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1
Alur, Kashyap. "Nonlinear mechanics of composite materials." Thesis, Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53600.
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Composite materials have been an area of active research in recent years due to the possibility of obtaining multifunctional structures. Viscoelastic layered composites with parallel plane layers consisting of a stiff constituent and a soft viscoelastic constituent are of particular interest as they have been shown to exhibit simultaneous high stiffness and high damping. Such materials would be useful in structural applications and in high vibration environments such as in a vehicle or machinery. They would provide the rigidity required while simultaneously dissipating mechanical energy.
The finite deformation mechanics of parallel plane viscoelastic layered composites has not been extensively studied. Under compressive loads they are very susceptible to instabilities. Buckling, for example is an elastic instability seen in load bearing materials. Since viscoelastic materials are rate and time dependent, the buckling modes for these composites not only depend on these factors, but also on the volume fraction of the stiff constituent. Three different cases are identified in the buckling and post-buckling response of these composites: non-dilute (high volume fraction), transition (intermediate volume fraction) and dilute (small volume fraction) cases.
Due to buckling from the application of prestrain, the stiffness and damping of these composites can be tuned by orders of magnitude. Adaptive and multifunctional materials can be designed taking advantage of this idea and the rate dependence of the modes of deformation.
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Li, Hong. "Experimental micromechanics of composite buckling strength." Thesis, Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/11719.
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Yilmaz, Suphi. "Buckling Driven Delamination Of Orthotropic Functionally Graded Materials." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/3/12607836/index.pdf.
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In today's technology severe working conditions increase demands on structural materials. A class of materials which are developed to meet these increased demands is Functionally Graded Materials (FGMs). These are inhomogeneous structural materials which are able to withstand large temperature gradients and corrosive environment. Application areas of FGMs are in aerospace industry, nuclear reactors, chemical plants and turbine systems. FGMs have gradual compositional variation from metal to ceramic which give them mechanical strength, toughness and heat resistance. However under high temperature gradients, cracking problems may arise due to thermal stresses. In layered structures the final stage of failure may be delamination due to crack extension. The objective of this study is to model a particular type of crack problem in a layered structure consisting of a substrate, a bond coat and an orthotropic FGM coating. There is an internal crack in the orthotropic layer and it is perpendicular to material gradation of coating. The position of the crack inside the coating is kept as a variable. The steady-state temperature distribution between the substrate and the coating causes a buckled shape along crack face. The critical temperature change, temperature distribution, mixed mode stress intensity values and energy release rates are calculated by using Displacement Correlation Technique. Results of this study present the effects of geometric parameters such as crack length, crack position, etc as well as the effects of the type of gradation on buckling behavior and mixed mode stress intensity factors.
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Grall, Bruno. "Structural analysis of geodesically stiffened composite panels with variable stiffener distribution." Thesis, This resource online, 1992. http://scholar.lib.vt.edu/theses/available/etd-12232009-020522/.
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Singhatanadgid, Pairod. "Buckling of composite plates subjected to biaxial loading /." Thesis, Connect to this title online; UW restricted, 2000. http://hdl.handle.net/1773/7067.
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Radloff, Harold David. "Buckling response of symmetrically laminated composite plates having a trapezoidal planform area." Thesis, This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-06112009-063507/.
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Vakiener, Adam R. "Preliminary investigation of flange local buckling in pultruded wide flange structural shapes." Thesis, Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/19562.
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Dávila, Carlos G. "Delamination initiation in postbuckled dropped-ply laminates /." This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-07282008-134842/.
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Lo, Patrick Kar-Leung. "Comparison of theory and experiment for flexural-torsional buckling of laminated composite columns." Thesis, Virginia Polytechnic Institute and State University, 1985. http://hdl.handle.net/10919/50051.
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Vlasov’s one-dimensional structural theory for thin-walled open section bars was originally developed and used for metallic elements. The theory was recently extended to laminated bars fabricated from advanced composite materials. The purpose of this research is to provide a study and assessment of the extended theory. The focus is on flexural and torsional-flexural buckling of thin-walled, open section, laminated composite columns. Buckling loads are computed from the theory using a linear bifurcation analysis, and are compared to available experimental data. Also, a geometrically nonlinear beam column analysis by the finite element method is developed from the theory. Results from the nonlinear compression response analysis are compared to limited available test data. The merits of the theory and its implementation are discussed.Master of Science
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Huang, Haiying. "Single and multiple delamination behavior in composite plates." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/12541.
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Giardina, Ronald Joseph Jr. "General Nonlinear-Material Elasticity in Classical One-Dimensional Solid Mechanics." ScholarWorks@UNO, 2019. https://scholarworks.uno.edu/td/2666.
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We will create a class of generalized ellipses and explore their ability to define a distance on a space and generate continuous, periodic functions. Connections between these continuous, periodic functions and the generalizations of trigonometric functions known in the literature shall be established along with connections between these generalized ellipses and some spectrahedral projections onto the plane, more specifically the well-known multifocal ellipses. The superellipse, or Lam\'{e} curve, will be a special case of the generalized ellipse. Applications of these generalized ellipses shall be explored with regards to some one-dimensional systems of classical mechanics. We will adopt the Ramberg-Osgood relation for stress and strain ubiquitous in engineering mechanics and define a general internal bending moment for which this expression, and several others, are special cases. We will then apply this general bending moment to some one-dimensional Euler beam-columns along with the continuous, periodic functions we developed with regard to the generalized ellipse. This will allow us to construct new solutions for critical buckling loads of Euler columns and deflections of beam-columns under very general engineering material requirements without some of the usual assumptions associated with the Ramberg-Osgood relation.
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Dávila, Carlos G. "Delamination initiation in postbuckled dropped-ply laminates." Diss., Virginia Tech, 1991. http://hdl.handle.net/10919/38915.
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Cardanini, Alisha Ann. "Finite Element Analysis of Bi-Metallic Structures with Adhesive Delamination." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu150185598849201.
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Koide, Rubem Matimoto. "Aplicação de regressão de vetores de suporte na otimização em flambagem e pós-flambagem de estruturas compósitas laminadas." Universidade Tecnológica Federal do Paraná, 2016. http://repositorio.utfpr.edu.br/jspui/handle/1/2655.
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CAPESMateriais compósitos laminados são utilizados em diversos setores da indústria, principalmente nas áreas automobilística de competição e aeroespacial, pois apresentam relações resistência-peso e rigidez-peso muito superiores aos materiais metálicos em geral. Estruturas fabricadas a partir desses materiais são normalmente finas e, consequentemente, estão sujeitas à flambagem. Requisitos tradicionais de projeto normalmente levam em conta a flambagem mas, para alguns casos, o projeto é conservador, visto que a estrutura pode ainda ser funcional no regime de pósflambagem. Entretanto, o comportamento nesse regime é não-linear, além da dificuldade de se estimar quando ocorre a falha da estrutura, o que torna a análise mais complexa e onerosa em relação à uma análise de flambagem linear. Nesse contexto está inserido o presente trabalho, que visa encontrar as orientações das fibras que maximizam as cargas de flambagem e de pós-flambagem de estruturas compósitas, usando no processo de otimização metamodelos para aliviar o custo computacional. Duas técnicas de metamodelagem são utilizadas e testadas: redes neurais artificiais e regressão de vetores de suporte, com ênfase para a última. Em combinação com os metamodelos são empregadas duas metaheurísticas de otimização desenvolvidas recentemente: o algoritmo harmony search e o algoritmo de vaga-lumes. Vários problemas com diferentes níveis de dificuldade são apresentados e discutidos. Os melhores resultados de otimização foram obtidos com o algoritmo de vaga-lumes associado ao metamodelo de regressão de vetores de suporte, mostrando que tais técnicas são promissoras na solução dessa classe de problemas. Como uma das principais contribuições desta tese tem-se a adaptação/implementação da técnica de regressão de vetores de suporte para problemas de empilhamento de lâminas em estruturas compósitas, particularmente na otimização em flambagem e pósflambagem. Além disso, foram realizados avanços na modelagem do comportamento e da otimização em pós-flambagem com a utilização de critérios de falha e de dano para compósitos.
Laminated composite materials are applied in many industrial sectors, particularly in competition automotive and aerospace fields, since they have strength-to-weight and stiffness-to-weight ratios much higher than the metals in general. Structures made by these materials are usually thin and hence they are subject to buckling. Traditional design requirements usually take into account the buckling, but in some cases the design is conservative since the structure can still be functional in the postbuckling regime. However, the behavior in this regime is nonlinear, in addition of being difficult to evaluate when the failure of the structure takes place, which makes the analysis more complex and computational expensive if compared to a linear buckling analysis. Within this context this work is inserted, which aims to find the orientations of the fibers that maximize the buckling and postbuckling load of composite structures using metamodels in the optimization process to alleviate the computational cost. Two metamodeling techniques are used and tested: artificial neural networks and support vector regression, with emphasis on the latter. In combination with the metamodels, two recently developed metaheuristics, the harmony search algorithm and the firefly algorithm, are employed. Several problems, with different levels of difficulty, are presented and discussed. The best optimization results were obtained with the firefly algorithm associated with the support vector regression metamodel, showing that these techniques are promising to solve this class of problems. One of the main contributions of this thesis is the adaptation/implementation of support vector regression for layup orientation sequence problems of composite structures, in particular for buckling and postbuckling optimizations. Moreover, advances were made in the modeling of the behavior and optimization in postbuckling regime using failure and damage criteria for composites.
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Pour, Shahid Saeed Abadi Parisa. "Mechanical behavior of carbon nanotube forests under compressive loading." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/47699.
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Carbon nanotube (CNT) forests are an important class of nanomaterials with many potential applications due to their unique properties such as mechanical compliance, thermal and electrical conductance, etc. Their deformation and failure in compression loading is critical in any application involving contact because the deformation changes the nature of the contact and thus impacts the transfer of load, heat, and charge carriers across the interface. The micro- and nano-structure of the CNT forest can vary along their height and from sample to sample due to different growth parameters. The morphology of CNTs and their interaction contribute to their mechanical behavior with change of load distribution in the CNT forest. However, the relationship is complicated due to involvement of many factors such as density, orientation, and entanglement of CNTs. None of these effects, however, are well understood. This dissertation aims to advance the knowledge of the structure-property relation in CNT forests and find methodologies for tuning their mechanical behavior. The mechanical behavior of CNT forests grown with different methodologies is studied. Furthermore, the effects of coating and wetting of CNT forests are investigated as methods to tailor the degree of interaction between CNTs. In situ micro-indentation of uncoated CNT forests with distinct growth-induced structures are performed to elucidate the effects of change of morphology along the height of CNT forests on their deformation mechanism. CNT aerial density and tortuosity are found to dictate the location of incipient deformation along height of CNT forests. Macro-compression testing of uncoated CNT forests reveals mechanical failure of CNT forests by delamination at the CNT-growth substrate. Tensile loading of CNT roots due to post-buckling bending of CNTs is proposed to be the cause of this failure and simple bending theory is shown to estimate the failure load to be on the same order of magnitude as experimental measurements. Furthermore, delamination is observed to occur in the in situ micro-indentation of CNT forests coated with aluminum on the top surface, which demonstrates the role of the mechanical constraints within the CNT forest in the occurrence of delamination at the CNT-substrate interface. In addition, this dissertation explores the mechanical behavior of CNT forests coated conformally (from top to bottom) with alumina by atomic layer deposition. In situ micro-indentation testing demonstrates that the deformation mechanism of CNT forests does not change with a thin coating (2 nm) but does change with a sufficiently thick coating (10 nm) that causes fracturing of the hybrid nanotubes. Ex situ flat punch and Berkovich indentations reveal an increase in stiffness of the CNT forests that are in range with those predicted by compression and bending theories. An increase in the recoverability of the CNTs is also detected. Finally, solvent infiltration is proposed as a method of decreasing stiffness of CNT forests and changing the deformation mechanism from local to global deformations (i.e., buckling in the entire height). Presence of solvents between CNTs decreases the van der Waals forces between them and produces CNT forests with lower stiffness. The results demonstrate the effect of interaction between CNTs on the mechanical behavior. This dissertation reveals important information on the mechanical behavior of CNT forests as it relates to CNT morphology and tube-to-tube interactions. In addition, it provides a framework for future systematic experimental and theoretical investigations of the structure-property relationship in CNT forests, as well as a framework for tuning the properties of CNT forests for diverse applications.
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Pyskir, Adrien. "Application de métamatériaux aux problématiques vibroacoustiques automobiles." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEC011.
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Les métamatériaux sont des matériaux architecturés de telle sorte qu’ils présentent des propriétés exotiques, issues non pas du matériau constitutif, mais de leur structure interne. Bien qu’ayant été étudiés depuis une vingtaine d’années, peu d’applications réelles ont été recensées, notamment dans le domaine industriel. Cette thèse est consacrée aux métamatériaux élastiques susceptibles de réduire les vibrations dans les véhicules automobiles. En effet, une meilleure isolation des principales sources vibratoires permettrait l’amélioration du confort vibratoire et la durée de vie des pièces mécaniques. Les résultats de calculs numériques et essais expérimentaux montrent que les métamatériaux peuvent satisfaire des contraintes contradictoires, et représentent donc des candidats intéressants pour la réalisation d’innovations industrielles. Ce type de solutions étant fondamentalement différent des systèmes d’isolation actuels, le premier chapitre dresse un état de l’art des métamatériaux, afin d’en comprendre les mécanismes et les méthodes numériques permettant d’en calculer les performances. Le deuxième chapitre aborde les techniques de caractérisation des matériaux employées pendant cette thèse. Les essais mécaniques ainsi que les résultats permettent de définir les modèles matériaux utilisés par la suite. Dans le troisième chapitre, des calculs numériques appliqués à différentes architectures aident à mieux comprendre certains mécanismes des métamatériaux et à choisir le meilleur candidat vis-à-vis des propriétés ciblées. Celui-ci est approfondi dans le quatrième chapitre, à travers des études paramétriques statiques et dynamiques. Des propositions d’améliorations géométriques sont proposées, y compris un métamatériau hybride aux propriétés supérieures. Afin de vérifier les résultats expérimentaux et d’acquérir une meilleure compréhension des mécanismes sous-jacents, le cinquième chapitre aborde finalement les essais expérimentaux effectués, l’analyse de leurs résultats, et leur confrontation avec les résultats numériquesMetamaterials are architectured materials exhibiting exotic properties due to their internal stucture rather than their constitutive material. They have now been studied for two decades, but have yet to make their mark outside laboratories, especially for industrial applications. This thesis focuses on elastic metamaterials that can contribute to fix vibration issues in the automotive field. Better isolation of the main vibration sources would increase both the vibroacoustic comfort in the vehicles and the safety of mechanical parts. Through computations and experimentations, it is shown that metamaterials can be designed to meet different criteria usually contradictory and as such, are strong candidates for innovative breakthroughs in industry. As this kind of solutions differs radically from existing ones, the first chapter is a state-of-the-art review, both to grasp the main mechanims behind the multitude of metamaterials designs that can be found in the literature, as well as the methods used to modelize them. The second chapter tackles the characterization of the materials used along this thesis. The mechanical tests and results presented allow to determine the material models then inserted in the computations. Through preliminary computations, the third chapter attempts to understand and select the most promising mechanisms to satisfy the expected specifications. The chosen design properties are further investigated in the fourth chapter, through static and dynamic computations, as well as parametric studies. A hybrid metamaterial with enhanced isolation properties is proposed. To finally assess the numerical results obtained and reach better undestanding of the underlying mechanisms, the fifth chapter deals with the performed experimental tests, their analysis, and their comparison with previous results
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Chan, Cameron D. "MECHANICAL OPTIMIZATION AND BUCKLING ANALYSIS OF BIO-COMPOSITES." DigitalCommons@CalPoly, 2012. https://digitalcommons.calpoly.edu/theses/877.
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Today’s environmental concerns have led a renewed search in industry to find new sustainable materials to replace non-renewable resources. President Barack Obama also quoted in the recent 2012 Presidential Debate “that there is a need to build the energy sources of the future and invest in solar, wind, and bio-fuels.” Bio-composites are believed to be the future and the new substitute for non-renewable resources. Bio-composites are similar to composites in that they are made up of two constituent materials; however the main difference is that bio-composites are made from natural fibers and a biopolymer matrix. This research investigates the buckling behavior of bamboo and will analyze and determine the slender ratio that will induce buckling when bamboo is used as a column. Along with the investigation of the bamboo under buckling, this study will also show the potential of bio-composites to replace non-renewable resources in industry through experimental and numerical analysis. However, in order to study the buckling behavior of the bamboo, the mechanical characteristics of the bamboo and optimal curing treatment first had to be established. This is because, in order for bamboo to acquire proper strength characteristics, the bamboo must first be treated.
Due to the scarcity of bamboo material in the lab, the obtainment of the mechanical properties of the bamboo as well as the optimal curing treatment was done in collaboration with Jay Lopez. In order for bamboo to acquire proper strength characteristics, the bamboo must be treated. In the first study, a total of four different types of natural treatments were analyzed to optimize the mechanical characteristics of bamboo. To assess each curing method, tensile and compression tests were performed to obtain the mechanical properties. Due to each bamboo culm having different thicknesses and cross sections, the specific strength property is used to normalize the data and allow for easy comparison and assessing of each curing method equally. The specific strength parameter is defined as the ultimate stress divided by the density of the material. These curing treatments consisted of four thermo-treatments, three different percentages of salt treatments, one lime treatment, and one oil treatment. The thermo-treatments consisted of heating the bamboo internodes in an autoclave with no pressure at 150oF, 180°F, 200°F, and 220°F. The experimental results of the thermo-treatments determined that bamboo obtains higher mechanical properties as well as reduced weight when heated at higher temperatures. This is explained by the increasing bound water extracted from the bamboo material at higher temperatures. In addition to finding the optimal heat treatment, the internodes of bamboo were soaked in natural additives that included a 3%, 6%, and 9% Instant Ocean sea salt solution, a Bonide hydrated lime solution, and a Kirkland canola oil solution for approximately five days and then heat treated at the optimal temperature of 220°F. The experimental results showed that all of the different additives had a significant effect on the mechanical properties.
After determining the mechanical properties of each curing method, the results were then analyzed through a trade study. The trade study parameters consisted of weight-drop of the material, the specific strength, and the ultimate stress for both compression and tension. Each parameter of the trade study is kept unbiased as the weighting of each parameter is set equal to each other. The results of the trade study indicated that the 3% salt solution was the optimal curing treatment, yielding a higher specific strength value for both compression and tension, along with a significantly lower weight-drop after curing.
After we came up with the optimal treatment, the buckling behavior of bamboo was investigated. The buckling analysis was investigated to determine at what slenderness ratio the bamboo would buckle when used as a column. A total of seven cases were investigated using different lengths, that ranged from 1.5” to 10”. Through experimental results, it was determined that a slenderness ratio above approximately 34.7 would induce global buckling to the bamboo column.
The last investigation of this study consisted of building a small prototype wall structure using bio-composites. The prototype wall structure was manufactured using a combination of bamboo and a bi-directional woven hemp fabric. The dimensions of the prototype were 15.13” long and 7.75” tall. The wall structure was tested under compression in the Aerospace Structures/Composites Lab and the Architectural Engineering Department’s high bay laboratory. The results of the experimental test on the wall showed great potential for bio-composites, as the structure withstood a force of 46,800 pounds.
A numerical analysis technique was also employed through the finite element method using the Abaqus software. The purpose of the finite element method was to validate the experimental results by comparing the buckling behavior of the tests. The numerical analysis showed very good agreement with the experimental results.
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Sotolongo, Wilfredo. "On the numerical implementation of cyclic elasto-plastic material models." Thesis, Georgia Institute of Technology, 1985. http://hdl.handle.net/1853/17594.
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Surano, Dominic E. "The Effectiveness of Damage Arrestment Devices in Delaying Fastener-Hole Interaction Failures in Carbon Fiber Polyurethane Foam Composite Sandwich Panels Subjected to Static and Dynamic Loading Under Increased Temperatures." DigitalCommons@CalPoly, 2010. https://digitalcommons.calpoly.edu/theses/436.
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A study was conducted to investigate simple, cost-effective manufacturing techniques to delay skin-core delamination, micro-buckling and bearing stress failures resulting from fastener-hole interactions. Composite sandwich panels, with and without damage arrestment devices (DADs), were subjected to monotonic compression at a rate of 5mm per second, and compression-compression fatigue at 50% yield at an amplitude of 65%, under temperatures of 75, 100, 125, 150, 175, and 200 °F.
The sandwiches tested were composed of two-layer cross-weave carbon fiber facesheets, a polyurethane foam core, and an epoxy film adhesive to join the two materials. The most successful method to delay the aforementioned failures involved milling rectangular slots in the foam core perpendicular to the holes and adding three additional layers of carbon fiber cross-weave. For the monotonic cases, the ultimate load increases were 97, 87, 100, 131, 96, and 119% for each of the respective temperatures listed above with a negligible weight increase. For the fatigue cases, the number of cycles for each test case was nearly identical. This still represents a large improvement because the yield used in the loading condition for the specimens with DADs was 97% greater than the specimens without DADs.
The experimental results were compared with a finite element model (FEM) built in Abaqus/CAE. The numeric and experimental results showed a strong correlation. All test specimens were manufactured and tested in the California Polytechnic State University Aerospace/Composites Laboratory.
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Rastgar, Agah Mobin. "Material Characterization of Aortic Tissue for Traumatic Injury and Buckling." Diss., Temple University Libraries, 2015. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/324268.
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Mechanical EngineeringPh.D.
While traumatic aortic injury (TAI) and rupture (TAR) continue to be a major cause of morbidity and mortality in motor vehicle accidents, its underlying mechanisms are still not well understood. Different mechanisms such as increase in intraluminal pressure, relative movement of aorta with respect to mediastinal structures, direct impact to bony structures have been proposed as contributing factors to TAI/TAR. At the tissue level, TAI is assumed to be the result of a complex state of supra-physiological, high rate, and multi-axial loading. A major step to gain insight into the mechanisms of TAI is a characterization of the aortic tissue mechanical and failure properties under loading conditions that resemble traumatic events. While the mechanical behavior of arteries in physiological conditions have been investigated by many researchers, this dissertation was motivated by the scarcity of reported data on supra-physiological and high rate loading conditions of aorta. Material properties of the porcine aortic tissue were characterized and a Fung-type constitutive model was developed based on ex-vivo inflation-extension of aortic segments with intraluminal pressures covering a range from physiological to supra-physiological (70 kPa). The convexity of the material constitutive model was preserved to ensure numerical stability. The increase in ë_è from physiological pressure (13 kPa) to 70 kPa was 13% at the outer wall and 22% at the inner wall while in this pressure range, the longitudinal stretch ratio ë_z increased 20%. A significant nonlinearity in the material behavior was observed as in the same pressure range, the circumferential and longitudinal Cauchy stresses at the inner wall were increased 16 and 18 times respectively. The effect of strain-rate on the mechanical behavior and failure properties of the tissue was characterized using uniaxial extension experiments in circumferential and longitudinal directions at nominal strain rates of 0.3, 3, 30 and 400 s-1. Two distinct states of failure initiation (FI) and ultimate tensile strength (UTS) were identified at both directions. Explicit direct relationships were derived between FI and UTS stresses and strain rate. On the other hand, FI and UTS strains were rate independent and therefore strain was proposed as the main mechanism of failure. On average, engineering strain at FI was 0.85±0.03 for circumferential direction and 0.58±0.02 for longitudinal direction. The engineering strain at UTS was not different between the two directions and reached 0.89±0.03 on average. Tissue pre-failure linear moduli showed an average of 60% increase over the range of strain rates. Using the developed material model, mechanical stability of aorta was studied by varying the loading parameters for two boundary conditions, namely pinned-pinned boundary condition (PPBC) and clamped-clamped boundary condition (CCBC). The critical pressure for CCBC was three times higher than PPBC. It was shown that the relatively free segment of aorta at the isthmus region may become unstable before reaching the peak intraluminal pressures that occur during a trauma. The mechanical instability mechanism was proposed as a contributing factor to TAI, where elevations in tissue stresses and strains due to buckling may increase the risk of injury.
Temple University--Theses
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Hadi, Bambang Kismono. "Buckling of anisotropic sandwich panels with and without holes." Thesis, Imperial College London, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307655.
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Wong, Ha Hang Aaron. "Buckling and stability of slender reinforced concrete deep beams." Thesis, University of Newcastle Upon Tyne, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279763.
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Saadé, Katy. "Finite element modeling of shear in thin walled beams with a single warping function." Doctoral thesis, Universite Libre de Bruxelles, 2005. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211043.
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The considerable progress in the research and development of thin-walled beam structures responds to their growing use in engineering construction and to their increased need for efficiency in strength and cost. The result is a structure that exhibits large shear strains and important non uniform warping under different loadings, such as non uniform torsion, shear bending and distortion.A unified approach is formulated in this thesis for 3D thin walled beam structures with arbitrary profile geometries, loading cases and boundary conditions. A single warping function, defined by a linear combination of longitudinal displacements at cross sectional nodes (derived from Prokic work), is enhanced and adapted in order to qualitatively and quantitatively reflect and capture the nature of a widest possible range of behaviors. Constraints are prescribed at the kinematics level in order to enable the study of arbitrary cross sections for general loading. This approach, differing from most published theories, has the advantage of enabling the study of arbitrary cross sections (closed/opened or mixed) without any restrictions or distinctions related to the geometry of the profile. It generates automatic data and characteristic computations from a kinematical discretization prescribed by the profile geometry. The amount of shear bending, torsional and distortional warping and the magnitude of the shear correction factor is computed for arbitrary profile geometries with this single formulation.
The proposed formulation is compared to existing theories with respect to the main assumptions and restrictions. The variation of the location of the torsional center, distortional centers and distortional rotational ratio of a profile is discussed in terms of their dependency on the loading cases and on the boundary conditions.
A 3D beam finite element model is developed and validated with several numerical applications. The displacements, rotations, amount of warping, normal and shear stresses are compared with reference solutions for general loading cases involving stretching, bending, torsion and/or distortion. Some examples concern the case of beam assemblies with different shaped profiles where the connection type determines the nature of the warping transmission. Other analyses –for which the straightness assumption of Timoshenko theory is relaxed– investigate shear deformation effects on the deflection of short and thin beams by varying the aspect ratio of the beam. Further applications identify the cross sectional distortion and highlight the importance of the distortion on the stresses when compared to bending and torsion even in simple loading cases.
Finally, a non linear finite element based on the updated lagrangian formulation is developed by including torsional warping degrees of freedom. An incremental iterative method using the arc length and the Newton-Raphson methods is used to solve the non linear problem. Examples are given to study the flexural, torsional, flexural torsional and lateral torsional buckling problems for which a coupling between the variables describing the flexural and the torsional degrees of freedom occurs. The finite element results are compared to analytical solutions based on different warping functions and commonly used in linear stability for elastic structures having insufficient lateral or torsional stiffnesses that cause an out of plane buckling.
Doctorat en sciences appliquées
info:eu-repo/semantics/nonPublished
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Tran, Tony D. "An Investigation of Initially Delaminated Composite Sandwich with Delamination Arrest Mechanism under Buckling Loading." DigitalCommons@CalPoly, 2010. https://digitalcommons.calpoly.edu/theses/428.
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This thesis involves the development of a fiberglass-foam composite sandwich structure with the introduction of delamination arrestment keys; therefore, a study of an initially delaminated composite sandwich structure was the experimental analysis on multiple configurations in how the arrestment keys are placed.
The first part of this thesis research was to the experimental design and manufacturing of the composite sandwich plates. These plates were later cut down to the specific test dimensions and manufacturing processes for the composite sandwich plates and test specimens were created. The composite sandwich plates were manufactured using a vacuum resin infusion process. The dimensions of the composite layup are 14 inches in length with a width of 10.75 inches. The width size has margin to account for machining. The actual dimensions of the test specimen after it is prepared are 14 inches by 0.75 inches. The test anvil length is 11 inches and is used to perform tests to determine mechanical characteristics of the structures under buckling loading. These plates provide approximately 9 to 13 specimens per each case. All the test specimens have 4 plies of 18 oz fiberglass woven roving fabric from Jamestown Distributors, a LAST-A-FOAM FR-6710 foam core, and 5 to 1 ratio of West Systems 105/206 epoxy. Also, a non-porous material was integrated into the structure to create an initial delamination in some of the case studies. The integration of the delamination arrestment keys involve milling the foam core to provide the necessary grooves for key placements before the structure is vacuumed and epoxy is flowed. The arrestment keys are made of unidirectional fiberglass strand and the West Systems 105/206 epoxy using a wet layup process. In addition, fiberglass woven roving specimens were created to see the material characteristics under compression and tensile loading. The same is created to determine the material properties of the foam core, wood boundary core, and arrestment keys under compression loading.
The second part to this thesis investigation is the experimental testing of the test specimens with all different variables considered. Those variables includes determining the final solid cure duration of the fiberglass skin, the geometric lengths between pure compression and pure buckling, behaviors of different initial delamination size, effects of continuous and discontinuous arrestment keys parallel and perpendicular to the in-plane loading, and material properties. The final solid cure duration differ from what the manufacturer gave on their epoxy. This experiment testing followed ASTM D-3039 standard to see the differences in elastic modulus over duration of 15 days. The resulting data shows that the test specimen fully cures after 13 to 14 days. The test specimens in search of the geometric buckling length for this investigation did not follow ASTM C-364 standard in full, but follows a variation of the ASTM C-364 standard in order to support buckling loading condition and the limited accessibility of the test equipments. Instead, the modifications are found with a different test jig design and test specimen configuration. The test jig was created to provide a pinned condition with a 0.25 inch diameter. The test specimen is laid up with a foam and wood cores. Two wood cores are laid at each edge of the foam core to increase loading capacity and holes are drilled through the wood cores to create a pinned-pinned case for the optimum buckling condition. The results detailing the geometric buckling show that after 9 inches anvil length there is no compression; only buckling occurs with a cross-sectional dimension of 0.75 inch by 0.575 inch. The 11 inch foam length was chosen for convenience of machining. This modified setup was also used for testing the different configuration with the embedded arrestment keys. The multiple different configurations completed for these test specimens under unstable loading, the experiment results show that a continuous arrestment key embedded significantly improve the loading capacity over a perfectly sound non-delaminated specimen and maintain the majority of loading capacity even with an introduced delamination. The embedded continuous key also provided a higher horizontal displacement capability before fracture in comparison to the initially delaminated test specimens. As for the test specimens used to determine the material characteristics, ASTM D-3410 and modified ASTM C-364 standards were followed. The test specimens had a fiber volume fraction of approximately 0.60, which details the brittle failure under tensile and compression loading. The results also show that the fibrous fiberglass test specimens have a higher ultimate strength in compression or buckling then in tension.
All of the experimental testing was completed in the Aerospace Engineering Structural/Composite Lab at California Polytechnic State University at San Luis Obispo, California. Therefore, an introduction of a continuous arrestment key parallel to the in-plane loading and embedded into the composite sandwich structure provided a significant increase in loading and buckling capabilities in comparison to the control test specimens with and without an initial delamination and no embedded key. The continuous key placed parallel to the load vector increased the structural strength with an increase of 126% from a 1-inch delaminated structures and only an 11% drop from non-delaminated structures. That is, 1-inch and 2-inch delaminated structures showed a 61% drop and 81% drop from non-delaminated structures. Some configurations have reduced or arrested of the delaminated region.
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Gunawidjaja, Ray. "Organic/inorganic nanostructured materials towards synergistic mechanical and optical properties /." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29733.
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Thesis (Ph.D)--Industrial and Systems Engineering, Georgia Institute of Technology, 2010.Committee Chair: Tsukruk, Vladimir; Committee Member: Bucknall, David; Committee Member: Kalaitzidou, Kyriaki; Committee Member: Shofner, Meisha; Committee Member: Tannenbaum, Rina. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Sears, Nicholas C. "Investigations into the Quasi-Static and Dynamic Properties of Flexible Hybrid Electronic Material Systems." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1525278328687427.
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Xu, Hailan. "Buckling, Postbuckling and Imperfection Sensitivity Analysis of Different Type of Cylindrical Shells by Hui's Postbuckling Method." ScholarWorks@UNO, 2013. http://scholarworks.uno.edu/td/1781.
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Hui and Chen (1986) were the first to show that the well-known Koiter’s General Theory of Elastic Stability of 1945 can be significantly improved by evaluating the postbuckling b coefficient at the actual applied load, rather than at the classical buckling load. Such improvement method was demonstrated to be (1) very simple to apply with no tedious algebra, (2) significant reduction in imperfection sensitivity and (3) although it is still asymptotically valid, there exists a significant extension of the range of validity involving larger imperfection amplitudes. Strictly speaking, Koiter’s theory of 1945 is valid only for vanishingly small imperfection amplitudes. Hence such improved method is termed Hui’s Postbuckling method. This study deals with the postbuckling and imperfection sensitivity of different kinds of cylinders, using the Hui’s postbuckling method. For unstiffened cylinder and laminate cylinder the results are compared with ABAQUS simulation results, and a parameter variation of stringer/ring stiffened cylinder is also evaluated. A significant positive shift of the postbuckling b coefficient is found which indicates that Koiter's general stability theory of 1945 has significantly overestimated the imperfection sensitivity of the structure. Also, compared with the Koiter's general stability theory, the valid region is significantly increased by using Hui's postbuckling method.
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Ananth, Sirija. "Stability Analysis of Additively Manufactured Isogrid." University of Dayton / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1449850405.
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Jackman, Henrik. "Mechanical behaviour of carbon nanostructures." Doctoral thesis, Karlstads universitet, Institutionen för ingenjörsvetenskap och fysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-32041.
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Abstract Carbon nanotubes (CNTs) have extraordinary mechanical and electrical properties. Together with their small dimensions and low density, they are attractive candidates for building blocks in future nanoelectromechanical systems and for many other applications. The extraordinary properties are however only attained by perfectly crystalline CNTs and quickly deteriorate when defects are introduced to the structure. The growth technique affects the crystallinity where in general CNTs grown by arc-discharge are close to perfectly crystalline, while CVD-grown CNTs have large defect densities. Mechanical deformation also affects these properties, even without introducing defects. When CNTs are bent they behave similarly to drinking straws, i.e. they buckle or ripple and their bending stiffness drops abruptly. In this thesis, the mechanical behaviour of individual CNTs and vertically aligned carbon nanofibers (VACNFs) has been studied by performing force measurements inside electron microscopes. Cantilevered CNTs, and VACNFs, were bent using a force sensor, yielding force-deflection curves while their structure was imaged simultaneously. We have found that CNTs grown by arc-discharge have a high enough crystallinity to possess a Young’s modulus close to the ideal value of 1 TPa. CVD-grown CNTs possess a Young’s modulus that is about one order of magnitude smaller, due to their large defect density. The VACNFs are yet another order of magnitude softer as a result of their cup-stacked internal structure. We also found that a high defect density will increase the critical strain for the rippling onset and the relative post-rippling stiffness. Multi-walled CNTs with a small inner diameter are less prone to ripple and have a larger relative post-rippling stiffness. Our findings show large variations in the onset of rippling and the bending stiffness before and after rippling. These variations open up possibilities of tailoring the mechanical properties for specific applications.Baksidetext Carbon nanotubes (CNTs) have extraordinary mechanical and electrical properties. Together with their small dimensions and low density, they are attractive candidates for building blocks in nanoelectromechanical systems (NEMS), and many other applications. In this thesis the mechanical behaviour of individual CNTs and vertically aligned carbon nanofibers has been studied by performing force measurements inside electron microscopes. We have found that the mechanical behaviour is very sensitive to the defect density and the internal structure of the CNTs. The extraordinary properties are only attained by defect free CNTs and quickly deteriorate if defects are introduced to the structure. Mechanical deformations also alter these properties. Single-walled CNTs behave similarly to drinking straws when bent, i.e. they buckle, while the inner tubes of multi-walled CNTs prevent buckling. Instead a more distributed rippling pattern is created for multi-walled CNTs. Both these deformation behaviours will cause an abrupt drop in the bending stiffness, which is detrimental for many applications. The findings in this work will have implications for the design of future NEMS.
Artikel 2 Image formation mechanisms tidigare som manuskript, nu publicerad: urn:nbn:se:kau:diva-16425 (MÅ 150924)
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Lima, e. Silva Rafael Augusto de 1984. "Projeto mecânico de biela automotiva baseado em otimização estrutural." [s.n.], 2013. http://repositorio.unicamp.br/jspui/handle/REPOSIP/265973.
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Orientador: Marco Lúcio BittencourtDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
Made available in DSpace on 2018-08-24T09:08:40Z (GMT). No. of bitstreams: 1 LimaeSilva_RafaelAugustode_M.pdf: 10555723 bytes, checksum: 1d037ebafbabc24e214c06ad76c5f54a (MD5) Previous issue date: 2013
Resumo: O trabalho descreve o projeto mecânico convencional de uma biela automotiva de motor de combustão interna e apresenta uma metodologia alternativa baseada em métodos de otimização estrutural com o objetivo de reduzir a massa do componente. Para tal, é feita a descrição de todos os parâmetros de projeto assim como a definição de critérios de projeto. Em virtude da criticidade da aplicação selecionada, motor de ciclo Diesel utilizado em caminhões de trabalho pesado com picos de pressão de combustão de até 240 bar, a biela foi projetada sem bucha utilizando-se extensivamente os métodos numéricos. O Método dos Elementos Finitos foi aplicado para cálculo de tensões, deslocamentos, pressões de contato, flambagem, fadiga e para a síntese modal de componentes utilizados na análise dinâmica de multicorpos com mancais elasto-hidrodinâmicos. Dois modelos de fadiga dos materiais foram estudados: o modelo americano baseado no diagrama de Goodman e tensões principais e o modelo alemão baseado no diagrama de Haigh e efeito do gradiente de tensões multiaxiais; dos quais concluiu-se que o modelo americano é suficiente para o projeto da biela ao passo que o modelo alemão traz oportunidades adicionais de redução de massa. A otimização topológica, otimização de forma e análise de sensibilidade permitiram a obtenção de uma biela 210g (3\%) mais leve e com melhor desempenho dos mancais hidrodinâmicos. Finalmente, concluiu-se que o Método da Otimização Topológica apresenta oportunidades interessantes aos projetistas na fase de conceituação de produtos como alternativa aos desenhos convencionais, no entanto, demanda esforço adicional para o atendimento de todos os critérios de projeto do componente
Abstract: The present work consists of the conventional mechanical design description of an internal combustion engine connecting rod and also the proposal of an alternative methodology based on optimization methods with the objective of reducing the component mass. Therefore, it is performed a detailed view of all design parameters as well as the definition of design criteria. Because of the critical application selected, Diesel engine of heavy duty truck with combustion pressure reaching up to 240bar, the connecting rod was designed without bushing and with extensive use of numerical methods. The Finite Element Method was applied to assess stresses, displacements, contact pressures, buckling, fatigue and to perform the Component Modal Synthesis for multi-body dynamics simulation with elasto-hydrodynamic bearings. Two fatigue models were studied: the American model based on Goodman's diagram and principal stresses and the German model based on Haigh's diagram and multiaxial stress gradient effect; from which it was concluded that the American model is enough to design the connecting rod, while the German model presents additional weight reduction opportunities. The topology optimization, shape optimization and sensitivity analysis enabled a 210g (3\%) lighter connecting rod with improved bearings performance. Finally, it was concluded that the Topology Optimization Method presents good opportunities for the design engineers in the conceptual phases of product development with alternatives to the conventional designs. However, additional effort is necessary to fit the concept into all design criteria
Mestrado
Mecanica dos Sólidos e Projeto Mecanico
Mestre em Engenharia Mecânica
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Piao, Kun. "An Elevated-Temperature Tension-Compression Test and Its Application to Mg AZ31B." The Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1316096630.
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Siéfert, Emmanuel. "Inflating to shape : from soft architectured elastomers to patterned fabric sheets." Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS018.
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Dans cette thèse à l'interface entre élasticité et géométrie, nous nous attachons à développer, étudier et programmer des structures gonflables élancées qui changent de forme. Une première stratégie consiste à fabriquer des plaques d'élastomère incluant des réseaux de canaux. L’expansion de ces canaux mis sous pression se produit presque exclusivement perpendiculairement à leur direction principale. Le choix de l'orientation et de la densité locales du réseau de canaux permet de contrôler la direction et l'intensité de la pseudo-croissance de cette plaque homogénéisée. Sous pression, la métrique cible de ces objets devient en général incompatible avec l'état plan. La structure flambe alors spontanément pour adopter une forme qui minimise l’énergie élastique, ce qui revient à suivre la métrique imposée par gonflement dans le cas d’objets minces. Une deuxième technique consiste à thermocoller entre elles deux feuilles minces inextensibles selon un réseau de lignes qui définit des canaux gonflables. Le système élémentaire, constitué d'un seul tube formé de deux rubans sinueux joints le long de leurs bords, voit sa courbure amplifiée lors du gonflement. Nous mesurons, expliquons et exploitons cet effet surprenant qui résulte de la maximisation du volume contenu dans le tube sous la contrainte de l'inextensibilité de l'enveloppe. Nous étendons l'étude à des structures bidimensionnelles qui voient la distance entre deux lignes de couture parallèles se contracter lors du gonflement. Le contrôle de la déformation homogénéisée dans le plan permet de programmer le déploiement dans l'espace de surfaces complexes à partir de ces structures initialement planesIn this thesis at the interface between geometry and mechanics, we aim at developing, studying and programming slender morphing inflatables structures. A first strategy consists in manufacturing elastomeric plates embedding a network of channels, which expand, when inflated, mainly perpendicular to their local orientation, similarly to simple elastic tubes. Playing with both the orientation and density of channels, we control the direction and intensity of the in-plane homogenized ``growth", in general incompatible with a flat geometry. The structure spontaneously buckles and adopts a shape which minimizes its elastic energy. For very thin slender bodies, this reduces to follow the target metric induced by inflation. We then study the inflation of structures made of two superimposed inextensible thin sheets, sealed together along a specific line network. Starting with flat curved ribbons, we observe and rationalize the surprising overcurvature upon inflation by maximizing the inner volume given the inextensibility constraint. We finally extend our investigation to two-dimensional structures and control the in-plane contraction upon inflation, which occurs perpendicular to the seam?s direction. We program the morphing of such stiff inflatable structures and investigate their mechanics
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Lainé, Christian. "Étude numérique et expérimentale du comportement mécanique de plaques et coques sandwich : développement d'un élément non-linéaire, grandes transformations et application au flambement de plaques raidies en construction navale." Université Joseph Fourier (Grenoble ; 1971-2015), 1998. http://www.theses.fr/1998GRE10037.
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Cette etude s'interesse au comportement mecanique des structures en materiaux composites epais, utilisees en construction navale (monolithiques epais et sandwichs). Une attention particuliere est apportee au comportement en flambement. La premiere etape consiste a identifier les caracteristiques materiaux. Des essais permettant de determiner la contrainte de compression, les modules de cisaillement transverse ainsi que le comportement non-lineaires sont examines. Une procedure de caracterisation en vue du calcul est proposee. La seconde etape aboutit au developpement de deux elements coques, non lineaires, grands deplacements et grandes deformations. Le champs de cisaillement transverse est decrit soit par une formulation variationnelle, soit par une evolution quadratique dans l'epaisseur. Une ecriture de la loi de comportement en reperage materiel est proposee. Lors de la troisieme etape deux essais sont proposes : plaque sous pression hydrostatique pour la validation du logiciel, flambement de plaques raidies par cisaillement afin de mettre au point une procedure de calcul.
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Catanho, R. V. "An extension to classical lamination theory for buckling and vibration of functionally graded plates." Thesis, 2019. https://hdl.handle.net/10539/29151.
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A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, August 2019An extension to classical lamination theory (CLT) is presented to analyse the natural fre- quencies and critical buckling loads of simply supported functionally graded plates. The variation of the through-thickness properties of the plate is governed by a power law which is subsequently represented by a polynomial series of su cient order and varies according to the law of mixtures or the Mori-Tanaka Homogenization method. The sti ness matrices are found, from which the position of the neutral plane is established which allows for the governing equations for the natural frequency and critical buckling load to be derived using the Rayleigh-Ritz method. The natural frequency and critical buckling loads are determ- ined for various volume indices, aspect and span ratios and the accuracy thereof is validated against 2D, 3D and quasi-3D solutions found in literature. A comparison with CLT found that the present study produces natural frequencies and critical bucking loads which are more accurate and which converge faster than CLT.
NG (2020)
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Aihua, Liang. "Finite element analysis of distortional buckling of cold-formed stainless steel columns." Thesis, 2012. http://hdl.handle.net/10210/4302.
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M.Ing.Because cold-formed stainless steel is a new type of light steel material and dose not have a long history of utilisation in structures, there are many issues that need to be researched and discussed. Making a more thorough investigation and study of cold-formed stainless steels is essential. As a numerical analysis tool, the finite element method proves to be useful in structural analysis. The buckling modes of cold-formed stainless steel members, such as local, flexural, torsional and torsional-flexural buckling, are well known and well documented in design specifications. Distortional buckling is a special kind of buckling mode, which is less well known. Researchers have recently paid more attention to this problem. For stainless steel structures, it is necessary to investigate their behaviour when distortional buckling occurs. In this project, the distortional buckling of cold-formed stainless steel columns under axial compression is investigated. The finite element method is used to analyse and calculate different buckling modes, especially distortional buckling. This is compared to experimental results and other theoretical predictions. The ABAQUS finite element code is used throughout. Finite element modelling is very important prior to processing and analysis. ABAQUS models are created to study distortional buckling. The initial imperfection of structural members is taken into account with these models, using specific sine wave descriptions with respect to different structural parameters. A dynamic processing approach is chosen in the finite element analysis. The effectiveness and accuracy of these models have been verified by both experimental tests and theoretical calculations. Buckling mode and behaviour are predicted and analysed in terms of the finite element models and processes. Suggestions are made for buckling analysis and design based on the research results.
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(9226604), Amin Joodaky. "MECHANICS AND DESIGN OF POLYMERIC METAMATERIAL STRUCTURES FOR SHOCK ABSORPTION APPLICATIONS." Thesis, 2020.
Find full textAbstract:
This body of work examines analytical and numerical models to simulate the response of structures in shock absorption applications. Specifically, the work examines the prediction of cushion curves of polymer foams, and a topological examination of a $\chi$ shape unit cell found in architected mechanical elastomeric metamaterials. The $\chi$ unit cell exhibits the same effective stress-strain relationship as a closed cell polymer foam. Polymer foams are commonly used in the protective packaging of fragile products. Cushion curves are used within the packaging industry to characterize a foam's impact performance. These curves are two-dimensional representations of the deceleration of an impacting mass versus static stress. The main drawback with cushion curves is that they are currently generated from an exhaustive set of experimental test data. This work examines modeling the shock response using a continuous rod approximation with a given impact velocity in order to generate cushion curves without the need of extensive testing. In examining the $\chi$ unit cell, this work focuses on the effects of topological changes on constitutive behavior and shock absorbing performance. Particular emphasis is placed on developing models to predict the onset of regions of quasi-zero-modulus (QZM), the length of the QZM region and the cushion curve produced by impacting the unit cell. The unit cell's topology is reduced to examining a characteristic angle, defining the internal geometry with the cell, and examining the effects of changing this angle.
However, the characteristic angle cannot be increased without tradeoffs; the cell's effective constitutive behavior evolves from long regions to shortened regions of quasi-zero modulus. Finally, this work shows that the basic $\chi$ unit cell can be tessellated to produce a nearly equivalent force deflection relationship in two directions. The analysis and results in this work can be viewed as new framework in analyzing programmable elastomeric metamaterials that exhibit this type of nonlinear behavior for shock absorption.
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"Study on Buckling of Stiff Thin Films on Soft Substrates as Functional Materials." Doctoral diss., 2014. http://hdl.handle.net/2286/R.I.24814.
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abstract: In engineering, buckling is mechanical instability of walls or columns under compression and usually is a problem that engineers try to prevent. In everyday life buckles (wrinkles) on different substrates are ubiquitous -- from human skin to a rotten apple they are a commonly observed phenomenon. It seems that buckles with macroscopic wavelengths are not technologically useful; over the past decade or so, however, thanks to the widespread availability of soft polymers and silicone materials micro-buckles with wavelengths in submicron to micron scale have received increasing attention because it is useful for generating well-ordered periodic microstructures spontaneously without conventional lithographic techniques. This thesis investigates the buckling behavior of thin stiff films on soft polymeric substrates and explores a variety of applications, ranging from optical gratings, optical masks, energy harvest to energy storage. A laser scanning technique is proposed to detect micro-strain induced by thermomechanical loads and a periodic buckling microstructure is employed as a diffraction grating with broad wavelength tunability, which is spontaneously generated from a metallic thin film on polymer substrates. A mechanical strategy is also presented for quantitatively buckling nanoribbons of piezoelectric material on polymer substrates involving the combined use of lithographically patterning surface adhesion sites and transfer printing technique. The precisely engineered buckling configurations provide a route to energy harvesters with extremely high levels of stretchability. This stiff-thin-film/polymer hybrid structure is further employed into electrochemical field to circumvent the electrochemically-driven stress issue in silicon-anode-based lithium ion batteries. It shows that the initial flat silicon-nanoribbon-anode on a polymer substrate tends to buckle to mitigate the lithiation-induced stress so as to avoid the pulverization of silicon anode. Spontaneously generated submicron buckles of film/polymer are also used as an optical mask to produce submicron periodic patterns with large filling ratio in contrast to generating only ~100 nm edge submicron patterns in conventional near-field soft contact photolithography. This thesis aims to deepen understanding of buckling behavior of thin films on compliant substrates and, in turn, to harness the fundamental properties of such instability for diverse applications.Dissertation/Thesis
Ph.D. Mechanical Engineering 2014
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Walker, Mark. "The optimal design of laminated plates for maximum buckling load using finite element and analytical methods." Thesis, 1994. http://hdl.handle.net/10413/9048.
Full textAbstract:
In the first part of the study, finite element solutions are presented for the optimal
design of symmetrically laminated rectangular plates subject to a combination of
simply supported, clamped and free boundary conditions. The design objective
is the maximisation of the biaxial buckling load by determining the fibre orientations
optimally with the effects of bending-twisting coupling taken into account.
The finite element method coupled with an optimisation routine is employed in
analysing and optimising the laminated plate designs. The effect of boundary
conditions, the number of layers and bending-twisting coupling on the optimal
ply angles and the buckling load are numerically studied.
Optimal buckling designs of symmetrically laminated rectangular plates under
in-plane uniaxial loads which have a nonuniform distribution along the edges
are presented in the second part of the study. In particular, point loads, partial
uniform loads and nonuniform loads are considered in addition to uniformly distributed
in-plane loads which provide the benchmark solutions. Poisson's effect
is taken into account when in-plane restraints are present along the unloaded
edges. Restraints give rise to in-plane loads at unloaded edges which lead to biaxial
loading, and may cause premature instability. The laminate behavior with
respect to fiber orientation changes significantly in the presence of Poisson's effect
as compared to that of a laminate where this effect is neglected. This change in
behavior has significant implications for design optimisation as the optimal values
of design variables with or without restraints differ substantially. In the present
study, the design objective is the maximisation of the uniaxial buckling load by
optimally determining the fiber orientations. Numerical results, determined using
the finite element method, are given for a number of boundary conditions
and for uniformly and non-uniformly distributed buckling loads.
In the third part of the study, finite element solutions are presented for the optimal
design of symmetrically laminated rectangular plates with central circular
cut-outs subject to a combination of simply supported, clamped and free boundary
conditions. The design objective is the maximisation of the biaxial buckling
load by determining the fiber orientations optimally. The effect of boundary conditions
and bending-twisting coupling on the optimal ply angles and the buckling
load are numerically studied. The results are compared to those for laminates
without holes.
The fourth part of the present study gives optimal designs of symmetrically
laminated angle-ply plates, which are obtained with the objective of maximising
the initial post buckling stiffness. The design involves optimisation over the ply
angles and the stacking sequence to obtain the best laminate configuration. The
stacking sequence is chosen from amongst five candidate designs. It is shown that
the best configuration depends on the ratio of the in-plane loads in the x and y
directions. Results are also given for two additional configurations which do not
exhibit bending-twisting coupling.
The final section of the present study deals with the optimal design of uniaxially
loaded laminated plates subject to elastic in-plane restraints along the unloaded
edges for a maximum combination of prebuckling stiffness, postbuckling
stiffness and buckling load. This multiobjective study illustrates that improved
buckling and post buckling performance can be obtained from plates which are
designed in this fashion. The multiobjective results are also compared to single
objective design results.Thesis (Ph.D.)-University of Natal, Durban, 1994.
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Frigeri, Ary Vinicius Nervis. "Thermal and mechanical behavior of railway tracks." Master's thesis, 2021. http://hdl.handle.net/10198/23684.
Full textAbstract:
Mestrado de dupla diplomação com a UTFPR - Universidade Tecnológica Federal do ParanáRailways are infrastructures subject to open weather conditions and also to temperature changes during the day and over the season. Due to this change, internal stresses may appear, whether tensile or compressive depending on the stress-free temperature and the current measure. High compressive stress may lead the track to buckle, meanwhile tensile stress can cause brittle failure. Given the importance of the temperature on railways, many models have been developed to correlate weather conditions and rail temperatures, in order to avoid the occurrence of mechanical instabilities which cause major problems in the operation of railroads. The present work validates one model developed by CNU university by comparing it with nite element solutions and also with experimental data of a rail track in the city of Mirandela-Portugal. A python package was developed to solve the model and is available to download. The model shows a good correlation between measured and simulated rail temperatures. In addition, by utilizing weather information of other locations in Portugal, the maximum expected rail temperatures were determined. Furthermore, mechanical analyses were made to analyze the critical temperature to reach the buckling mode of instability without the e ect of rolling loads and also the important parameters that a ect this phenomenon. The simulations show that the quality of the ballast and the initial miss-alignment of the track are the most important. Keywords:
Caminhos de ferro s~ao estruturas expostas a uma grande variedade de condi c~oes clim aticas e, concretamente a varia c~oes de temperatura durante o dia e ao longo das esta c~oes durante o ano. Devido a estas varia c~oes tens~oes internas ocorrem, podendo ser esfor cos de compress~ao ou tra c~ao, dependendo da temperatura neutral do per l. A ocorr^encia de tens~oes de compress~ao elevadas pode causar encurvadura da via, enquanto que os esfor cos de tra c~ao podem ocasionar a fratura fr agil. Devido a import^ancia das temperaturas nas vias f erreas, muitos modelos t^em sido desenvolvidos para correlacionar condi c~oes clim aticas com a temperatura da via tendo em vista o seu uso como ferramenta de preven c~ao de acidentes na opera c~ao. O presente trabalho utiliza um destes modelos, desenvolvido pela universidade CNU e valida-o utilizando solu c~oes com o m etodo dos elementos nitos e, tamb em, com dados experimentais de uma via f errea localizada na cidade de Mirandela-Portugal. Foi ainda desenvolvido um software utilizando a linguagem Python para facilitar a solu c~ao do modelo, estando ainda dispon vel para download. O modelo demonstrou boa correla c~ao entre as temperaturas simuladas e medidas. Al em disso, utilizando informa c~oes meteorol ogicas de outras localidades em Portugal, as temperaturas m aximas esperada das vias foram determinadas. Posteriormente, an alises mec^anicas de encurvadura foram realizadas para determinar em quais temperaturas uma via ferra pode sofrer encurvadura e, tamb em, quais par^ametros que in uenciam este fen^omeno. As simula c~oes mostram que a qualidade do balastro e as imperfei c~oes iniciais da via s~ao os mais importantes.
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Χόρτης, Δημήτριος. "Nonlinear mechanics and finite element with material damping for the static and dynamic analysis of composite wind turbine blades." Thesis, 2012. http://hdl.handle.net/10889/5437.
Full textAbstract:
The aim of the current dissertation is the development of finite element models capable of predicting the damping and the damped structural dynamic response of laminated composite blades and beams. The present thesis is divided into two main parts, of which the first one studies the material coupling effect on the static and modal characteristics of composite structures. New damping coupling terms are formulated and incorporated into a linear beam finite element to better capture the composite material and structural coupling effects.
The second part describes the theoretical framework for predicting the nonlinear damping and damped vibration of laminated composite structures due to large in-plane tensile and compressive forces. A nonlinear beam finite element for composite strips is developed capable of capturing the effects of geometric nonlinearity on the damping of composite laminates. The damping mechanics consider a strain based Kelvin viscoelastic model and Green-Lagrange nonlinear strain expressions, which introduce geometric nonlinearity into the formulation. Incorporation of first-order shear deformation theory into the equations of motion provides the linear and new nonlinear cross-section stiffness and damping terms. Within each element, the stain field is approximated by linear interpolation shape functions. An incremental-iterative methodology is formulated into the finite element solver, based on the Newton-Raphson technique in order to obtain the system solution at each iteration, till the final convergence is achieved. For the sake of completeness, a series of experimental measurements were carried out for the composite strip, subject to tensile and buckling loads. Correlations with theoretical predictions gave credence to the ability of the nonlinear finite element to predict damping of composite structures undergoing large displacements and rotations in the nonlinear regime. The finite element was further extended to include the nonlinear analysis of large-scale hollow composite structures. New first- and second-order stiffness and damping terms were developed and incorporated into an updated nonlinear beam finite element, capable of capturing the effect of rotational stresses on the static and modal characteristics of composite beams and blades.Σκοπός της παρούσας διδακτορικής διατριβής με τίτλο: "Ανάπτυξη Μη-Γραμμικού Προτύπου Πεπερασμένου Στοιχείου με Απόσβεση για τη Στατική και Δυναμική Ανάλυση Πτερυγίων Ανεμογεννητριών" είναι η ανάπτυξη προτύπων πεπερασμένων στοιχείων με απόσβεση ικανών να προβλέπουν τη στατική και δυναμική απόκριση δοκών και πτερυγίων από σύνθετα υλικά. Η εργασία επικεντρώνεται σε δύο κύριες κατευθύνσεις, που αφορούν τόσο την εισαγωγή νέων όρων στο μητρώο απόσβεσης ενός πεπερασμένου στοιχείου δοκού, όσο και την ανάπτυξη ενός μη-γραμμικού κώδικα πεπερασμένου στοιχείου για τη μελέτη της μη-γραμμικής συμπεριφοράς δοκών και πτερυγίων από σύνθετα υλικά που υπόκεινται σε μεγάλες μετατοπίσεις και περιστροφές. Στο πρώτο μέρος της διατριβής μελετάται η επίδραση της σύζευξης, λόγω της ανισοτροπίας του σύνθετου υλικού, τόσο στη στατική απόκριση όσο και στα μορφικά χαρακτηριστικά κατασκευών από σύνθετα υλικά, διαφόρων διατομών και γεωμετριών. Διατυπώνονται νέοι όροι απόσβεσης που εκφράζουν την εν λόγω σύζευξη και οι οποίοι καθιστούν το γραμμικό πεπερασμένο στοιχείο δοκού πιο πλήρες στην επίλυση προβλημάτων όπου η σύζευξη υλικού επηρεάζει τη συμπεριφορά της κατασκευής. Στο δεύτερο και πλέον σημαντικό μέρος της παρούσας διατριβής αρχικά περιγράφεται το θεωρητικό υπόβαθρο για την πρόβλεψη της μη-γραμμικής δυναμικής απόσβεσης λεπτών δοκών κατασκευασμένα από σύνθετα υλικά οι οποίες υπόκεινται σε μεγάλα συν-επίπεδα εφελκυστικά φορτία ή φορτία λυγισμού. Αναπτύσσεται νέο πεπερασμένο στοιχείο ικανό να περιγράψει την επίδραση της γεωμετρικής μη-γραμμικότητας στην απόσβεση και τη δυσκαμψία της δοκού. Εφαλτήριο για την ανάπτυξη αυτής της μεθοδολογίας ήταν η ανάγκη της πρόβλεψης της δυναμικής απόσβεσης σε κατασκευές από σύνθετα υλικά με πιο πολύπλοκη και εύκαμπτη γεωμετρία, όπως αυτή των πτερυγίων ανεμογεννητριών. Η ανάπτυξη του μη-γραμμικού πεπερασμένου στοιχείου ξεκινά από το επίπεδο της στρώσης του υλικού, όπου διατυπώνονται οι καταστατικές εξισώσεις θεωρώντας το ιξωδοελαστικό πρότυπο του Kelvin για το υλικό της κατασκευής. Στη συνέχεια εισάγονται οι Green-Lagrange εξισώσεις συμβιβαστού οι οποίες εκφράζουν τη γεωμετρική μη-γραμμικότητα καθώς και οι εξισώσεις κίνησης. Σε επίπεδο διατομής, οι κινηματικές υποθέσεις βασίζονται στις παραδοχές της διατμητικής θεωρίας δοκού πρώτης τάξης. Η πρόβλεψη της μη-γραμμικής απόκρισης μιας κατασκευής από σύνθετα υλικά επιτυγχάνεται μέσω της προσομοίωσης της με έναν επαρκή αριθμό πεπερασμένων στοιχείων. Στο εσωτερικό κάθε στοιχείου οι παραμορφώσεις προσεγγίζονται από γραμμικές συναρτήσεις μορφής, οι οποίες οδηγούν στη μητρωική μορφή των μη-γραμμικών εξισώσεων του συστήματος. Λόγω του γεγονότος ότι οι εξισώσεις αυτές εξαρτώνται από τη λύση, δεν μπορούν να λυθούν απευθείας κάτι που καθιστά αναγκαία τη χρήση μιας σταδιακής-επαναληπτικής τεχνικής. Στην παρούσα διατριβή εισάγεται στο λύτη του μη-γραμμικού κώδικα η Newton-Raphson τεχνική. Το επόμενο βήμα αφορά τη σύνθεση των ολικών δομικών μητρών του συστήματος και την εφαρμογή των συνοριακών συνθηκών. Σε κάθε επανάληψη λαμβάνει χώρα η επίλυση των γραμμικοποιημένων εξισώσεων και ο υπολογισμός των πραγματικών και εφαπτομενικών μη-γραμμικών μητρώων δυσκαμψίας και απόσβεσης της κατασκευής, τα οποία τελικώς επιλύονται με τη μέθοδο της αριθμητικής ολοκλήρωσης κατά Gauss. Το πεπερασμένο στοιχείο δοκού εξελίχθηκε περαιτέρω ώστε να συμπεριλάβει τη μη-γραμμική ανάλυση μεγάλων λεπτότοιχων κατασκευών από σύνθετα υλικά, όπως αυτά των πτερυγίων ελικοπτέρων και ανεμογεννητριών. Η εισαγωγή της πλήρους έκφρασης της αξονικής μη-γραμμικής Green-Lagrange παραμόρφωσης στη διατύπωση των κινηματικών υποθέσεων οδηγεί στην πλήρη έκφραση των πραγματικών και εφαπτομενικών δομικών μητρών της κατασκευής. Οι νέοι μη-γραμμικοί όροι δυσκαμψίας και απόσβεσης πρώτης και δεύτερης τάξης μπορούν να περιγράψουν την επίδραση των εσωτερικών εφελκυστικών τάσεων στα μορφικά χαρακτηριστικά δοκών και πτερυγίων. Το μη-γραμμικό πεπερασμένο στοιχείο είναι ικανό να χαρακτηρίσει τη στατική συμπεριφορά και την αποσβενυμένη ταλάντωση δοκών από σύνθετα υλικά. Η επαλήθευση του μη-γραμμικού κώδικα πραγματοποιήθηκε μέσω μιας σειράς πειραματικών μετρήσεων που αφορούσαν τη μέτρηση της φυσικής συχνότητας και της μορφικής απόσβεσης σε λεπτές δοκούς από σύνθετα υλικά τόσο σε εφελκυσμό όσο και σε συνθήκες λυγισμού. Τα πειραματικά αποτελέσματα έρχονται σε πολύ καλή συμφωνία με τις θεωρητικές προβλέψεις του κώδικα κάτι που εξασφαλίζει την αξιοπιστία του μη-γραμμικού πεπερασμένου στοιχείου.
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Βαρέλης, Δημήτρης. "Ανάπτυξη μεθόδου πεπερασμένων στοιχείων για την επίλυση της σύζευξης μη γραμμικής συμπεριφοράς ευφυών πλακών και κελυφών με πιεζοηλεκτρικά στοιχεία." 2004. http://nemertes.lis.upatras.gr/jspui/handle/10889/283.
Full textAbstract:
Περίληψη Σκοπός της παρούσας διδακτορικής διατριβής είναι η διατύπωση µοντέλων µηχανικής και η ανάπτυξη µεθοδολογίας πεπερασµένων στοιχείων, για τηv αριθµητική επίλυση τoυ προβλήµατος της συζευγµένης µη-γραµµικής απόκρισης πιεζοηλεκτρικών κελυφών και πλακών µε εµφυτευµένα πιεζοηλεκτρικά στοιχεία. Η ανάπτυξη της παρούσας µεθόδου στηρίχθηκε σε θεωρίες µεσοµηχανικής για τη ανάλυση στρωµατοποιηµένων πιεζοηλεκτρικών κελυφών και κατά επέκταση πλακών και δοκών. Πιο συγκεκριµένα διατυπώνονται σε επίπεδο στρώσης, οι καταστατικές εξισώσεις του ηλεκτροµηχανικού πεδίου, οι εξισώσεις συµβιβαστού των παραµορφώσεων-µετατοπίσεων, που εµπεριέχουν την γεωµετρική µη γραµµικότητα, καθώς και οι γενικευµένες εξισώσεις κίνησης (εξισώσεις ισορροπίας των τάσεων στο µηχανικό και διατήρησης ηλεκτρικού φορτίου στο ηλεκτρικό πεδίο). Στη συνέχεια δύναται να γραφούν οι παραπάνω εξισώσεις κίνησης σε ολοκληρωτική µορφή, µε την βοήθεια της αρχής των φανταστικών µετατοπίσεων, ώστε να ισχύουν για ολόκληρη την πιεζοηλεκτρική πολύστρωτη δοµή. Τα παραπάνω ολοκληρώµατα όγκου υποβιβάζονται σε ολοκληρώµατα επιφάνειας µε την εισαγωγή των κινηµατικών υποθέσεων για τις ελαστικές και ηλεκτρικές µεταβλητές κατάστασης. Για την επίλυση των παραπάνω συζευγµένων µη γραµµικών ολοκληρωτικών εξισώσεων αναπτύχθηκε µέθοδος πεπερασµένων στοιχείων. ∆υο 8-κοµβα συζευγµένα µη γραµµικά ισοπαραµετρικά πεπερασµένα στοιχεία κελύφους και πλάκας αναπτύσσονται. Στο εσωτερικό των στοιχειών το παραµορφωσιακό πεδίο προσεγγίζεται µε πολυώνυµικές εξισώσεις δευτέρου βαθµού, που ονοµάζονται συναρτήσεις µορφής. Με την βοήθεια των συναρτήσεων µορφής προκύπτουν οι συζευγµένες µη γραµµικές εξισώσεις σε µητρωική µορφή, και λόγω του ότι εξαρτώνται από τη λύση δεν µπορούν να λυθούν απευθείας αλλά χρησιµοποιείται µια σταδιακή- επαναληπτική µέθοδος βασισµένη στη Newton-Raphson τεχνική. Αφού πραγµατοποιηθεί η σύνθεση των ολικών µητρώων, εφαρµοστούν οι µηχανικές και ηλεκτρικές συνοριακές συνθήκες τελικά επιλύονται οι προκύπτουσες γραµµικοποιηµένες συζευγµένες εξισώσεις σε κάθε επανάληψη εως ότου επιτευχθεί σύγκλιση της λύσης. Σε κάθε επανάληψη υπολογίζονται ταπραγµατικά και εφαπτοµενικά µη γραµµικά µητρώα καθώς επίσης και τα διανύσµατα ανισορροπίας µεταξύ των εξωτερικών και εσωτερικών δυνάµεων και ηλεκτρικών φορτίων. Τα µη γραµµικά ελαστικά και πιεζοηλεκτρικά µητρώα, που εµπεριέχουν τη γεωµετρική µη γραµµικότητα, καθώς και τα γραµµικά επιλύονται αριθµητικά µε τη µέθοδο Gauss. Η παρούσα µέθοδος µπορεί να εφαρµοστεί για τη διερεύνηση και αριθµητική επίλυση µιας σειράς προβληµάτων ευφυών πιεζοηλεκτρικών κατασκευών, όπου η γεωµετρική µη γραµµικότητα (µεγάλες µετατοπίσεις και περιστροφές σε σχέση µε το πάχος, αλλά µικρές παραµορφώσεις) παίζει σηµαντικό ή πρωτεύοντα ρόλο, µε ιδιαίτερη έµφαση στα εξής προβλήµατα: Μοντελοποίηση ευφυών κατασκευών υπό µεγάλη κάµψη. Εφαρµογές σε κατασκευές, στις οποίες επιδιώκονται µεγάλες αλλαγές στο σχήµα τους µέσω µεγάλων ενεργών µετατοπίσεων και περιστροφών, υπό την επιβολή ηλεκτρικού πεδίου στους πιεζοηλεκτρικούς διεγέρτες (morphing structures) . Πρόβλεψη κρίσιµων επίπεδων µηχανικών δυνάµεων και ηλεκτρικών τάσεων λυγισµού, οι οποίες µπορεί να οδηγήσουν τις ευφυείς πλάκες και τα κελύφη σε συνθήκες λυγισµού και απώλειας ευστάθειας. Πρόβλεψη και προσοµοίωση του λυγισµού και µετα-λυγισµού σε panel αεροναυπηγικών κατασκευών, µέσω παρακολούθησης της µεταβολής των φυσικών συχνοτήτων της κατασκευής ή της αναπτυσσόµενης ηλεκτρικής τάσης στους πιεζοηλεκτρικούς αισθητήρες. Την ενεργή µεταβολή της δυσκαµψίας (αύξηση ή µείωση) ευφυών κατασκευών µε την επιβολή κατάλληλου ηλεκτρικού δυναµικού στους πιεζοηλεκτρικούς διεγέρτες. Πρόβλεψη της µετάβασης των πιεζοηλεκτρικών κελυφών από τη µια θέση ισορροπίας σε άλλη (snap-through), υπό την επιβολή µηχανικού φορτίου ή πιεζοηλεκτρικής καµπτικής ροπής µέσω των πιεζοηλεκτρικών διεγερτών.πραγµατικά και εφαπτοµενικά µη γραµµικά µητρώα καθώς επίσης και τα διανύσµατα ανισορροπίας µεταξύ των εξωτερικών και εσωτερικών δυνάµεων και ηλεκτρικών φορτίων. Τα µη γραµµικά ελαστικά και πιεζοηλεκτρικά µητρώα, που εµπεριέχουν τη γεωµετρική µη γραµµικότητα, καθώς και τα γραµµικά επιλύονται αριθµητικά µε τη µέθοδο Gauss. Η παρούσα µέθοδος µπορεί να εφαρµοστεί για τη διερεύνηση και αριθµητική επίλυση µιας σειράς προβληµάτων ευφυών πιεζοηλεκτρικών κατασκευών, όπου η γεωµετρική µη γραµµικότητα (µεγάλες µετατοπίσεις και περιστροφές σε σχέση µε το πάχος, αλλά µικρές παραµορφώσεις) παίζει σηµαντικό ή πρωτεύοντα ρόλο, µε ιδιαίτερη έµφαση στα εξής προβλήµατα: Μοντελοποίηση ευφυών κατασκευών υπό µεγάλη κάµψη. Εφαρµογές σε κατασκευές, στις οποίες επιδιώκονται µεγάλες αλλαγές στο σχήµα τους µέσω µεγάλων ενεργών µετατοπίσεων και περιστροφών, υπό την επιβολή ηλεκτρικού πεδίου στους πιεζοηλεκτρικούς διεγέρτες (morphing structures) . Πρόβλεψη κρίσιµων επίπεδων µηχανικών δυνάµεων και ηλεκτρικών τάσεων λυγισµού, οι οποίες µπορεί να οδηγήσουν τις ευφυείς πλάκες και τα κελύφη σε συνθήκες λυγισµού και απώλειας ευστάθειας. Πρόβλεψη και προσοµοίωση του λυγισµού και µετα-λυγισµού σε panel αεροναυπηγικών κατασκευών, µέσω παρακολούθησης της µεταβολής των φυσικών συχνοτήτων της κατασκευής ή της αναπτυσσόµενης ηλεκτρικής τάσης στους πιεζοηλεκτρικούς αισθητήρες. Την ενεργή µεταβολή της δυσκαµψίας (αύξηση ή µείωση) ευφυών κατασκευών µε την επιβολή κατάλληλου ηλεκτρικού δυναµικού στους πιεζοηλεκτρικούς διεγέρτες. Πρόβλεψη της µετάβασης των πιεζοηλεκτρικών κελυφών από τη µια θέση ισορροπίας σε άλλη (snap-through), υπό την επιβολή µηχανικού φορτίου ή πιεζοηλεκτρικής καµπτικής ροπής µέσω των πιεζοηλεκτρικών διεγερτών.πραγµατικά και εφαπτοµενικά µη γραµµικά µητρώα καθώς επίσης και τα διανύσµατα ανισορροπίας µεταξύ των εξωτερικών και εσωτερικών δυνάµεων και ηλεκτρικών φορτίων. Τα µη γραµµικά ελαστικά και πιεζοηλεκτρικά µητρώα, που εµπεριέχουν τη γεωµετρική µη γραµµικότητα, καθώς και τα γραµµικά επιλύονται αριθµητικά µε τη µέθοδο Gauss. Η παρούσα µέθοδος µπορεί να εφαρµοστεί για τη διερεύνηση και αριθµητική επίλυση µιας σειράς προβληµάτων ευφυών πιεζοηλεκτρικών κατασκευών, όπου η γεωµετρική µη γραµµικότητα (µεγάλες µετατοπίσεις και περιστροφές σε σχέση µε το πάχος, αλλά µικρές παραµορφώσεις) παίζει σηµαντικό ή πρωτεύοντα ρόλο, µε ιδιαίτερη έµφαση στα εξής προβλήµατα: Μοντελοποίηση ευφυών κατασκευών υπό µεγάλη κάµψη. Εφαρµογές σε κατασκευές, στις οποίες επιδιώκονται µεγάλες αλλαγές στο σχήµα τους µέσω µεγάλων ενεργών µετατοπίσεων και περιστροφών, υπό την επιβολή ηλεκτρικού πεδίου στους πιεζοηλεκτρικούς διεγέρτες (morphing structures) . Πρόβλεψη κρίσιµων επίπεδων µηχανικών δυνάµεων και ηλεκτρικών τάσεων λυγισµού, οι οποίες µπορεί να οδηγήσουν τις ευφυείς πλάκες και τα κελύφη σε συνθήκες λυγισµού και απώλειας ευστάθειας. Πρόβλεψη και προσοµοίωση του λυγισµού και µετα-λυγισµού σε panel αεροναυπηγικών κατασκευών, µέσω παρακολούθησης της µεταβολής των φυσικών συχνοτήτων της κατασκευής ή της αναπτυσσόµενης ηλεκτρικής τάσης στους πιεζοηλεκτρικούς αισθητήρες. Την ενεργή µεταβολή της δυσκαµψίας (αύξηση ή µείωση) ευφυών κατασκευών µε την επιβολή κατάλληλου ηλεκτρικού δυναµικού στους πιεζοηλεκτρικούς διεγέρτες. Πρόβλεψη της µετάβασης των πιεζοηλεκτρικών κελυφών από τη µια θέση ισορροπίας σε άλλη (snap-through), υπό την επιβολή µηχανικού φορτίου ή πιεζοηλεκτρικής καµπτικής ροπής µέσω των πιεζοηλεκτρικών διεγερτών.-