[pt] Valkó e Oligney propuseram um modelo que estima a evolução da fratura utilizando uma interpretação direta da curva de pressão de fundo medida durante uma operação de fracpack. Os únicos dados de entrada necessários para a aplicação do modelo são os registros geralmente disponíveis durante e após a operação. Considerando uma fratura de geometria radial e utilizando equações simples de fluxo e de geomecânica, o modelo obtém raios de empacotamento a partir da inclinação positiva da curva de pressão de fundo nos períodos de tip screenout. Nesta dissertação o modelo de Valkó e Oligney é aprimorado com a inclusão e o ajuste das equações de estado para o crescimento da fratura e para o processo de filtração, respectivamente. O modelo é também estendido para outras geometrias bidimensionais de fratura, PKN e KGD. A aplicação do modelo foi realizada utilizando os registros de pressão de oito operações de fracpack. Os resultados obtidos são a curva de propagação da fratura, o crescimento da abertura, a eficiência ao longo do tempo e a distribuição final do agente de sustentação na fratura. Para a validação desses resultados foram utilizados dois simuladores comerciais com modelos tridimensionais. Os estudos de caso indicaram que os ajustes realizados aproximaram os resultados do modelo aos obtidos nos simuladores comerciais. Além disso, a aplicação dos modelos desenvolvidos para cada geometria de fratura e a comparação com os resultados dos simuladores comerciais, permitiu confirmar a tendência esperada que, durante uma operação de fracpack, a geometria da fratura se aproxima da radial. / [en] Valkó and Oligney developed a model to estimate fracture evolution using a direct interpretation of the measured bottom hole pressure curve during a fracpack operation. The only input data needed to use the model are the usual records of the job, available during and after the operation. Considering radial fracture geometry and using simplified equations of flow and geomechanics, the model estimates a packing radius of the fracture using the slope of the increasing bottom hole pressure curve during the tip screenout period. In this work, Valkó and Oligney method is enhanced with the inclusion of state equations for the fracture growth and for the leakoff process in order to improve the model, but still maintaining minimum input data. The method is also extended to other two-dimensional fracture geometries, PKN and KGD. To apply the enhanced method, eight fracpack operation data were used. The results obtained are fracture propagation, width growth and fluid efficiency in time as well as the final proppant distribution in the fracture. To validate these results, this work used two commercial simulators with three-dimensional models. The case studies show that the modifications done to Valkó and Oligney method approximate the two-dimensional model results to the ones obtained using the commercial simulators with threedimensional models. Furthermore, the comparison between the application of the model for each fracture geometry and the commercial simulators results confirmed the expected tendency for the fracture geometry during a fracpack operation, which is a radial fracture.

Les évènements de convection profonde sont importants car ils forment les masses d'eau intermédiaires et profondes qui nourrissent la circulation globale. La mer du Labrador, qui forme la Labrador Sea Water (LSW), est le site le plus documenté de l'océan Atlantique Nord. La mer d'lrminger a également été citée mais n'est pas entièrement reconnue à cause du manque d'observations directes. Cette thèse fournit la première description de la convection profonde en mer d'lrminger à l'échelle du bassin grâce aux données Argo. Trois évènements de convection se sont produits en mer d'lrminger depuis 2010. Au cours de l'hiver 2O11-2O12, la convection atteint 1000 m et est expliquée par la séquence d'apparition des tip jets groenlandais.La convection de l'hiver 2O13-2O14, qui atteint 1300 m, est caractérisée par un préconditionnement particulièrement important et un forçage par les tip jets faible. La convection de l'hiver 2O14-2O15, qui atteint 1700 m, montre des tip jets très nombreux et persistants. L'advection de LSW provenant de la mer du Labrador explique les profondeurs exceptionnelles observées au cours de ces deux derniers hivers. Les résultats montrent que la convection n'est pas rare en mer d'lrminger et qu'elle joue un rôle non négligeable sur l'équilibre climatique. / The deep convection events are important because they form the intermediate and deep water masses feeding the global circulation. The Labrador Sea is the main site of deep convection in the North Atlantic Ocean and produces the intermediate Labrador Sea Water (LSW). The lrminger Sea was also cited but was forgotten during decades because of the lack of direct observations. This thesis provides the first description of the lrminger Sea deep convection at basin scale, thanks to the Argo data. Three convective events occurred in the lrminger Sea since 2010. During the 2011-2012 winter, the convection reached 1000 m and is explained by the sequence of the Greenland tip jets. The event of the 2O13-2O14 winter, reaching 130O m, is characterized by a strong preconditioning and a weak forcings by the Greenland tip jets.The convection event of the 2O14-2015 winter, reaching 1700 m, shows many of persistant tip jets. The advection of LSW from the Labrador Sea explains the deepest mixed layers observed during the last two winters. The results show that deep convection in the lrminger Sea is not a rare isolated event and plays a significant role on the climate balance.

L’écoulement de jeu dans les compresseurs axiaux est étudié à l’aide de la Zonal Detached Eddy Simulation (ZDES). L’objectif consiste à évaluer la capacité de méthodes hybrides URANS/LES à simuler l’écoulement de jeu d’un compresseur axial réaliste afin de mieux comprendre la physique de cet écoulement, notamment son comportement au vannage ainsi que l’effet de sillages venant du stator amont sur le rotor aval. Après avoir choisi la méthode hybride ZDES, un banc d’essai numérique est défini afin de simuler le premier rotor du compresseur de recherche CREATE. Ce banc a la particularité de pouvoir prendre en compte les effets instationnaires venant de la roue directrice d’entrée (RDE), notamment son sillage ainsi que les tourbillons générés en pied et en tête. Basé sur des critères de maillage ZDES, il est utilisé pour évaluer cette méthode comparativement aux méthodes classiques RANS et URANS. La ZDES est validée par étape jusqu’à une analyse spectrale de l’écoulement de jeu se basant sur des données expérimentales. Elle s’est révélée capable de capturer plus précisément l’intensité et la position des phénomènes instationnaires rencontrés en tête du rotor, notamment le tourbillon de jeu. Les densités spectrales de puissance analysées montrent que cela est dû en partie à une meilleure prise en compte du transfert d’énergie des grandes vers les petites structures de l’écoulement avant leur dissipation. De plus, l’écart entre les approches s’accentue lorsque le tourbillon de jeu traverse le choc en tête. Proche pompage, les effets d’interaction entre le choc, le tourbillon de jeu, la couche limite carter et le tourbillon venant de la tête de la RDE sont amplifiés. Le décollement de la couche limite carter s’accentue et une inversion locale de l’écoulement est observée. De plus, le tourbillon de jeu s’élargit et est dévié vers la pale adjacente, ce qui intensifie le phénomène de double écoulement de jeu. L’interaction du tourbillon venant de la tête de la RDE avec le choc et le tourbillon de jeu du rotor est ensuite étudiée au point de dessin. Un battement du tourbillon de jeu est rencontré lors de l’interaction de ce tourbillon avec le tourbillon de tête de la RDE, ce qui diminue le double écoulement de jeu. / The tip-leakage flow in axial compressors is studied with the Zonal Detached Eddy Simulation (ZDES). This study aims at evaluating the capability of hybrid URANS/LES methods to simulate the tip-leakage flow within a realistic axial compressor in order to better understand the involved physics, especially the behaviour of the flow near surge and the effects of stator wakes on the downstream rotor. Once the ZDES method is chosen, a numerical test bench is defined to simulate the first rotor of the research compressor CREATE. This bench takes into account the unsteady effects of the Inlet Guide Vane (IGV), such as its wake as well as vortices generated at the IGV hub and tip. It is based upon ZDES meshing criteria and is used to evaluate this method compared to classic RANS and URANS approaches. A method validation is carried out up to a spectral analysis compared to experimental data. The ZDES is capable to capture more accurately the intensity and position of the unsteady phenomena encountered in the tip region, especially the tip-leakage vortex. The power spectral densities highlight that this partly originates from a better capture of the energy transfer from large to small structures until their dissipation. The discrepancy between the methods is accentuated as the tip-leakage vortex crosses the shock. Near the surge line, the interactions between the shock, the tip-leakage vortex, the boundary layer developing on the shroud and the vortex generated by the IGV tip are amplified. The boundary layer on the shroud separates earlier and a local flow inversion occurs. Besides, the tip-leakage vortex widens and is deflected toward the adjacent blade. This strengthens the double leakage. At the design operating point, the interaction of the IGV tip vortex with the shock and the rotor tip vortex is studied. A vortex flutter is observed as the IGV tip vortex arrives on the rotor blade and stretches the rotor tip vortex. This phenomenon decreases the double leakage.

An understanding of crack tip fields and fracture mechanisms in single crystals can help in developing better polycrystalline alloys and manufacturing processes. To this end, the effects of loading rate, material inertia and strain rate sensitivity on crack tip fields and their influence on fracture mechanisms in FCC single crystals are examined in this work by performing finite element analysis. It is shown that, in the absence of inertial effects, high loading rates elevate the stresses ahead of a crack tip and decrease the plastic strains in rate dependent single crystals. Also, it is found that the quasi-static near-tip stress field can be adequately characterized by the energy release rate J and a constraint parameter Q. Similar two-parameter characterization is possible even under dynamic loading. It is observed that if a suitable reference solution is used, the role of inertia manifests as a loss of constraint with increasing loading rate irrespective of strain rate sensitivity and lattice orientation. Thus, at very high loading rates, inertial effects oppose the role of rate sensitivity and cause a decrease in stresses near the tip. The relative influence of these two factors depends on rate sensitivity index. For a mildly rate dependent single crystal, the predicted cleavage fracture toughness remains constant up to a certain loading rate and thereafter increases sharply. On the other hand, for a strongly rate dependent single crystal, fracture toughness drops initially up to a certain loading rate beyond which it increases marginally. The loss of crack tip constraint is found to retard the ductile fracture mechanisms of void growth and coalescence. However, this is dependent on lattice orientation. In-situ experimental observation of void growth near a notch tip also shows strong orientation dependence. In addition, 3D finite element results indicate though-thickness dependence of equivalent plastic slip and hydrostatic stress leading to variations in void growth along the thickness direction of the specimens. The predicted load-displacement curves, lattice rotation, slip traces and void growth using finite element analysis are found to be in good agreement with the experimental observations. Thus, the present study has provided an understanding of the role of several factors such as constraint level, rate sensitivity, material inertia, lattice orientation and 3D effects on the mechanics of fracture of ductile single crystals.

No description available.

The integrity of nearly all engineering structures are threatened by the presence of cracks. Structural failure occurs if a crack larger than a critical size exists. Although most well designed structures initially contain no critical cracks, subcritical cracks can grow to failure under fatigue loading, called fatigue crack growth (FCG). Because it is impossible or impractical to prevent subcritical crack growth in most applications, a damage tolerant design philosophy was developed for crack sensitive structures. Design engineers have taken advantage of the FCG threshold concept to design for long fatigue lives. FCG threshold (DKth) is a value of DK (crack-tip loading), below which no significant FCG occurs. Cracks are tolerated if DK is less than DKth. However, FCG threshold is not constant. Many variables influence DKth including microstructure, environment, and load ratio. The current research focuses on load ratio effects on DKth and threshold FCG. Two categories of load ratio effects are studied here: extrinsic and intrinsic. Extrinsic load ratio effects operate in the crack wake and include fatigue crack closure mechanisms. Intrinsic load ratio effects operate in the crack-tip process zone and include microcracking and void production. To gain a better understanding of threshold FCG load ratio effects (1) a fatigue crack closure model is developed to consider the most likely closure mechanisms at threshold, simultaneously, and (2) intrinsic load ratio mechanisms are identified and modeled. An analytical fatigue crack closure model is developed that includes the three closure mechanisms considered most important at threshold (PICC, RICC, and OICC). Crack meandering and a limited amount of mixed-mode loading are also considered. The rough crack geometry, approximated as a two-dimensional sawtooth wave, results in a mixed-mode crack-tip stress state. Dislocation and continuum mechanics concepts are used to determine mixed-mode crack face displacements. Plasticity induced crack closure is included by modifying an existing analytical model, and an oxide layer in the crack mouth is modeled as a uniform layer. Finite element results were used to verify the analytical solutions for crack-tip stress intensity factor and crack face displacements. These results indicate that closure for rough cracks can occur at two locations: (1) at the crack-tip, and (2) at the asperity nearest the crack-tip. Both tip contact and asperity contact must be considered for rough cracks. Tip contact is more likely for high Kmax levels, thick oxide layers, and shallow asperity angles, a. Model results indicate that closure mechanisms combine in a synergistic manner. That is, when multiple closure mechanisms are active, the total closure level is greater than the sum of individual mechanisms acting alone. To better understand fatigue crack closure where multiple closure mechanisms are active (i.e. FCG threshold), these interactions must be considered. Model results are well supported by experimental data over a wide range of DK, including FCG threshold. Closure-free load ratio effects were studied for aluminum alloys 2024, 7050, and 8009. Alloys 7050 and 8009 were selected because load ratio effects at FCG threshold are not entirely explained by fatigue crack closure. It is believed that closure-free load ratio mechanisms occur in these alloys. Aluminum alloy 2024 was selected for study because it is relatively well behaved, meandering most load ratio effects are explained by fatigue crack closure. A series of constant Kmax threshold tests on aluminum alloys were conducted to eliminate fatigue crack closure at threshold. Even in the absence of fatigue crack closure load ratio (Kmax) effects persist, and are correlated with increased crack-tip damage (i.e. voids) seen on the fatigue crack surfaces. Accelerated FCG was observed during constant Kmax threshold testing of 8009 aluminum. A distinct transition is seen the FCG data and is correlated with a dramatic increase in void production seen along the crack faces. Void production in 8009 aluminum is limited to the specimen interior (plane-strain conditions), promoting crack tunneling. At higher values of Kmax (+_ 22.0 MPaà m), where plane-stress conditions dominate, a transition to slant cracking occurs at threshold. The transition to slant cracking produces an apparent increase in FCG rate with decreasing DK. This unstable threshold behavior is related to constraint conditions. Finally, a model is developed to predict the accelerated FCG rates, at higher Kmax levels, in terms of crack-tip damage. The effect of humidity (in laboratory air) on threshold FCG was studied to ensure that environmental effects at threshold were separated from load ratio effects. Although changes in humidity were shown to strongly affect threshold FCG rates, this influence was small for ambient humidity levels (relative humidity between 30% and 70%). Transient FCG behavior, following an abrupt change in humidity level, indicated environmental damage accumulated in the crack-tip monotonic plastic zone. Previous research implies that hydrogen (a component of water vapor) is the likely cause of this environmental damage. Analysis suggests that bulk diffusion is not a likely hydrogen transport mechanism in the crack-tip monotonic plastic zone. Rather, dislocation-assisted diffusion is presented as the likely hydrogen transport mechanism. Finally, the (extrinsic) fatigue crack closure model and the (intrinsic) crack-tip damage model are put in the context of a comprehensive threshold model. The ultimate goal of the comprehensive threshold model is to predict fatigue lives of cyclically loaded engineering components from (small) crack nucleation, through FCG, and including failure. The models developed in this dissertation provide a basis for a more complete evaluation of threshold FCG and fatigue life prediction. The research described in this dissertation was performed at NASA-Langley Research Center in Hampton, Virginia. Funding was provided through the NASA GSRP program (Graduate Student Researcher Program, grant number NGT-1-52174). / Ph. D.

<p dir="ltr">Gas turbines are widely used to provide propulsion, electrical-power, and mechanical power. Though tremendous advances have been made since Frank Whittle’s patent of a turbojet in 1930 and Hans von Ohain’s patent of the first operational turbojet in 1936, industry still has aggressive goals on improvements in efficiency and service life. One area where further advances are needed is better control of the flow across the gap between the blade tip and the shroud, referred to as tip-leakage flow (TLF). This is because TLF accounts for up to one-third of the aerodynamic losses in a turbine stage.</p><p dir="ltr">In this study, hybrid LES-RANS based on IDDES and steady RANS based on the SST turbulence model were used to study the compressible flow in a 1.5-stage turbine with geometry and operating conditions that are relevant to power-generation gas turbines. The focus is on the flow in the tip-gap region that account for the flow features created by the upstream stator vanes, stator-rotor interactions, and downstream stator vanes. Results obtained reveal the flow structures about the tip-gap region and the flow mechanisms that create them. Results obtained also show where steady RANS with mixing plane could predict correctly when compared with results from IDDES that resolve the unsteadiness of the turbulence and the motion of the rotor blades passing the stator vanes. Turbulent statistics from the IDDES were generated to guide the development of better RANS models. Results were also obtained by using RANS to examine the effects of blade loading, where mass flow rate through the 1.5 stage turbine was varied with the rotor’s rotational speed fixed at 3,600 RPM – the speed at which power-generation gas turbines operate in the U.S.</p><p dir="ltr">Key findings are as follows: In the first-stage stator, horseshoe, passage, and corner vortices were found to be confined within 10 to 15% span from the hub and shroud, and both steady RANS and IDDES generated similar results. Steady RANS and IDDES, however, differed considerably in how they predicted the wake downstream of the vane’s trailing edge. This coupled with the use of mixing plane, steady RANS was unable to account for effects of stator-rotor interactions and their effects on the tip-leakage flow. In the rotor, steady RANS predicted passage vortices that extended up to 50% span from the hub and 25% span from the shroud. The flow through the tip gap was found to induce a separation bubble on the blade tip and one large and two small vortical structures on the suction side of the blade and a vortical structure next to the shroud. These structures were found to grow along the axial chord of the blade. Steady RANS also predicted the large tip leakage vortex that contained the fluid from the tip-leakage flow to breakdown. IDDES did not predict the vortex breakdown because all of the coherent vortical structures identified including the separated region on the blade tip were unsteady and constantly shedding. As a result, IDDES predicted much smaller mean passage vortices – albeit the instantaneous structures were nearly as large as those predicted by steady RANS.</p>

Low-temperature magnetic force microscopy was used to study the phase diagram of a La1/3Pr1/3Ca1/3MnO3 thin film grown on a (110) NdGaO3 (NGO) substrate by pulsed laser deposition. Traditionally, one can observe the phase change at the nanoscale level as the sample is cooled from room temperature through the transition temperature to liquid nitrogen temperatures, but in this case a fixed voltage ranging from 0 V to 31 V was applied before each cooling cycle. From in and ex situ transport measurements, it is observed that the temperature of the peak of the transition increases with applied field; however, the MFM images show that the magnetic transition begins at a lower temperature with the same increase in field. Thus, this dissertation shows that a new voltage control exists for the phase transition in certain manganites. / text

The power plant and aerospace industries have been facing a huge loss, due to structural failure. The structural failure occurs due to the presence of the crack in it. Hence, it becomes necessary to monitor the structural health under operating condition. Most of the techniques, for structural health monitoring are used for a specific purpose. Some of these techniques require structure dismantling, which is very much expensive and time consuming. So the vibration based structural health monitoring is advantageous, compared to other techniques. Most of the vibration based Structural Health Monitoring (SHM) approaches, use linear vibration theory. But, these linear vibration based procedures, have inherently low sensitivity to crack. Since crack introduces nonlinearities in the system, their merits in damage detection need to be investigated for SHM. In this thesis, the problem is focused on studying nonlinear dynamics of cracked structures for Structural Health Monitoring. For this, simulations and experiments are performed. The new procedure for the simulation is developed using Matlab-Simulink. It uses the numerical approximation for dynamic compliance operators and a nonlinear model of cracks contact faces interaction to study the dynamic behaviour of the cracked bar. Furthermore, the finite element model of the cracked cantilever bar with crack- tip plasticity is developed and the dynamic behaviour of the elasto-plastic bar is studied. Additionally, numerous experiments are performed to study the dynamics of cantilever bar with the fatigue crack in it. The results from Matlab-Simulink simulation shows the distribution of higher harmonics generated along the bar length, as a function of distance from the crack. In finite element simulation, comparison is made between the resonance frequency of cracked cantilever bar with and without crack-tip plasticity. It is found that, there is decrease in resonance frequency of the cracked bar with cracked tip plasticity, when compared with the resonance frequency of cracked bar without crack-tip plasticity. This reduction in resonance frequency is due to the crack-induced plasticity near the crack tip which affects the overall stiffness of bar. In experiments, the response is measured at four different points on the cracked cantilever bar at a given resonant frequency of excitation at lower and higher vibration amplitude. For lower vibration amplitude, it is found that the response obtained near the vicinity of the crack shows the presence of higher harmonics of resonant frequency, which disappears in the response obtained far away from the crack. For higher vibration amplitude, it is found that the response obtained near the vicinity of the crack shows the presence of higher harmonics along with the low frequency component. This low frequency component causes modulation, which leads to the generation of side band frequencies near the resonant frequency. The occurrence of low frequency component and side band frequencies is due to the vibro-impact behaviour of crack. The amplitude of these side band frequencies and higher harmonics are reduced in the response obtained far away from the crack. This indicates that crack-induced nonlinearity has a localized effect on the dynamics of bar. It is also observed that the magnitude of low frequency component is proportional to the magnitude of resonant frequency of excitation. This indicates that crack behaves like a signal modulator, detector of low frequency component and amplifier as the magnitude of low frequency component is proportional to the magnitude of resonant frequency excitation. From the Matlab-Simulink simulation and experimental results, it is concluded that crackinduced nonlinearity affects the dynamic behaviour of the cracked bar significantly, which will be effective in structural health monitoring. Keywords: vibro-impact, crack, dynamic compliance, harmonics, modulator, detector, amplifier, crack-tip plasticity, resonance frequency, structural health monitoring.

The telescoping cantilever beam structure is applied in many different engineering sectors to achieve weight/space optimisation for structural integrity. There has been limited theory and analysis in the public domain of the stresses and deflections involved when applying a load to such a structure. This thesis proposes (a) The Tip Reaction Model, which adapts classical mechanics to predict deflection of a two and a three section steel telescoping cantilever beam; (b) An equation to determine the Critical buckling loads for a given configuration of the two section steel telescoping cantilever beam assembly derived from first principles, in particular the energy methods; and finally (c) the derivation of a design optimization methodology, to tackle localised buckling induced by shear, torsion and a combination of both, in the individual, constituent, hollow rectangular beam sections of the telescopic assembly. Bending stress and shear stress is numerically calculated for the same structure whilst subjected to inline and offset loading. An FEA model of the structure is solved to verify the previous deflection, stress and buckling predictions made numerically. Finally an experimental setup is conducted where deflections and stresses are measured whilst a two section assembly is subjected to various loading and boundary conditions. The results between the predicted theory, FEA and experimental setup are compared and discussed. The overall conclusion is that there is good correlation between the three sets of data.