Добірка наукової літератури з теми "3D beam element"

Le nouvel élément de poutre est une évolution d'un élément de Timoshenko poutre avec un nœud supplémentaire situé à mi-longueur. Ce nœud supplémentaire permet l'introduction de trois composantes supplémentaires de contrainte afin que la loi constitutionnelle 3D complète puisse être utilisée directement. L'élément proposé a été introduit dans un code d'éléments finis dans Matlab et une série d'exemples de linéaires/petites contraintes ont été réalisées et les résultats sont systématiquement comparés avec les valeurs correspondantes des simulations ABAQUS/Standard 3D. Ensuite, la deuxième étape consiste à introduire le comportement orthotrope et à effectuer la validation de déplacements larges / petites contraintes basés sur la formulation Lagrangienne mise à jour. Une série d'analyses numériques est réalisée qui montre que l'élément 3D amélioré fournit une excellente performance numérique. En effet, l'objectif final est d'utiliser les nouveaux éléments de poutre 3D pour modéliser des fils dans une préforme composite textile. A cet effet, la troisième étape consiste à introduire un comportement de contact et à effectuer la validation pour un nouveau contact entre 3D poutres à section rectangulaire. La formulation de contact est dérivée sur la base de formulation de pénalité et de formulation Lagrangian mise à jour utilisant des fonctions de forme physique avec l'effet de cisaillement inclus. Un algorithme de recherche de contact efficace, qui est nécessaire pour déterminer un ensemble actif pour le traitement de contribution de contact, est élaboré. Et une linéarisation constante de la contribution de contact est dérivée et exprimée sous forme de matrice appropriée, qui est facile à utiliser dans l'approximation FEM. Enfin, on présente quelques exemples numériques qui ne sont que des analyses qualitatives du contact et de la vérification de l'exactitude et de l'efficacité de l'élément de 3D poutre proposé<br>The new beam element is an evolution of a two nodes Timoshenko beam element with an extra node located at mid-length. That extra node allows the introduction of three extra strain components so that full 3D stress/strain constitutive relations can be used directly. The second step is to introduce the orthotropic behavior and carry out validation for large displacements/small strains based on Updated Lagrangian Formulation. A series of numerical analyses are carried out which shows that the enhanced 3D element provides an excellent numerical performance. Indeed, the final goal is to use the new 3D beam elements to model yarns in a textile composite preform. For this purpose, the third step is introducing contact behavior and carrying out validation for new 3D beam to beam contact with rectangular cross section. The contact formulation is derived on the basis of Penalty Formulation and Updated Lagrangian formulation using physical shape functions with shear effect included. An effective contact search algorithm is elaborated. And a consistent linearization of contact contribution is derived and expressed in suitable matrix form, which is easy to use in FEM approximation. Finally, some numerical examples are presented which are only qualitative analysis of contact and checking the correctness and the effectiveness of the proposed 3D beam element

This dissertation developed a method that can accurately and efficiently capture the response of a structure by rigorous combination of a reduced-dimensional beam finite element model with a model based on full two-dimensional (2D) or three-dimensional (3D) finite elements. As a proof of concept, a joint 2D-beam approach is studied for planar-inplane deformation of strip-beams. This approach is developed for obtaining understanding needed to do the joint 3D-beam model. A Matlab code is developed to solve achieve this 2D-beam approach. For joint 2D-beam approach, the static response of a basic 2D-beam model is studied. The whole beam structure is divided into two parts. The root part where the boundary condition is applied is constructed as a 2D model. The free end part is constructed as a beam model. To assemble the two different dimensional model, a transformation matrix is used to achieve deflection continuity or load continuity at the interface. After the transformation matrix from deflection continuity or from load continuity is obtained, the 2D part and the beam part can be assembled together and solved as one linear system. For a joint 3D-beam approach, the static and dynamic response of a basic 3D-beam model is studied. A Fortran program is developed to achieve this 3D-beam approach. For the uniform beam constrained at the root end, similar to the joint 2D-beam analysis, the whole beam structure is divided into two parts. The root part where the boundary condition is applied is constructed as a 3D model. The free end part is constructed as a beam model. To assemble the two different dimensional models, the approach of load continuity at the interface is used to combine the 3D model with beam model. The load continuity at the interface is achieved by stress recovery using the variational-asymptotic method. The beam properties and warping functions required for stress recovery are obtained from VABS constitutive analysis. After the transformation matrix from load continuity is obtained, the 3D part and the beam part can be assembled together and solved as one linear system. For a non-uniform beam example, the whole structure is divided into several parts, where the root end and the non-uniform parts are constructed as 3D models and the uniform parts are constructed as beams. At all the interfaces, the load continuity is used to connect 3D model with beam model. Stress recovery using the variational-asymptotic method is used to achieve the load continuity at all interfaces. For each interface, there is a transformation matrix from load continuity. After we have all the transformation matrices, the 3D parts and the beam parts are assembled together and solved as one linear system.

The main purpose of this thesis is to develop a 3D beam element in order to model wind turbine wings made of composite materials. The proposed element is partly based on the formulation of the classical beam element of constant cross-section without shear deformation (Euler-Bernoulli) and including Saint-Venant torsional effects for isotropic materials, similarly to the one presented in Batoz &amp; Dhatt (1990, pp.147-190). The main novelty consists in the addition of the coupling between axial and bending with torsional effects that may arise when using composite materials. PreComp, a free access code developed by the National Renewable Energy Laboratory (NREL) to provide structural properties for composite blades, is used to obtain the section properties for the beam element. Its performance is assessed, showing its inaccuracy especially when calculating torsional related constants when webs are present in the cross-section. Shell models of constant cross-section cantilever blades are developed to assess the performance of the beam elements, including or not coupling terms. Natural frequencies and displacements under static loads are compared for different study cases of increasing complexity. For fiber-reinforced materials, elements with coupling terms show good agreement with the shell model, especially for the dynamic problem. Elements without coupling terms are unable to capture the dynamic behavior, as these terms seem to have a higher effect on the results when compared to the static case (especially the FT term).

Mid-rise to tall timber buildings are internationally gaining popularity. Reaching heights greater than 5 to 6 storeys has been made possible by the availability of engineered wood products, such as Laminated Veneer Lumber (LVL), Glued laminated timber (Glulam) and Cross Laminated Timber (CLT). These buildings are referred to as “mass timber buildings”. As the height of timber buildings increases, so do their potential risks of progressive collapse. Progressive collapse is characterised by a local failure of a load-bearing structural element which may propagate through the whole building, and ultimately causes its partial or entire collapse. While progressive collapse mechanisms of reinforced concrete and steel buildings have been widely researched, limited studies have been carried out on mass timber buildings. Their ability to resist progressive collapse and their load transfer mechanisms after the loss of a load-bearing element are currently unclear. First, to gain an initial understanding of the progressive collapse behaviour of post-and-beam mass timber buildings, a series of scaled-down 1×2-bay (2D) timber frame substructures were tested under a middle column removal scenario. The behaviour of the frames and the ability of three types of commercially used beam-to-column connections and a proposed novel connection, to develop catenary action under large deformations was measured. The system capacity in terms of the Uniformly Distributed Pressure (UDP) was also quantified. The test results showed that only the proposed novel connector was able to sustain the design pressure in international design specifications if no dynamic increase factor was considered, and therefore presented a potential solution to improve the robustness of post-and-beam mass timber buildings. Furthermore, progressive collapse of post-and-beam mass timber buildings cannot be resisted by the frame alone using the investigated currently used connections and alternative load paths must be found. Second to further explore the mechanisms of post-and-beam mass timber buildings against progressive collapse, four scaled-down 2×2-bay (3D) substructures, with CLT panels, were constructed and tested in the laboratory. Three substructures were tested under an edge column removal scenario, with substructures manufactured from two different types of beam-to-column connections. Namely, two tests were performed with a connection type commonly used in Australia, and one test with the proposed novel connection investigated earlier. The last substructure was subjected to two different corner column removal scenarios, with the substructure tested twice under different CLT panels configurations. The substructure was assembled from the commonly used in Australia beam-to-column connection. In all tests, two Uniformly Distributed Pressures (UDP) were applied to the floors in two stages: (i) a constant UDP of 4.8 kPa was first applied to the bays not adjacent to the removed column and (ii) an idealised UDP was then increasingly applied to the remaining bay(s) through a hydraulic jack connected to a six-point loading tree. The load redistribution mechanisms (alternative load paths), the structural response and failure modes were recorded. In general, experimental test results showed that the applied load was principally transferred to the three columns the closest to the removed column and that the CLT panels spanning over two bays were efficient in resisting the load. The layout of the CLT panels plays a critical role in resisting progressive collapse. A simplified analytical model, consistent with the current industry design practice and pre-defined alternative load paths, was used to predict the ultimate resistance capacity of the tested specimens and compared to the experimental capacities. Overall, the simplified methodology was found to be conservative. Third, finite element (FE) models were developed using the component model and validated against the 2D and 3D experimental results. The properties of the springs to be used in the component model were obtained from additional experimental component tests. CLT panels were simulated using layered shell elements while beam elements were used for the beams and columns. In the 2D numerical model, the ultimate load was accurately predicted and the development of compressive arch and catenary actions were well reproduced. The validated 2D model was then used to build the 3D model. For all tests, the 3D numerical models accurately predicted the overall load-displacement responses, load redistribution mechanisms, failure modes and strain developments in the beams and CLT panels. The validated numerical models were used to conduct a series of parametric studies to further examine the structural responses of the post-and-beam mass timber buildings. The results indicated that the structural capacity would be reduced when only using onebay long CLT panels, compared to using either staggered or all two-bay long CLT panels. Also, beam-to-column connections of the frames connected to the removed column could locally support the CLT panels above, providing an additional alternative load path for the structure in the context of progressive collapse, which is normally neglected in industry design practice.<br>Thesis (PhD Doctorate)<br>Doctor of Philosophy (PhD)<br>School of Eng & Built Env<br>Science, Environment, Engineering and Technology<br>Full Text

Utbredningen av en brand inuti en byggnad kan leda till global eller lokal strukturell kollaps, särskilt i stålramkonstruktioner. Faktum är att stålkonstruktioner är särskilt utsatta för termiska angrepp på grund av ett högt värde av stålkonduktivitet och tvärsnitten med små tjockleken. Som en viktig aspekt av konstruktionen bör brandsäkerhetskrav uppnås antingen enligt föreskrivande regler eller enligt antagande av prestationsbaserad brandteknik. Trots möjligheten att använda enkla metoder som involverar membersanalys kombinerat med nominella brandkurvor, är en mer exakt analys av det termomekaniska beteendet hos en stålkonstruktion ett tilltalande alternativ eftersom det kan leda till mer ekonomiska och effektiva lösningar genom att ta hänsyn till möjliga gynnsamma mekanismer. Denna analys kräver vanligtvis utredning av delar av strukturen eller till och med av hela strukturen. För detta ändamål och för att få en djupare kunskap om strukturelementens beteende vid förhöjd temperatur bör numerisk simulering användas. I denna avhandling utvecklades och användes termomekaniska finita element som är lämpliga för analys av stålkonstruktioner utsätta för brand. Relevanta fallstudier utfördes. Utvecklingen av både ett termomekaniskt skal- och 3D balkelement baserade på en korotationsformulering presenteras. De flesta relevanta strukturfall kan undersökas på ett adekvat sätt genom att antingen använda något av dessa element eller kombinera dem. Korotationsformuleringen är väl lämpad för analyser av strukturer där stora förskjutningar, men små töjningar förekommer, som i fallet med stålkonstruktioner i brand. Elementens huvuddrag beskrivs, liksom deras karakterisering i termomekaniskt sammanhang. I detta avseende övervägdes materialnedbrytningen på grund av temperaturökningen och den termiska expansionen av stål vid härledningen av elementen. Dessutom presenteras en grenväxlingsprocedur för att utföra preliminära instabilitetsanalyser och få viktig inblick i efterknäckningsbeteendet hos stålkonstruktioner som utsätts för brand. Tillämpningen av de utvecklade numeriska verktygen ges i den del av avhandlingen som ägnas åt det publicerade forskningsarbetet. Flera aspekter av knäckningen av stålkonstruktionselement vid förhöjd temperatur diskuteras. I Artikel I tillhandahålls överväganden om påverkan av geometriska imperfektioner på beteendet hos komprimerade stålplattor och kolonner vid förhöjda temperaturer, liksom implikationer och resultat av användningen av grenväxlingsprocedur. I Artikel II valideras det föreslagna 3D-balkelementet genom meningsfulla fallstudier där torsionsdeformationer är signifikanta. De utvecklade balk- och skalelementen används i en undersökning av knäckningsmotstånd hos komprimerade vinkel-, Tee- och korsformade stålprofiler vid förhöjd temperatur som presenteras i Artikel III. En förbättrad knäckningskurva för design presenteras i detta arbete. Som ett exempel på tillämpningen av principerna för brandsäkerhetsteknik presenteras en omfattande analys i Artikel IV. Två relevanta brandscenarier identifieras för den undersökta byggnaden, som modelleras och analyseras i programmet SAFIR.<br>The ignition and the propagation of a fire inside a building may lead to global or local structural collapse, especially in steel framed structures. Indeed, steel structures are particularly vulnerable to thermal attack because of a high value of steel conductivity and of the small thickness that characterise the cross-sections. As a crucial aspect of design, fire safety requirements should be achieved either following prescriptive rules or adopting performance-based fire engineering. Despite the possibility to employ simple methods that involve member analysis under nominal fire curves, a more accurate analysis of the thermomechanical behaviour of a steel structural system is an appealing alternative, as it may lead to more economical and efficient solutions by taking into account possible favourable mechanisms. This analysis typically requires the investigation of parts of the structure or even of the whole structure. For this purpose, and in order to gain a deeper knowledge about the behaviour of structural members at elevated temperature, numerical simulation should be employed. In this thesis, thermomechanical finite elements, suited for the analyses of steel structures in fire, were developed and exploited in numerical simulation of relevant case studies. The development of a shell and of a 3D beam thermomechanical finite element based on a corotational formulation is presented. Most of the relevant structural cases can be adequately investigated by either using one of these elements or combining them. The corotational formulation is well suited for the analyses of structures in which large displacements, but small strains occur, as in the case of steel structures in fire. The main features of the elements are described, as well as their characterization in the thermomechanical context. In this regard, the material degradation due to the temperature increase and the thermal expansion of steel were considered in the derivation of the elements. In addition, a branch-switching procedure to perform preliminary instability analyses and get important insight into the post-buckling behaviour of steel structures subjected to fire is presented. The application of the developed numerical tools is provided in the part of the thesis devoted to the published research work. Several aspects of the buckling of steel structural elements at elevated temperature are discussed. In paper I, considerations about the influence of geometrical imperfections on the behaviour of compressed steel plates and columns at elevated temperatures are provided, as well as implications and results of the employment of the branch-switching procedure. In Paper II, the proposed 3D beam element is validated for meaningful case studies, in which torsional deformations are significant. The developed beam and shell elements are employed in an investigation of buckling resistance of compressed angular, Tee and cruciform steel profiles at elevated temperature presented in Paper III. An improved buckling curve for design is presented in this work. Furthermore, as an example of the application of Fire Safety Engineering principles, a comprehensive analysis is proposed in Paper IV. Two relevant fire scenarios are identified for the investigated building, which is modelled and analysed in the software SAFIR.

<p>This report investigates the feasibility and accuracy of determining fastener flexibility with 3D FE and representing fasteners in FE load distribution models with simple elements such as springs or beams. A detailed study of 3D models compared to experimental data is followed by a parametric study of different shell modelling techniques. These are evaluated and compared with industry semi-empirical equations.</p><p>The evaluated 3D models were found to match the experimental values with good precision. Simulations based on these types of 3D models may replace experimental tests. Two different modelling techniques were also evaluated for use in load distribution models. Both were verified to work very well with representing fastener installations in lap-joints using the ABAQUS/Standard solver. Further improvement of one of the models was made through a modification scale factor. Finally, the same modelling technique was verified using the NASTRAN solver.</p><p>To summarize, it is concluded that:</p><p>• Detailed 3D-models with material properties defined from stress-strain curves correspond well to experiments and simulations may replace actual flexibility tests.</p><p>• At mid-surface modelling of the connecting parts, beam elements with a circular cross section as a connector between shell elements is an easy and accurate modelling technique, with the only data input of bolt material and dimension.</p><p>• Using connector elements is accurate only if the connecting parts are modelled in the same plane, i.e. with no offset. Secondary bending due to offset should only be accounted for once and only once throughout the analysis, and it is already included in the flexibility input.</p>

Les éléments de poutres classiques (Euler-Bernoulli, Timoshenko, Vlassov…), sont tous basés sur certaines hypothèses simplificatrices, qui ont pour conséquence de fixer la forme de la cinématique de l'élément. Ceci revient à réduire un modèle ayant par définition une infinité de d.d.l., à un modèle avec un nombre fini de d.d.l.. Quel que soit donc le chargement auquel sera soumise la poutre, elle se déformera toujours selon la cinématique adoptée au départ. L'objectif de cette thèse est de s'affranchir des hypothèses inhérentes aux modèles de poutres classiques, pour développer un nouveau modèle de poutre enrichie, capable de représenter d'une manière précise les déformations globales aussi bien que locales. Ce type d'élément, permettra la représentation de la flexion transversale dans une poutre, de capturer des effets locaux, produits par exemple par un câble d'ancrage ou de précontrainte sur un tablier de pont, ou encore de traiter le traînage de cisaillement sur des poutres à grandes largeurs. Après un bref rappel de quelques théories de poutres classiques, on présentera dans les deux premiers articles, une nouvelle méthode pour la détermination de modes transversaux et de gauchissements, basée sur une analyse aux valeurs propres d'un modèle mécanique de la section pour l'obtention de la base des modes transversaux, et un procédé d'équilibre itératif pour la détermination de la base des modes de gauchissements. La cinématique ainsi définie, le PTV sera utilisé pour obtenir les équations d'équilibre de la poutre, pour ensuite en déduire la matrice de raideur à partir de leur solution analytique. Dans le troisième article, une nouvelle méthode est proposée pour l'obtention d'une cinématique plus appropriée, où les bases des modes transversaux et de gauchissements sont obtenues en fonction des chargements extérieurs. Cette méthode est basée sur l'application de la méthode des développements asymptotiques à la résolution des équations fortes décrivant l'équilibre d'une poutre<br>The available classical beam elements (such as Euler-Bernoulli, Timoshenko, Vlassov…), are all based on some hypothesis, that have the effect of defining the kinematic of the beam. This is equivalent to reducing a model with an infinity of d.o.f., to a model with a finite d.o.f.. Thus, for arbitrary loadings, the beam will always deform according to the adopted kinematics. The objective of this thesis, is to completely overcome all the hypothesis behind the classical beam models, to develop a new higher order beam model, able to represent precisely the global and local deformations. This kind of element will also allow the derivation of the transversal bending of the beam, to capture the local effects due to anchor or prestressing cables, or to treat the shear lag phenomenon in large width spans. After a brief review of some classical beam theories, we will develop in the two first articles a new method to obtain a basis for the transverse deformation and warping modes. The method is based on an eigenvalue analysis of a mechanical model of the cross section, to obtain the transverse deformation modes basis, and an iterative equilibrium scheme, to obtain the warping modes basis. The kinematic being defined, the virtual work principle will be used to derive the equilibrium equations of the beam, then the stiffness matrix will be assembled from their analytical solution. In the third article, a new method is proposed for the derivation of a more appropriate kinematic, where the transverse deformation and warping modes are obtained in function of the external loadings. The method is based on the application of the asymptotic expansion method to the strong form of the equilibrium equations describing the beam equilibrium

L'objectif principal visé par cette thèse est de modéliser les poutres gonflables en textiles techniques orthotropes. Les approches statiques font l'objet de ce rapport. Avant d'aborder ce problème, nous avons été amenés à identifier tous les paramètres qui ont un effet direct sur les propriétés mécaniques effectives de ces composites. Ainsi, nous avons développé un modèle micro mécanique de prédiction de ces propriétés mécaniques. Le modèle proposé est basé sur l’analyse d'un volume élémentaire représentatif (VER) prenant en compte non seulement les propriétés mécaniques et la. fraction de volume de chaque phase dans le VER mais également leur géométrie et leur architecture. Chaque fil dans le VER a été modélisé comme un matériau isotrope transverse (contenant les fibres et la résine). La méthode dite d’assemblage de cylindres a été utilisée pour l’homogénéisation au niveau des fils. Une deuxième homogénéisation est ensuite réalisée. Elle prend en compte la fraction de volume de chaque constituant (fils de chaîne, fils de trame et résine non prise en compte dans les fils). Le modèle a été validé par des résultats expérimentaux existant dans la littérature. Une élude paramétrique a été menée afin d'étudier les effets des divers paramètres géométriques et mécaniques sur ces propriétés mécaniques. Dans l'analyse structurale, un modèle poutre gonflable 3D de Timoshenko en tissu orthotrope a été proposé. Il prend en compte les non-linéarités géométriques et l'effet de la force suiveuse générée par la pression de gonflage. Les équations d'équilibre non-linéaires dérivent du principe des travaux virtuels en configuration lagrangienne totale. Dans une première approche, une linéarisation a été faite autour de la configuration de référence précontrainte pour obtenir les équations adaptées aux problèmes linéaires. A titre d'exemple, le problème de flexion plane a été abordé. Quatre cas de conditions aux limites ont été traités et les résultats obtenus améliorent les modèles existants dans le cas de tissu isotrope. Les charges de plissage ont été également proposées dans chaque cas traité. Dans une deuxième approche, les équations non-linéaires ont été discrétisées par la méthode des éléments finis. Deux types de solutions ont été alors proposées : les solutions aux problèmes éléments finis linéaires obtenues par une linéarisation des équations discrétisées autour de la configuration de référence précontrainte et les solutions aux problèmes éléments finis non-linéaires réalisées en adoptant une méthode Quasi-Newton sous sa forme incrémentale. A titre d’exemple, la flexion d’une poutre encastrée-libre a été étudiée et les résultats améliorent les modèles théoriques. Le modèle éléments finis non-linéaire a été comparé favorablement à un modèle éléments finis coque mince 3D. Une étude paramétrique a été ensuite effectuée. Elle a porté sur l'influence des propriétés mécaniques et sur de la pression de gonflage sur la réponse de la poutre. Les solutions éléments finis linéaires se sont avérées proches des résultats théoriques linéarisés d'une part et les résultats du modèle éléments finis non-linéaire se sont avérés proches des résultats du modèle linéaire dans le cas des propriétés mécaniques élevées alors que le modèle éléments finis non-linéaire est indispensable pour modéliser ces poutres lorsque les propriétés mécaniques du tissu sont faibles<br>The main objective of this thesis was to model inflatable beams made frorn orthotropic woven fabric composites. The static aspects were investigated in this report. Before planning to develop these models, it was necessary to know all the parameters which have a direct effect on the effective mechanical properties these composites. Thus, a micro­ mechanical model was performed for predicting the effective mechanical properties. The proposed model was based on the analysis of the representative volume element (RVE). The model took into account not only the mechanical properties and volume fraction of each components in the RVE but also their geometry and architecture. Each yarn in the RVE was modelled as a transversely isotropic material (containing fibres and resin) using the concentric cylinders model (CCIVI). A second volumetric averaging which took into account the volume fraction of each constituent (warp yarn, weft yarn and resin), was performed. The model was validated favorably against experimental available data. A parametric study was conducted in order to investigate the effects of various geometrical and mechanical parameters on the elastic properties of these composites. ln the structural analysis, a 3D Timoshenko airbeam with a homogeneous orthotropic woven fabric (OWF) was addressed. The model took into account the geometrical nonlinearities and the inflation pressure follower force effect. The analytical equilibrium equations were performed using the total Lagrangian form of the virtual work principle. As these equations were nonlinear, in a first approach, a linearization was performed at the prestressed reference configuration to obtain the equations devoted to linearized problems. As example, the bending problem was investigated. Four cases of boundary conditions were treated and the deflections and rotations results improved the existing models in the case of isotropic fabric. The wrinkling load in every case was also proposed. In a second approach, the nonlinear equilibrium equations of the 3DTimoshenko airbeam were discretized by the finite element method. Two finite element solutions were then investigated : finite element solutions for linearized problems which were obtained by the means of the linearization around the prestressed reference configuration of the nonlinear equations and nonlinear finite element solutions which were performed by the use of an optimization algorithm based on the Qua.si-Newton method. As an example, the bending problem of a cantilever inflated beam under concentrated load was considered and the deflection results improve the theoretical models. As these beams are made from fabric, the beam models were validated through their comparison with a 3D thin-shell finite element model. The influence of the material effective properties and the inflation pressure on the beam response was also investigated through a parametric study. The finite element solutions for linearized problems were found to be close to the theoretical linearized results. On the other hand, the results for the nonlinear finite element model were shown to be close to the results for the linearized finite element model in the case of high mechanical properties and the non linear finite element model was used to improve the linearized model when the mechanical properties of the fabric are low

This paper is intended to present wavelet Galerkin schemes for the boundary element method. Wavelet Galerkin schemes employ appropriate wavelet bases for the discretization of boundary integral operators. This yields quasisparse system matrices which can be compressed to O(N_J) relevant matrix entries without compromising the accuracy of the underlying Galerkin scheme. Herein, O(N_J) denotes the number of unknowns. The assembly of the compressed system matrix can be performed in O(N_J) operations. Therefore, we arrive at an algorithm which solves boundary integral equations within optimal complexity. By numerical experiments we provide results which corroborate the theory.

La presente tesi tratta dello studio di concetti volti all'ottenimento di strutture meccaniche a rigidezza variabile per applicazioni in ambito di ricerca scientifica, in particolare per una futura applicazione in un robot aereo ad ala battente, al fine di studiare l'interazione tra ala elastica ed aria. Vengono riassunti i metodi per ottenere rigidezza variabile ed, in seguito ad una fase di confronto basato su requisiti ed obiettivi di progetto, vengono scelte due soluzioni. Il lavoro mostra che il concetto "sliding segments" funziona bene per una trave composta da un'asta interna ed un tubo esterno, entrambi formati da segmenti rigidi e flessibili alternati, di due materiali differenti. La rigidezza flessionale della trave varia grazie ad una traslazione dell'asta interna. Viene inoltre mostrato come un'asta ed un tubo possono essere combinati per ottenere una trave rotante con diversi livelli di rigidezza flessionale in una direzione, riducendo gli effetti della flessione deviata.