Dissertations / Theses on the topic 'Multistable structures'

A composite laminate with unsymmetric layup may exhibit two distinct stable configurations at room temperature due to the residual thermal stresses imparted during curing. This bistability leads unsymmetric composite laminates to be good candidates for adaptive flexible structures in particular for the fields of aerospace and aircraft. To extend the application potential of bistable composite laminates, the multistable behaviour of continuous compound composite surfaces are investigated. Two connection approaches are presented in this research. By tailoring the asymmetric bistability of indi- vidual composite elements, continuous surfaces composed of series-connected bistable composite shells demonstrate a high degree of multistability. This model can be developed to design longer composite surfaces possessing more stable shapes. In addition, a high degree of multistability is achieved by connecting square composite elements in a tessellated feature. By reducing the interactions between adjacent elements, a tessellated surface composed of n bistable elements shows a theoretical maximum 2n stable shapes. Finally, potential applications of highly multistable composite surfaces are introduced. The proposed multistable designs improve the flexible functionality of the adaptive structures.

'Compliant' structures, i. e., structures that undergo large deformations as part of their normal behaviour, can perform the function of a complex mechanism with just a single, or very few, components. When multistability is introduced into compliant structures, an even greater simplification can be made, with actuators and/ or locking mechanisms becoming redundant. Corrugated shells are commonly encountered at all scales of engineering, providing shell structures of a dramatically increased stiffness at little extra cost or weight. What is historically less understood is the compliant behaviour of corrugated shells. The topic of this thesis, therefore,' is the development and analysis of new categories of structure incorporating corrugations, utilising their compliancy to achieve morphing behaviours that are not otherwise possible. Three distinct categories of shell are studied: (i) 'flat' corrugated sheets, which are multistable under the action of internal stresses, 'snapping' between their flat state and a cylindrically coiled state: these shells are also capable of developing 'twisted' stable states. An algebraic model based on internal strain energy determines the conditions for the various modes of multistability for uniform changes in curvature throughout the shell, and a plastic flow model describes the formation of the internal stresses; (ii) 'curved' corrugated sheets, 'which can morph between shapes of differing Gaussian curvature without plastic deformation. Again, a simplified algebraic model describes their behaviour, as does a numerical model. These models consider the strong coupling that occurs between bending and stretching in corrugated shells; and (iii) 'doubly-corrugated ' sheets, similar to an eggbox in concept, which , again, can morph between shapes of different Gaussian curvature, and also exhibit synclastic bending behaviour. Some of the analyses performed on the curved corrugated sheets are also applied to these shells. The qualitative behaviour of the analytical models matches, in every detail considered, the behaviour observed in prototypes. Natural-mode analyses on the category (ii) and (iii) shells demonstrate that the interesting behaviour occurs at relatively low stiffnesses, implying that t his behaviour is potentially useful. Quantitatively, however, the analyses for categories (i) and (iii) above do not prove simple to verify. Physical tests compare the internal stresses of the category (i) shells to those predicted by the plastic flow analysis: the predicted stresses exceed those measured by, typically, 50 %, but it is not established whether the discrepancy is due to the simplifications in the model, inaccuracies in the crude test method or a flmv in om understanding. On its prediction of the curvatures created, the model is more successful. A test is more easily devised for the category (ii) shells, and measurements of the shape change are within 4 % of both the finite-element simulations and the simplified algebraic models over 90 % of the test range. In measuring forces, to test the constitutive relations produced; the accuracy is lower, largely due to the difference between our material model and the real properties of the polymer used: there is strong agreement between the finite element model and the algebraic model. In conclusion, this thesis is successful in generating new varieties of structure, which have a a wide potential for application. A broad range of designs and analyses are presented, describing the behaviour of these structures well enough that our understanding of them seems justified. Nonetheless, this work merely scratches the surface of what may be achieved in this field, demonstrating some of the potential of such shells and leaving much to be done in their further evaluation and development.

The field of vibrational energy harvesting aims to transform ambient mechanical energy into electrical energy. For example, this energy can be used to operate autonomous sensor units for structural health monitoring or to supply low power electronic devices. Robust energy harvesters that allow to harvest sufficient energy over a broad frequency range are crucial for these applications. One strategy to increase the bandwidth of energy harvesters and, thus, the robustness is the exploitation of multistable structures. This is due to their feature of showing large amplitude oscillations that result from snap-through actions (inter-well oscillations) in a significant frequency range. The aim of this thesis is to analyze different multistable energy harvester designs in order to optimize their performance and formulate design criteria. The considered designs are a bistable electromechanical beam, a bistable electromechanical composite plate and a newly proposed design of a multistable plate with four equilibria. Firstly, analytical models for the multistable energy harvesters are presented in order to assess their broad bandwidth harvesting capabilities. Analytical methods are applied to these models to investigate the underlying bifurcation behavior. Based on the analytical investigations, design criteria are formulated to describe the favorable harvesting domain. Numerical simulations are performed to supplement the analytical investigations. The differences of the considered structures are highlighted concerning robust and efficient harvesting by means of numerical simulations for different types of excitation. Experiments are carried out to complement the analytical and numerical analysis. The experiments establish the transferability of the numerical and analytical findings to real-world applications.

The field emission measurements for the multistage structured nanotubes (i.e., thin-multiwall and single wall carbon nanotubes grown on multiwall carbon nanotubes) were carried out and a low turn-on field of ~0.45 V/ μm, high emission current of 450 μA at a field of IV/μm and a large field enhancement factor of ~26200 were obtained. The thin multiwall carbon nanotubes (thin-MWNTs) and single wall carbon nanotubes (SWNTs) were grown on the regular arrays of vertically aligned multi wall carbon nanotubes (MWNTs) on porous silicon substrate by Chemical Vapor Deposition (CVD) method. The thin-MWNTs and SWNTs grown on MWNTs in this way have a multistage structure which gives higher enhancement of the electric field and hence the electron field emission.

Unbound granular materials (UGMs) used in the base and sub-base layers of flexible pavements play a significant role in the overall performance of the structure. Proper understanding and characterization of the deformation behaviour of UGMs in pavement structures are, therefore, vital for the design and maintenance of flexible pavements. In this study, the resilient deformation (RD) and the permanent deformation (PD) behaviour of UGMs were investigated for the better understanding and improved modelling of these deformation characteristics. The study is based on a series of repeated-load triaxial (RLT) tests carried out on several UGMs commonly used in pavement structures. Here, the influences of stress level and moisture content - two of the most significant factors affecting the deformation behaviour of UGMs - were analysed. The effects of the grain size distribution and the degree of compaction were also considered. The study on the RD behaviour indicated that the resilient stiffness (MR)of UGMs increases with the increased bulk stress level, which can be satisfactorily described by the k-θ model. Moisture was found to negatively impact the MR as long as the deformation was mostly resilient with a negligible amount of accumulated PD. Analysis of the influence of moisture on the parameters k1 and k2 of the k-θ model showed that k1 decreases with increased moisture and k2 is relatively insensitive to moisture. Based on these observations, a simple model was developed for the impact of moisture on MR. The performance of this model was comparable to an existing moisture dependent MR model. In contrast, it was further observed that at the later stages of the RLT tests, after a relatively large number of load applications, the MR increased with increased moisture up to the optimum moisture content. This occurred when the RD was accompanied by a significant amount of PD. Further investigation suggested that moisture aided the post-compaction (PC) and possible particle rearrangement that resulted in the increased PD and increased MR. In this case k1 decreased, whereas k2 increased, with increased moisture. The existing MR-moisture model did not work for this behaviour. This suggests that the effect of PC on MRshould be considered in modelling. However, although not explored in this study, it may be possible to simulate this effect of increase in MR with increased moisture due to PC using the proposed model if k2 is expressed as a function of moisture. The PD characteristics of UGMs were investigated based on the multistage (MS) RLT test. In contrast with the single stage (SS) RLT test, the MS RLT test accounts for the effect of stress history and enables a comprehensive study of the material behaviour under cyclic stresses of various magnitudes. Since the existing PD models cannot be directly applied for the MS loading procedure, a general formulation based on the time hardening concept was derived that can be used to extend the models for the MS loading conditions. Based on this formulation, some of the current models were calibrated and their performance in predicting the PD behaviour in MS RLT tests was compared. The investigation regarding the impact of moisture on PD showed that moisture significantly increases the accumulation of PD. Generally, materials with finer grading showed more sensitivity to moisture with regards to both PD and RD. To characterize the impact of moisture, moisture sensitivity of different grain size distributions and the impact of the degree of compaction on PD with reduced effort, a simple model was proposed. Unlike some of the well-performing existing models, this model can be calibrated using a single MS RLT test without requiring any separate static failure triaxial tests. This model was validated using the MS RLT test data with satisfactory results. The sensitivity of the parameters of this model was studied with respect to moisture content, degree of compaction and grain size distribution. Some reasonable trends for the sensitivity of the parameters to these influential factors were obtained, which suggests that these may be further developed to incorporate into the model.

QC 20150325

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 241-245).
This thesis describes the effect of increasing multistage axial compressor rotor blade tip clearance on embedded stage performance and flow structure for clearance-to-span ratios ranging from 1.4% to 5.6% using steady and unsteady three-dimensional viscous flow multistage computations. Embedded stage efficiency displays decreased sensitivity as rotor tip clearance increases with two flow regimes. For clearance-to-span ratios less than 3.6%, a nearly linear decrease in stage efficiency of 1.6 points per 1% increase in clearance-to-span is identified, in agreement with published literature. For clearance-to-span ratios greater than 3.6%, the computed stage efficiency decreases at a rate of 0.5 points per 1% increase in clearance-to-span. A parameter is developed that correlates with rotor tip section loss generation over a range of rotor tip clearance-to-span ratios and flow coefficients. The blade row relative streamwise tip section blockage increases in both rotor and stator passages and follows trends in rotor and stator tip section loss generation with rotor tip clearance. The tip section velocity deficit into the stator increases with tip clearance resulting in stator suction side corner flow separation, creating a challenge to design a high efficiency stage with larger tip clearance.
by Andrew Koff.
S.M.

Ces travaux de thèse sont basés principalement sur le phénomène de multistabilité des structures minces de type plaques et coques ainsi que quelques applications associées. Les travaux sont divisés en deux parties. La première partie a pour objet l’étude théorique, numérique et expérimentale de la multistabilité des coques minces orthotropes peu profondes à courbures uniformes. On montre notamment qu’une telle coque, lorsqu’elle est soumise à la combinaison d’une courbure initiale et d’une précontrainte suffisamment élevées, possède jusqu’à trois configurations stables vis-à-vis des propriétés matériaux. Dans un premier temps, nous proposons des critères de conception et fabrication de coques multistables allant jusqu’à la tristabilité, validés numériquement et expérimentalement. Ensuite, nous appliquons ces critères à la conception et à la fabrication de coques multistables cylindriques dont la différence de niveau énergétique entre les deux états stables est minime. Sur ce support, la deuxième partie est consacrée à des applications exploitant la bistabilité des coques cylindriques minces à faible différence énergétique. Nous effectuons tout d’abord une application au contrôle de forme via l’utilisation de matériaux actifs que l’on attache à la structure. Cela comprend une première phase théorique de conception de la structure et de la loi d’actionnement, et une seconde phase de mise en œuvre expérimentale. Ensuite, nous étudions théoriquement et expérimentalement les propriétés de dynamique non-linéaire de ce type de coques dans le but de mettre en évidence les modes d’oscillations intrinsèques à une source d’excitation externe. Enfin, nous proposons une application à la récupération d’énergie vibratoire non-linéaire de coques multistables cylindriques métalliques par voie piézoélectrique
This work is essentially based on the phenomenon of multistability of thin structures as plates and shells and some associated applications. The work is divided in two parts. The first part aims to study theoretically, numerically and experimentally the multistability of thin orthotropic shallow shells with uniform curvature. We show notably that such a shell, when submitted to the combination of initial curvature and pre-stresses sufficiently high, possesses up to three stable states towards the choice of the material. First, we propose criteria to design and manufacture multistable shells up to tristability ; this work is validated by finit element simulations and experiments. After, we apply those criteria to the design and manufacture ofcylindrical multistable shells for which the energetic gap between stable states is minimal. The second part is dedicated to direct applications of bistability of thin cylindrical bistable shells with low energetic gap. We first propose an application on shape control via the use of active materials which we bond on the structure. This includes a first phase of theoretical design of both the structure and the actuation law, and a second phase of experimental demonstration. After, we study both theoretically and experimentally the non-linear dynamic properties of such structures with the aim to highlight the different modes of oscillations intrinsic to an external excitation source. Finally, we propose an application to non-linear broadband energy harvesting from vibrations based on multistable piezoelectric excited shells

Les futures générations de réseaux sans fil posent beaucoup de défis pour la communauté de recherche. Notamment, ces réseaux doivent être en mesure de répondre, avec une certaine qualité de service, aux demandes d'un nombre important de personnes et d'objets connectés. Ce qui se traduit par des exigences assez importantes en termes de capacité. C'est dans ce cadre que les méthodes d'accès multiple non orthogonaux (NOMA) ont été introduit. Dans cette thèse, nous avons étudié et proposé une méthodes d'accès multiple basé sur la technique compute and forawrd et sur les réseaux de point (Lattice codes) tout en considérant différentes constructions de lattice. Nous avons également proposé des amélioration de l'algorithme de décodage de la méthode SCMA (Sparse code multiple access) basé sur les réseaux de points. Afin de simplifier les décodeurs multi-niveaux utilisés, nous avons proposé des expressions simplifiées de LLRs ainsi que des approximations. Finalement, nous avons étudié la construction D des lattices en utilisant les codes polaires. Cette thèse était en collaboration avec le centre de recherche de Huawei France
Future generations of wireless networks pose many challenges for the research community. In particular, these networks must be able to respond, with a certain quality of service, to the demands of a large number of connected people and objects. This drives us into quite important requirements in terms of capacity. It is within this framework that non-orthogonal multiple access methods (NOMA) have been introduced. In this thesis, we have studied and proposed a multiple access method based on the compute and forward technique and on Lattice codes while considering different lattice constructions. We have also proposed improvements to the algorithm for decoding the Sparse code multiple access (SCMA) method based on Lattice codes. In order to simplify the multi-stage decoders used in here, we have proposed simplified expressions of LLRs as well as approximations. Finally, we studied the construction D of lattices using polar codes. This thesis was in collaboration with the research center of Huawei France

Le marché des anticorps monoclonaux représente actuellement la majorité des protéines recombinantes à intérêt thérapeutique. Les difficultés liées à la production de tels anticorps dans des cellules de mammifères, ont poussé la communauté scientifique à envisager des organismes alternatifs pour leur production. Récemment, les microalgues ont montré leur capacité à produire des anticorps monoclonaux fonctionnels. Il est cependant nécessaire de modifier la glycosylation naturelle de ces microalgues afin qu’elles produisent des anticorps pouvant être utilisés pour certaines applications thérapeutiques chez l’Homme. Dans ce contexte, l’étude de la N-glycosylation de la diatomée Phaeodactylum tricornutum a été réalisée. Cette étude a consisté dans un premier temps à déterminer la structure du précurseur oligosaccharidique des N-glycannes des protéines formées dans le réticulum endoplasmique. Les résultats ont permis de montrer la synthèse par la diatomée d’un précurseur tronqué d’un glucose comparé à la majorité des eucaryotes. Dans un second temps, il a été nécessaire de déterminer la structure détaillée des N-glycannes oligomannosidiques. Ces résultats ont permis d’établir que les structures Man9GlcNAc2 et Man5GlcNAc2 synthétisées sont similaires à celles retrouvées chez les mammifères. De plus, le Man5GlcNAc2 synthétisé est l’isomère substrat accepteur de la N-acétylglucosaminyltransférase I (GnT I) qui catalyse l’étape primordiale initiant la synthèse des glycannes complexes. Cette étude a donc permis d’initier l’humanisation de la voie de la Nglycosylation par la sur-expression de la GnT I chez la diatomée afin d’amplifier les proportions en Nglycannes complexes portant des résidus N-acétylglucosamine en position terminale
Currently, monoclonal antibodies represent the major class of recombinant proteins used for therapeutical applications in humans. The constraints due to the production of these monoclonal antibodies in mammalian cells led scientists to develop alternative expression system for the production of monoclonal antibodies. Recently, microalgae have been shown to be able to produce functional monoclonal antibodies. However, the engineering of the N-glycosylation pathway in these microalgae is required in order to optimize the effector functions of the algae-made antibody for some therapeutic applications. In this context, this PhD thesis focused on the study of the N-glycosylation of the diatom Phaeodactylum tricornutum. Initially, we focused on the structural analysis of the oligosaccharide precursor synthesized in the endoplasmic reticulum. The results showed that P.tricornutum synthesizes a truncated precursor as compared to the other eukaryotes. Then, I have investigated the detailed structure of the oligomannosidic N-glycans and demonstrated that the Man9GlcNAc2 and Man5GlcNAc2 synthesized were similar to those found in mammals. In addition, the Man5GlcNAc2 synthesized is the acceptor isomer for the N-cetylglucosaminyltransferase I (GnT I) which initiates in eukaryotes the synthesis of complex-type N-glycans. Humanization of the N-glycosylation pathway was finally iinitiated by over-expression of the GnT I in the diatom in order to increase the amounts of complex N-glycans carrying N-acetylglucosamine residues in the terminal position

碩士
國立交通大學
電機與控制工程系所
96
In multiple-input multiple-output (MIMO) communication systems Maximum- Likelihood detection is the preferred detection method which achieves the optimal performance. However, the main drawback of Maximum-Likelihood detection is that it suffers from exponential computational complexity against the number of antennas and signal modulation methods. Thus we consider K-Best algorithm based on the breadth-first tree search. The K-Best algorithm provides near-ML performance while its computational complexity is reduced compared to the Maximum-Likelihood detection. This thesis proposes the multistage K-Best algorithm in MIMO systems. In the proposed method, we decompose higher order constellation into several lower order constellations and apply ordering of lower order constellation to reduce the probability of missing the Maximum-Likelihood solution. It can be seen that the multistage K-Best algorithm and conventional K-Best algorithm achieve almost identical PER performance at the same K value. However, the multistage K-Best algorithm is shown to achieve such performance with a significantly lower computational complexity compared to the conventional K-Best algorithm. At the same time the VLSI architecture for the implementation of the multistage K-Best algorithm based on 0.18-μm technology is presented.

碩士
國立交通大學
資訊科學學系
82
For a shared-memory multiprocessor system, the multistage inter- connection network ( MIN )is a high-bandwidth and cost- effective choice among the various interconnection schemes. However , the pure MIN has two properties : (i) There is only a single path between any PP-MM (processing element and memory module ) pair. ( ii ) A link is always shared by multiple PE-MM paths . The problems of the fault-tolerance and the hot-spot contention caused by the above two properties may seriously degrade the system performance.A binary tree based combining network for MIN was proposed to solvethe hot spot contention.This cost-effective scheme can solve hot spot contention efficiently, but the binary tree sturcture has a poor fault-tloerant capability . In this thesis a modified fault-tolerant binary tree called Jump Tree is proposed. The reliability of the tree structure is enhanced by adding some redundant links and multiplexers. Furthermore , we propose an integrated MIN design which has a fault-tolerant MIN (chained network ) and a fault-tolerant and cost-effective combining network (Jump Tree).The chained network can handle the nonuniform traffic spots for the regular routing the Jump Tree can combine the hot-spot packets efficiently and reliably. The Jump Tree based combining MIN has a five time increases in the reliabilty compared with that based on a binary tree. The average delay of our design is also shorter than the chained combining network's.

Although the design of Multistage testing (MST) has received increasing attention, previous studies mostly focused on comparison of the psychometric properties of MST with CAT and paper-and-pencil (P&P) test. Few studies have systematically examined the number of items in the routing test, the number of subtests in a stage, or the number of stages in a test design to achieve accurate measurement in MST. Given that none of the studies have identified an ideal MST test design using polytomously-scored items, the current study conducted a simulation to investigate the optimal design for MST using generalized partial credit model (GPCM). Eight different test designs were examined on ability estimation across two routing test lengths (short and long) and two total test lengths (short and long). The item pool and generated item responses were based on items calibrated from a national test consisting of 273 partial credit items. Across all test designs, the maximum information routing method was employed and the maximum likelihood estimation was used for ability estimation. Ten samples of 1,000 simulees were used to assess each test design. The performance of each test design was evaluated in terms of the precision of ability estimates, item exposure rate, item pool utilization, and item overlap. The study found that all test designs produced very similar results. Although there were some variations among the eight test structures in the ability estimates, results indicate that the performance overall of these eight test structures in achieving measurement precision did not substantially deviate from one another with regard to total test length and routing test length. However, results from the present study suggest that routing test length does have a significant effect on the number of non-convergent cases in MST tests. Short routing tests tended to result in more non-convergent cases, and the presence of fewer stage tests yielded more of such cases than structures with more stages. Overall, unlike previous findings, the results of the present study indicate that the MST test structure is less likely to be a factor impacting ability estimation when polytomously-scored items are used, based on GPCM.
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