Este trabalho tem o objetivo de determinar as constantes elásticas de laminados compósitos de carbono/epóxi através do método não destrutivo de ultrassom, utilizando a técnica da imersão. Foram implementados dois métodos de imersão em água para a determinação das constantes elásticas. Um que utiliza o percurso na água sem a amostra como referência (FRM), e outro que utiliza o percurso com incidência normal na amostra como referência (SRM). Laminados unidirecionais e biaxiais foram ensaiados em um goniômetro com elevada precisão de movimento angular, e a aquisição de sinais feita com alta taxa de amostragem e grande resolução de amplitude. Uma análise da influência da variação da temperatura e espessura foi feita e comprovada a superioridade do método SRM sobre o FRM. A compensação da temperatura através de um par de transdutores, bem como através de um termômetro com resolução de 0.01°C foi implementada e comparada. As curvas de velocidade foram obtidas com transdutores de diferentes frequências centrais e comparadas. Foi realizada uma análise detalhada da sensibilidade das constantes elásticas com relação as velocidades em cada plano de medição, e através desta análise foram utilizados os planos mais adequados para reconstrução das constantes elásticas. Também foi verificada a polarização das ondas de cisalhamento para cada plano de medição, uma vez que é necessária a correta associação da velocidade experimental com a velocidade calculada para o procedimento de otimização. Um algoritmo de otimização foi utilizado para reconstruir as constantes elásticas dos laminados compósitos a partir da medição experimental das velocidades, onde foi avaliada a convergência utilizando a equação de Christoffel na forma implícita e explícita, para os materiais de simetria tetragonal e hexagonal. / The objective of this work is to obtain the elastic constants of carbon/epoxy composite laminates using ultrasonic non destructive testing, with immersion technique. Two immersion methods were implemented. One method uses as reference the path without sample, called fluid reference method (FRM), the second method uses the path with the specimen in normal incidence, called self-reference method (SRM). Unidirectional and biaxial laminates were tested using a goniometer with high angular resolution, and the signals were acquired with a high sample rate and amplitude resolution. Analyses of the influence of temperature and thickness variations were performed and concluded the superiority of the SRM over the FRM method. Temperature compensation using a pair of transducers and a 0.01° resolution thermometer was implemented and compared. Velocity curves were obtained and compared using transducers with different frequencies. A detailed analysis was performed to evaluate the sensitivity of the elastic constants to each velocity in different acquisition planes, and the results were used to reconstruct the elastic constants in the planes/velocities which they were most sensible to. As the correct association of each experimental velocity with the calculated velocities from Christoffel\'s equation is necessary, an analysis of the polarization mode of each velocity in the acquisition planes was also performed. An optimization algorithm was used to reconstruct the elastic constants of composite laminates from experimental velocities. Its convergence was evaluated for the implicit and explicit form of Christoffel\'s equation, and for materials with tetragonal and hexagonal symmetry.

No description available.

Este trabalho tem o objetivo de determinar as constantes elásticas de laminados compósitos de carbono/epóxi através do método não destrutivo de ultrassom, utilizando a técnica da imersão. Foram implementados dois métodos de imersão em água para a determinação das constantes elásticas. Um que utiliza o percurso na água sem a amostra como referência (FRM), e outro que utiliza o percurso com incidência normal na amostra como referência (SRM). Laminados unidirecionais e biaxiais foram ensaiados em um goniômetro com elevada precisão de movimento angular, e a aquisição de sinais feita com alta taxa de amostragem e grande resolução de amplitude. Uma análise da influência da variação da temperatura e espessura foi feita e comprovada a superioridade do método SRM sobre o FRM. A compensação da temperatura através de um par de transdutores, bem como através de um termômetro com resolução de 0.01°C foi implementada e comparada. As curvas de velocidade foram obtidas com transdutores de diferentes frequências centrais e comparadas. Foi realizada uma análise detalhada da sensibilidade das constantes elásticas com relação as velocidades em cada plano de medição, e através desta análise foram utilizados os planos mais adequados para reconstrução das constantes elásticas. Também foi verificada a polarização das ondas de cisalhamento para cada plano de medição, uma vez que é necessária a correta associação da velocidade experimental com a velocidade calculada para o procedimento de otimização. Um algoritmo de otimização foi utilizado para reconstruir as constantes elásticas dos laminados compósitos a partir da medição experimental das velocidades, onde foi avaliada a convergência utilizando a equação de Christoffel na forma implícita e explícita, para os materiais de simetria tetragonal e hexagonal. / The objective of this work is to obtain the elastic constants of carbon/epoxy composite laminates using ultrasonic non destructive testing, with immersion technique. Two immersion methods were implemented. One method uses as reference the path without sample, called fluid reference method (FRM), the second method uses the path with the specimen in normal incidence, called self-reference method (SRM). Unidirectional and biaxial laminates were tested using a goniometer with high angular resolution, and the signals were acquired with a high sample rate and amplitude resolution. Analyses of the influence of temperature and thickness variations were performed and concluded the superiority of the SRM over the FRM method. Temperature compensation using a pair of transducers and a 0.01° resolution thermometer was implemented and compared. Velocity curves were obtained and compared using transducers with different frequencies. A detailed analysis was performed to evaluate the sensitivity of the elastic constants to each velocity in different acquisition planes, and the results were used to reconstruct the elastic constants in the planes/velocities which they were most sensible to. As the correct association of each experimental velocity with the calculated velocities from Christoffel\'s equation is necessary, an analysis of the polarization mode of each velocity in the acquisition planes was also performed. An optimization algorithm was used to reconstruct the elastic constants of composite laminates from experimental velocities. Its convergence was evaluated for the implicit and explicit form of Christoffel\'s equation, and for materials with tetragonal and hexagonal symmetry.

Recently, the development of electronic technology towards higher frequencies and larger band widths has led to interest in finding new piezoelectric materials, which could be used to make filters with larger pass band widths and oscillators with better frequency stability. Langasite (La3Ga5SiO14, LGS) and its isomorphs have enticed considerable attention of researchers as a potential substrate material for piezoelectric device applications because of its high frequency stability and fairly good electromechanical coupling factors for acoustic wave devices. Nonlinear effect including drive level dependence, mode coupling, force-frequency effect and electroelasic effect are critical for the design of these devices. Third-order elastic constants (TOEC) play an important role in a quantitative analysis of these nonlinear effects. In particular these elastic constants are of great importance when the BAW (Bulk Acoustic Wave) and SAW (Surface Acoustic Wave) sensors of force, acceleration and so on are designed. Until now Langasite (LGS) and Langatate (LGT) crystal resonators have been qualified in terms of quality factor, temperature effect, isochronism defect and material quality. One of the most important advantages of those crystals is that they will not undergo phase transitions up to its melting temperature of 1450°. Presently there is no data on TOEC of LGT crystals. Our objective is to create an experimental procedure to measure and collect the complete set of third-order elastic constants of Langasite (La3Ga5SiO14) and Langatate (La3Ga5.5Ta0.5O14) crystals and compare the new values for langasite with values previously reported.

No description available.

The aim of this study is to determine the mechanical characteristics of the filament wound composite tubes working under internal pressure loads, generating data for further investigation with a view of estimating the remaining life cycle of the tubes during service. Data is generated experimentally by measuring the mechanical behavior like strains in hoop direction, maximum hoop stresses that are formed during internal pressure loading. Results have been used to identify and generate the necessary data to be adopted in the design applications. In order to determine these parameters, internal pressure tests are done on the filament wound composite tube specimens according to ASTM D 1599-99 standard. The test tubes are manufactured by wet filament winding method, employing two different fiber types, two different fiber tension settings and five different winding angle configurations. The internal pressure test results of these specimens are studied in order to determine the mechanical characteristics, and the effects of the production variables on the behavior of the tubes. Pressure tests revealed that the carbon fiber reinforced composite tubes exhibited a better burst performance compared to the glass fiber reinforced tubes, and the maximum burst performance is achieved at a winding angle configuration of [&plusmn / 54&deg / ]3[90&deg / ]1. In addition, the tension setting is found not to have a significant effect on the burst performance. The burst pressure data and the final failure modes are compared with the results of the ASME Boiler and Pressure Vessel Code laminate analysis, and it was observed that there is a good agreement between the laminate analysis results and the experimental data. The stress &ndash / strain behavior in hoop direction are also studied and hoop elastic constants are determined for the tubes.

Ab initio alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to determine the effect of long range order on the elastic properties. The lattice parameters and single-crystal elastic constants of Cu3Au are calculated for different partially ordered structures ranging from the fully ordered L12 to the random face centered cubic lattice. Special attention is put on the chemical and magnetic effects and on the interplay between these two degrees of freedom. For non-magnetic Cu3Au, it was found that the lattice parameters and single-crystal elastic constants follow a clear trend with the degree of chemical order: namely, C11 and C12 decrease slightly, whereas C44 remains nearly constant with increasing disorder. As none of the single-crystal elastic constants were modified significantly due to the long-range-order, the polycrystalline elastic moduli of Cu3Au also keep nearly constant upon ordering. The Debye temperature does not show a strong chemical order dependence either. Using the calculated Debye temperatures, we find that for the entropy change upon order-disorder transition varies between −0.018 kB and 0.022 kB, the upper value being surprisingly close to 0.023 kB observed in experiments. However, some of the elastic constants of ferromagnetic Ni3Fe, adopting the same crystal lattice as Cu3Au, were affected considerably. For the lattice parameter a, the main effect of magnetism is concentrated in the chemically dis-ordered region, with long-range order parameter S below ∼ 0.6 and the effect gradually disappears with increasing S. In the ferromagnetic state, the lattice parameter is almost constant as a function of the degree of order. Out of the three single-crystal elastic constants, only C11 and C12 are found to be affected by magnetism in the ordered state, however, their combined effect results in a nearly constant bulk modulus as a function of S. C44 changes slightly with S and magnetic state. The tetragonal shear elastic constant C´ , the Young’s modulus E and the shear modulus G increase significantly with the degree of order in the ferromagnetic state, but the effect becomes weak as the system approaches the random regime. Especially the C´ shear elastic constant depends strongly on the magnetic state and the degree of order. As a result, the Zener anisotropy ratio C44/C´ and the Possion’s ratio are strongly affected by the long-range order in the ferromagnetic state. Nevertheless, the actual values for the Pugh ratio and the Cauchy pressure remain far from their critical values, indicating that the ductility of Ni3Fe is not influenced by the chemical/magnetic ordering. Interestingly, the ferromagnetic L12 system possesses ∼5.4% larger elastic Debye temperature than the paramagnetic one, which in turn has similar ΘD as the chemically disordered face centered cubic phase being in either ferro-or paramagnetic state. The implications of the chemical/magnetic order on the mechanical properties and order-disorder transition is discussed. / <p>QC 20140929</p>

<p>The present thesis deals with the application of <i>ab-initio</i> electronic structure calculations based on density functional theory for material design.</p><p>The correlation between magnetic properties and electronic structures has been investigated in detail for diluted magnetic semiconductors (DMS), which have promising application for spintronics devices. The point defects, acting as electron donor or electron acceptor, have been studied for their key role in mediating the long ranged ferromagnetic interaction between transition metal (TM) ions. The electron holes induced by electron acceptor are completely spin polarized in semiconductor and exhibit a very significant efficiency to the ferromagnetic coupling between TM ions. While the electron donor shows a negative effect to the ferromagnetism in the system. The common trend of magnetic interaction and electron charge transfer between TM ions and electron acceptors or electron donators have been outlined. The Coulomb correlation <i>U</i> of <i>d</i> electrons, which could change the energy levels of TM <i>d</i> band respective to the host semiconductor band, also shows a significant influence on the magnetic behavior in DMS. </p><p>The crystallography phase transition under high pressure has also been studied for the iron doped with light element, carbon. Our calculated results show that interstitial carbon defect has little effect on the iron's bcc to hcp phase transition under high pressure. The other carbon iron phases, like Fe₃C, has also been studied in a wide pressure range.</p><p>We also present a first-principles description on the temperature dependence of elastic constant for solids. The total temperature effects are approximated as a sum of two separated parts, the thermal expansion contribution, which gives the normal linearly decreasing effect on the elastic constant with increasing temperature, and the electronic band contribution, which could lead anomalous behavior for thermal elastic constants.</p>

The present thesis deals with the application of ab-initio electronic structure calculations based on density functional theory for material design. The correlation between magnetic properties and electronic structures has been investigated in detail for diluted magnetic semiconductors (DMS), which have promising application for spintronics devices. The point defects, acting as electron donor or electron acceptor, have been studied for their key role in mediating the long ranged ferromagnetic interaction between transition metal (TM) ions. The electron holes induced by electron acceptor are completely spin polarized in semiconductor and exhibit a very significant efficiency to the ferromagnetic coupling between TM ions. While the electron donor shows a negative effect to the ferromagnetism in the system. The common trend of magnetic interaction and electron charge transfer between TM ions and electron acceptors or electron donators have been outlined. The Coulomb correlation U of d electrons, which could change the energy levels of TM d band respective to the host semiconductor band, also shows a significant influence on the magnetic behavior in DMS. The crystallography phase transition under high pressure has also been studied for the iron doped with light element, carbon. Our calculated results show that interstitial carbon defect has little effect on the iron's bcc to hcp phase transition under high pressure. The other carbon iron phases, like Fe3C, has also been studied in a wide pressure range. We also present a first-principles description on the temperature dependence of elastic constant for solids. The total temperature effects are approximated as a sum of two separated parts, the thermal expansion contribution, which gives the normal linearly decreasing effect on the elastic constant with increasing temperature, and the electronic band contribution, which could lead anomalous behavior for thermal elastic constants.

No description available.