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Articoli di riviste sul tema "620.1/1"
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Mormann, Thomas J., e Wolfgang Jeitschko. "The Silver(I) Mercury(II) Oxide Nitrate with the Empirical Formula AgHg2NO5". Zeitschrift für Naturforschung B 54, n. 12 (1 dicembre 1999): 1489–94. http://dx.doi.org/10.1515/znb-1999-1202.
Testo completoAbstract (sommario):
The title compound was prepared by solid state reaction of Ag2O with Hg(NO3)2· H2O in air at 350 °C. Its crystal structure was determined from single-crystal diffractometer data: Pnma, a = 620.1(1) pm, b = 670.1(1) pm, c = 1267.5(2) pm, Z = 4, R = 0.026 for 586 structure factors and 33 variable parameters. The compound may be represented by the formula Ag(HgO)2NO3 . The mercury(II) together with the oxygen atoms form zig-zag chains with linear coordination of the mercury atoms. The oxygen atoms of these chains are linked via silver(I) atoms, thus forming two-dimensionally infinite nets, which contain the trigonal planar nitrate groups in interstices. Thus, only secondary Hg-O and Ag-O bond are formed between the nets.
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Balagizi, Akonkwa, Ahouansou Montcho Simon, Nshombo Muderhwa e Lalèyè Philippe. "Caractérisation De La Pêche Au Lac Kivu". European Scientific Journal, ESJ 13, n. 21 (31 luglio 2017): 269. http://dx.doi.org/10.19044/esj.2017.v13n21p269.
Testo completoAbstract (sommario):
This study was focused on the description of fishing gears on Lake Kivu. Data were collected from March 2012 to February 2014 in Goma, Bukavu, and Kibuye fishing stations. The study findings indicate that gill nets, beach seine, lift net, trammel net composed of two panels of small and larger meshes (mosquito net attached to lift net parts), mosquito net, longline, single line, pots, and cast net were identified as fishing gears used on Lake Kivu. Longline was more selective (2 species) in terms of species selection. The lift net presented the best catch (19.4 ± 11 kg), while the trammel net showed the highest CPUE (7.9 ± 6.1 kg/h). The large dimensions of gears and the long netting duration characterized gillnets (1023.2 ± 620.1 m2 ) and longline (10.3 ± 4.6 hours) respectively. Cast-nets (20mm) showed the largest mesh size, while mosquito nets (1 ± 0.3 mm) were the smallest.
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Tekerli, M., e S. Koçak. "Relationships between production and fertility traits in first lactation and life time performances of Holstein cows under subtropical condition". Archives Animal Breeding 52, n. 4 (10 ottobre 2009): 364–70. http://dx.doi.org/10.5194/aab-52-364-2009.
Testo completoAbstract (sommario):
Abstract. The objective of this study was to determine the relationships between milk yield and calving interval in first lactation and breeding efficiency, herd life and length of productive life of 1 293 Holstein cows maintained at Ceylanpinar State Farm in Southeast Anatolia from 1990 to 2002. The least-squares means of first lactation milk yield, lactation length and calving interval were 5 620.1 ± 76.6 kg, 315.5 ± 2.0 and 386.9 ± 2.6 days. The values for breeding efficiency, herd life and length of productive life were 94.9 %, 2 095.9 ± 34.7 and 1 257.2 ± 34.4 days. The year of first lactation had significant effect (P<0.05) on all traits except for calving interval and lactation length, and breeding efficiency was only affected by age at first insemination and age at first calving (P<0.05). Heritabilities were 0.27 ± 0.08 for milk yield, 0.02 ± 0.05 for lactation length, 0.06 ± 0.05 for calving interval, 0.05 ± 0.05 for breeding efficiency, 0.30 ± 0.08 for herd life and 0.29 ± 0.08 for length of productive life. Genetic and phenotypic correlations between different traits ranged from −0.49 to 0.99 and −0.37 to 0.99 respectively.
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Asil, Esma, Mustafa Volkan Yılmaz e Hulya Yardimci. "Effects of black tea consumption and caffeine intake on depression risk in black tea consumers". African Health Sciences 21, n. 2 (2 agosto 2021): 858–65. http://dx.doi.org/10.4314/ahs.v21i2.47.
Testo completoAbstract (sommario):
Background: The aim of this study was to compare black tea consumption and caffeine intake with depression status.
Subjects and Methods: This study was conducted on 491 adults (M:169, F:322). The average daily caffeine intake of individuals was calculated using the amounts of caffeinated beverages they consumed daily and the caffeine contents of these beverages. The participants’ depression status was determined using the Beck Depression Inventory (BDI). All of the research data were evaluated using STATA.
Results: According to BDI scores, 30.1% of participants had depression. Black tea was consumed by all of the partici- pants and also had the highest consumption level of 620.1±90.4mL and the mean caffeine intake of the participants was 629.5±418.8 mg. Multivariate regression analyses showed that consuming more than 1 cup was protective against depression up to 4 cups. Moreover, a 450-600 mg caffeine intake also reduces the risk of depression than lower or higher intake levels.
Conclusion: Our study suggests that black tea consumption up to 4 cups and caffeine intake between 450-600 mg can help protect against depression. Further studies are needed to better understand the protective effects of black tea and caffeine on depression.
Keywords: Caffeine; black tea; beck depression inventory; depression.
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Shalish, Miriam, Leslie A. Will, Naomi Fukai, Bo Hou e Bjorn R. Olsen. "Role of polycystin-1 in bone remodeling:Orthodontic tooth movement study in mutant mice". Angle Orthodontist 84, n. 5 (settembre 2014): 885–90. http://dx.doi.org/10.2319/082313-620.1.
Testo completo
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Shayeb, M. Abu, A. Majid e S. Zobidi. "Distribution of natural radioactivity and radiological hazard using a NaI(Tl) gamma-ray spectrometric system". Journal of Building Physics 40, n. 4 (27 luglio 2016): 324–33. http://dx.doi.org/10.1177/1744259116631048.
Testo completoAbstract (sommario):
The concentrations of 226Ra, 232Th, and 40K are measured in the material collected from two locations. The collected materials are analyzed using gamma-ray spectrometry. The activity concentration of the naturally occurring radionuclides 226Ra, 232Th, and 40K in building material varies from 12.6 to 121.4, 13.6 to 142, and 69.5 to 620.6 Bq kg−1, respectively. The radium equivalent activity, absorbed dose rate, annual effective dose, and hazard index are also calculated.
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WANG, WEI-CHI, YANBIN LI, MICHAEL F. SLAVIK e HUA XIONG. "Trisodium Phosphate and Cetylpyridinium Chloride Spraying on Chicken Skin to Reduce Attached Salmonella typhimurium". Journal of Food Protection 60, n. 8 (1 agosto 1997): 992–94. http://dx.doi.org/10.4315/0362-028x-60.8.992.
Testo completoAbstract (sommario):
Spraying treatments with trisodium phosphate (TSP) and cetylpyridinium chloride (CPC) were evaluated for their effectiveness in reducing Salmonella typhimurium attached to chicken skins. Chicken skins with an area of 38.5 cm2 were cut from the breast areas of pre-chill chicken carcasses, mounted in a plastic holder, and inoculated with S. typhimurium. The inoculated skins were sprayed with tap water, 10% (wt/vol) TSP, or 0.1 % CPC solutions at 10, 35, or 60°C and 206.8, 413.7, 620.5, 827.4, or 1034.2 kPa for 30 s. After spraying, each skin was rinsed with tap water, transferred to a plastic bag containing 50 ml buffered peptone water, and stomached for 1 min. The stomaching water was collected, diluted serially, plated on xylose lysine tergitol 4 (XLT4) agar and Petrifilm aerobic count plates, and incubated for 18 to 24 h at 37°C. The results showed that tap water spraying reduced S. typhimurium by 0.7 to 1.6 log, while the reduction ranges for TSP and CPC spraying treatments were 1.6 to 2.3 and 1.5 to 2.5 log, respectively. Greater reductions in the numbers of S. typhimurium were obtained in TSP spraying treatments in the high pressure range (620.5 to 1034.2 kPa) and in CPC spraying treatments at 10°C.
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Ling, Peng, Fred G. Gmitter, Larry W. Duncan e S. Y. Xiao. "363 INHERITANCE OF CITRUS NEMATODE RESISTANCE AND ITS LINKAGE WITH RAPD MARKERS IN CITRUS". HortScience 29, n. 5 (maggio 1994): 483a—483. http://dx.doi.org/10.21273/hortsci.29.5.483a.
Testo completoAbstract (sommario):
A family of 63 citrus intergenemic backcross hybrids was used for this study. The parents and hybrids were multiplied by rooted cuttings, with 6 uniform replicates selected per hybrid, and each plant was inoculated with citrus nematodes (Tylenchulus semipenetrans) 5 times over 2 mo. The number of nematode female larvae per gram of fine fresh root was determind 2 mo after the last inoculation. The phenotypic variation of the hybrids was continuous and wide-ranged, from 8.0 females· g-1 of root tissue (resistant parent Swingle citrumelo=15.6) to 620.0 females· g-1 of root tissue (susceptible parent LB 6-2=540.5). Bulked segregant analysis (BSA), using RAPD fragments, was conducted with 2 DNA bulks of individuals from the extremes of the phenotypic distribution. Three hundred twenty primers were screened and 5 were found to generate repeatedly single RAPD fragments specific to the resistant bulk. The segregation of resistance-associated fragments among the individuals was examined, and the linkage between these markers and potential nematode resistance loci was estimated.
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El-Salam, Asmaa Reda Abd, K. E. Rady, Ezzat A. ELFadaly e Mobarak Hassan Aly. "Enhanced Structural and Morphological Properties of Doped Cobalt Zinc Ferrite". Journal of Nanotechnology and Nanomaterials 4, n. 2 (22 novembre 2023): 89–93. http://dx.doi.org/10.33696/nanotechnol.4.046.
Testo completoAbstract (sommario):
In this study, Mn2+ substituted Co0.8−x Mnx Zn0.2 (where x = 0.0, 0.1, 0.2, and 0.3) ferrites are prepared by a coprecipitation method to study the effect of Mn2+ions on the structural and morphological properties. These ferrites are characterized by X-ray powder diffraction (XRD), and Fourier transform infrared. X-ray diffraction patterns of the prepared samples confirm partial substitution of Mn2+ ions that does not change the basic structure of Co0.8 Zn0.2 Fe2O4. It also provides information about the formation of a single-phase spinel structure without any secondary phase. It is concluded that Co0.6 Mn0.2 Zn0.2 Fe2O4 has a spherical shape with an average particle size of 22.51 nm based on TEM, as confirmed by the XRD analysis. FT-IR analysis confirms the formation of vibrational frequency bands associated with the entire spinel structure. The IR spectra of ferrites show two clear and sharp absorption bands in the range of 442.09 and 620.21 cm-1 in the range of 200–1000 cm-1, which confirms the formation of the ferrite composite.
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Hussain, Muhammad Iftikhar, Zafar Iqbal Khan, Taimoor Hassan Farooq, Dunia A. Al Farraj e Mohamed Soliman Elshikh. "Comparative Plasticity Responses of Stable Isotopes of Carbon (δ13C) and Nitrogen (δ15N), Ion Homeostasis and Yield Attributes in Barley Exposed to Saline Environment". Plants 11, n. 11 (5 giugno 2022): 1516. http://dx.doi.org/10.3390/plants11111516.
Testo completoAbstract (sommario):
Salinity is a major threat to agricultural productivity worldwide. The selection and evaluation of crop varieties that can tolerate salt stress are the main components for the rehabilitation of salt-degraded marginal soils. A field experiment was conducted to evaluate salinity tolerance potential, growth performance, carbon (δ13C) and nitrogen isotope composition (δ15N), intrinsic water use efficiency (iWUE), harvest index, and yield stability attributes in six barley genotypes (113/1B, 59/3A, N1-10, N1-29, Barjouj, Alanda01) at three salinity levels (0, 7, and 14 dS m−1). The number of spikes m−2 was highest in Alanda01 (620.8) while the lowest (556.2) was exhibited by Barjouj. Alanda01 produced the highest grain yield (3.96 t ha−1), while the lowest yield was obtained in 59/3A (2.31 t ha−1). Genotypes 113/1B, Barjouj, and Alanda01 demonstrate the highest negative δ13C values (−27.10‰, −26.49‰, −26.45‰), while the lowest values were obtained in N1-29 (−21.63‰) under salt stress. The δ15N was increased (4.93‰ and 4.59‰) after 7 and 14 dS m−1 as compared to control (3.12‰). The iWUE was higher in N1-29 (144.5) and N1-10 (131.8), while lowest in Barjouj (81.4). Grain protein contents were higher in 113/1B and Barjouj than other genotypes. We concluded that salt tolerant barley genotypes can be cultivated in saline marginal soils for food and nutrition security and can help in the rehabilitation of marginal lands.
Tesi sul tema "620.1/1"
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Delgado, Sánchez Clara. "Nouvelles méthodes d’optimisation et de caractérisation de mousses à base de tanins pour l’isolation thermique du bâtiment". Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0246/document.
Testo completoAbstract (sommario):
Dans cette thèse, des mousses produites à plus de 90% à partir de produits naturels et à très faible conductivité thermique ont été étudiées en détail. L’objectif principal de ce travail était d’améliorer certaines faiblesses de ces matériaux et de résoudre les problèmes qui pourraient être rencontrés lors de leur utilisation, pour leur permettre de concurrencer d’autres mousses synthétiques actuellement sur le marché de l’isolation thermique. De nouvelles méthodologies ont été proposées pour optimiser les mousses à base de tanin de différents points de vue. Tout d’abord, des mousses liquides ont été analysées en termes de stabilité et de processus de polymérisation à l’aide d’un analyseur de lumière rétrodiffusée, afin de les transformer en mousses rigides de tanin plus performantes. Des plans d’expériences ont également été utilisés pour améliorer les propriétés mécaniques de mousses rigides, produites par moussage physique, sans porter préjudice à leur conductivité thermique. D’autre part, des traitements d’hydrophobisation ont été réalisés pour réduire la sensibilité de ces mousses à l’eau, qu’elle soit sous forme liquide ou vapeur, et l’effet des ingrédients des formulations sur leurs propriétés au feu a été élucidé. Enfin, deux techniques de caractérisation mécanique ont été étudiées et comparées, ce qui a permis de déterminer le coefficient de Poisson et le facteur de perte, et de mettre en évidence les précautions à prendre pour caractériser les mousses fragilesIn this thesis, foams based on more than 90% of natural products and with an exceptionally low thermal conductivity have been studied in depth. The main objective of this work was to improve some of the weaknesses of those materials and to solve problems that might be encountered during use, for allowing them to compete with other synthetic foams that are currently on the thermal insulation market. New methodologies have been proposed to optimise tannin-based foams from different points of view. First, liquid foams were analysed in terms of stability and polymerisation process using a backscattered light analyser, in order to convert them into improved rigid tannin-based foams. Experimental design was also used to improve the mechanical properties of physically blown rigid foams without prejudicing their thermal conductivity. On the other hand, hydrophobisation treatments were suggested for reducing the sensitivity of those foams to water in liquid or vapour form, and the effect of formulations’ ingredients on their fire properties were elucidated. Finally, two techniques of mechanical characterisation were investigated and compared, leading to Poisson’s coefficient and loss factor, and evidencing the precautions to be taken for characterising brittle foams
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Daly, Michael Andre John. "Advanced imaging and mechanistic modelling of ductile fracture". Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/advanced-imaging-and-mechanistic-modelling-of-ductile-fracture(6d00e179-cb90-4225-b334-b9a21e2b95f2).html.
Testo completoAbstract (sommario):
Nuclear Reactor Pressure Vessels (RPV) are manufactured from medium strength low alloy ferritic steel, specifically selected for its high toughness and good weldability. The ability of the RPV material to resist crack growth is crucial given that it is one of the fundamental containment safety systems of nuclear power plants. For most of their lifetime, the RPV operates at sufficiently elevated temperatures to ensure the material is ductile. However, the development of ductile damage, in the form of voids, and the ability to predict ductile tearing in RPV materials using a mechanistically-based model remains difficult. The Gurson-Tvergaard-Needleman (GTN) model of ductile tearing provides one such tool for predicting ductile damage development in RPV materials. The difficulty in using the GTN model lies in the ability to calibrate the model parameters in a robust manner. The parameters are typically calibrated data, derived from fracture tests and relying on an iterative “trial and error” procedure of numerical simulations and comparison with test data until the model reproduces the experimental behaviour with sufficient accuracy. This research has addressed the development of a mechanistically-based approach to the calibration of the GTN model by developing a new understanding of the ductile fracture mechanism in RPV material through conventional metallography and 3D X-ray computed tomography to image the initiation, growth and coalescence of ductile voids. The metallographic and tomographic data were analysed in a quantitative manner to establish a direct link between the microstructural features and void evolution and the key parameters of the GTN model. This approach has established a more robust mechanistically based method for the calibration of the GTN model that will enhance the conventional iterative calibration procedure. The calibrated model was applied to predict ductile tearing behaviour in compact-tension and notched-tensile specimens. The results showed good agreement with test data and also reproduced the morphology and branching of crack extension observed in practise. Whilst these observations were due, in part, to the numerical solving procedure, they enabled new insights to be gained regarding the development of non-uniform void volume fraction distributions in tested specimensThe results from this research will strengthen the guidance provided to structural integrity engineers in industry regarding the calibration and application of ductile damage mechanics models such as the GTN model for predicting ductile initiation and growth in RPV materials.
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Papantonis, Stergios. "Investigation of passive electromagnetic components with metamaterials". Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/34313.
Testo completoAbstract (sommario):
The main goal of this work is the design and analysis of passive components employing metamaterial structures and in particular the wire medium metamaterial. Although there has been a lot of research interest in the physics of such metamaterial structures, there are not many resources available describing the behaviour of classical components, such as waveguides and cavity resonators, that are formed by metamaterials. Therefore, the aforementioned widely used devices, are realized with the deployment of the "Fakirs bed of nails" and their performance is analyzed. Our motivation is to expand existing analytical models and their applications to commonly used passive electromagnetic components, with a view to explore potentially new applications. As a means of study analytical techniques together with numerical simulations and measurements were used. This thesis is structured in the following chapters. The first chapter is an introduction to the basic principles of electromagnetics and their use on the framework of metamaterials; as illustrations some state of the art applications are presented. The next chapter is a literature review covering the work that has been done in the area of our main research interest (i.e., the Fakir's bed of nails as a metamaterial). An overview of the mathematics describing its behaviour is given as well as applications of the proposed structure. Attention has been paid on the latest studies because they provide complete physical insight. Some results from this chapter are used later as background knowledge for the analysis of passive components. This chapter is intended to lay the foundations for the reader to continue reading the rest of this work without the need to look in the literature. Chapter three investigates the dispersion effects in parallel-plate waveguides with both plates being realized by the Fakir's bed of nails. This chapter serves as an example as to how the Fakir's bed of nails can be used to form components. An analytical solution describing the behaviour of the waveguide is presented and compared against full wave numerical simulations. Chapter four presents a theoretical study of the resonant behaviour of metallic nanorods. A clear analogy between the coupled rods and the split rings/split squares is shown. The decline in the resonant frequency as the gap decreases, previously described in terms of self-capacitance, is interpreted by surface plasmons coupled across the gap. Chapter five presents a new enabling technology for implementing tunable rectangular waveguide components and circuits with the use of 2D and 3D metamaterials; a holey metal surface and wire media, respectively. As proof of concepts, results for tunable rectangular waveguide filters are presented with the use of pin block inductive irises and capacitive posts. Furthermore, by adapting the traditional metal-pipe rectangular waveguide for tunability, regions of the solid metal walls are replaced by holey metasurfaces. Prototype tunable structures were measured for verification and good agreement is achieved between full-wave numerical simulations and measurements. Chapter six analyzes a radically new design of waveguide verification device, suitable for measuring instruments such as Vector Network Analyzers. The device is designed to enable its roperties to be changed, by known amounts, after the device has been connected to the system that requires verification. The performance of the device is based on introducing relative changes in the transmitted and reflected signals and so is insensitive to errors introduced by waveguide flange imperfections. This makes the technique, in principle, ideally suited for waveguide VNAs operating at millimeter- and submillimeter-wave frequencies where these flange errors can dominate the measurements. A verification device is designed, simulated and tested in WR-15 waveguide (50-75 GHz). The last part of this thesis presents a rigorous analysis of lossy spherical cavity resonators starting from first principles. The electromagnetic field inside the spherical cavity is expanded in normal waveguide modes and the eigenfrequencies of the cavity resonator are obtained analytically by enforcing the appropriate boundary conditions at the cavity wall. Unlike perturbation techniques, used when low losses are present, there are no inherent limitations in the presented analysis and, therefore, its applicability range is much broader. Exact analytical results, acting as a benchmark reference standard, are compared to those generated independently by two commercial full-wave simulation software packages (HFSS and COMSOL). When the wall transforms from being a perfect electrical conductor to free space, as its intrinsic conductivity decreases from infinity to zero, it is found that the eigenmode solvers with both software packages increasingly fail. With both software packages, all possible modeling strategies have been investigated and their associated limitations identified. Moreover, a plane-wave approximation model is proposed that accurately predicts the numerical simulation results.
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Zhao, Fan. "Modelling of gas-solid flows with non-spherical particles". Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/34398.
Testo completoAbstract (sommario):
Dispersed multiphase flows are common in nature and industry and are governed by complex physical phenomena. The complex features of the turbulence continuity carrier phase and the dispersed phase make the problem of a dispersed multiphase flow much more complex than a single phase flow. This research work focuses on modelling and analysing one type of dispersed multiphase flows: solid particles suspended in a turbulent channel flow. The aim of this thesis is to numerically investigate the effects of Stokes number, particle shape and particle volume fraction on the behaviour of gas-solid turbulent channel flows with non-spherical particles. This study not only considers spherical particles but also studies non-spherical fibre-like ellipsoids suspended in the channel flow. To fully describe the complex dynamics of non- spherical particles, the rotational motion and orientation is efficiently and accurately re- solved by applying unit Quaternions. To address inevitable numerical errors caused by the Quaternion integration algorithms in previous studies, a novel Quaternion integration method is derived, validated and applied for more accurately updating the unit Quaternions. This work also derives a new Quaternion equation to relate second order tensor variables between different frameworks. This research work applies four-way coupling to accurately model the complex gas-solid turbulent channel flows, and the fluid-particle, particle-particle and particle-wall interactions are all taken into account. Important conclusions from this work are summarized as follows. In four-way coupled simulations, the average viscosity of the fluid flow is not affected by the particles, whereas the turbulence intensity is reduced by adding small heavy particles. The average direct dissipation by the particles is negligible, and the primary mechanism by which the particles affect the flow is by altering the turbulence structure near and around the turbulence kinetic energy peak. For non-spherical particles, the distributions of the orientation angles clearly demonstrate that ellipsoids tend to align within the plane that lies perpendicular to the span-wise direction in the very near wall region, follow the stream-wise direction in the buffer layer, and almost randomly distribute in the central region of the channel.
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Nejati, Morteza. "Finite element modeling of frictional contact and stress intensity factors in three-dimensional fractured media using unstructured tetrahedral meshes". Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/34348.
Testo completoAbstract (sommario):
This thesis introduces a three-dimensional (3D) finite element (FE) formulation to model the linear elastic deformation of fractured media under tensile and compressive loadings. The FE model is based on unstructured meshes using quadratic tetrahedral elements, and includes several novel components: (i) The singular stress field near the crack front is modeled using quarter-point tetrahedral finite elements. (ii) The frictional contact between the crack faces is modeled using isoparametric contact discretization and a gap-based augmented Lagrangian method. (iii) Accurate stress intensity factors (SIFs) of 3D cracks computed using the two novel approaches of displacement correlation and disk-shaped domain integral. The main contributions in the FE modeling of 3D cracks are: (i) It is mathematically proven that quarter-point tetrahedral finite elements (QPTs) reproduce the square root strain singularity of crack problems. (ii) A displacement correlation (DC) scheme is proposed in combination with QPTs to compute SIFs from unstructured meshes. (iii) A novel domain integral approach is introduced for the accurate computation of the pointwise $J$-integral and the SIFs using tetrahedral elements. The main contributions in the contact algorithm are: (i) A square root singular variation of the penalty parameter near the crack front is proposed to accurately model the contact tractions near the crack front. (ii) A gap-based augmented Lagrangian algorithm is introduced for updating the contact forces obtained from the penalty method to more accurate estimates. The results of contact and stress intensity factors are validated for several numerical examples of cubes containing single and multiple cracks. Finally, two applications of this numerical methodology are discussed: (i) Understanding the hysteretic behavior in rock deformation; and (ii) Simulating 3D brittle crack growth. The results in this thesis provide significant evidence that tetrahedral elements are efficient, reliable and robust instruments for accurate linear elastic fracture mechanics calculations.
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Pham, Kien Cuong. "Nano-structured carbon materials for energy generation and storage". Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/33734.
Testo completoAbstract (sommario):
A nano-structured carbon material referred to as Graphene-Carbon Nanotube hybrid is developed for electrochemical energy conversion and storage devices. The hybrid is obtained by catalyst-free growth of free-standing graphene on CNT scaffolds. The hybrid combines the advantageous properties of constituent materials, including an ultra-high density of graphitic edges of graphene and a porous structure of CNTs. As a catalyst support for platinum in PEM fuel cells, the hybrid shows both enhanced catalytic activity and superior stability compared to a commercial carbon black-supported platinum catalyst. The hybrid is also used as a support material for amorphous molybdenum sulfide in supercapacitor and hydrogen evolution reaction catalyst applications. As a supercapacitor electrode material, the hybrid shows high specific capacitance and good stability. As a hydrogen evolution reaction catalyst, the hybrid is one of the most active non-precious catalysts ever reported. FIB-SEM tomography is used to reconstruct the porous 3D structure of carbon electrodes.
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Hernando, Quintanilla Francisco. "Pseudospectral collocation method for viscoelastic guided wave problems in generally anisotropic media". Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/34915.
Testo completoAbstract (sommario):
In Non-Destructive Evaluation (NDE) applications guided waves are attractive to perform rapid inspections of long lengths and large areas. However, they are complicated, therefore it is important to have as much information and understanding about their physical properties as possible in order to design the most efficient and robust inspection process as well as to draw the correct conclusions from the measurement results. The main piece of information to gain insight into the guided wave's properties is dispersion curves which, for isotropic structures such as plates and cylinders, have been available for many years. There are many robust algorithms which are currently used to compute them: finite element simulations, partial wave based root finding routines (PWRF) and semi-analytical finite element simulations (SAFE). These methodologies have been generalized and also used to study and compute dispersion curves of more complicated anisotropic materials though the range of tractable cases was limited. Although robust, all these approaches present several challenges, mostly computational, such as missing modes (PWRF), the so called "large-fd" problem (PWRF), artificially increased stiffness (FE, SAFE) or improvement of dispersion curve tracing routines (FE, PWRF, SAFE). In addition, when studying complicated anisotropic materials with a low degree of symmetry or unusual axes configurations where propagation does not take place along any of the principal axes, PWRF routines are frequently unreliable and one must resort to specific SAFE simulations which also present their own challenges and, depending on the SAFE scheme used, can yield spurious modes which need to be carefully filtered. Recently, Pseudospectral Methods (Galerkin and Collocation schemes), were introduced in the field of elastic guided waves, providing a powerful, yet strikingly and conceptually simple alternative to the above algorithms by successfully finding the dispersion curves in isotropic structures and in some simple anisotropic problems. However, a systematic and general approach for accurately and robustly computing dispersion curves of guided waves in anisotropic media, up to the most general case of triclinic symmetry, has not yet been developed. The goal of the work presented in this thesis is to develop such a tool by means of the Pseudospectral Collocation Method (SCM) and to take advantage of its particular features to make it as robust as possible. Firstly, a PSCM scheme is developed for computing dispersion curves of guided waves in anisotropic elastic media by finding all the frequencies for a given value of the real wavenumber. The results are validated with the existing literature as well as with the results provided by the software DISPERSE developed in the NDT group at Imperial College London. Many of the most remarkable features of the PSCM (spectral accuracy, speed, and its failure to miss modes for instance) are already observed in this simple, yet important, class of problems in elastic media. Secondly, guided waves in viscoelastic anisotropic media are studied. In this case, modes present attenuation due to material damping which is reflected in the wavenumber being complex. In order to handle complex wavenumbers the PSCM schemes developed for elastic materials are appropriately extended by means of the Companion Matrix Method. It will be seen that, apart from lowly attenuated propagating modes, all the other highly attenuated modes are found, yielding the full three-dimensional spectrum of the problem under consideration. Moreover, when the PSCM schemes for viscoelastic media are used to compute the dispersion curves of guided waves in an elastic medium, all the remaining, imaginary as well as complex, roots of the elastic problem which were not computed by the simpler PSCM elastic schemes are found, providing the full three-dimensional picture of the dispersion curves. These PSCM schemes, as any other of the aforementioned approaches, only find pairs (\omega,k). If dispersion curves are to be plotted, those pairs must be linked correctly in order to plot the desired dispersion curves, which is non-trivial when crossings amongst modes occur. Motivated by this, an investigation of the parity and coupling properties of guided wave solutions is carried out in detail for all crystal classes. This investigation provides a robust alternative to conventional tracing routines and avoids the problem of mode crossings by exploiting the parity and coupling properties of the solutions. Finally, the most complicated problems involving embedded structures are investigated by including a Perfectly Matched Layer (PML) in the previously developed PSCM schemes for viscoelastic media. The dispersion curves for leaky and trapped modes in an isotropic elastic plate and in a similar cylinder immersed in an infinite ideal fluid are found, showing very good agreement with the results given by PWRF routines in a large range of frequencies. Last, but not least, an illustration of a two-dimensional PSCM scheme is presented to study a vibrating membrane. The results are compared with the available analytical solution showing again excellent agreement.
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Shah, Saurabh Mahesh Kumar. "Multi-scale imaging of porous media and flow simulation at the pore scale". Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/34323.
Testo completoAbstract (sommario):
In the last decade, the fundamental understanding of pore-scale flow in porous media has been undergoing a revolution through the recent development of new pore-scale imaging techniques, reconstruction of three-dimensional pore space images, and advances in the computational methods for solving complex fluid flow equations directly or indirectly on the reconstructed three-dimensional pore space images. Important applications include hydrocarbon recovery from - and CO2 storage in - reservoir rock formations. Of particular importance is the consideration of carbonate reservoirs, as our understanding of carbonates with respect to geometry and fluid flow processes is still very limited in comparison with sandstone reservoirs. This thesis consists of work mainly performed within the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC) project, focusing on development of three dimensional imaging techniques for accurately characterizing and predicting flow/transport properties in both complex benchmark carbonate and sandstone rock samples. Firstly, the thesis presents advances in the application of Confocal Laser Scanning Microscopy (CLSM), including the improvement of existing sample preparation techniques and a step-by step guide for imaging heterogeneous rock samples exhibiting sub-micron resolution pores. A novel method has been developed combining CLSM with sequential grinding and polishing to obtain deep 3D pore-scale images. This overcomes a traditional limitation of CLSM, where the depth information in a single slice is limited by attenuation of the laser light. Other features of this new method include a wide field of view at high resolution to arbitrary depth; fewer grinding steps than conventional serial sectioning using 2D microscopy; the image quality does not degrade with sample size, as e.g. in micro-computed tomography (micro- CT) imaging. Secondly, it presents two fundamental issues - Representative Element of Volume (REV) and scale dependency which are addressed with qualitative and quantitative solutions for rocks increasing in heterogeneity from beadpacks to sandpacks to sandstone to carbonate rocks. The REV is predicted using the mathematical concept of the Convex Hull, CH, and the Lorenz coefficient, LC, to investigate the relation between two macroscopic properties simultaneously, in this case porosity and absolute permeability. The effect of voxel resolution is then studied on the segmented macro-pore phase (macro-porosity) and intermediate phase (micro-porosity) and the fluid flow properties of the connected macro-pore space using lattice-Boltzmann (LB) and pore network (PN) modelling methods. A numerical coarsening (up-scaling) algorithm have also been applied to reduce the computational power and time required to accurately predict the flow properties using the LB and PN methods. Finally, a quantitative methodology has been developed to predict petrophysical properties, including porosity and absolute permeability for X-ray medical computed tomography (CT) carbonate core images of length 120 meters using image based analysis. The porosity is calculated using a simple segmentation based on intensity grey values and the absolute permeability using the Kozeny-Carman equation. The calculated petrophysical properties were validated with the experimental plug data.
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Fert, Marcin Maciej. "An investigation of the mechanical performance of Z-pin reinforced composites". Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/33729.
Testo completoAbstract (sommario):
Fibrous composites, having excellent mechanical properties in the direction of the fibres, have lower mechanical properties in the through thickness direction, controlled by resin. Z-pinning improves the delamination toughness (up to 500%) with a relatively modest reduction to the in-plane mechanical properties (typically 5-15%). This experimental study investigates the mechanical performance of Z-Pins bridging an existing delamination in fibre reinforced resin composites under pull-out (Mode I), shear-out (Mode II) and mixed mode loading conditions using a specially designed testing rig. In Mode II the opening displacement was restricted and measured by springs of three different stiffnesses. A new technique of needle assisted Z-Pin insertion was developed, in which prepreg panels were perforated with a steel needle in order to insert Z-Pins. This technique ensured the desired orientation of Z-Pins, improved pinning quality and removed the necessity of costly preforms used in the traditional UAZ method. Test specimens were blocks (15 mm x 15 mm x 6mm thick) of carbon-epoxy IM7/8552 composite in unidirectional (UD) and quasi-isotropic (QI) stacking sequences, with PTFE delamination film in the mid-plane recreating an existing crack, bridged with a single T300/9310 Z-Pin or a group of four pins of either 0.28 mm or 0.51 mm diameter. Three phases of pull-out were identified: Linear Phase (linear force-displacement curve), Crack Formation (unstable crack propagation phase) and Frictional Sliding (friction-controlled pull-out). Two phases of shear-out were identified: Linear Phase (with no energy loss) and Breaking Phase (where the fibrous structure of the Z-Pins is fractured, ending with Z-Pin breakage). In mixed mode specimens behaved similarly to pull-out for the pin angles up to 45°. For higher angles the behaviour was more similar to pure shear-out. The influence of the Z-Pin diameter, z-pinning depth, distance between adjacent Z-Pins, composite stacking sequence and pull-out speed on the Z-Pins behaviour were investigated. The results will be useful in the formulation of improved Z-Pin bridging laws for use in finite element models.
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Moghaddasi-Tafreshi, Azamolsadat. "Design optimization using the boundary integral equation method". Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46451.
Testo completoLibri sul tema "620.1/1"
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P, Schaffer James, a cura di. The science and design of engineering materials. 2a ed. Boston: WCB McGraw-Hill, 1999.
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L, Thomas Edwin, a cura di. The structure of materials. New York: J. Wiley, 1999.
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G, Rethwisch David, a cura di. Materials science and engineering: An introduction. 8a ed. Hoboken, NJ: John Wiley, 2009.
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1970-, Brownell Blaine Erickson, a cura di. Transmaterial 2: A catalog of materials that redefine our physical environment. New York, N.Y: Princeton Architectural Press, 2008.
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P, Webster, a cura di. The science and engineering of materials. 2a ed. London: Chapman and Hall, 1990.
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Frank, Haddleton, Green Phil e Robertson Howard, a cura di. The science and engineering of materials. 3a ed. London: Chapman & Hall, 1996.
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1960-, Phulé Pradeep Prabhakar, a cura di. The science and engineering of materials. 5a ed. Toronto, Ont: Thomson, 2006.
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Lalitha, Subramanian, e Maiti Amitesh, a cura di. Molecular modeling techniques in material sciences. New York: Dekker, 2005.
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Engineering Materials Volume 1. 2a ed. Butterworth-Heinemann, 1996.
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Physical Foundations of Materials Science. Springer, 2004.
Cerca il testo completoAtti di convegni sul tema "620.1/1"
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Bouzid, Sofiane, Abdel-Hakim Bouzid e Anh Dung Ngô. "Stress Analysis of ASME Section X Flanges Using Classical Lamination Theory". In 2022 14th International Pipeline Conference. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/ipc2022-89130.
Testo completoAbstract (sommario):
Abstract Fiber Reinforced Plastic composite flanges are extensively used in pressure vessels and pipping systems working under harsh and corrosive environment. ASME section X gives the design procedure to follow to verify the mechanical integrity and leak tightness of these flanges. Unfortunately, this method treats FRP bolted flange joints using the same analytical approach as that of ASME section VIII div. 1 which is derived for metallic flanges. The approach was developed in the late 30’s for flanges having isotropic material properties while the flexibility of the flange different elements and their elastic interaction is ignored. In the absence of a suitable approach such as one based on classical laminate theory, the structural integrity of FRP bolted flange joints remains an issue and some flange classes and sizes will suffer structural failure. It is proposed to study ASME section X flanges using a recently developed model based on the classical laminate theory and the flexibility of the different elements to identify those critical ones that needs particular attention with regards to composite material selection and bolting and pressure limits. Parameters such as flange ring rotation and stresses in the bolts, flange and gaskets will be investigated. The study on the strength of FRP flanges described in ASME section X RD-620.1 table, will reveal the most critical size and class flanges and their highly stressed locations. The critical stresses and their locations on the flange will be given for each flange. The study will be conducted on flanges of NPS 1 to 48 and classes from 25 to 150. The analysis will be conducted on flanges fabricated out of Fiberglass Reinforced Plastic (FRP) laminates, manufactured with vinyl ester resins and various types of fibrous glass reinforcing. The study shows that FRP flanges of class 25 and 50 are most vulnerable and should be less loaded.