No description available.

The thermodynamics of three-dimensional, single-component elastic crystalline solids was developed by Slattery and Lagoudas (2005). Considering the in¯nitesimal deformations, the stress can be expressed as a function of the lattice vectors and density in the reference configuration and ¹(I;mn), which is defined as the derivative of specific Helmoholtz free energy with respect to the I(mn). Using the Cauchy - Born rule to connect the interatomic potential energy and the specific Helmholtz free energy, it is possible to calculate the elastic properties of both nano-scale materials such as carbon nanotubes and macro-scale materials such as diamond and silicon. In this study, we used Terso® (1988a) - Brenner (1990b) Potential, Terso® (1988b) potential and Finnis and Sinclair (1984) potential for carbon, silicon, and vanadium systems respectively. Using the interatomic potentials to describe the specific Helmholtz free energy, the elastic properties of graphite, diamond, silicon and vanadium were calculated. This method was also extended to the calculation of Young's modulus of single-walled carbon nanotubes (SWCNTs), which are composed of a two dimensional array of carbon atoms. For SWCNT, we get good agreement with the available experimental data. For diamond and silicon, C11 and C12 were consistent with both the superelastic model and the experimental data. The difference of C44 between the calculation and experimental data was due to accuracy of the potential functions.

Aluminum alloys are a suitable substitution for heavy ferrous alloys in automobile structures. The purpose of this study was to investigate the flow stress behavior of as-cast and homogenized A535 aluminum alloy under various deformation conditions. A hot compression test of A535 alloy was performed in the temperature range of 473-673 K (200-400˚C) and strain rate range of 0.005-5 s-1 using a GleebleTM machine. Experimental data were fitted to Arrhenius-type constitutive equations to find material constants such as n, nʹ, β, A and activation energy (Q). Flow stress curves for as-cast and homogenized A535 alloy were predicted using an extended form of the Arrhenius constitutive equations. The dynamic shock load response of the alloy was studied using a split Hopkinson pressure bar (SHPB) test apparatus. The strain rate used ranged from 1400 s-1 to 2400 s-1 for as-cast and homogenized A535 alloy. The microstructures of the deformed specimens under different deformation conditions were analyzed using optical microscopy (OM) and scanning electron microscopy (SEM). Obtained true stress-true strain curves at elevated temperatures showed that the flow stress of the alloy increased by increasing the strain rate and decreasing the temperature for both as-cast and homogenized specimens. The homogenization heat treatment showed no effect on the mechanical behavior of the A535 alloy under hot deformation conditions. Hot deformation activation energy for both as-cast and homogenized A535 alloy was calculated to be 193 kJ/mol, which is higher than that for self-diffusion of pure aluminum (142 kJ/mol). The calculated stress values were compared with the measured ones and they showed good agreement by the correlation coefficient (R) of 0.997 and the average absolute relative error (AARE) of 6.5 %. The peak stress and the critical strain at the onset of thermal softening increased with strain rate for both the as-cast and homogenized A535 alloy. Homogenization heat treatment affected the high strain-rate deformation of the alloy, by increasing the peak stress and the thermal softening onset strain compared to those obtained for as-cast specimens. Deformed shear bands (DSBs) were formed in both the as-cast and homogenized A535 alloy in the strain rate range of 2000-2400 s-1.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23196号 / 工博第4840号 / 新制||工||1756(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 田中 功, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The thesis deals with molecular dynamic simulation of the influence of water on the deformation of hydrogels. Hydrogels are model materials formed from macromolecular networks solvated with water. It was found that water can form bridges between macromolecules that take the form of temporary ionic crosslinks. These bridges affect the behavior of the network during deformation. Water bridges are water molecules that have a limited radius of motion in the space between two macromolecules. The concentration of the water bridges was regulated by a partial charge on the macromolecular chain in the organic network. Bridges are a type of interaction that is relatively strong but significantly delocalized. It is not possible to dissociate the water bridge, after dissociation it will be re-created in another place in a short time. The influence of water bridges was compared with other types of network crosslinks, especially covalent and physical bonds. Covalent crosslinks are modeled as a simple binding interaction between two macromolecules. They are undissociable and are local throughout the simulation. Physical bonds are modeled as micelles, where hydrophobic groups form the core and hydrophilic groups form the micelle shell. Physical bonds have the nature of dissociable bonds that are local. Different types of crosslinks have different effects on deformation properties. The deformation of a network containing a combination of two types of crosslinks was simulated: (i) physically-covalent, (ii) ionically-covalent, and (iii) physically-ionic networks and (iv) ternary physically-covalent-ion networks. For individual and combined networks, the behavior depending on simple networks was verified. The number of water bridges was fundamentally affected by the primary structure of the chains. When the PEG chain was replaced with hydrophobic polyoxymethylene (POM) or polyoxytrimethylene (POTM), their solvation and mechanical behavior deteriorated.

Indiana University-Purdue University Indianapolis (IUPUI) / Nanoscale materials have become an active area of research due to the enhanced mechanical properties of the nanomaterials in comparison to their respective bulk materials. The effect that the size and shape of a nanomaterial has on its mechanical properties is important to understand if these materials are to be used in engineering applications. This thesis presents the results of molecular dynamics (MD) simulations on copper, gold, nickel, palladium, platinum, and silver nanowires of three cross-sectional shapes and four diameters. The cross-sectional shapes investigated were square, circular, and octagonal while the diameters varied from one to eight nanometers. Due to a high surface area to volume ratio, nanowires do not have the same atomic spacing as bulk materials. To account for this difference, prior to tensile loading, a minimization procedure was applied to find the equilibrium strain for each structure size and shape. Through visualization of the atomic energy before and after minimization, it was found that there are more than two energetically distinct areas within the nanowires. In addition, a correlation between the anisotropy of a material and its equilibrium strain was found. The wires were then subjected to a uniaxial tensile load in the [100] direction at a strain rate of 108 s-1 with a simulation temperature of 300 K. The embedded-atom method (EAM) was employed using the Foiles potential to simulate the stretching of the wires. The wires were stretched to failure, and the corresponding stress-strain curves were produced. From these curves, mechanical properties including the elastic modulus, yield stress and strain, and ultimate strain were calculated. In addition to the MD approach, an energy method was applied to calculate the elastic modulus of each nanowire through exponential fitting of an energy function. Both methods used to calculate Young’s modulus qualitatively gave similar results indicating that as diameter decreases, Young’s modulus decreases. The MD simulations were also visualized to investigate the deformation and yield behavior of each nanowire. Through the visualization, most nanowires were found to yield and fail through partial dislocation nucleation and propagation leading to {111} slip. However, the 5 nm diameter octagonal platinum nanowire was found to yield through reconstruction of the {011} surfaces into the more energetically favorable {021} surfaces.

Le procédé de projection à froid (cold spray en anglais) est un procédé fondé sur l’accélération de particules qui restent à l’état solide pour former des dépôts. L’un des forts potentiels applicatifs de ce procédé réside dans la réalisation de dépôts composites car l'incorporation des particules céramiques dans des poudres métalliques influence la microstructure et les propriétés des dépôts. Néanmoins, le principe de construction du dépôt composite n’est pas encore parfaitement établi. En conséquence, les recherches menées dans cette étude sur la fabrication de dépôts composites s’articulent autour de plusieurs domaines, à savoir :• La science des matériaux avec des études sur l’effet de la taille et de la teneur (15 vol.% - 60 vol.%) de la particule du renfort (SiC);• La mécanique des fluides avec des modélisations des vitesses des particules céramiques (SiC) et alliage d’aluminium (Al5056) et les simulations du comportement à la déformation de la particule;• Les caractérisations des dépôts avec des analyses de microstructure et de microdureté, de la cohésion du dépôt et de comportement en frottement des dépôts;Les résultats montrent que la température du gaz n'a aucun effet sur la teneur en SiC dans les dépôts mais provoque une amélioration du rendement de dépôt. La teneur en SiC dans les dépôts composites d’Al5056/SiCp augmente avec l’augmentation de la teneur en SiC dans les poudres initiales. L’ajout de SiC dans les dépôts d’Al5056 augmente la dureté et améliore la résistance à l'usure des dépôts, et puis l’amélioration dépend de la teneur en SiC dans les dépôts composites. La force de cohésion des dépôts augmente dans un premier temps avec l’augmentation de la teneur en SiC puis diminue à partir d’environ 26-27%. Les dépôts composites renforcés par SiC-67 et SiC-27 ont une teneur en SiC semblable dans les dépôts ; Pourtant la microdureté, la force de cohésion et la résistance à l'usure des dépôts formés par Al5056/SiC-67 sont supérieures à celles des dépôts construits par Al5056/SiC-27. Ce phénomène relève l’importance de l’énergie cinétique des particules renforts.Les résultats expérimentaux ont montré que les particules de SiC ne se déforment pas plastiquement mais qu’elles sont susceptibles de créer des cratères sur le substrat ou le revêtement déjà formé ou encore rebondir ou bien de s’insérer mécaniquement dans le revêtement déposé. Finalement, un modèle eulérien a été développé pour prédire la vitesse critique à partir de la morphologie de l’éjection de matière au moment de l’impact. Ce modèle a également été étendu au dépôt composite pour représenter le procédé d’empilement des particules pendant la projection. Les résultats calculés montrent la plus grande déformation des particules de la matrice grâce à l’impact des renforts. / In cold spraying, particles are accelerated in the gas jet to achieve a high velocity and deposit on the substrate with a solid state. One of potential and important applications of cold spray is realizing the composite coatings. The incorporated ceramic particles in the composite coating can greatly influence the microstructure and properties of the coatings. The objective of this thesis was to investigate factors influencing the reinforcement content in the coatings and especially the formation mechanism in cold spraying. Al5056/SiC composite coatings were prepared by cold spraying. The effect of particle size and the reinforcement content in the powders on the reinforcement content in the coatings and thus on the microstructure and the properties of the coatings were studied. A search on the particle deformation and the formation mechanism of the composite coating was also carried out by using software of fluent and Abaqus.The results show that the addition of the SiC particles in the coating increases the hardness and improves the wear resistance of the coatings. However, the cohesion strength of the coatings first increases with the increase of the SiC content in the coating and then at a certain fraction, it decreases. Moreover, under the condition of having a similar SiC content in the coating, larger SiC particles lead to better properties of the coatings.Finally, an eulerian model was used for predicting the critical velocity by the morphology of the material jet. This model has also been extended to the composite model to demonstrate the built-up process of the composite coating during cold spraying. The calculation results show that the matrix particles deform more greatly after being impacted by the reinforcements.

Solid Oxide Fuel Cells (SOFCs) are emerging as a potential breakthrough energy conversion technology for clean and efficient production of electricity and heat from hydrogen and hydrocarbon fuels. Sc0.1Ce0.01ZrO2 electrolytes for Solid Oxide Fuel Cells are very promising materials because their high ionic conductivity in the intermediate temperature range 700°C-800°C. The vibration response of cubic and rhombohedral (β) 10 mol%Sc2O3 - 1 mol%CeO2 - ZrO2(Sc0.1Ce0.01ZrO2 ) both at room and high-temperatures is reported. The in-situ heating experiments and ex-situ indentation experiments were performed to characterize the vibrational behavior of these important materials. A temperature and stress-assisted phase transition from cubic to rhombohedral phase was detected during in-situ Raman spectroscopy experiments. While heating and indentation experiments performed separately did not cause the transition of the cubic phase into the rhombohedral structure under the performed experimental conditions and only broadened or strained peaks of the cubic phase could be detected, the heating of the indented (strained) surface leaded to the formation of the rhombohedral Sc0.1Ce0.01ZrO2. Both temperature range and strained zone were estimated by in situ heating and 2D mapping, where a formation of rhombohedral or retention of cubic phase has been promoted. The mechanical properties, such as Young’s modulus, Vickers hardness, indentation fracture resistance, room and high temperature four point bending strength and SEVNB fracture toughness along with the stress – strain deformation behavior in compression, of 10 mol% Sc2O3 – 1 mol % CeO2 - ZrO2 (ScCeZrO2) ceramics have been studied. The chosen composition of the ScCeZrO2 has very high ionic conductivity and, therefore, is very promising oxygen ion conducting electrolyte for the intermediate temperature Solid Oxide Fuel Cells. Therefore, its mechanical behavior is of importance and is presented in this study.

No description available.

Das Zugtragverhalten von bewehrten und unbewehrten Bauteilen wird von einer Vielzahl von Faktoren beeinflusst. Maßgeblich wird es von der Festigkeit des verwendeten Betons, dem Verbundverhalten zwischen Bewehrung und Beton sowie vom vorhandenen Spannungszustand im Bauteil bestimmt. In der Regel werden im täglichen Planungsgeschäft des Ingenieurs einaxiale Spannungszustände unter Berücksichtigung der Materialeigenschaften des Betons aus den Standardprüfungen betrachtet. Jedoch treten in einer Vielzahl von Anwendungen mehraxiale Spannungszustände auf. Beispielhaft sollen hier Bereiche von zweiachsig spannenden Deckenplatten, in Bereichen von Rahmenecken, rotationssymmetrischen Bauwerkshüllen sowie bei Brückenbauwerken mit durchlaufender Fahrbahn im Bereich der Stützen genannt werden. Normative Regelungen sehen bisher im Falle einer zweiaxialen Druckbeanspruchung lediglich die Erhöhung der Druckfestigkeit bzw. Verbundspannung vor. Regelungen zur Festigkeit des Betons unter zweiaxialer Zugbelastung existieren dagegen nicht. Daraus abgeleitet stellt sich die Frage, welchen Einfluss eine zweiaxiale Zugbeanspruchung auf das Festigkeits- und Verformungsverhalten von unbewehrten und bewehrten Bauteilen ausübt. Mit Blick auf übliche Konstruktionsbetone sollen diese Fragestellungen für einen Beton C20/25 und C40/50 geklärt werden. Im Rahmen eines Forschungsvorhabens wurden hierzu Versuche an unbewehrten Betonsscheiben und gemischt bewehrten Bauteilen durchgeführt. Das im CEB-FIP MODELL CODE 90 vorgestellte Modell zur Beschreibung des einaxialen Spannungs-Dehnungs-Verhaltens bildet das reale Verhalten von Beton unter zweiaxialer Zugbelastung nur ungenügend ab. Hierfür werden Modelle zur Beschreibung des Verformungsverhaltens von Beton unter Berücksichtigung von zweiaxialen Spannungszuständen für einen Beton C20/25 und C40/50 entwickelt. Weiterhin werden Bruchkriterien für die zwei Betonsorten vorgestellt, mit denen die Zugfestigkeit des Betons unter zweiaxialer Zugbelastung bestimmt werden kann. Während bei einem Beton C20/25 die zweiaxiale Zugfestigkeit annähernd der einaxialen Zugfestigkeit entspricht, so nimmt die Zugfestigkeit des Betons C40/50 unter zweiaxialer Zugbelastung um ca. 25% ab. Hinsichtlich der Bruchdehnungen unter zweiaxialer Zugbelastung wurde festgestellt, dass diese mit steigendem Spannungsverhältnis 1 : 2 abnehmen. Darüber hinaus bilden die Modelle zur Bestimmung des Spannungs-Dehnungs-Verhaltens des unbewehrten Betons die Versuchsergebnisse sehr gut ab. Mit Hilfe der hier vorliegenden Ergebnisse können somit das Verformungs- und Festigkeitsverhalten von Beton unter zweiaxialer Zugbelastung sehr gut abgebildet werden. In Bauteilversuchen wurde das Verformungsverhalten unter zweiaxialer Zugbelastung von gemischt bewehrten Bauteilen untersucht. Die Bestimmung der Verformungen erfolgte hierbei mittels Dehnmessstreifen auf der Betonoberfläche, dem schlaffen Bewehrungsstahl und dem im nachträglichen Verbund liegenden Spannglied. Ein indirekter Nachweis des Einflusses auf das Verbundverhalten des Spanngliedes erfolgte. Es wurde aufgezeigt, dass unter zweiaxialer Zugbelastung die Dehnungen im Spannstahl infolge der Längsrissbildung über dem Hüllrohr abnehmen. Dies lässt die Aussage zu, dass die Verbundwirkung des Spanngliedes durch eine orthogonal wirkende Zugbelastung negativ beeinflusst wird. Aufbauend auf den Versuchsergebnissen wird eine Empfehlung für den Einsatz von Dehnmessstreifen zur Bestimmung der Verformungen auf einbetonierten Betonstählen gegeben. Die Berechnung der Erstrisslasten aus den Bauteilversuchen mit den entwickelten Bruchkriterien hat eine sehr gute Übereinstimmung ergeben. / The tensile load-bearing characteristics of structural elements made of reinforced or non-reinforced concrete is influenced by a number of factors. Mainly it depends on the strength of the concrete, the interaction between the concrete and the rebar, and the state of stress in the concrete element. Traditionally the designing engineer examines uni-axial stress conditions under consideration of the material properties of the concrete based on standard tests. However, multiple-stress conditions apply for a number of application of such elements, e.g. in concrete slabs designed for bi-axial load bearing, in the joints of frames, in axial symmetrical constructions, or in the intersections of column and deck of multi-span bridges. The commonly used design standard recommends the increase of the compression strength of the concrete or the bond stress for cases of bi-axial load-bearing caused by compression. However, no recommendations are given for the design strength of a concrete under bi-axial tensile stress. Therefore it is interesting to know how a bi-axial tensile stress is influencing the load-bearing and deformation behaviour of structural elements made of reinforced or non-reinforced concrete. This has been investigated for two commonly used concretes (C20/25 and C40/50). Part of an earlier research programme was to perform trials on slabs made of reinforced and non-reinforced concrete. In result a model CEB-FIP MODELL CODE 90 was introduced to describe the deformation of the slab due to a uni-axial stress. However, the model does not satisfactory describe the real behaviour of the slab under a bi-axial tensile stress. In this dissertation a new model will be presented to describe the deformation behaviour of a Concrete C20/25 and a Concrete C40/50 under bi-axial tensile stress. Furthermore, criteria for the two concretes are introduced to describe the ultimate limit state under bi-axial tensile stress. It has been found the bi-axial tensile strength of a Concrete C20/25 is comparable to its uni-axial strength. In difference, the tensile strength of a Concrete C40/50 is decreased by 25% when subject to bi-axial stress. The ultimate limit stress due to bi-axial tensile stress decreases with increasing ratio of the stress 1 : 2. The Strains 1 and 2 are the strains as a result of the biaxial tensile forces in the main directions. The presented model to describe the strain-stress behaviour of an unreinforced concrete is found to agree well with the observations from the trials. Based on the results of this thesis it is possible to describe the strain-stress behaviour of concrete under bi-axial tensile stress. The stress-strain behaviour of structural elements has been investigated under bi-axial tensile stresses. Strains have been monitored with strain-gauges fixed to the surface of the concrete, to the rebars and to the post-tensioning tendons. Therefore, the influence to the interaction of tendon and concrete has been demonstrated indirectly. Furthermore, it has been shown the strain of the tendon decreases following the development of cracks along the grout tube due to the application of bi-axial tensile stress. It can be concluded the bound of the tendon is influenced adversely by tensile stresses applied in perpendicular direction. Recommendations are given for the application of strain-gauges to measure strains of rebars set in concrete. Based on these trials, the estimation of the critical stress to develop initial cracks has been found in good agreement to the presented criteria.