Dissertations / Theses on the topic 'Dimensional stability'
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1
Keskin, Suleyman Bahadir. "Dimensional Stability Of Engineered Cementitiouscomposites." Phd thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614713/index.pdf.
Full textAbstract:
Cementitious materials with strain-hardening property and high tensile ductility are
promising materials on account of their mechanical and durability performances.
These materials require special ingredients which make it costly to be used in
conventional constructions. Hence, potential applications of Engineered
Cementitious Composites (ECC) generally focus on layered systems or repairs
which require the use of ECC together with another material. For it to be used
especially as a repair material, it should have sufficient dimensional compatibility
for preventing restrained shrinkage cracking. In this thesis, a strain-hardening fiberreinforced
cementitious composite, named Engineered Cementitious Composites,
was produced with local ingredients and their mechanical performance, dimensional
stability properties were investigated.
For investigating the effect of materials and mix proportions on mechanical
properties, compressive strength, flexural strength with mid-span beam deflections
and matrix fracture toughness tests were conducted. For determining the dimensional compatibility properties, autogenous, drying and restrained shrinkage
tests were conducted along with tensile creep tests. As a result it was shown that,
mechanical and dimensional stability properties are affected by the ingredients and
mix proportions. It was shown that especially autogenous shrinkage of mixtures was
relatively high which can cause early age cracking. In order to mitigate the adverse
effect of autogenous shrinkage, the effect of pre-soaked expanded perlite aggregate
replacement on mechanical, shrinkage and dimensional compatibility properties
was investigated. As a result it was found out that autogenous shrinkage can be
mitigated by the use of pre-soaked expanded perlite aggregate replacement.
2
Raslan, M. S. "Three-dimensional stability of steel frameworks." Thesis, University of Southampton, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379317.
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3
Wang, Mengjie. "Stability analysis of three-dimensional boundary layers with parabolized stability equations /." The Ohio State University, 1994. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487858106119026.
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4
Nanri, Yasunori. "Dimensional stability of superheated steam dried paper." Thesis, McGill University, 1995. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=29768.
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The dimensional stability of superheated steam dried paper made from groundwood, thermomechanical, chemithermomechanical and bleached kraft pulps were investigated in terms of hygroexpansivity, drying shrinkage and wet expansion. For all the pulps tested these characteristics of dimensional stability were found to be improved by high temperature superheated steam drying.The hygroexpansion coefficient, the dimensional change relative to paper moisture content, was reduced about 15% by superheated steam drying at 320°C, compared to paper dried in low temperature air. Irreversible shrinkage was reduced by up to 30% for sheets dried in superheated steam. However, these effects on hygroexpansivity were caused mainly by the high drying temperature and the resulting high drying rate regardless of the drying fluid. Drying shrinkage and wet expansion as well were reduced by about 10% and 15% respectively for paper dried in superheated steam.
In superheated steam drying the elevated web temperature at the start of drying is believed to affect paper properties through thermal softening of the lignin and relaxation of drying and wet-pressing stresses. Increased fiber bonding with softened lignin accounts for both the increased elastic modulus and decreased light scattering coefficient observed. These were obtained only for sheets from mechanical pulps with initial moisture content of about 0.4 kg-water/kg-fiber or more, consistent with the mechanism of thermal softening of plasticized lignin. High sheet temperature in the falling rate period improves hygroscopicity of paper through reducing its equilibrium moisture content.
Superheated steam drying improves the dimensional stability of paper from such pulps, including less anisotropy, thereby enhancing paper quality.
5
Navaranjan, N. "Modelling warp and dimensional stability in softwood." Thesis, University of Canterbury. Mechanical Engineering, 2002. http://hdl.handle.net/10092/7854.
Full textAbstract:
Seasoned softwood board exposed to in-service moisture variations is subject to dimensional changes and distortions. In-service dimensional instability of a board is a critical issue since it causes serious problems in buildings and other engineering structures.
This thesis presents research involving the development of numerical models to describe the in-service changes in dimension, at a board scale, for small variations in the moisture content. The models used input data defining the properties of the microstructural components that make up the cellular structure of the wood resulting in the cells having heterogeneous anisotropic properties. The application of full cyclic constraint to the boundaries of the elements that made up the representative volume element was investigated and used to determine the equivalent homogenised properties. A cellular model was developed to describe the behaviour of the wood as a function of density, microfibril angle, spiral grain angle and moisture content. The resultant finite element board models used a database generated from the cellular model to describe the dimensional changes associated with small in-service changes in moisture content.
In conjunction with the development of this model the candidate carried out an experimental programme using a specific supplied pinus radiata log. The determination of the cellular shrinkage/swelling coefficients, using an experimental technique developed for this research, verified the accuracy of the homogenization method and its subsequent application to the cellular model. The results from the sawn boards were compared with the results from the finite element board models.
6
OLIVEIRA, DENISE DE. "WEAK STABILITY FOR INFINITE DIMENSIONAL LINEAR SYSTEMS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1990. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=9384@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOO objetivo deste trabalho é o estudo das condições para a estabilidade de sistemas lineares discretos de dimensão infinita invariantes no tempo, evoluindo em um espaço de Hilbert. Apresentaremos uma vasta coleção de resultados sobre estabilidade assintótica uniforme, incluindo uma condição espectral equivalente. Em relação à estabilidade assintótica fraca, analisaremos tanto a dificuldade de se estabelecer uma condição necessária e suficiente sobre o espectro do operador, como também sua relação com similaridade a contração. Por último, apresentaremos alguns resultados disponíveis sobre estabilidade assintótica forte para algumas classes específicas de operadores.
The purpose of this work is to analyse stability conditions for infinity-dimensional linear discrete systems operating in a Hilbert space. Whe shall present a wide collections of results on uniform asymptotic stability, incluiding an equivalent spectral condition. Concerning the weak asymptotic stability, we shall analyse the dificulty associated to the problem of attempting to establish a necessary and sufficient condition involving the spectral of the system operator. The relation between weak asymptotic stability and similarity to a contraction will be analysed as well. Finally, we shall present some of the available results concerning strong asymptotic stability for particular classes of operators.
7
Punyamurthula, Sujan. "Three-dimensional analysis of waste impoundment stability /." Thesis, Connect to this title online; UW restricted, 1996. http://hdl.handle.net/1773/10210.
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8
Lagnado, Ronald Robert Leal L. Gary. "The stability of two-dimensional linear flows /." Diss., Pasadena, Calif. : California Institute of Technology, 1985. http://resolver.caltech.edu/CaltechETD:etd-03272008-105253.
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9
Tempelmann, David. "Stability and Receptivity of Three-Dimensional Boundary Layers." Licentiate thesis, KTH, Mechanics, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11579.
Full textAbstract:
The stability and the receptivity of three-dimensional flat plate boundary layers is studied employing parabolised stability equations. These allow for computationally efficient parametric studies. Two different sets of equations are used. The stability of modal disturbances in the form of crossflow vortices is studied by means of the well-known classical parabolised stability equations (PSE). A new method is developed which is applicable to more general vortical-type disturbances. It is based on a modified version of the classical PSE and describes both modal and non-modal growth in three-dimensional boundary layers. This modified PSE approach is used in conjunction with a Lagrange multiplier technique to compute spatial optimal disturbances in three-dimensional boundary layers. These take the form of streamwise oriented tilted vortices initially and develop into streaks further downstream. When entering the domain where modal disturbances become unstable optimal disturbances smoothly evolve into crossflow modes. It is found that non-modal growth is of significant magnitude in three-dimensional boundary layers. Both the lift-up and the Orr mechanism are identified as the physical mechanisms behind non-modal growth. Furthermore, the modified PSE are used to determine the response of three-dimensional boundary layers to vortical free-stream disturbances. By comparing to results from direct numerical simulations it is shown that the response, including initial transient behaviour, is described very accurately. Extensive parametric studies are performed where effects of free-stream turbulence are modelled by filtering with an energy spectrum characteristic for homogeneous isotropic turbulence. It is found that a quantitative prediction of the boundary layer response to free-stream turbulence requires detailed information about the incoming turbulent flow field. Finally, the adjoint of the classical PSE is used to determine the receptivity of modal disturbances with respect to localised surface roughness. It is shown that the adjoint approach yields perfect agreement with results from Finite-Reynold-Number Theory (FRNT) if the boundary layer is assumed to be locally parallel. Receptivity is attenuated if nonlocal and non-parallel effects are accounted for. Comparisons to direct numerical simulations and extended parametric studies are presented.
10
Hosseini, Seyed Mohammad. "Stability and transition of three-dimensional boundary layers." Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-123175.
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A focus has been put on the stability characteristics of different flow types existing on air vehicles. Flow passing over wings and different junctions on an aircraft face numerous local features, ranging from different pressure gradients, to interacting boundary layers. Primarily, stability characteristics of flow over a wing subject to negative pressure gradient is studied. The current numerical study conforms to an experimental study conducted by Saric and coworkers, in their Arizona State University wind tunnel experiments. Within that framework, a passive control mechanism has been tested to delay transition of flow from laminar to turbulence. The same control approach has been studied here, in addition to underling mechanisms playing major roles in flow transition, such as nonlinear effects and secondary instabilities. Another common three-dimensional flow feature arises as a result of streamlines passing through a junction, the so called corner-flow. For instance, this flow can be formed in the junction between the wing and fuselage on a plane. A series of direct numerical simulations using linear Navier-Stokes equations have been performed to determine the optimal initial perturbation. Optimal refers to a perturbation which can gain the maximum energy from the flow over a period of time. Power iterations between direct and adjoint Navier- Stokes equations determine the optimal initial perturbation. In other words this method seeks to determine the worst case scenario in terms of perturbation growth. Determining the optimal initial condition can help improve the design of such surfaces in addition to possible control mechanisms.QC 20130604
RECEPT
11
Sainak, Ala Naeem. "Three dimensional finite element analysis of slope stability." Thesis, Oxford Brookes University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320546.
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12
Flanagan, Ronan. "The dimensional stability of composite laminates and structures." Thesis, Queen's University Belfast, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263396.
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13
Murata, Keiju. "Stability Analysis of Higher Dimensional Rotating Black Holes." 京都大学 (Kyoto University), 2010. http://hdl.handle.net/2433/120656.
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14
Monteiro, Ricardo. "Classical and thermodynamic stability of black holes." Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/227571.
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We consider the stability of black holes within both classical general relativity and the semiclassical thermodynamic description. In particular, we study linearised perturbations and their contribution to the gravitational partition function, addressing technical issues for charged (Reissner-Nordstrom) and rotating (Kerr-AdS) black holes. Exploring the connection between classical and thermodynamic stability, we find classical instabilities of Myers-Perry black holes and bifurcations to new black hole families.
15
Loiseau, Jean-Christophe. "Dynamics and global stability analysis of three-dimensional flows." Thesis, Paris, ENSAM, 2014. http://www.theses.fr/2014ENAM0016/document.
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Comprendre, prédire et finalement retarder la transition vers la turbulence dans les écoulements sont d'importants problèmes posés aux scientifiques depuis les travaux pionniers d'Osborne Reynolds en 1883. Ces questions ont été principalement adressées à l'aide de la théorie des instabilités hydrodynamiques. A cause des ressources informatiques limitées, les analyses de stabilité linéaire reposent essentiellement sur d'importantes hypothèses simplificatrices telles que celle d'un écoulement parallèle. Dans ce cadre, connu sous le nom de stabilité locale, seule la stabilité d'écoulement ayant un fort intérêt académique mais relativement peu d'applications pratiques a pu être étudiée. Néanmoins, au cours de la décennie passée, l'hypothèse d'écoulement parallèle a été relaxée au profit de celle d'un écoulement bidimensionnel conduisant alors à ce que l'on appelle la stabilité globale. Ce nouveau cadre permet alors d'étudier les mécanismes d'instabilité et de transition ayant lieu au sein d'écoulements plus réalistes. Plus particulièrement, la stabilité d'écoulements fortement non-parallèles pouvant présenter des décollements massifs, une caractéristique fréquente dans les écoulements d'intérêt industriel, peut maintenant être étudiée. De plus, avec l'accroissement constant des moyens de calcul et le développement de nouveaux algorithmes de recherche de valeurs propres itératifs, il est aujourd'hui possible d'étudier la stabilité d'écoulements pleinement tridimensionnels pour lesquels aucune hypothèse simplificatrice n'est alors nécessaire. Dans la continuité des travaux présentés par Bagheri et al. en 2008, le but de la présente thèse est de développer les outils nécessaires à l'analyse de la stabilité d'écoulements 3D. Trois écoulements ont été choisis afin d'illustrer les nouvelles capacités de compréhension apportées par l'analyse de la stabilité globale appliquée à des écoulements tridimensionnels réels : i) l'écoulement au sein d'une cavité entraînée 3D, ii) l'écoulement se développant dans un tuyau sténosé, et enfin iii) l'écoulement de couche limite se développant au passage d'une rugosité cylindrique montée sur une plaque plane. Chacun de ces écoulements a différentes applications pratiques allant d'un intérêt purement académique à une application biomédicale et aérodynamique. Ce choix d'écoulements nous permet également d'illustrer les différents aspects des outils développés au cours de cette thèse ainsi que les limitations qui leur sont inhérentesUnderstanding, predicting and eventually delaying transition to turbulence in fluid flows have been challenging issues for scientists ever since the pioneering work of Osborne Reynolds in 1883. These problems have mostly been addressed using the hydrodynamic linear stability theory. Yet, due to limited computational resources, linear stability analyses have essentially relied until recently on strong simplification hypotheses such as the “parallel flow” assumption. In this framework, known as “local stability theory”, only the stability of flows with strong academic interest but limited practical applications can be investigated. However, over the course of the past decade, simplification hypotheses have been relaxed from the “parallel flow” assumption to a two-dimensionality assumption of the flow resulting in what is now known as the “global stability theory”. This new framework allows one to investigate the instability and transition mechanisms taking place in more realistic flows. More particularly, the stability of strongly non-parallel flows exhibiting separation, a common feature of numerous flows of practical interest, can now be studied. Moreover, with the continuous increase of computational power available and the development of new iterative eigenvalue algorithms, investigating the global stability of fully three-dimensional flows, for which no simplification hypothesis is necessary, is now feasible. Following the work presented in 2008 by Bagheri et al., the aim of the present thesis is thus to develop the tools mandatory to investigate the stability of 3D flows. Three flow configurations have been chosen to illustrate the new investigation capabilities brought by global stability theory when it is applied to realistic three-dimensional flows: i) the flow within a cuboid lid-driven cavity, ii) the flow within an asymmetric stenotic pipe and iii) the boundary layer flow developing over a cylindrical roughness element mounted on a flat plate. Each of these flows have different practical applications ranging from purely academic interests to biomedical and aerodynamical applications. They also allow us to put in the limelight different aspects and possible limitations of the various tools developed during this PhD thesis
16
Nichols, Paul Vincent. "An Investigation of the Dimensional Stability of Dental Alginates." Thesis, The University of Sydney, 2006. http://hdl.handle.net/2123/1270.
Full textAbstract:
Dimensional stability was defined by Nicholls (1977) as “the ability (of a material) to maintain accuracy over time”, and the result of loss of accuracy, “distortion”, as “the relative movement of a single point, or group of points, away from some originally specified reference position such that permanent deformation is apparent”. Maintaining dimensional stability of dental impression materials is vital if the impression cannot be cast (in stone) soon after removal from the mouth. Dental irreversible hydrocolloid (alginate) is a major dental impression material used worldwide in many clinical procedures. However, alginate is dimensionally unstable and changes its dimensions (suffers “distortion”) after removal from the mouth. With storage times of more than ten minutes, alginate begins to distort, and after one to three hours (depending on the product and storage conditions) cannot be used for many clinical purposes, especially fixed prosthodontics such as crowns and bridges (Hampson 1955, Skinner & Hoblit 1956, Wilson & Smith 1963, Rudd et al. 1969, Miller 1975, Inohara 1977, Schoen et al. 1978, Coleman et al. 1979, Linke et al. 1985, Habu et al. 1986, Peutfeldt & Asmussen 1989, Mathilde & Peters 1992, Khan & Aziz Sahu 1995, Eriksson et al. 1998, Schleier et al. 2001, and Donovan & Chee 2004). This loss of accuracy, due to dimensional instability, manifests as a time-dependent distortion of the poured stone cast, and thus any prosthesis fabricated will not fit in the mouth. With the introduction of the more stable elastomers in the 1950s (Stackhouse 1970, Glenner 1997, Brown 2003) that could be stored for days if necessary, without loss of accuracy, the alginates fell out of favour for fixed prosthodontics. Recently, there has been a resurgence of interest in alginate for use in dental procedures where dimensional stability is critical (Peutzfeldt and Asmussen 1989, Eriksson et al. 1998). This in part is due to the favourable properties of alginate not found in the elastomers. Of greatest significance is that alginate hydrocolloid is hydrophilic, whereas elastomers are hydrophobic (Phillips & Ito 1958, Glenner 2004). Thus, alginate materials are able to reproduce wet oral areas with greater precision and to produce a superior "fit" of, say, a gold casting produced by the Lost Wax technique (Skinner and Phillips 1982). A number of reports have been published which investigate newer alginate materials that are claimed to be more dimensionally stable than older formulations. Puetzfeldt and Asmussen (1989) found that a newer alginate , if stored at 100% relative humidity, retained accuracy over 24 hours that was equivalent to that of the elastomers. More recently, the manufacturer of another alginate has claimed equivalent dimensional stability to the elastomers for up to 100 hours, and, whilst this claim has not been reported on in the literature, the present thesis will show that, under favourable conditions of storage, the material maintained clinically useful accuracy for up to 100 hours. Another approach to improving the accuracy of alginate impressions has been to combine reversible hydrocolloid with alginate (the “Bilaminar” technique). Frederick and Caputo (1997) confirmed that the new agar reversible hydrocolloids are just as accurate (at the time of removal from the mouth) as the new elastomers. Mathilde et al. (1992) and Eriksson et al. (1998) have shown that several of the “bilaminar” impression techniques for fixed prosthodontics, where alginate is used as a tray material supporting a reversible hydrocolloid (agar) wash, are as accurate and dimensionally stable as elastomers for up to three hours. However, these studies are difficult to interpret due to lack of uniformity in the testing methods, and the fact that there is no regulatory standard available to measure dimensional stability for dental alginates. The International Standard (IS) for alginate impression materials (ISO 1563:1990E) contains no specification for dimensional stability, and thus places no requirement for manufacturers to state dimensional stability properties on their labels. In contrast, ISO 4823:1992(E) specifies the IS for elastomeric dental impression materials, and it does specify a requirement for dimensional stability (less than 1.5% distortion after 24 hours). Further, the IS sets a method for determination of dimensional stability. Briefly, this method (the Optical Method) uses a travelling optical microscope to measure the accuracy of the distance between score lines on an impression of a test grid, at various time periods. The American Dental Association Specification No. 19 for dental elastomeric impression materials is identical to the IS. There is currently no specific Australian Standard (AS) for the dimensional stability of any dental impression material. Overview of Experimental Methods A. The Optical Method The aim of Part A of this investigation was to: 1. Adapt the Optical Method of the IS for elastomers to be reproducible for dental alginates. This was achieved by using a perforated test tray (to simulate clinical conditions), and measuring the grid pattern on a dental stone button after casting the test impression, rather than direct measurement of the impression, as for the IS. 2. To measure and rank the dimensional stability of a number of locally available dental alginates. Measurements of the test stone buttons proved reproducible, and the results were different for each sample, allowing them to be ranked according to dimensional stability after 50 and 100 hours of storage. The results show that the traditional optical method for measuring dimensional stability, as specified in the IS for dental elastomers, can be adapted to measure the dimensional stability of dental alginates However, the Optical Method of measuring dimensional stability of dental alginates is cumbersome and time-consuming. It was hypothesised that dimensional stability of dental alginates could be measured more conveniently by finding a thermal property that is directly proportional to dimensional stability. This method could be useful for the rapid determination of relative performance, and allow comparison with a determined benchmark. B. The Thermal Method Recently, modern methods of Thermal Analysis, Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) have been used to rapidly age various polymers, including food alginates (Chinachoti 1996), in order to measure thermal stability. This thesis shows that thermal stability is an indicator of dimensional stability. The aim of Part B of this investigation was therefore to adapt thermal analysis techniques to dental alginates, and develop a method to measure their thermal stability. These results were then compared with those for dimensional stability measured by the Optical Method to determine the relationship between thermal and dimensional stability for dental alginates. The results show that current thermal analysis methods of TGA and DSC can be adapted to measure relative dental alginate dimensional stability, and are both rapid and convenient. This study also provides evidence that commercial products differ as regards the property of dimensional stability, and can be ranked accordingly. C. Practical Application of the Methods The aim of part C of this thesis was to validate the methods (both optical and thermal) developed in this study by using them to investigate the effect of varying the water/powder ratio on the dimensional stability of dental alginates. It was shown that dimensional stability is affected by changes to the recommended water/powder ratio, that both the methods detected and measured the changes, and that the results were proportional, in that any percentage change detected by the optical method, was mirrored by the thermal method, confirming that the more convenient thermal methods can be used to measure dimensional stability.
17
Nichols, Paul Vincent. "An Investigation of the Dimensional Stability of Dental Alginates." University of Sydney, 2006. http://hdl.handle.net/2123/1270.
Full textAbstract:
Master of Science (Dentistry)Dimensional stability was defined by Nicholls (1977) as “the ability (of a material) to maintain accuracy over time”, and the result of loss of accuracy, “distortion”, as “the relative movement of a single point, or group of points, away from some originally specified reference position such that permanent deformation is apparent”. Maintaining dimensional stability of dental impression materials is vital if the impression cannot be cast (in stone) soon after removal from the mouth. Dental irreversible hydrocolloid (alginate) is a major dental impression material used worldwide in many clinical procedures. However, alginate is dimensionally unstable and changes its dimensions (suffers “distortion”) after removal from the mouth. With storage times of more than ten minutes, alginate begins to distort, and after one to three hours (depending on the product and storage conditions) cannot be used for many clinical purposes, especially fixed prosthodontics such as crowns and bridges (Hampson 1955, Skinner & Hoblit 1956, Wilson & Smith 1963, Rudd et al. 1969, Miller 1975, Inohara 1977, Schoen et al. 1978, Coleman et al. 1979, Linke et al. 1985, Habu et al. 1986, Peutfeldt & Asmussen 1989, Mathilde & Peters 1992, Khan & Aziz Sahu 1995, Eriksson et al. 1998, Schleier et al. 2001, and Donovan & Chee 2004). This loss of accuracy, due to dimensional instability, manifests as a time-dependent distortion of the poured stone cast, and thus any prosthesis fabricated will not fit in the mouth. With the introduction of the more stable elastomers in the 1950s (Stackhouse 1970, Glenner 1997, Brown 2003) that could be stored for days if necessary, without loss of accuracy, the alginates fell out of favour for fixed prosthodontics. Recently, there has been a resurgence of interest in alginate for use in dental procedures where dimensional stability is critical (Peutzfeldt and Asmussen 1989, Eriksson et al. 1998). This in part is due to the favourable properties of alginate not found in the elastomers. Of greatest significance is that alginate hydrocolloid is hydrophilic, whereas elastomers are hydrophobic (Phillips & Ito 1958, Glenner 2004). Thus, alginate materials are able to reproduce wet oral areas with greater precision and to produce a superior "fit" of, say, a gold casting produced by the Lost Wax technique (Skinner and Phillips 1982). A number of reports have been published which investigate newer alginate materials that are claimed to be more dimensionally stable than older formulations. Puetzfeldt and Asmussen (1989) found that a newer alginate , if stored at 100% relative humidity, retained accuracy over 24 hours that was equivalent to that of the elastomers. More recently, the manufacturer of another alginate has claimed equivalent dimensional stability to the elastomers for up to 100 hours, and, whilst this claim has not been reported on in the literature, the present thesis will show that, under favourable conditions of storage, the material maintained clinically useful accuracy for up to 100 hours. Another approach to improving the accuracy of alginate impressions has been to combine reversible hydrocolloid with alginate (the “Bilaminar” technique). Frederick and Caputo (1997) confirmed that the new agar reversible hydrocolloids are just as accurate (at the time of removal from the mouth) as the new elastomers. Mathilde et al. (1992) and Eriksson et al. (1998) have shown that several of the “bilaminar” impression techniques for fixed prosthodontics, where alginate is used as a tray material supporting a reversible hydrocolloid (agar) wash, are as accurate and dimensionally stable as elastomers for up to three hours. However, these studies are difficult to interpret due to lack of uniformity in the testing methods, and the fact that there is no regulatory standard available to measure dimensional stability for dental alginates. The International Standard (IS) for alginate impression materials (ISO 1563:1990E) contains no specification for dimensional stability, and thus places no requirement for manufacturers to state dimensional stability properties on their labels. In contrast, ISO 4823:1992(E) specifies the IS for elastomeric dental impression materials, and it does specify a requirement for dimensional stability (less than 1.5% distortion after 24 hours). Further, the IS sets a method for determination of dimensional stability. Briefly, this method (the Optical Method) uses a travelling optical microscope to measure the accuracy of the distance between score lines on an impression of a test grid, at various time periods. The American Dental Association Specification No. 19 for dental elastomeric impression materials is identical to the IS. There is currently no specific Australian Standard (AS) for the dimensional stability of any dental impression material. Overview of Experimental Methods A. The Optical Method The aim of Part A of this investigation was to: 1. Adapt the Optical Method of the IS for elastomers to be reproducible for dental alginates. This was achieved by using a perforated test tray (to simulate clinical conditions), and measuring the grid pattern on a dental stone button after casting the test impression, rather than direct measurement of the impression, as for the IS. 2. To measure and rank the dimensional stability of a number of locally available dental alginates. Measurements of the test stone buttons proved reproducible, and the results were different for each sample, allowing them to be ranked according to dimensional stability after 50 and 100 hours of storage. The results show that the traditional optical method for measuring dimensional stability, as specified in the IS for dental elastomers, can be adapted to measure the dimensional stability of dental alginates However, the Optical Method of measuring dimensional stability of dental alginates is cumbersome and time-consuming. It was hypothesised that dimensional stability of dental alginates could be measured more conveniently by finding a thermal property that is directly proportional to dimensional stability. This method could be useful for the rapid determination of relative performance, and allow comparison with a determined benchmark. B. The Thermal Method Recently, modern methods of Thermal Analysis, Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) have been used to rapidly age various polymers, including food alginates (Chinachoti 1996), in order to measure thermal stability. This thesis shows that thermal stability is an indicator of dimensional stability. The aim of Part B of this investigation was therefore to adapt thermal analysis techniques to dental alginates, and develop a method to measure their thermal stability. These results were then compared with those for dimensional stability measured by the Optical Method to determine the relationship between thermal and dimensional stability for dental alginates. The results show that current thermal analysis methods of TGA and DSC can be adapted to measure relative dental alginate dimensional stability, and are both rapid and convenient. This study also provides evidence that commercial products differ as regards the property of dimensional stability, and can be ranked accordingly. C. Practical Application of the Methods The aim of part C of this thesis was to validate the methods (both optical and thermal) developed in this study by using them to investigate the effect of varying the water/powder ratio on the dimensional stability of dental alginates. It was shown that dimensional stability is affected by changes to the recommended water/powder ratio, that both the methods detected and measured the changes, and that the results were proportional, in that any percentage change detected by the optical method, was mirrored by the thermal method, confirming that the more convenient thermal methods can be used to measure dimensional stability.
18
Qu, Lei, and 瞿磊. "Multiplicity and stability of two-dimensional reaction-diffusion equations." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B31226656.
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Hussain, Zahir. "Stability and transition of three-dimensional rotating boundary layers." Thesis, University of Birmingham, 2010. http://etheses.bham.ac.uk//id/eprint/839/.
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The flow over a rotating cone in still fluid is susceptible to crossflow and centrifugal instability modes of spiral nature, depending on the cone half-angle. For parameters ranging from propeller nose cones to rotating disks, the instability triggers co-rotating vortices, whereas for slender spinning missiles, counter-rotating vortices are observed. Upon introduction of an oncoming flow, the problem essentially becomes a battle between the streamwise and azimuthal shear flow, due to the rotating surface. The boundary layer instability is again visualized by the formation of spiral vortices, which wrap around the cone surface in a helical nature. For both crossflow and centrifugal instabilities, we derive the mean flow boundary layer equations and investigate the high Reynolds number asymptotic linear stability of the flow to inviscid crossflow modes (type I), type II modes, which arise from a viscous-Coriolis force balance, and neutral modes for a slender cone. The influence of the cone half-angle (ψ) and axial flow strength (s or Ts) on the number and orientation of the spiral vortices is examined, with comparisons made with previous experimental and numerical results.
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Quartey-Papafio, Neils. "Optical Properties and Dimensional Stability of Dental Resin-Composites." Thesis, University of Manchester, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.532200.
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Kamm, James Russell Saffman P. G. "Shape and stability of two-dimensional uniform vorticity regions /." Diss., Pasadena, Calif. : California Institute of Technology, 1987. http://resolver.caltech.edu/CaltechETD:etd-06302004-093810.
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Olsen, Gregory R. "Dimensional Stability of Geosynthetic Clay Liners in Landfill Applications." DigitalCommons@CalPoly, 2011. https://digitalcommons.calpoly.edu/theses/651.
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An investigation was conducted related to the dimensional stability of geosynthetic clay liners (GCLs) in landfill applications. Multiple occurrences of panel separation of overlap seams in GCLs have been documented; however, explanation for the relative contribution of various mechanisms causing shrinkage has been limited. A systematic test program was conducted to determine the effects of a variety of conditions on GCL dimensional stability.
Effects of initial moisture content, permeant type, and overburden pressure were tested by subjecting various GCL products to wet-dry cycles and measuring the dimensional change with each cycle. Different GCL types were each tested under various combinations of initial moisture content (as-received, 50, 75, 100, and 125%), permeant types (tap water, distilled water, and 0.1 M CaCl2), and overburden pressures (0, 6, and 20 kPa). Thermal expansion tests were conducted by heating or cooling GCL test specimens to temperatures of 0, 10, 40, 60, and 80°C at constant moisture content. Subgrade tests were conducted by placing GCL test specimens on compacted clay and sand subgrades in different orientations and hydration conditions in sealed containers and measuring dimensional change over time. Mechanical necking tests were conducted by subjecting GCL specimens to varying levels of tension and measuring the longitudinal and transverse strains at each load increment. Field simulation tests were conducted by placing specimens on a compacted sand subgrade beneath an exposed geomembrane liner outdoors in late summer.
Initial moisture content tests resulted in shrinkage strains as high as 20% after 20 wet-dry cycles. GCLs ranged from slightly anisotropic [approximately 1.1:1 ratio of machine (MD) to cross-machine (XMD) shrinkage] to highly anisotropic (approximately 3:1 ratio of MD to XMD shrinkage). Most combinations of GCL type and initial moisture content resulted in GCL MD shrinkage strains greater than a value that would cause panel separation (termed panel separation threshold, PST) at roll ends during the first wet-dry cycle. All test specimens contracted beyond the PST in the MD within 3 wet-dry cycles. GCL specimens without attached geomembranes contracted beyond the PST in the XMD within 5 cycles. Permeant type tests demonstrated that hydration with 0.1 M CaCl2 reduced shrinkage by 50-80% compared to permeation with tap water. Overburden tests demonstrated that applying 6 kPa and 20 kPa reduced specimen shrinkage by at least 60% and 80%, respectively. Thermal expansion tests indicated that temperature changes at constant moisture content had little effect on GCL dimensional stability. Subgrade tests demonstrated that subgrade type and moisture as well as GCL type and orientation had effects on dimensional stability. Tensile necking tests demonstrated that transverse shrinkage occurred due to tensile forces in GCLs, but shrinkage was nearly always less than PST. Field simulation tests demonstrated that wet-dry cycles in the field were less intense and/or less frequent than in the laboratory. Results of this testing provide a basis for GCL overlap specifications necessary to maintain full coverage and future research to confirm a suggested method of preconditioning bentonite to prevent shrinkage.
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Brynjell-Rahkola, Mattias. "Global stability analysis of three-dimensional boundary layer flows." Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-175353.
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This thesis considers the stability and transition of incompressible boundary layers. In particular, the Falkner–Skan–Cooke boundary layer subject to a cylindrical surface roughness, and the Blasius boundary layer with applied localized suction are investigated. These flows are of great importance within the aviation industry, feature complex transition scenarios, and are strongly three-dimensional in nature. Consequently, no assumptions regarding homogeneity in any of the spatial directions are possible, and the stability of the flow is governed by an extensive three-dimensional eigenvalue problem. The stability of these flows is addressed by high-order direct numerical simulations using the spectral element method, in combination with a Krylov subspace projection method. Such techniques target the long-term behavior of the flow and can provide lower limits beyond which transition is unavoidable. The origin of the instabilities, as well as the mechanisms leading to transition in the aforementioned cases are studied and the findings are reported. Additionally, a novel method for computing the optimal forcing of a dynamical system is developed. This type of analysis provides valuable information about the frequencies and structures that cause the largest energy amplification in the system. The method is based on the inverse power method, and is discussed in the context of the one-dimensional Ginzburg–Landau equation and a two-dimensional flow case governed by the Navier–Stokes equations.QC 20151015
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Spence, Timothy. "The Effect of Machining Residual Stresses on the Dimensional Stability of Aluminum Alloys used in Optical Systems." Digital WPI, 2010. https://digitalcommons.wpi.edu/etd-dissertations/104.
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Dimensional stability of materials is of critical importance in the fabrication of precision components used for applications such as optical systems. One source of dimensional instability is residual stress produced on the surfaces of parts due to machining operations. A creep model is proposed in this research that describes how these stressed layers affect the overall geometry of a component as they creep over time and temperature. Depth corrected XRD stress measurements and a bimetallic strip model were utilized to quantify the residual stress layer of two machining operations. The creep model parameters were determined by monitoring curvature over time and temperature. This model can be used for two purposes, the prediction of long term storage effects on part geometry for purposes of reliability assessment, and the design of short term, moderate temperature stress relief treatments. Two alloys were investigated, aluminum 6061-T6, and aluminum 4032-0. A methodology for applying the observed creep strain to complex parts using a finite element analysis is proposed.
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Nakashima, Akira, and Yoshikazu Hayakawa. "Stability analysis of grasped object by soft-fingers with 3-dimensional deformation based on moment stability." IEEE, 2009. http://hdl.handle.net/2237/13955.
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Todd, Michael. "One dimensional dynamics : cross-ratios, negative Schwarzian and structural stability." Thesis, University of Warwick, 2003. http://wrap.warwick.ac.uk/50608/.
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This thesis concerns the behaviour of maps with a unique critical point which is either a maximum or a minimum: so-called unimodal maps. Our first main result proves that for C2+η unimodal maps with non-flat critical point we have good control on the behaviour of cross-ratios on small scales. This result, an improvement on a result of Kozlovski in [K2], proves that in many cases the negative Schwarzian condition (which is not even defined if a map is not C3) is unnecessary. This result follows recent work of Shen, van Strien and Vargas. The main tools are standard cross-ratio estimates, the usual principal nest, the Koebe Lemma, the real bounds from [SV] and the 'Yoccoz Lemma'. Our second main result concerns questions of structural stability. Prompted by the final section of Kozlovski's thesis [K1], we prove that in some cases we can characterise those points at which a small local perturbation changes the type of the map. We prove for these cases that this set of 'structurally sensitive points' is precisely the postcritical set. The main tools are the Koebe Lemma, the real bounds of [LS1], and the quasiconformal deformation argument of [K3]. The thesis is arranged in the form of two chapters dealing with each of the main results separately, followed by an appendix to prove an auxiliary result. The chapters may be read independently of each other.
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Bredt, James Frederic. "Binder stability and powder/binder interaction in three dimensional printing." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/10999.
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Cui, Lin. "Three-dimensional Numerical Model for Seabed Foundation Stability around Breakwaters." Thesis, Griffith University, 2020. http://hdl.handle.net/10072/395539.
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With the increasing demand for coastal zones from human activities, a growing number of breakwaters have been constructed around the main beach and major estuaries to defend against wave erosion and damage. The vulnerability of the breakwater foundation can be associated with dynamic soil responses in the vicinity of structure when subjects to the consecutive ocean wave loading. For the severe situations, soil liquefaction may occur around the breakwater foundation, which is considered as a significant cause of catastrophic failures of many marine structures. Therefore, understanding and predicting soil responses and liquefaction potential around breakwaters have become one of the main concerns when design and maintain these marine structures.
The traditional models used to analyse the soil responses and liquefaction potential in the neighbourhood of breakwaters were mostly limited to two-dimensional (2D) frameworks, in which only the middle cross-section of the breakwaters under perpendicular waves can be investigated. However, the natural environment is three-dimensional (3D) that involves much more complicated fluid-seabed-structure interactions, which requires a 3D model. What’s more, most of the existing models assumed the seabed foundation as poro-elastic medium, which only the oscillatory soil responses and momentary liquefaction can be studied. Nevertheless, the residual soil responses and liquefaction within the poro-elastoplastic soil are more significant and can cause more severe damage to the marine structure foundations. Another deficiency of the traditional models is the lack of advanced Computational Fluid Dynamic (CFD) model to accurately simulate more realistic conditions, for example, including the interactions of ocean currents.
According to the gaps in previous literature, the main objective of this thesis is defined as numerically predicting the soil responses and examining the breakwater foundation stability (i.e., liquefaction potential) under combined waves and currents loading within both poroelastic and poro-elastoplastic seabed foundation from both two- and three-dimensional perspectives for different engineering conditions. One of the main novel contributions of this study is to develop the integrated numerical model that make up for the deficiency of the fluid-seabed-structure interactions problems mentioned above: the wider application ranges including complicated 3D situations; the consideration of poro-elastoplastic soil behaviour and corresponding soil liquefaction; the inclusion of an advanced flow model to precisely predict the hydrodynamic behaviour around the structures. In the future, the models can be further developed and applied to practical engineering analyses, providing preliminary results for the design of the projects.
The integrated numerical model consists of the flow sub-model, the seabed sub-model and the coupling module between two sub-models. The flow model is developed based on the Finite Volume Method (FVM) by solving the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations for simulating the two incompressible phases (i.e., water and air) inside and outside the porous medium. The seabed model is governed by the dynamic Biot’s equations known as the u− p approximations, in which the relative displacements of pore fluid to soil particles are ignored and the acceleration of pore fluid and solid particles is included. Two constitutive models: poro-elastic model for oscillatory soil responses and momentary liquefaction; and poro-elastoplastic model for residual soil responses and residual liquefaction, are incorporated into the seabed model. An integration module is developed between flow sub-model and seabed sub-model through pressure continuity on the common faces. A set of validation works have been done to prove the capability of simulating the fluid-seabed-breakwater interactions in an accurate way.
By adopting the integrated numerical model, three numerical studies have been conducted in this thesis, including one 2D study (soil responses around submerged breakwaters with Bragg reflection) and two 3D studies (seabed foundation stability around breakwaters at river mouth; seabed foundation stability around offshore detached breakwaters). A series of results, including the hydrodynamic properties of flow domain, variation of pore pressure, effective stresses and soil displacements, and characteristics of soil liquefaction within both poro-elastic and poro-elastoplastic seabed foundation have been obtained. Numerical results revealed that the construction of breakwaters can dramatically change the flow pattern and stress state in the vicinity, which will further affect the assessment of foundation stability. Besides, compared to the poro-elastic seabed foundation, the liquefaction is much easier to occur in the poro-elastoplastic seabed foundation and usually will develop to a much more significant level, which can cause critical failure of the structures. Furthermore, the effects of wave characteristics and soil properties on the breakwater foundation stability have been examined through parametric studies: the soil liquefaction is more serious within the loosely deposited seabed with poor drainage conditions under large wave height and wave period. It was also found that the currents have remarkable effects on foundation stability that aggravate with the increase of currents velocity.Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Eng & Built Env
Science, Environment, Engineering and Technology
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Nielsen, Ingelise. "Dimensional stability of paper in relation to paper conservation procedures." Thesis, University of Manchester, 1994. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.674633.
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Carlu, Mallory. "Instability in high-dimensional chaotic systems." Thesis, University of Aberdeen, 2019. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=240675.
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In this thesis I make extensive use of the Lyapunov analysis formalism to unravel fundamental mechanisms of instability in two different systems : the Kuramoto model of globally coupled phase-oscillators and the Lorenz 96 (L96) atmospheric "toy" model, portraying the evolution of a physical quantity along a latitude circle. I start by introducing the relevant theoretical background, with special attention on the main tools I have been using throughout this work : Lyapunov Exponents (LEs), which quantify the asymptotic growth rates of infinitesimal perturbations in a system, and by extension, its degree of chaoticity, and Covariant Lyapunov Vectors (CLVs), which indicate the phase space direction (or the geometry) associated with these growth rates. The Kuramoto model is central in the study of synchronization among oscillatory units characterized by their various natural frequencies, but little is known on its chaotic dynamics in the unsynchronized state. I thus investigate the scaling behavior of the first LE, upon different assumptions on the natural frequencies, and make use of educated structural simplifications to analyze the origin of chaos in the finite size model. On the other hand, the L96 model has been devised to gather the main dynamical ingredients of atmospheric dynamics, namely advection, damping, external (solar) forcing and transfers across different scales of motion, in a minimalist and functional way. It features two coupled dynamical layers : the large scale variables, representing synoptic scale atmospheric dynamics, and the small scale variables, faster and more numerous, associated with convective scale dynamics. The core of the study revolves around geometrical properties of CLVs, in the aim of understanding the processes underlying the observed multiscale chaoticity, and an exhaustive study of a non-trivial ensemble of CLVs featuring relevant projection on the slow subspace.
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Citro, Vincenzo. "Unsteady and three-dimensional fluid dynamic instabilities." Doctoral thesis, Universita degli studi di Salerno, 2016. http://hdl.handle.net/10556/2220.
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Olds, Shana D. "Modeling and LQR Control of a Two-Dimensional Airfoil." Thesis, Virginia Tech, 1997. http://hdl.handle.net/10919/36668.
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In this paper we develop a mathematical model of a two-dimensional aeroelastic airfoil. This model is used to design a flutter suppression controller. Flutter is a vibration in a wing caused by airstream energy being absorbed by the lifting surface. Flutter increases with increasing speed. For simplicity, we consider a flat plate in a two-dimensional flow. The model is developed in the frequency domain and then transformed into the time domain. The uncontrolled model is numerically simulated using MATLAB. Linear Quadratic Regulator (LQR) theory is used to design a state feedback controller. The LQR control scheme consists of using a full state feedback controller of the form u=-Kx, where K is a control gain matrix. The goal is to use LQR theory to supress flutter and to maintain stability of the closed loop system.Master of Science
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Parisini, Enrico. "On the dynamics and stability of rotating higher dimensional black holes." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amslaurea.unibo.it/16386/.
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We study the stability of fast-spinning D=5 black rings making use of the blackfold formalism. We also construct a new effective theory involving blackfolds with transverse spins, able to describe rotating higher dimensional black holes with a set of finite angular momenta. The blackfold approach is a perturbative formalism that allows to capture the behaviour of higher dimensional black holes with high angular momenta. We use it to discuss the dynamic instabilities of boosted black strings, obtaining the expressions for longitudinal and elastic quasinormal modes up to contributions proportional to the square of the horizon scale. A longitudinal Gregory-Laflamme instability is found, as expected. We also study the instabilities of black rings, and we compute the quasinormal frequencies for high mode numbers m= k R and up to contributions proportional to the horizon scale. They agree with the corresponding large-D frequencies, and they signal the presence of a Gregory-Laflamme instability. Finally, we construct a new effective theory describing blackfolds with transverse angular momenta, considering explicitly Kerr black strings, doubly-spinning Myers-Perry black strings and black ring strings. We use these results to build six-dimensional solutions with horizon topology consisting in the product of 2-spheres with 2-tori, and black ring p-balls with even p, endowed with horizon geometry S^2 x S^1 x B^p. The range of validity of our calculation includes relatively low reduced transverse momenta, down to j ∼ 0.1, considerably improving the original blackfold description of a large class of configurations. Finally, as an example, we analyse the leading order stability of black ring p-branes and of D=5 Myers-Perry black branes. In both cases, any accessible configuration turns out to be unstable.
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Li, An-Jui. "Two and three dimensional stability analyses for soil and rock slopes." University of Western Australia. School of Civil and Resource Engineering, 2009. http://theses.library.uwa.edu.au/adt-WU2010.0016.
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Slope stability assessments are classical problems for geotechnical engineers. The predictions of slope stability in soil or rock masses play an important role when designing for dams, roads, tunnels, excavations, open pit mines and other engineering structures. Stability charts continue to be used by engineers as preliminary design tools and by educators for training purposes. However, the majority of the existing chart solutions assume the slope problem is semi-infinite (plane-strain) in length. It is commonly believed that this assumption is conservative for design, but non-conservative when a back-analysis is performed. In order to obtain a more economical design or more precise parameters from a back-analysis, it is therefore important to quantify three dimensional boundary effects on slope stability. A significant aim of this research is to look more closely at the effect of three dimensions when predicting slope stability. In engineering practice, the limit equilibrium method (LEM) is the most popular approach for estimating the slope stability. It is well known that the solution obtained from the limit equilibrium method is not rigorous, because neither static nor kinematic admissibility conditions are satisfied. In addition, assumptions are made regarding inter slice forces for a two dimensional case and inter-column forces for a three dimensional case in order to find a solution. Therefore, a number of more theoretically rigorous numerical methods have been used in this research when studying 2D and 3D slope problems. In this thesis, the results of a comprehensive numerical study into the failure mechanisms of soil and rock slopes are presented. Consideration is given to the wide range of parameters that influence slope stability. The aim of this research is to better understand slope failure mechanisms and to develop rigorous stability solutions that can be used by design engineers. The study is unique in that two distinctly different numerical methods have been used in tandem to determine the ultimate stability of slopes, namely the upper and lower bound theorems of limit analysis and the displacement finite element method. The limit equilibrium method is also employed for comparison purposes. A comparison of the results from each technique provides an opportunity to validate the findings and gives a rigorous evaluation of slope stability.
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Hosseini, Seyed Mohammd. "On stability, transition and turbulence in three-dimensional boundary-layer flows." Doctoral thesis, KTH, Stabilitet, Transition, Kontroll, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-177617.
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A lot has changed since that day on December 17, 1903 when the Wright brothers made the first powered manned flight. Even though the concepts behind flying are unaltered, appearance of stat-of-the-art modern aircrafts has undergone a massive evolution. This is mainly owed to our deeper understanding of how to harness and optimize the interaction between fluid flows and aircraft bodies. Flow passing over wings and different junctions on an aircraft faces numerous local features, for instance, acceleration or deceleration, laminar or turbulent state, and interacting boundary layers. In our study we aim to characterize some of these flow features and their physical roles. Primarily, stability characteristics of flow over a wing subject to a negative pressure gradient are studied. This is a common condition for flows over swept wings. Part of the current numerical study conforms to existing experimental studies where a passive control mechanism has been tested to delay laminarturbulent transition. The same flow type has also been considered to study the receptivity of three-dimensional boundary layers to freestream turbulence. The work entails investigation of effects of low-level freestream turbulence on crossflow instability, as well as interaction with micron-sized surface roughness elements. Another common three-dimensional flow feature arises as a resultof stream-lines passing through a junction, the so-calledcorner-flow. For instance, thisflow can be formed in the junction between the wing and fuselage on aplane.A series of direct numerical simulations using linear Navier-Stokes equationshave been performed to determine the optimal initial perturbation. Optimalrefers to perturbations which can gain the maximum energy from the flow overa period of time. In other words this method seeks to determine theworst-casescenario in terms of perturbation growth. Here, power-iterationtechnique hasbeen applied to the Navier-Stokes equations and their adjoint to determine theoptimal initial perturbation. Recent advances in super-computers have enabled advance computational methods to increasingly contribute to design of aircrafts, in particular for turbulent flows with regions of separation. In this work we investigate theturbulentflow on an infinite wing at a moderate chord Reynolds number of Re= 400,000 using a well resolved direct numerical simulation. A conventional NACA4412 has been chosen for this work. The turbulent flow is characterizedusing statistical analysis and following time history data in regions with interesting flow features. In the later part of this work, direct numerical simulation has been chosen as a tool to mainly investigate the effect of freestream turbulence on the transition mechanism of flow from laminar to turbulent around a turbine blade.QC 20151125
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Canavan, James W. "The effects of drying on the dimensional stability of spruce wood." Thesis, University of Glasgow, 2017. http://theses.gla.ac.uk/8134/.
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Sitka spruce (Picea sitchensis (Bong.) Carr.) has been the most common forestry species in UK plantations for many decades. It is generally fast grown in short rotation, which has implications for quality. The harvested timber has a number of end-use applications. Of these, sawn construction grade timber is at the high end of market value. Currently, under one third of home grown sawn timber processed by sawmills achieves the required C16 or higher grading for construction class timber. Presently there is significant standing stock of 40 years old or under. Improvements in timber quality would create significant benefits for growers and processors. Distortion in kiln dried sawn timber is a considerable barrier to improving grading classification and marketability. Twist in particular accounts for around six percent of the rejections after kiln drying. Additional twist when timber dries further; in storage, at construction sites or in-service, is perceived as a marketing issue. The focus of this work was to better understand the evolution of twist during the drying process and seek to improve quality by examining the key properties in sawn timber that contribute to the formation of twist. Samples of regularly processed full size (3 m in length) sawn timber were kiln dried with no restraint to promote maximum twist. This was done in three stages to assess the impact of intermittent drying. Measurements of twist, moisture content, acoustic velocity and basic dimensions were taken before and after each drying cycle. The results showed highly significant differences between battens containing the pith and those that did not. Drying in stages did not seem to affect the linearity of the moisture content against twist relationship. Twist was seen to develop at a consistent rate throughout the process regardless of the final amount of twist accrued or the position from which the batten was sawn. Where timber is likely to dry to lower moisture content after sale it is therefore possible to predict the additional twist from the moisture content expected and allow for it by stricter grading than the twist standard demands. Twist was seen to decrease slightly during a lengthy period of storage in controlled conditions between drying cycles. This may allude to a relaxation effect which could have implications for timber distortion in-service but this would require a more extensive study before any such conclusions could be drawn. A second batch of unrelated samples, were kiln dried to 12% MC under restraint. All of these samples contained the pith along their length and all developed significant amounts of twist. This suggests that battens containing the pith will twist regardless of restraint. A method was developed to measure longitudinal shrinkage. This required the batten size to be reduced to fit the method and battens were sawn into 3 x 1 m sections. Battens were re-saturated and measured daily throughout air drying until an equilibrium moisture content was achieved. Measurements of transverse shrinkage were also taken concurrently. No relationship could be found between the longitudinal and transverse results. When plotted against the data for twist a positive relationship was found with longitudinal shrinkage. Acoustic tools were used to calculate dynamic stiffness. As the values for acoustic velocity squared correlated well with dynamic stiffness, other possible relationships were explored. Good agreement was found with twist and longitudinal shrinkage. This suggests it may be possible to predict twist in sawn timber directly or as a function of microfibril angle, using acoustic tools. A simple method for measuring grain angle was devised. Discs taken from the trees used in the drying experiment were measured for grain angle to compare with other measured properties. The measurement method worked well but the sample size was small and the discs too thin to provide good quality data quality. The results were not as instructive as expected. A vague relationship with twist was found. A repeat of the experiment with an improved method could yield more conclusive results. A method was developed to measure radial and tangential shrinkage in discs. Discs of Sitka spruce from a different source were used. Discs were saturated and allowed to dry with the expectation of cracks developing. The intention was to explore the nature of internal stresses present in intact discs and why tangential and radial shrinkage differs. As only one disc cracked the scope of comparison was limited but it was apparent that the greater shrinkage tendency in the tangential direction was responsible for radial cracking.
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Wang, Tzyy-Ming. "Secondary stability and three dimensional natural convection in a shallow cavity /." The Ohio State University, 1991. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487683756124784.
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Collier, Fayette. "Curvature effects on the stability of three-dimensional laminar boundary layers." Diss., Virginia Polytechnic Institute and State University, 1988. http://hdl.handle.net/10919/53679.
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The linear stability equations which govern the growth of small periodic disturbances for compressible, three-dimensional laminar boundary layer flow are derived in an orthogonal curvilinear coordinate system. The parallel flow assumption is utilized in the derivation. The system of equations is solved using a finite difference scheme similar to that in a current state-of-the-art stability analysis code, COSAL. The LR method and the inverse Rayleigh iteration procedure are used to calculate the eigenvalues.
The stability of the three-dimensional compressible laminar boundary layer including the effects of streamline and surface curvature for flows past swept wings where crossflow type disturbances dominate is calculated. A parametric study is performed varying Reynolds number and sweep angle on an airfoil with a concave cutout in the leading edge region of the lower surface. It is known that convex curvature has a stabilizing effect on the laminar boundary layer. Conversely, concave curvature has a destabilizing effect. The magnitude of these effects for swept wing flows is determined. Non-stationary as well as stationary disturbances are calculated, and the most amplified frequencies are identified.
N-factor correlations at the measured location of transition are made utilizing flight test data. Results indicate that amplification rates and hence, N-factors, for swept wing flows over convex surfaces are reduced by about 30 to 50 percent when curvature effects are included in the linear stability analysis. In addition, comparisons are made with some experimental results on a swept concave-convex surface. Calculated velocity vector plots show good agreement with observed disturbances in the laminar boundary layer over the concave surface. The results of the calculations show that concave curvature destabilizes "crossflow” type disturbances with a 30 percent increase in amplification rate.Ph. D.
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Muller, Susan Sanette. "The effect of temperature on the linear dimensional stability of elastomers." University of the Western Cape, 2012. http://hdl.handle.net/11394/4624.
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Masters of ScienceSometimes, dental impressions need to be transported to distant laboratories. It has been reported that the temperature in a vehicle can reach up to 66C when the outdoor temperature is 38C. These temperatures may be reached during South African summers. The objective of this in vitro study was to investigate the effect of temperature and time on the dimensional stability of two elastomeric impression materials. Methodology: Specimens consisted of impressions made of an ISO-specified test-block featuring a pattern of grooves. Materials used were polyether (Impregum Penta) and polyvinylsiloxane (Affinis Precious regular body). Using an SLR camera and standardized technique, the specimens were photographed at 2 different temperatures (21°C and 66°C) and 3 time intervals (30min, 8hrs and 14 days). This resulted in a total of 12 groups (n=10) to be compared. Digital images of the impressions were calibrated and measured using digital analyzing software. These distances were used to evaluate the mean % dimensional change (%DC) for each group. VEPAC module of Statistica 10 was used for the statistical analysis. To analyze exactly where the differences lied, a Fisher LSD correction was applied to correct for multiple pair wise comparisons. Results: Comparing polyether with silicone, there was no difference in the mean %DC for specimens kept at 21°C for 8hrs (polyether=0.364; silicone=0.237). Neither was there a difference between polyether and silicone when heated to 66°C, cooled off, and measured after 8hrs (polyether=0.306; silicone=0.297) or after 14 days (polyether=-0.272; silicone=-0.093). For both polyether and silicone, the mean %DC of the groups exposed to 66°C, cooled off and measured after 8hrs (polyether=0.306; silicone=0.297) differed significantly when compared to the group measured after 14 days (polyether=-0.2723; silicone=-0.092) (P<0.0001 and P=0.0029 resp). For both polyether and silicone, the mean %DC of the groups exposed to 66°C, cooled off and measured after 8hrs (polyether=0.306; silicone=0.297) did not differ when compared to the 21°C (polyether=0.364; silicone=0.237). Conclusions: Within limitations of this study, both materials were heat-sensitive. It is recommended that materials return to 21°C before casting. Despite statistical differences, all results were within ISO specifications of maximum 1.5%DC.
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Almeida, Tiago Hendrigo de. "Estudo da estabilidade dimensional de madeiras tropicais brasileiras." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/18/18158/tde-05082015-091914/.
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A madeira é um material de origem biológica, renovável, com papel importante no ciclo do carbono, que contribui para o desenvolvimento sustentável. As madeiras tropicais brasileiras são apreciadas na indústria da construção civil em peças estruturais, componentes de edificação e mobiliário. A melhor utilização da matéria-prima nos diversos processos depende do conhecimento das suas propriedades, e nesse aspecto, os estudos sobre estabilidade dimensional são escassos, em sua maioria, desenvolvidos para coníferas do hemisfério Norte. Este trabalho tem como objetivo geral aprofundar o estudo da estabilidade dimensional, trabalhando com propriedades físicas de madeiras tropicais brasileiras, com ênfase nas densidades, ponto de saturação das fibras, retratibilidade e inchamento. Utilizando madeira de quinze espécies tropicais brasileiras, foram realizados ensaios, de acordo com a NBR 7190/97 para determinar: densidade aparente 0% e 12% de umidade, densidade básica, ponto de saturação das fibras, retração, inchamento, coeficiente de anisotropia, coeficiente de retratibilidade volumétrica e coeficiente de inchamento volumétrico. Investigaram-se as relações entre as propriedades determinadas de modo a poder inferir sobre: a estabilidade dimensional baseando-se nas densidades; as precisões de relações desenvolvidas em pesquisas correlatas. Além disso, foi desenvolvida (e investigada sua precisão, ao nível de significância 5%) uma relação entre densidade básica e densidade aparente via retratibilidade/inchamento. De acordo com os resultados, o melhor coeficiente de determinação ajustado atingido na tentativa de relacionar densidades com os parâmetros da estabilidade dimensional foi 19,58% indicando que a densidade não é um bom estimador da estabilidade dimensional da madeira. Algumas das relações desenvolvidas em pesquisas correlatas não foram consideradas válidas, de acordo com a análise estatística. Para a relação desenvolvida no presente trabalho, embora a mesma apresente média e coeficiente de variação próximos aos dos resultados experimentais, a análise de representatividade não confirmou sua significância.Wood is an organic and renewable material, with important role in carbon cycle, which contributes to sustainable development. Brazilian tropical woods are considered in building industry in structural ant other components and in furniture. The better use of raw materials in different processes depends on knowledge of their properties and, in this respect, studies of dimensional stability are scarce, mostly developed for Northern Hemisphere conifers. This work aims to extend the study of dimensional stability, now working with physical properties of Brazilian tropical woods, emphasizing densities, fiber saturation point; shrinkage and swelling. Using wood of fifteen Brazilian tropical species, tests were carried out, according to NBR 7190/97 to determine: density (0% and 12% moisture content); basic density; fiber saturation point; shrinkage; swelling; anisotropy coefficient; volumetric shrinkage coefficient; and volumetric swelling coefficient. Correlations between certain properties were investigated in order to be able to infer: dimensional stability based on densities; precision of relationships developed in related research. Furthermore, it was developed (and investigated their accuracy, 5% significance level) a relationship between specific gravity and bulk density via shrinkage/ swelling. According to results, the best determination coefficient, obtained in attempt to correlate density with parameters of dimensional stability, was 19.58%. This indicates that density is not a good estimator of dimensional stability for tropical wood. Based on statistical analysis, some of the relationships developed in related researches were not considered valid. For the relationship developed in this research, although it presents average and coefficient of variation close to experimental results, analysis of accuracy did not confirm its significance.
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Butler, Sheneve. "Exfoliation and Air Stability of Germanane." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1366295898.
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Winstanley, Elizabeth. "Aspects of four-dimensional black holes with hair : stability and entropy considerations." Thesis, University of Oxford, 1996. https://ora.ox.ac.uk/objects/uuid:bcb07e40-9538-42a4-a3e9-7bbbd834c7c4.
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In recent years, a large number of black holes have been presented as candidates for an evasion of the "no-hair" conjecture. These examples typically have two features: a non-Abelian gauge field and instability. A large part of this thesis is devoted to a detailed study of the Einstein-Yang-Mills-Higgs (EYMH) black holes, including the analytic proof of the evasion of the "nohair" theorem in this case and proving that the black holes are unstable. We also consider an example of a "hairy" black hole not involving a non-Abelian gauge field, which arises in a higher derivative model of gravity derived from string theory, and prove analytically how the "no-hair" theorem is evaded. The rest of this thesis is concerned with the thermodynamics and quantum field theory of these black holes. In a first order approximation to the unknown theory of quantum gravity, we calculate the entropy of the "hairy" black holes. This turns out to be divergent, and parts of the divergences are attributed to the effect of hair on information loss processes occurring as the black hole evolves in time. We pursue this idea further by making a preliminary estimate of the magnitude of the quantum de-coherence effects on the state of the quantum field as time proceeds. These processes may be of interest phenomenologically in the future. The extension of the theory to non-static geometries is also discussed, by describing the results of bringing rotation into the picture. We prove that the Hartle-Hawking state is not regular everywhere outside the event horizon of a Kerr black hole, with the result that quantum field theory on rotating black hole space-times is more complicated than on static geometries.
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SILVA, FREDERICO MARTINS ALVES DA. "LOW DIMENSIONAL MODELS FOR NONLINEAR VIBRATION ANALYSIS AND STABILITY OF CYLINDRICAL SHELLS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2008. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=11688@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIORNesta tese, as vibrações não-lineares e a estabilidade de uma casca cilíndrica contendo um fluido são estudadas com base em modelos de dimensão reduzida, isto é, modelos com um número reduzido de graus de liberdade. A partir dos funcionais de energia potencial e cinética de uma casca cilíndrica, deduzem-se suas equações de movimento. O campo de deformações da casca cilíndrica segue a teoria não- linear de Donnell para cascas abatidas. O fluido é considerado interno à casca irrotacional, não-viscoso e incompressível, sendo descrito a partir de um potencial de velocidade que leva em consideração a interação entre o fluido e a estrutura. Para resolver o sistema de equações de equilíbrio da casca, desenvolve-se um procedimento analítico que permite obter os campos de deslocamento axial e circunferencial em função dos deslocamentos laterais, além de atender as condições de contorno do problema. Desta forma, reduz-se o sistema de equações de equilíbrio a uma única equação diferencial parcial que é resolvida com o método de Galerkin. A determinação dos deslocamentos laterais é feita a partir de técnicas de perturbação que ordena os modos não-lineares de acordo com sua importância na solução da casca cilíndrica. Comprova-se essa ordenação através do método de Karhunen-Loève que fornece, também, uma expansão ótima para os deslocamentos laterais. Além dessas técnicas, apresenta-se uma redução polinomial que relacionam as amplitudes dos modos não-lineares com a amplitude do modo linear, criando uma expansão modal com 1 GDL. Apresentam-se respostas no tempo, fronteiras de instabilidade e diagramas de bifurcação para uma casca cilíndrica submetida a dois tipos de carregamentos harmônicos, pressão lateral e carga axial. A seguir, são propostos alguns critérios para a análise da a integridade do sistema dinâmico tanto para um sistema com 1 GDL quanto para um sistema multidimensional através da evolução e erosão das bacias de atração. Por fim, estuda-se o comportamento de cascas cilíndricas parcialmente cheias, mostrando a influência da altura do fluido nas fronteiras de instabilidade e curvas de ressonância da casca cilíndrica.
The nonlinear vibrations and stability of a fluid-filled cylindrical shell is investigated using reduced order models. First, the nonlinear equations of motion of the cylindrical shell are deduced based on the expressions for the potential and kinetic energy, which are obtained using Donnell shallow shell theory. The internal fluid is considered to be irrotational, non- viscous and incompressible. It is described by a velocity potential that takes into account the fluid-shell interaction. A procedure is proposed to obtain analytically the axial and circumferential displacements of the shell, satisfying the in-plane equations of motion and the associated boundary conditions. So, the problem is reduced to one partial differential equation of motion which is solved by the Galerkin method. The transversal displacement field is obtained by perturbation techniques. This enables one to identify the relevance of each term in the nonlinear expansion of the vibration modes. Then, the Karhunen-Loève method is employed to investigate de relative importance of each mode obtained by the perturbation analysis on the nonlinear response and to deduce optimal interpolation function to be used in the Galerkin procedure. A SDOF model is also obtained by relating the modal amplitudes of the nonlinear modes to the vibration amplitude of the linear mode. Time responses, instability boundaries and ifurcation diagrams are obtained for cylindrical shells subjected to harmonic lateral and axial loads. Different procedures for the analysis of the shell integrity are proposed based on the evolution and erosion of the basins of attraction in state-space. Finally, the influence of the fluid height on the stability boundaries and resonance curves is studied.
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Kalamas, James. "A three-dimensional flow stability analysis of the particle bed nuclear reactor." Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/47342.
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Fekkai, Zakia. "Dimensional stability of biaxially drawn PET : effects of processing and material composition." Thesis, Loughborough University, 1991. https://dspace.lboro.ac.uk/2134/32890.
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Biaxial orientation of PET for the production of high strength films for demanding applications, such as slot liners for electrical motors and sound and audio visual tapes, is a well established process. More recently biaxial orientation of PET has been utilised for the production of carbonated beverages, bottles and cans for processed food packaging to achieve high strength and impact resistance. These containers, however, are not suitable for hot filling and high temperature sterilization purposes owing to the lack of dimensional stability.
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DePolo, Wade Scott. "Dimensional Stability and Properties of Thermoplastics Reinforced with Particulate and Fiber Fillers." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/29068.
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This work has been concerned with the dimensional stability and the structure-property relationships of thermoplastics reinforced with particulate and fiber fillers. The first part of this study was concerned with ascertaining the main causes of warpage observed for injection-molded thermoplastics reinforced with high aspect ratio fibers. Typically, warpage in injection-molded fiber reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow-induced stresses generated during mold filling do not play a significant role in the warpage. The warpage of PP composites reinforced with TLCP fibers was found to increase with an increase in fiber loading. The shrinkage and the thermal expansion of the TLCP/PP composites and, hence, the thermally induced stresses decreased with an increase in fiber loading while the flow-induced stresses increased. The increase in the flow-induced stresses was attributed to an inhibition of stress relaxation and greater generation of orientation of the polymer chains with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber reinforced thermoplastics, the flow-induced residual stresses must be accounted for.
The second part of this work was concerned with minimizing the particle loading of reinforced PC/PBT composites while maintaining the stiffness, i.e. modulus, and the dimensional stability of injection molded flat panels. This was accomplished by using high aspect ratio (≈100-150) nano-clays as opposed to micron-size talc (≈5-10). It was found that by using nano-clays surface modified with a quaternary ammonium salt that contained two hydroxyl groups as opposed to fine talc particles, the level of particle reinforcement could be reduced from 6 to 1 wt% without sacrificing the modulus of the reinforced PC/PBT composites. Further benefits included a 26% increase in flexural strength, 77% increase in the tensile toughness and 3% reduction in the density of the reinforced PC/PBT composites. An increase in the modulus and tensile toughness was observed even though there was evidence of loss in molecular weight of the PC/PBT matrix, which was supported by the rheological behavior of the composites.Ph. D.
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Rafeh, Faten. "Three dimensional modelling for the long term stability analysis of underground cavities." Thesis, Lille 1, 2015. http://www.theses.fr/2015LIL10156/document.
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La présence de carrières souterraines abandonnées dans le nord de la France, à proximité de zones urbanisées, est à l’origine de problèmes importants touchant à la sécurité des personnes et des constructions, d’une part, et à l’élaboration de plans d’urbanisme d’autre part. Dans ce cadre, cette thèse présente une étude numérique de comportement mécanique et stabilité de carrières souterraines creusées il y a longtemps dans la craie du nord de la France. Elle a pour objectif de proposer une approche complémentaire aux observations et investigations naturalistes. Ces dernières confirment la présence de familles des joints stratifiées dans la craie du nord de la France et d’autres fractures qui se développent due à la dégradation. Ainsi, le premier objectif est de rendre compte du comportement anisotrope induit par la présence de ces joints. Un critère de rupture orienté est développé et différents mécanismes de plasticité sont considérés pour générer cet effet. Une modélisation numérique tridimensionnelle d’une excavation est mise en œuvre. Une procédure de réduction des paramètres de résistance au cisaillement est proposée pour les joints afin d’étudier leur influence sur les mécanismes de déformation et de rupture et sur la stabilité. Les effets du temps et les mécanismes de dégradation induits par différents facteurs aggravants ont aussi un rôle important. Une approche de dégradation dépendante du temps a été développée pour l’analyse des mécanismes de déformation et de rupture en fonction du temps et pour l’estimation de la stabilité à long terme. Enfin, pour l’estimation préliminaire de la stabilité des carrières, des formules empiriques et des abaques sont proposésThe presence of unexploited underground cavities in the region of North France regarding their close proximity to urbanized areas, poses serious problems in terms of security of both people and constructions on one hand, and induces a strong constraint against the planning and urban development on the other hand. In this framework, this thesis presents a numerical study of the mechanical behaviour and the long term stability of underground cavities excavated in the chalk substratum of North France. It aims to provide a complementary understanding to geological investigations and observations carried out. This latter shows that stratified joints are present in the chalk layers as well as fractures developing due to the degradation of chalk with time. Hence, the first target is to account for the anisotropic behaviour induced due to the presence of these joints. An oriented yield criterion is developed where several mechanisms of plasticity are considered to generate this effect. A three dimensional numerical model of the excavation is executed. An approach for reducing shear strength properties of the joints is proposed to study the joints influence on the strain and failure mechanisms and on the stability state of the cavity. Degradation mechanisms induced upon time in the presence of different aggravating factors create also a critical effect. A non-homogeneous time-dependent-degradation model is developed to analyse the time-dependent deformation and failure mechanisms and to assess the long term stability of these cavities. At the end, empirical formulae and abacuses are developed for a preliminary estimation of the stability of underground cavities of North France
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Ichiguchi, Katsuji. "Numerical Studies of Three-Dimensional Equilibrium and Stability for Stellarator/Heliotron Configurations." Kyoto University, 1989. http://hdl.handle.net/2433/74743.
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Larsson, Per. "Dimensional Stability of Paper Influence of Fibre-Fibre Joints and Fibre Wall Oxidation." Licentiate thesis, KTH, Fibre and Polymer Technology, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4635.
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Papper är ett mycket mångsidigt material. Trots detta finns det ett flertal egenskaper som begränsar papperets användbarhet. Ett av de större problemen med cellulosa- och lignocellulosafibrer är att de sänker sin fria energi genom att sorbera vatten, och denna sorption förändrar papperets dimensioner. Detta fenomen kallas vanligtvis för bristfällig dimensionsstabilitet och uppträder i form av registerfel vid flerfärgstryck eller som krullning, buckling och vågiga papperskanter vid utskrift, kopiering och lagring, eller med en vidare definition som förkortad livslängd hos lådor på grund av mekanosorptivt kryp.
Avsikten med denna avhandling har varit att studera och kvantifiera vilka egenskaper som styr, och hur de påverkar, den vatteninducerade dimensionsförändringen som sker hos ett fibernätverk, samt hur dess dimensionsstabilitet kan förbättras. Detta har studerats både genom att ändra fiberns fuktsorptionsegenskaper och genom att förändra adhesionen och kontaktgraden mellan fibrerna i fiber-fiberfogarna. Fogegenskaperna har också varierats genom att tillverka laboratorieark torkade under inspänning samt ark torkade fritt för att minimera mängden inbyggda spänningar i arket.
Blekt kraftmassa har behandlats med polyelektrolytmultilager (PEM) för att förbättra adhesionen mellan fibrerna och för att öka kontaktgraden mellan fibrerna i fogen. Kontaktgraden har även minskats genom förhorning av fibrerna före arkformning. För de ark som fick torka fritt gav PEM-behandlingen en ökad hygroexpansionskoefficient, det vill säga dimensionsförändringen normaliserad mot förändringen i fuktinnehåll, vid samma förändring i relativ luftfuktighet medan förhorningen minskade hygroexpansionskoefficienten något. Om arken emellertid torkades under inspänning observerades ingen skillnad i hygroexpansionskoefficient mellan de olika fibermodifieringarna. Detta tolkades som ett resultat av en ökad kontaktzon och en större utbredning ut ur fogens plan, när arken torkades utan inspänning. En utbredning som medför att en större del av fiberns transversella expansion överförs som expansion i pappersplanet.
Fibrernas fuktsorptionsegenskaper förändrades genom natriumperjodatoxidering av 1,4-glukanernas C2-C3-bindning. Detta skapade sannolikt tvärbindningar i fiberväggen som förbättrade fiberväggens tålighet både genom att låsa fibrillerna närmare varandra och genom att ta bort potentiella adsorptionssäten som annars är tillgängliga för vattenadsorption. Perjodatoxidationen minskar också fibrernas kristallinitet och således frigjorde oxidationen hydroxylgrupper där vattenmolekyler kan adsorbera. Detta innebar att oxidationen både minskade och ökade interaktionen mellan vatten och fibervägg, men dock på olika strukturell nivå. Tvärbindningarna visade sig också märkbart reducera sorptionshastigheten när arken utsattes för en förändrad luftfuktighet så länge de inte tidigare utsatts för relativa luftfuktigheter nära mättnad. Som ett resultat av den lägre förändringen i fuktinnehåll vid en förändring i luftfuktighet från 20 till 85 % RF minskade dimensionsförändringens amplitud för de tvärbundna arken upp till 30 %. Emellertid uppvisade de tvärbundna arken en högre hygroexpansionskoefficeint, vilket innebär att de blev mer känsliga för absoluta förändringar i fuktinnehåll.
Paper is a very versatile material. Nevertheless, there are several factors limiting its usefulness, and one of the major issues is that cellulosic and ligno-cellulosic fibres lower their free energy by sorbing water and this water changes the dimensions of the paper. This phenomenon is usually referred to as a lack of dimensional stability and is often evident as misregister during multicolour printing or curl, cockle and wavy edges during printing, copying and storage or, with a wider definition, also as a shortened life-time of boxes during storage due to mechano-sorptive creep.
The work described in this thesis aims to study and quantify the importance of the different mechanisms causing water-induced dimensional changes in a fibre network and to investigate how to improve the dimensional stability of ligno-cellulosic materials. This has been done both by altering the fiber properties such as the moisture sorptivity and by changing the adhesion and degree of contact within the fibre-fibre joints. The properties of the fibre-fibre joints have been varied by drying laboratory sheets both under restraint and freely to minimise the generation of built-in stresses.
Bleached kraft fibres were treated using the polyelectrolyte multilayer (PEM) technique to improve the adhesion between the fibres and to increase the molecular contact within the joints. In contrast, the degree of contact was impaired by hornifying the fibres before sheet preparation. For sheets allowed to dry freely, the PEM-treatment increased the hygroexpansion coefficient, i.e. the dimensional movement normalised with respect to the change in moisture content, when subjected to changes in relative humidity whereas the hornification process resulted in a slightly lowered hygroexpansion coefficient. However, when the sheets were dried under restraint, the different joint and fibre modifications led to no difference in hygroexpansion coefficient. This was interpreted as being a result of an increase in the total contact zone between the fibres when the sheets were dried under restraint, with a greater extension in the outof- plain direction of the joint resulting in a transfer of a larger part of the transverse swelling to the in-plane expansion.
The sorptivity of the fibres was changed by oxidising the C2-C3 bond of the 1,4- glucans with periodate. This most likely created covalent cross-links in the fibre wall both improved the integrity of the fibre wall by locking adjacent fibril lamellae to each other and also removed possible sites for water sorption onto the cellulose surfaces. Periodate oxidation also led to a decrease in the crystallinity of the cellulose within the fibres, making more cellulose hydroxyl groups available for the adsorption of water molecules. This means that the oxidation both decreased and increased the interaction between the fibre wall and moisture but, on two different structural levels. The crosslinks significantly reduced the sorption rate when the papers was subjected to changes in relative humidity, as long as the fibres were not subjected to humidities close to saturation. The smaller change in moisture content when the relative humidity was changed between 20 and 85 % RH meant that the dimensional stability of the crosslinked sheets was increased. On the other hand, the hygroexpansion coefficient was increased in the case of papers made from fibres with the highest degree of oxidation, i.e. the sheets became more sensitive to absolute changes in moisture content.
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Strömberg, Malin. "Paper dimensional stability in sheet-fed offset printing : Papperets dimensionsstabilitet i en arkoffsetpress." Thesis, Högskolan Dalarna, Grafisk teknik, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:du-1400.
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In offset printing, dampening solution is used to create a good balance in the process. If too much water is transferred to the paper, the sheet can change its size between the printing units, due to water absorption, and cause a problem with the colour register. This phenomenon is usually referred to as fanout. In this degree project, an investigation was made to see if the paper dimensions changed through its way in the sheet-fed printing process. The instrument Luchs Register Measuring Systems (Lynx) was used, and a method for measuring if the paper changed its dimensions with this instrument, was developed. Paper qualities with three different grammages were used, 90, 130 and 250 gsm. This investigation showed that all paper qualities changed their size with widening in the gripper edge in the range of 10 - 70 µm and in the trailing edge the increase was 10 - 130 µm. The elongations of the papers were in the range of 10- 300 µm. The papers with lowest grammage changed more than the heavier. To see if the print had been affected of the widening and elongation, print quality parameters like relative contrast, dot gain and mottle were correlated with the Lynx data from the sheets. The group of papers that gave correlations were in 130 gsm. The sheets had visual doubling and the combined standard deviation from the Lynx marks K3, K5 and K21 correlated with dot gain. When the variations increased so did the dot gain and this indicates that the doubling was due to the widening. There was also a correlation between the standard deviation from K3 and Mottle. The sheets widened with an average of 30 µm in the gripper edge and since there probably were doubling due to widening it also affected the Mottle values. What the widening depends on is hard to tell. Since widening was so small, it could be due to water absorption, papers being ironed out or maybe the sheets have been flattened out. It probably needs a more detailed investigation to find out what causes the widening. Further investigations about how print quality is affected by the register accuracy of a printing machine should include a print form with measuring areas close to the Lynx marks. The measuring areas should contain fine hairlines, negative text printed with at least two colours and some pictures to evaluate together with standard measuring should give a good knowledge about the subject.