent deformation and weakening of intact material has also been reported. But a lack of complete understanding of the effects of short duration current pulses hinders the assessment of the reliability of such conductors in high energy applications. Therefore, an investigation was undertaken to examine the behavior of electromagnetically and mechanically loaded conductors. This work investigates the effects of short-duration, high-current-density pulses in combination with viii mechanical loading. The aim is to develop a theoretical model to describe the resulting mechanical response. The model is to provide a characterization of the possible effects of thermally-induced plastic strains on metals loaded beyond or just below their yield strength or below the critical stress intensity factor. In the experiments reported here, two types of specimens, undamaged and damaged, were subjected to combined electromechanical loads. Undamaged specimens were used to observe thermally-induced plastic strains - strains not caused by an increase in mechanical loading, but rather resulting from the reduction of yield strength and post-yield stiffness due to the increase in temperature. The experiments were conducted such that it would be possible to develop a model that would conclusively account for the observed material behavior. The second sets of specimens were weakened a priori by the introduction of a crack in order to study the influence of such crack-like defects on the electrical and mechanical fields, and to produce a safe design envelope with respect to the loading conditions. Failure was found to occur due to melting triggered by joule heating; a quantitative criterion based on current concentration and heat accumulation near the crack tip has been developed based on these experimental results.Item Estimating production and cost for clamshell mechanical dredges(Texas A&M University, 2005-02-17) Adair, Robert FletcherClamshell dredges are used around the United States for both navigational and environmental dredging projects. Clamshell dredges are extremely mobile and can excavate sediment over a wide range of depths. The object of this thesis is to develop a methodology for production and cost estimation for clamshell dredge projects. There are current methods of predicting clamshell dredge production which rely on production curves and constant cycle times. This thesis calculates production estimation by predicting cycle time which is the time required to complete one dredge cycle. By varying the cycle time according to site characteristics production can be predicted. A second important component to predicting clamshell dredge production is bucket fill factor. This is the percent of the bucket that will fill with sediment depending on the type of soil being excavated. Using cycle time as the basis for production calculation a spreadsheet has been created to simplify the calculation of production and project cost. The production calculation also factors in soil type and region of the United States. The spreadsheet is capable of operating with basic site characteristics, or with details about the dredge, bucket size, and region. Once the production is calculated the project cost can be determined. First the project length is found by dividing the total amount of sediment that is to be excavated by the production rate. Once the project length is calculated the remainder of the project cost can be found. The methods discussed in this thesis were used to calculate project cost for 5 different projects. The results were then compared to estimates by the government and the actual cost of the project. The government estimates were an average of 39% higher than the actual project cost. The method discussed in this thesis was only 6% higher than the actual cost.Item Mechanical and thermal properties of kenaf/polypropylene nonwoven composites(2013-05) Hao, Ayou; Chen, Jonathan Yan; Koo, Joseph H.; Kovar, Desiderio; Krifa , Mourad; Shi, Li; Xu, BugaoThe objectives of this research are to characterize the mechanical and thermal performance of natural fiber nonwoven composites and to predict the composite strength and long-term creep performance. Three natural fibers: kenaf, jute, and sunn hemp as potential candidates were compared in terms of physical, thermal and mechanical properties. In order to see the effects of fiber surface chemical treatment, sunn hemp fiber was treated with sodium hydroxide (NaOH) agent. Kenaf fiber was selected for the following study due to the higher specific modulus and the moderate price of kenaf fiber. After alkaline treatment, the moisture content, glass-transition temperature, and decomposition temperature of sunn hemp fiber increased but not significantly. The mechanical performance of kenaf/polypropylene nonwoven composites (KPNCs) in production of automotive interior parts was investigated. The uniaxial tensile, three-point bending, in-plane shearing, and Izod impact tests were performed to evaluate the composite mechanical properties. The thermal properties were evaluated using TGA, DSC, and DMA. An adhesive-free sandwich structure was found to have excellent impact resistance performance. Based on the evaluation of mechanical and vii thermal properties, manufacturing conditions of 230 C and 120 s for 6 mm thick sample and 230 C and 60 s for 3 mm thick samples were selected. The open-hole and pin filled-hole effects on the tensile properties of KPNCs in production of automotive interior parts were investigated. Three specimen width-to-hole diameter (W/D) ratios of 6, 3 and 2 were evaluated. A preliminary model by extended finite element method (XFEM) was established to simulate the composite crack propagation. Good agreement was found between experimental and simulation results. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. By calculating the stress concentration factor Kt for brittle materials, the net section stress factor Kn for ductile materials, and the strength reduction factor Kr, it was found that KPNC was relatively ductile and insensitive to the notch. The strain rate effects on the tensile properties of KPNC were studied. The strain rate effects confirmed the time-dependence of KPNCs. Afterward, the creep behavior of KPNC and PP performed by DMA was investigated extensively. The linear viscoelastic limit (LVL) was found to be 1 MPa in this study. The long-term creep behavior of KPNC compared to virgin PP plastic was predicted using the time-temperature superposition (TTS) principle. Three-day creep tests were also conducted to verify the effectiveness of TTS prediction. It was found that the master curve for PP fit better with the three-day creep data than KPNC, due to the multiphase thermo-rheological complexity of KPNC. The creep recovery, stress effects and cyclic creep performance were also evaluated. Two popular creep models: the four-element Burgers model and the Findley power law model were used to simulate the creep behavior in this study. It was found that KPNC had higher creep resistance and better creep recoverability than virgin PP plastics.Item Thermal and mechanical analysis of interconnect structures in 3D stacked packages(2010-05) Wakil, Jamil Abdul; Chen, Shaochen; Ho, Paul S.; Shapiro, Michael; Shi, Li; Sikka, KamalPhysical scaling limits of microelectronic devices and the need to improve electrical performance have driven significant research and development into 3D architecture. The development of die stacks in first level packaging is one of the more viable short-term options for improved performance. Placement of memory die above or below processors in a traditional flip chip C4 package with through-silicon vias (TSVs) has significant benefits in reducing data and power transmission paths. However, with the electrical performance benefits come great thermal and mechanical challenges. There are two key objectives for this work. The first is understanding of the die-die interface resistance, R[subscript dd], composed of the back end of line (BEOL) layers and micro-C4 interconnects. The interfacial resistance between BEOL material layers, the impact of TSVs and the impact of strain on R[subscript dd] are subtopics. The second key objective is the understanding of package thermal and mechanical behavior under operating conditions, such as local thermal disturbances. To date, these topics have not been adequately addressed in the literature. It is found that R[subscript dd] can be affected by TSVs, and that the interfacial contributions predicted by theoretical sub-continuum models can be significantly different than measurements. Using validated finite element models, the significance of the power distribution and R[subscript dd] on the temporal responses of 2D vs. 3D packages is highlighted. The results suggest local thermal hotspots can greatly exacerbate the thermal penalty due to the R[subscript dd] and that no peaks in stress arise in the transient period from power on to power off.