Bridges with external post-tensioned tendons are considered to be more durable than bridges with internal tendons (tendons within the webs and flanges), because external tendons are easier to inspect. In addition, in the event that extensive corrosion damage is detected, it is possible to replace an external tendon. However, an appropriate inspection for detecting damage needs to be determined for external tendons. This investigation focuses on the vibration technique, which uses the dynamic properties of the external tendon to infer the effective prestress force. Four large-scale external tendons, designed to simulate one section of an external tendon between two deviators in a post-tensioned bridge, were tested. In the study, damage to the tendons was induced in a quantifiable fashion at a specific location and the tensile force was measured directly. In addition, free-vibration tests were conducted periodically. This provided a direct means of measuring the sensitivity of measured natural frequencies and measured tensile force to local damage. The measured data were correlated with an approximation of the stiff string vibration model. In addition to the laboratory specimens, field testing was conducted on a bridge with external post-tensioned tendons. The findings from the study show that a loss in tensile force was not linear with a loss in the cross-sectional area of the strand, which results from stress redistribution within the tendon. Also, the natural frequencies were much less sensitive to the level of induced damage than the tensile force. While the measured data from the laboratory data compared very well with the analytical model, the field measurements exhibited a much greater deviation from the model. Due to several factors, the difference between the laboratory specimens and the bridge tendons are believed to be caused by larger levels of inherent error in the model. The findings from the investigation support the notion that vibration testing is most appropriately used in comparing relative differences between peer tendons. / text

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Close coupling scattering calculations have been conducted for the para spin modification of H₂-{He, Ne and Ar}. The XC(fit) potential energy surfaces for H₂-Ne and H₂-Ar have been used for calculations for these two systems, while a newly fitted version of the Schaefer and Kohler potential energy surface was used for the H₂-He system. The fitting procedure employs nine modified Lennard-Jones oscillator functions to describe accurately 90% of the original tabulated potential energy surface to better than 12% error. The scattering calculations for H₂-Arfailed at higher energies due to the presence of a previously undocumented potential energy surface turn-over at R less than 1. 0 Angstroms. Manifold-to-manifold v=1 vibrational relaxation calculations for each of these systems are compared with other experimental and theoretical calculations. These comparisons demonstrate a common discrepancy between previous calculations and the current calculations for each system. The current vibrational relaxation rate constants are generally too small when compared to low temperature values of Audibert et al. and Orlikowski, and the high temperature values obtained by Flower et al. and Dove andTeitelbaum. The current calculations indicate the presence of a dramatic up-turn in the low temperature H₂-He rate constants. Other experimental and theoretical treatments do not exhibit this same up-turn, which is puzzling. A set of follow-up calculations featuring a larger basis set (such as the {16,12,10,8} Flower et al. basis set) and a larger manifold of included relaxation pathways are needed to improve these calculations.

On account of the high temperatures and pressures experienced by energetic materials during deflagration and detonation, it is important to know not only the physical properties of these materials at ambient temperatures and pressures, but also to understand how their structure and properties are affected by extreme conditions. Combined computational and experimental investigations of the effects of high pressures on the structure and properties of several energetic materials are described herein. A comparison of the performances of different pseudopotentials and density functional theory (DFT) dispersion correction schemes in calculating crystal geometries and vibrational frequencies of crystalline ammonium perchlorate at high pressure is described. The results highlight the fact that care must be taken when choosing pseudopotentials for high-pressure studies. A comprehensive comparison of calculated vibrational modes (including symmetry) with experiment has been performed, with the frequencies of all internal modes predicted to lie within 5% of experimental values. This study established that no significant improvements in the calculation of crystal geometries of ammonium perchlorate are obtained by employing DFT-D corrections. The enthalpy of fusion (ΔHfus) of the highly metastable β-form of RDX (cyclotrimethylenetrinitramine) was determined to be 12.63 ± 0.28 kJ mol-1. DFT-D calculations of the lattice energies of the α- and β-forms of RDX are described. Furthermore, the response of the lattice parameters and unit-cell volumes to pressure for the α-, γ- and ε-forms of RDX calculated using DFT-D are in very good agreement with experimental data. Phonon calculations provide good agreement with vibrational frequencies obtained from Raman spectroscopy, and a predicted inelastic neutron scattering (INS) spectrum of α-RDX shows excellent agreement with experimental INS data recorded as part of this study. The results of the high-pressure phonon calculations have been used to show that the heat capacities of the α-, γ- and ε- forms of RDX are only weakly affected by pressure. DFT-D calculations have been utilised to describe accurately the structure and properties of both β-HMX (Cyclotetramethylenetetranitramine) and α-FOX-7 (1,1-Diamino-2,2-dinitroethylene) as a function of pressure. This work presents data for the experimental hydrostatic compression of both deuterated β-HMX and α-FOX-7 performed using neutron powder diffraction at the ISIS Neutron and Muon facility, in addition to experimental determinations of the INS spectra of both β-HMX and α-FOX-7. The DFT-D hydrostatic compression studies for both materials reproduce the experimental compression trends. Furthermore, the calculated vibrational properties as a function of pressure were in very good agreement with available experimental data. The results of the phonon calculations were then used to predict the effect of pressure on the heat capacities of β-HMX and α-FOX-7. These predictions suggest a very weak pressure dependence of heat capacities (approximately -1 J K-1 mol-1 GPa-1) for these materials. This work demonstrates that the DFT-D model performs extremely well over a range of conditions, and is able to describe accurately intramolecular and intermolecular interactions, and thus the structure and properties of organic molecular nitramine crystals. The computational model was therefore used to predict the high-pressure hydrostatic compression behaviour of a related nitramine, CL-20 (2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), the results of which highlighted possible discrepancies in the experimental high-pressure X-ray diffraction data recorded for ε-CL-20. This prompted a high-pressure neutron powder diffraction study, which showed good agreement with the computational results, thereby highlighting radiation damage in the X-ray experiments.

Neste trabalho as propriedades físicas e químicas das nanopartículas (NPs) de prata de diferentes tamanhos e formatos são investigadas teoricamente utilizando cálculos de primeiros princípios dentro da Teoria do Funcional da Densidade (DFT), descrita em termos da Aproximação do Gradiente Generalizado (GGA), com o uso de Pseudopotenciais Ultrassuaves (USPP). Uma investigação sistemática das propriedades estruturais, energéticas, eletrônicas e vibracionais (faixa do infravermelho - IR) é realizada para as nanopartículas passivadas e não passivadas. Seguindo as observações experimentais construímos as nanopartículas livres utilizando o empacotamento cristalino FCC nas simetrias octaédrica, cubo-octaédrica, octaédrica truncada e esférica, e, também, na simetria icosaédrica não cristalina. Para avaliar a inuência da dimensão das nanopoartículas em suas propriedades, são investigadas diferentes NPs com diâmetros variando entre 0,3 e 2,9 nm (entre 6 e 561 átomos). No presente estudo, são identicados três regimes característicos associados às dimensões do nanopartículas: (i) inferior a 1,5 nm (até 100 átomos), apresentando forte característica molecular; (ii) entre 1,5 e 2,0 nm (100 e 300 átomos), onde o comportamento molecular é inuenciado pelo caroço cristalino; e (iii) acima de 2,0 nm (mais de 300 átomos), onde as propriedades do cristal são preponderantes, mas com modulação de superfície. Os efeitos da passivação são investigados considerando a adsorção dos radicais metil (CH3) e vinil (C2H3) em NPs menores que 2,0 nm. Nossos resultados sugerem que as propriedades físicas consideradas (energia de formação, comprimentos de ligação e espectros IR) estão fortemente correlacionadas com a coordenação do sítio adsorvido. Os nossos cálculos da energia total sugerem que as adsorções realizadas sobre os sítios de menor coordenação são mais propensas a passivação pelas moléculas dos surfactantes considerados. Outros modelos de simplicação possíveis para a simulação das nanopartículas, como um único átomo ou secções transversais em hemisférios, também são investigados. Os nossos cálculos sugerem que os modelos simplicados devem ser usados com certo cuidado, por não descreverem adequadamente todas as propriedades físicas analisadas. / In this work, physical and chemical properties of silver nanoparticles (NPs) of different sizes and shapes are investigated theoretically using rst principles calculations in the Density Functional Theory (DFT) framework, described in terms of the Generalized Gradient Approximation (GGA), and by using Ultra-Soft Pseudopotentials (USPP). A systematic investigation of energetics, structural, electronic and vibrational (infrared range, IR) properties of both the passivated and nonpassivated nanoparticles are performed. Following experimental observations, free NPs are constructed using the crystalline FCC packing in octahedral, cube-octahedral, truncated octahedral and spherical symmetries, and also in noncrystalline icosahedral symmetry. In order to evaluate the inuence of the nanoparticless size in its properties, different NPs with diameters ranging between 0.3 and 2.9 nm (corresponding to 6 to 561 atoms) are investigated. In this study we identied three characteristics regimes associated with the nanoparticles dimensions: (i) below 1.5 nm (100 atoms) where remarkably molecular aspects are observed; (ii) between 1.5 and 2.0 nm (100 and 300 atoms) where the molecular behavior is inuenced by the core crystallinity; and (iii) above 2.0 nm (more than 300 atoms) where the crystal properties are preponderant, but with surface modulation. Passivation effects are investigated considering the adsorption of the methyl (CH3) and vinyl (C2H3) radicals on NPs smaller than 2.0 nm. Our results suggests that the considered physical properties (formation energy, bond lengths, IR spectra) are strongly correlated to the adsorbed site coordination. Our total energy calculations suggest that lower coordinated adsorption sites are more likely to be passivated by the surfactant molecules considered. Other possible simplied models for simulating NPs, such as a single atom or polar cross sections are also investigated. Our calculations suggest that simplied models might be used with some care, as not all considered physical properties are well described by them.

A presente tese descreve e discute as síntese de alguns mercaptais e mercaptóis e apresenta atribuições a várias frequências do espectro vibracional. Um apanhado bibliográfico sôbre a absorção no lnfra-Vermelho e Raman de tióis, sulfetos, dissulfetos e alguns mercaptais indica que acham-se descritas as frequências para os seguintes modos de vibração: estiramento de C-H deformação de CH3, CH2 e CH estiramento de S-H, C-S e S-S deformação de C-S-S, C-S-C, C-C-S e C-S-H. Para fins de comparação incluem-se também os dados da literatura referentes às frequências vibracionais de CH3, CH2 e CH nos hidrocarbonetos e compostos oxigenados. São descritas as sínteses de 20 dialquil e diarilmer-captais de aldeídos e cetonas alifáticas e aromáticas pelo emprêgo de dois métodos: (1) Reação entre tiol e composto carbonílico na presença de gás clorídrico sêco. (2) Reação entre ortotioformiato de alquila ou arila e composto carbonílico na presença de ácido sulfúrico concentrado. É fornecida uma prova da formação do intermediário catiônico pela catálise ácida do ortotioformiato de etila como primeiro passo de (2) por meio da reação tipo Friedel Crafts entre anisol e ortotioformiato, em presença de ácido sulfúrico concentrado, que conduziu ao dietilmercaptal do anisaldeído. É sugerido também um provável mecanismo para a reação do intermediário catiônico com o composto carbonílico. A reação do ortotioformiato com anisol, ainda não descrita na literatura, poderá se constituir f\'uturamente num novo método de obtenção de mercaptaisde aldeídos aromáticos. Observa-se que: Os espectros no Infra-Vermelho de compostos que contém os grupos C6H5-CH2S, C6H5-CHS2 e (C6H5)2CS2, nos quais o anel aromático não está ligado diretamente ao enxôfre, mostram, como a maioria dos compostos aromáticos monosubstituídos, um grupo de quatro bandas na região de 1610-1450cm-1. Entretanto, o grupo C6H5S, presente em difenilmercaptais, difenilsulfeto e difenildissulfeto, é caracterizado por três bandas na região de 1590-1435cm-1. As bandas correspondentes às vibrações de deformação no plano de C-H do anel aromático, na região de 1100-1000cm-1 do Infra-Vermelho são geralmente de fraca intensidade e de limitado uso analítico. Entretanto, nos espectros dos compostos contendo os grupos C6H5-CHS2, (C6H5)2CS2 e C6H5CH2S nota-se a presença de duas bandas de média ou forte intensidade. No caso de compostos que contém os grupos C6H5S, o número de bandas de média ou forte intensidade é aumentado para três. Os grupos SC2H5 e SCH2C6H5 nos mercaptais semelhantemente aos sulfetos e dissulfetos diferem entre si pelo wagging de CH2, em 1265 e 1240cm-1 respectivamente. A banda em 1050cm-1 de fraca intensidade atribuída na literatura ao rocking de CH3 em C2H5S, é também característica do grupo CH3 CHS2, sendo porém neste último caso, de forte intensidade. A banda de forte intensidade em 1110cm-1 nos espectros de Infra-Vermelho dos mercaptóis da acetona, que é fraca ou às vêzes omitida nos sulfetos e disulfetos contendo o grupo (CH3)2CHS, é característica do grupo (CH3)2C ligado a dois átomos de enxôfre. Os mercaptais de aldeídos alifáticos diferem dos correspondentes mercaptóis, no Infra-Vermelho pela presença de duas bandas em 1260 e 1180cm-1 de fraca e média intensidade respectivamente, correspondentes aos modos de deformação de C-H. As bandas em 1225 e 1155cm-1, presentes no dietilmercaptal do benzaldeído, mas ausentes no composto deuterado correspondente, são devidas aos modos de deformação de C-H nos mercaptais de aldeídos aromáticos. As bandas em 1260 e 1230cm-1, presentes em mercaptais de aldeídos alifáticos e aromáticos respectivamente, que se mostraram polarizados no espectro Raman, correspondem à deformaçao simétrica do grupo-C-H. As bandas em 1180 e 1155cm-1, não polarizadas no espectro Raman, são correspondentes à deformação assimétrica do mesmo grupo. Faz-se um estudo comparativo de: frequências de deformação de C-H vizinho aos átomos de enxôfre com as de C-H vizinho aos átomos de carbono, oxigênio e cloro. frequências de deformação no plano de C-H do anel aromático em compostos de enxôfre com as frequências correspondentes em compostos clorados. / Abstract not available