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Dissertations / Theses on the topic 'Shells'
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1
Scott, Joline L. "Shells." Cleveland State University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=csu1285194565.
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Caresta, Mauro Mechanical & Manufacturing Engineering Faculty of Engineering UNSW. "Structural and acoustic responses of a submerged vessel." Publisher:University of New South Wales. Mechanical & Manufacturing Engineering, 2009. http://handle.unsw.edu.au/1959.4/44404.
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Excitation of the low frequency vibrational modes of a submerged vessel can generate significant radiated noise levels. Vibrational modes of a submarine hull are excited from the transmission of fluctuating forces through the shaft and thrust bearings due to the propeller rotating in an unsteady fluid. The focus of this work is to investigate the structural and acoustic responses of a submarine hull under axial excitation. The submarine hull is modelled as a cylindrical shell with internal bulkheads and ring stiffeners. The cylindrical shell is closed by truncated conical shells, which in turn are closed at each end using circular plates. The entire structure is submerged in a heavy fluid medium. The structural responses of the submerged vessel are calculated by solving the cylindrical shell equations of motion using a wave approach and the conical shell equations with a power series solution. The displacement normal to the surface of the structure in contact with the fluid medium was calculated by assembling the boundary/continuity matrix. The far field radiated sound pressure was then calculated by means of the Helmholtz integral. Results from the analytical model are compared with computational results from a fully coupled finite element/boundary element model. The individual and combined effects of the various influencing factors, corresponding to the ring stiffeners, bulkheads, conical end closures and fluid loading, on the structural and acoustic responses are characterised by examining the contribution by the circumferential modes. It is shown that equally spaced internal bulkheads generate a periodic structure thus creating a grouping effect for the higher circumferential modes, but do not have strong influence on the sound radiation. Stiffeners are found to have an important effect on both the dynamic and acoustic responses of the hull. The contribution of the conical end closures on the radiated sound pressure for the lowest circumferential mode numbers is also clearly observed. This work shows the importance of the bending modes when evaluating the sound pressure radiated by a submarine under harmonic excitation from the propulsion system.
3
Loukaides, Evripides George. "Elementary morphing shells." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/269724.
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Multistable shells are not yet completely understood. Even under the widespread Uniform Curvature (UC) assumption, the limits of this behaviour have not been established and the influence of individual material and geometric parameters has not been described conclusively; this research explores these open questions. In addition, this project was motivated by the need for practical design guidelines and the pursuit of alternative construction and actuation methods for multistable shells. Our analysis is based on an expression for the strain energy of a shell under a set of simplifying assumptions— primarily the aforementioned UC assumption. We extend this concept beyond the work of previous authors by admitting a more diverse range of anisotropic materials. Furthermore, we take advantage of some aspects of the mathematical field of Catastrophe Theory (CT) to maximise the generality of available results. When appropriate, we examine aspects of our predictions by constructing relevant shell structures, with particular focus on material considerations. A commercial Finite Element Analysis package provides additional means of analysis and comparison. On the theoretical front, the influence of certain control parameters on the availability of multistability is described in closed-form while a unique graphical overview of the limits of this behaviour is provided. In the lab, a novel tristable shell is constructed from a laminate and the use of specialized materials is scrutinised. In a subsequent project, a bistable spherical cap made from a customized material is actuated by a magnetic field—the ensuing snap-through event is recorded with a high-speed camera, leading to valuable insights on the transition geometry. Furthermore, we confirm the possibility of bistability for developable, non-prestressed shells, composed of a single material, using grid shells and thin honeycomb shells.
4
McDaniel, James Gregory. "A new higher-order shell theory for vibration and viscoelastically-coated circular cylindrical shells." Diss., Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/15825.
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Li, Jian-Min [Verfasser], and Jan [Akademischer Betreuer] Knippers. "Timber shell structures : form-finding and structural analysis of actively bent grid shells and segmental plate shells / Jian-Min Li ; Betreuer: Jan Knippers." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2017. http://d-nb.info/1138234893/34.
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6
Sadr-Hashemi, Farshid. "Buckling of conical shells." Thesis, University College London (University of London), 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.685403.
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7
Htet, Aung Thu. "Full-body Shell Creation for CAD Virtual Humans including Tightly-Spaced, Enclosed Shells." Digital WPI, 2017. https://digitalcommons.wpi.edu/etd-theses/1264.
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Computational human models have become essential in several different biomedical and electrical engineering research areas. They enable scientists to study, model, and solve complex problems of human body responses to various external stimuli including electromagnetic and radio-frequency signals.
This study describes the algorithms and procedures of creating multi-tissue full-body Computer-Aided Design (CAD) human models. An emphasis is made on full-body shells of variable thicknesses, e.g. skin, fat, and average body container shells. Such shells, along with internal organs, are useful for multiple high- and low-frequency simulations in a variety of applications.
Along with the creation of full-body models, an automatic algorithm to selectively decimate the meshes based on average surface curvature is developed. The algorithm will significantly reduce model size while keeping the same interpolation accuracy.
8
Kolahi, Abdolahad Salehi. "Co-rotational methods for small and large strain beams, sheets and shells." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299133.
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Shin, Haksik. "Nonlinear analysis of axisymmetric shells." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape4/PQDD_0017/MQ58505.pdf.
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10
Oudmaijer, René Dick. "Evolved stars with circumstellar shells." [S.l. : [Groningen] : s.n.] ; [University Library Groningen] [Host], 1995. http://irs.ub.rug.nl/ppn/141387866.
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11
Zhao, Jiabin. "Natural theory of nonlinear shells." Thesis, McGill University, 1996. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=40299.
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This thesis is devoted to the analysis of nonlinear shell problems. First, using the tangential differential calculus and the oriented boundary distance function, we derive two intrinsic nonlinear models for thin/shallow shells made up of a Saint Venant-Kirchhoff material or a homogeneous, isotropic, elastic material. The models extend the natural theory of plates to thin/shallow shells. Next, we prove the regularity property for the solutions of the linear model, and differentiability of the nonlinear operators associated with the nonlinear models in the given spaces. Finally, we prove existence and uniqueness theorems for the solution of the two nonlinear models using the inverse function theorem. Moreover, we discuss the Love-Kirchhoff theory and the Naghdi's and Koiter's conditions for nonlinear shells.
12
Ifayefunmi, Olawale Friday. "Combined stability of conical shells." Thesis, University of Liverpool, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.569897.
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This thesis is concerned with the study of the elastic-plastic buckling of short and relatively thick conical shells subjected to combined loading, i.e., axial compression and external pressure acting simultaneously. This is both numerical and experimental study. Within the context of numerical study, a nonlinear finite element calculations were carried out in order to obtain: (i) the failure loads of cones under axial compression only, external pressure only and under combined loading, (ii) the spread of plastic strain and the effect of strain hardening of the material on failure loads, and (iii) the sensitivity of buckling loads to initial geometric imperfections or to structural defects under various loading conditions. The thesis provides results of extensive FE calculations. An experimental programme involved tests on thirteen conical specimens CNC machined with integral top and bottom flanges from 252mm diameter steel billet. The specimens were made from mild steel material with average yield stress of 230.6 MPa, Young's modulus of 21 0490 MPa and Poisson' s ratio of 0.28l. Prior to tests, the existing test rig had to be significantly modified and instrumented in order to accommodate independent/combined loadings. The test procedure has been developed and successfully implemented. Two models were subjected to axial compression, with further two subjected to external pressure. The remaining nine cones were subjected to combined action of axial compression and external pressure. Experimental results were compared with predictions of failure loads obtained from the existing design codes. For the case of axial compression an extension of the design rules is outlined in order to widen the range of applicability. For the case of external pressure, the test data compared well with the theoretical work by Esslinger and Van Impe, [40]. At the same time the test data highlighted how inadequate estimates of the load carrying capacity are given by the design codes. The case of combined loading, i.e., axial compression and external pressure is only covered by ASME code case 2286-2, [157], and experimental data does not exist. The current study provides the first and much needed test data. The thesis also looks into the concept of equivalent cylinder. Numerical results point out to the fact that this approach is unsuitable for combined stability scenario (axial compression and external pressure). Experimental data is also compared with predictions given by the Finite Element calculations. Details about various approaches to modeling material properties, shape, wall thickness distribution, and boundary conditions are discussed. The quality of FE models is assessed by comparing the FE predictions of the load carrying capacity with the test data.
13
Lee, S. J. "Analysis and optimization of shells." Thesis, Swansea University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637865.
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Three computer programs are written, implemented and tested in this study: (a) shell finite element analysis program, LFEAP, for linear static, free vibration and geometrically nonlinear analyses, (b) a shape and size optimization program, LIBRA-DS, based on mathematical programming methods, and (c) a topology optimization program, LIBRA-OTS, based on optimality criteria method with layered artificial material model. A rigorous study of various shell element formulations is presented. In particular, an assumed natural strain shell element is developed and two well-known and reliable shell elements are employed to provide benchmarks for linear static and free vibration analyses. In addition, a shell element based on the element-based Lagrangian formulation is developed for post-optimization analysis where the nonlinear buckling loads of optimized shells are investigated. For the shape and size optimizations, consideration is given to the role of automatic mesh generation in conjunction with various surface definitions such as Coons patch and Bezier surface representations. Design sensitivity analysis is carried out using finite difference and semi-analytical methods. Appropriate perturbation step sizes are determined from a series of benchmark tests. The basic concepts of mathematical programming methods available in the adopted optimizer are explained. Various benchmark tests are then carried out to prove the capability of the developed shell design optimization system. For the topology optimization, a layered artificial material model is proposed and a standard resizing algorithm based on an optimality criteria method is employed. Numerous benchmark examples are presented to show the performance of the proposed methodology for shell topology optimization in various situations. The influence of various optimization parameters used in the adopted resizing algorithm on topology optimization process is also investigated. An algorithm for integrated design optimization is proposed and implemented to fine the stiffest shape, size or topology of various shells under linear static conditions.
14
Strutt, Jonathan Howard. "Spherical thin shells in relativity." Thesis, University of York, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.258371.
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15
Mesnil, Romain M. Eng Massachusetts Institute of Technology. "Stability of elastic grid shells." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82716.
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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 83-86).
The elastic grid shell is a solution that combines double curvature and ease of mounting. This structural system, based on the deformation of an initially at grid without shear stiffness was invented more than fifty years ago. The apparition of new materials such as GFRP increased the potential of such structures whose properties depend on the deformation, or equivalently pre-stress of an initial structure. Elastic grid shells seem particularly promising as shelters, lightweight roofs, or kinetic structures. Although fundamental to the behavior of the strucure, the influence of the pre-stress on the stability of elastic grid shells has yet to be studied. Understanding this phenomenon could allow engineers to design more efficiently elastic grid shells. This thesis studies the influence of pre-stress on the stability of elastic grid shells. The research conducts a parametric study that focuses both a pre-buckled arch and initially at circular elastic grid shells with dierent grid spacing and levels of pre-stress. Realistic values of the parameters are determined from existing projects. The buckling analysis as well as the form-finding of the different structures are performed using finite element analysis. The tools are validated with comparison of the shape and buckling capacity of a pre-buckled arch with existing experiments. The parametric studies lead to recommendations aiming to facilitate the design of elastic grid shells. Keywords Elastic grid shell, Low-Speed Dynamics, form-finding, linear buckling analysis
by Romain Mesnil.
M.Eng.
16
Smits, Derck Peter. "Atomic processes in nova shells." Doctoral thesis, University of Cape Town, 1990. http://hdl.handle.net/11427/16000.
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Bibliography: pages 155-165.The unusual spectra of the spatially-resolved nova shells of DQ Her, T Aur and CP Pup are reviewed. Because calculations for these conditions have not been made, recombination models of HI, HeI and CNO from a neutral to a doubly ionized state were constructed. The models are described and the results presented for densities at various temperatures. A photoionization model is also described. The effects of non-uniform density distributions in nova shells have been investigated and the observational consequences discussed. A model of the CP Pup shell is described and the evolution of this plasma during the nova's constant luminosity phase traced. The properties of neon novae are summarized and compared with the features seen in the optical spectra of old classical novae. Enhanced neon abundances are used in the model of the CP Pup Shell to investigate its effect on the evolution of the shell. Finally, the results of some near-infrared observations made on the shells of CP Pup, RR Pic and T Pyx are reported.
17
Goh, Julian Kok Seng. "Analysis of Pressurized Arch-Shells." Thesis, Virginia Tech, 1998. http://hdl.handle.net/10919/35576.
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A pressurized arch-shell structural component made of flexible material is considered.
The component is inflated with high internal pressure. The behavior of similar types
of structures, such as a pair of leaning pressurized arches and pressurized arch-supported
membrane shelters, has been investigated in the past. More recently, several types of
pressurized structures have been incorporated as part of the framework for a variety of
structural systems. Particularly, the U.S. Army has been investigating the use of large
lightweight and transportable pressurized arch-shell structures to be used as maintenance
shelters for vehicles, helicopters, and airplanes.
The formulated equations using thin shell theory are applied to a pressurized arch-shell
component. A numerical investigation based on the Rayleigh-Ritz method is utilized to
determine the behavior of arch-shells under various types of loading. The types of loading
include a uniformly distributed vertical load representing snow, a wind load, and a
horizontal side load distributed along the arc length. Deflections, stress resultants, and
moments at various locations are computed for two types of shapes: circular and non-circular
arch-shells.Master of Science
18
Mohamedien, Mohamed Ahmed. "The buckling of grid shells." Thesis, University of Bath, 1990. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.254000.
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19
Chebili, Rachid. "The boundary support of shells." Thesis, University of Bath, 1991. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279412.
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20
Al, lawati Hussain Ali Redha Mohammed. "The buckling of axially compressed cylindrical shells under different conditions." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/25723.
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Civil Engineering thin cylindrical shells such as silos and tanks are normally subjected to axial compression that arises from a stored solid, wind, earthquake, self-weight or roof loads. The walls of these shells are very thin, generally of the order of 6 to 25 mm, and massively less than the radius, which is typically 5 to 30 m. They are thus very thin shell structures, like those of rockets, spacecraft, motor vehicles and aircraft. The commonest failure mode is elastic buckling under axial compression. It has long been known that the buckling strength of a thin cylindrical shell under axial compression is very sensitive to tiny deviations of geometry, reducing the buckling strength to perhaps 10 or 20% of the value for the perfect structure. A normal internal pressure usually accompanies the axial compression, caused by stored granular solids or fluids. At relatively low pressures, the elastic buckling strength under axial compression rises, but an elastic-plastic buckling phenomenon intervenes at higher pressures, causing a dramatic decrease in buckling resistance associated with an elephant’s foot collapse mode. To construct such large shells, the fabrication technique is generally the assembly of many rolled plates or panels, joined by short longitudinal welds and continuous circumferential welds. The process of welding produces a distinctive geometric imperfection form at each weld joint, which in turn is extremely detrimental to the shell axial buckling carrying capacity. The strength may be further reduced by slight misalignments between adjacent panels, or in bolted construction, by vertical and horizontal lap splices. Due to the pattern of loading, both the axial compression and internal pressure increase progressively down the wall. Accordingly, practical construction usually uses a stepped wall, formed from panels of uniform thickness, but with larger thicknesses at lower levels. Since the loading varies smoothly, but each panel has constant thickness, the critical location for buckling lies at the base of a panel. But the greater thickness of the lower panel can usefully enhance the buckling strength of the critical panel above it. This thesis presents an extensive computational study that examines all the above influences, divided into chapters that are outlined here. A full exploration of the effect of the cylinder length on the perfect and imperfect elastic buckling strength is presented in Chapter 3. In Chapter 4, the elastic-plastic buckling resistance of imperfect cylinders is described, including strain hardening. These lead to many capacity curves, for which the key parameters are extracted. The effect of internal pressure on the buckling resistance of imperfect elastic cylinders is explored in Chapter 5. Chapter 6 studies the effect of high pressures that produce elastic-plastic elephant’s foot buckling at circumferential welds in imperfect shells. Next, a step change in plate thickness is studied in Chapter 7 for imperfect butt jointed cylinders with and without the internal pressure. Chapter 8 presents an exploration of the effect of plate misalignments at a circumferential joint, as well as the full misalignment of a circumferential lap joint in bolted construction. These are investigated in both the elastic and elastic-plastic domains. The entire thesis is conceived in the context of EN 1993-1-6 (2007) and the ECCS Recommendations on Shell Buckling (2008). This research has shown significant weaknesses in both the concepts and the detailed rules of these standards. Many conditions are found where either the standard is unnecessarily conservative, or its safety margin may be too low. Thus, some new provisions are proposed for each of the above practical problems. These are expected to provide useful knowledge for the design of such structures against buckling in the future.
21
Drachuk, Irina. "Cytocompatible coatings to control cell activity." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52220.
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Cell-surface engineering has been attracting increased interest in the field of biotechnology, tissue engineering, cell therapy, or biosensors/bioelectronics. Thin nanocoatings or sometimes referred as nanoshells allow for modifying and controlling variety of cell properties, specifically retardation of cell division or growth, masking immunological properties, providing chemical and mechanical resistance to external stressors, and ability to further functionalize shells in order to guide cells attachment, their proliferation and function in artificial environment.
Bottom-up approach, utilizing layer-by-layer (LbL) assembly of wide variety of different components (synthetic and natural polyelectrolytes, nanoparticles, and other nano-structures) has been introduced and elaborated to modify cell surfaces. Despite successful examples of the LbL-based cell encapsulation with polyelectrolytes, cytotoxicity of their polycation components possesses severe limitations for this approach. Additionally, by constructing rigid non-permeable shells can suppress the essential properties of cells.
In this view, the goal of this research is to explore the formation of cyto-compatible ultrathin coatings from synthetic and natural polymers through utilization of non-cationic counterparts, with possibility to actively control cell division, provide protection from external environment, and temper shell properties in order to elicit or change specific cell response.
22
Willoughby, David Scott. "An Investigation into Isogeometric Blended Shells." BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6565.
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Improvements to isogeometric blended shells are introduced which blend traditional Reissner-Mindlin shells, and Kirchhoff-Love shells, with an exact interpolation of the shell director increment. A gradient extraction operator is introduced which allows derivatives of basis functions to be exactly expressed as a linear combination of the basis functions themselves. Several benchmarks are investigated and the new blended shell is compared with different shell elements in ABAQUS and NASTRAN. In addition, the effect of different quadrature schemes is included in the comparisons. The new isogeometric blended shell performs comparably in some benchmarks, and even outperforms commercial shell finite elements in some benchmarks. Future improvements to the formulation are discussed.
23
Glandier, Christian Y. "Wave-vector analysis of the vibrations of thin cylindrical shells." Thesis, Georgia Institute of Technology, 1991. http://hdl.handle.net/1853/16797.
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Shaw, Christopher Edward. "The effects of imperfections on the acoustic scattering of a coated shell." Thesis, Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/17903.
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Garcia, Gonzalez Ferran. "Thermal convection in rotating spherical shells." Doctoral thesis, Universitat Politècnica de Catalunya, 2012. http://hdl.handle.net/10803/285778.
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The study of thermal convection in rotating spherical geometry is fundamental to explain many geophysical and astrophysical phenomena such as the generation of the magnetic fields, or the differential rotation observed in the atmospheres of the major planets. The difficulties associated with the experimental studies enhance the importance of the three-dimensional numerical simulations, such as those presented in this dissertation.
In order to obtain the evolution equations, the Boussinesq approximation is applied to the mass, momentum and energy conservation equations, which are rewritten in terms of toroidal and poloidal potentials. Together with the temperature field, they are expanded in spherical harmonics over the sphere, and in the radial direction a collocation method is used. Semi- implicit schemes, based in backward differentiation formulae (IMEX-BDF), implemented with order and time step control (VSVO), are used for time integration.
Applying non-slip boundary conditions with internal heating and very low Prandtl numbers (ratio between the thermal diffusive and the viscous time scales), one of the first exhaustive analysis of the linear stability of the conductive state has been performed. In addition, the existence of preferred polar antisymmetric modes at the onset of convection for high rotation rates has been described.
A study of the efficiency of different high order time integration schemes, either with fixed time-step or VSVO, has been carried out. In our own time evolution codes we apply the IMEX-BDF formulae with an explicit treatment of the nonlinear terms of the equations. The use of 'matrix-free' methods allows the implicit treatment of the Coriolis term, and makes the implementation of a step and order control easier. The results show that the use of high order methods, especially those with time-step and order control, increase the efficiency of the time integration, and allows to obtain more accurate solutions.
Finally, at low Prandtl number, and with non-slip boundary conditions, the nonlinear dynamics is deeply explored by means of temporal evolutions. The type of solutions is described, and the nonlinear mean flow properties are studied. Using parameters as close as possible to those of the Earth's outer core, the numerical simulations are compared with laboratory experiments and realistic measurements.L'estudi de la convecció tèrmica en geometria esfèrica en rotació es fonamental per explicar molts fenòmens geofísics i astrofísics, com la generació de camps magnètics, o la rotació diferencial observada en l'atmosfera dels planetes majors. Les dificultats associades amb els estudis experimentals afavoreixen que les simulacions numèriques tridimensionals, com les que es presenten en aquesta memòria, siguin una eina molt important en aquest camp. Per a l'obtenció de les equacions d'evolució, s'aplica l'aproximació de Boussinesq a les equacions de conservació de la massa, la quantitat de moviment i l'energia, i es reescriuen en funció dels potencials toroidal i poloidal. Els potencials i la temperatura es desenvolupen, sobre l'esfera, en harmònics esfèrics i en la variable radial s'usa col·locació. Per a la integració es fan servir esquemes semi-implícits, que en el nostre cas, estan basats en les fórmules de diferenciació regressiva (IMEX-BDF), que s'han implementat amb control d'ordre i pas (VSVO). En primer lloc, sota condicions de contorn d'adherència, calentament intern i nombre de Prandtl (quocient entre les escales de temps de difusió tèrmica i viscosa) molt baix s'ha realizat un dels primers anàlisis exhaustius de l'estabilitat lineal de l'estat conductiu, gràcies a la millora dels mètodes numèrics emprats. Així mateix, s'ha descrit per primera vegada l'existència de modes polars antisimètrics a l'inici de la convecció amb rotacions elevades. En segon lloc s'ha realizat un estudi de l'eficiència de diferents integradors temporals d'ordre alt, amb pas fix o VSVO. En els nostres propis codis temporals apliquem les fórmules IMEX-BDF amb un tractament explícit dels termes no lineals de les equacions. L'ús de mètodes 'matrix-free' fa rentable el tractament implícit del terme de Coriolis i facilita la implementació d'un control d'ordre i pas temporal adequat. Els resultats mostren que amb ordre elevat, amb o sense control de pas i ordre, s'incrementa l'eficiència de la integració i s'obtenen solucions més acurades. Finalment, amb nombre de Prandtl baix i condicions de contorn d'adherència, s'explora exhaustivament la dinàmica no lineal mitjançant evolucions temporals, tot descrivint el tipus de solucions. També s'estudien les propietats mitjanes de fluxos no lineals. Utilizant paràmetres el més similars possible als del nucli extern de la Terra es comparen els resultats de les simulacions numèriques amb experiments de laboratori i amb medicions de situacions reals.
26
Lorenzani, Silvia. "Fluid instabilities in precessing ellipsoidal shells." [S.l. : s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=964398702.
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Olcay, Yasemin. "Progressive Failure Analysis Of Composite Shells." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614202/index.pdf.
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The objective of this thesis is to investigate the progressive failure behavior of laminated fiber reinforced composite shell structures under different loading conditions. The laminates are assumed to be orthotropic and the first order shear
deformation theory is applied. Three-node layered flat-shell elements are used in the analysis. To verify the numerical results obtained, experimental and analytical results
found in literature are compared with the outputs of the study, and the comparison is found to have shown good agreement with the previous work. Rectangular graphite/epoxy composite laminates under transverse loading are analyzed through several boundary conditions and stacking sequences. Maximum stress criteria, Hashin&rsquos criteria and Tsai Wu criteria are employed to detect the failure and progressive failure methodology is be implemented according to instantaneous degradation approach. First ply failure, final failure loads, corresponding deformations and failure patterns are presented and compared.
28
Mseikeh, Camille Hanna. "Wrinkling of membranes, plates, and shells." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ30343.pdf.
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Molster, Frans Johan. "Crystalline silicates in circumstellar dust shells." [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 2000. http://dare.uva.nl/document/91412.
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Zhong, Hongzhi. "Behaviour of shells under localised impact." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308648.
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Holst, J. M. F. G. "Large deflection phenomena in cylindrical shells." Thesis, University of Cambridge, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604195.
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In the past, much research effort has been directed towards the problem of buckling of thin cylindrical shell structures under axial loading. Thus far, however, no wholly satisfactory solutions have been attained. In this thesis a new approach to shell buckling is proposed. It is suggested that a careful examination of a heavily deformed shell structure in the post-buckling regime - and the processes leading up to this state of deformation - may clarify some of the principles involved in its loss of stability. The complex deformation patterns can be reduced to a more elementary description consisting of inextensional and transitional regions. Corresponding features are found in shell inversions; and in this dissertation two relatively simple examples of inversion under a radial load are studied. Experiments investigating the inversion of a cylindrical shell under a radial point load are reported. Some straightforward empirical formulae are obtained that well describe the characteristics of the deformed geometry; and the applied load is established as a function of the deformation. The inversion of a spherical shell under a radial point load is considered as means of examining some of the basic features observed in the experiments performed, whilst eliminating the complexities arising from the lack of symmetry in the cylindrical case. A "simple model" is detailed which clearly highlights the major components of the total strain energy of the deformed surface and their origin. The results from this model are confirmed using a finite-element analysis, which also provides a more comprehensive description of the overall stress state of the shell. The finite-element method is likewise applied to the case of the cylindrical shell as an extension of the experiments. Findings from the latter are employed to confirm the validity of the numerical analysis. A detailed account is given of the distribution of stresses and strains over the deformed surface; and the similarities with the case of the spherical shell are outlined. Finally, an analytical model is established for the case of the cylindrical shell using the empirical formulae. The derivation of a condensed expression for the total strain energy is presented. It is shown that the model represents well the relevant data obtained numerically.
32
Randall, Richard John. "Fluid-structure interaction of submerged shells." Thesis, Brunel University, 1990. http://bura.brunel.ac.uk/handle/2438/5446.
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A general three-dimensional hydroelasticity theory for the evaluation of responses has been adapted to formulate hydrodynamic coefficients for submerged shell-type structures. The derivation of the theory has been presented and is placed in context with other methods of analysis. The ability of this form of analysis to offer an insight into the physical behaviour of practical systems is demonstrated. The influence of external boundaries and fluid viscosity was considered separately using a flexible cylinder as the model. When the surrounding fluid is water, viscosity was assessed to be significant for slender structural members and flexible pipes and in situations where the clearance to an outer casing was slight. To validate the three-dimensional hydroelasticity theory, predictions of resonance frequencies and mode shapes were compared, with measured data from trials undertaken in enclosed tanks. These data exhibited differences due to the position of the test structures in relation to free and fixed boundaries. The rationale of the testing programme and practical considerations of instrumentation, capture and storage of data are described in detail. At first sight a relatively unsophisticated analytical method appeared to offer better correlation with the measured data than the hydroelastic solution. This impression was mistaken, the agreement was merely fortuitous as only the hydroelastic approach is capable of reproducing-the trends recorded in the experiments. The significance of an accurate dynamic analysis using finite elements and the influence of physical factors such as buoyancy on the predicted results are also examined.
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Timarci, Taner. "Vibrations of composite laminated cylindrical shells." Thesis, University of Nottingham, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.283227.
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Haynie, Waddy. "Torsion of Elliptical Composite Cylindrical Shells." Diss., Virginia Tech, 2007. http://hdl.handle.net/10919/28547.
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The response of elliptical composite cylindrical shells under torsion is studied. The torsional condition is developed by rotating one end of the cylinder relative to the other. Prebuckling, buckling, and postbuckling responses are examined, and material failure is considered. Four elliptical cross sections, defined by their aspect ratio, the ratio of minor to major radii, are considered: 1.00 (circular), 0.85, 0.70, and 0.55. Two overall cylinder sizes are studied; a small size with a radius and length for the circular cylinder of 4.28 in. and 12.85 in., respectively, and a large size with radii and lengths five times larger, and thicknesses two times larger than the small cylinders. The radii of the elliptical cylinders are determined so the circumference is the same for all cylinders of a given size. For each elliptical cylinder, two lengths are considered. One length is equal to the length of the circular cylinder, and the other length has a sensitivity of the buckling twist to changes in the length-to-radius ratio the same as the circular cylinder. A quasi-isotropic lamination sequence of a medium-modulus graphite-epoxy composite material is assumed. The STAGS finite element code is used to obtain numerical results. The geometrically-nonlinear static and transient, eigenvalue, and progressive failure analysis options in the code are employed. Generally, the buckling twist and resulting torque decrease with decreasing aspect ratio. Due to material anisotropy, the buckling values are generally smaller for a negative twist than a positive twist. Relative to the buckling torque, cylinders with aspect ratios of 1.00 and 0.85 show little or no increase in capacity in the postbuckling range, while cylinders with aspect ratios of 0.70 and 0.55 show an increase. Postbuckling shapes are characterized by wave-like deformations, with ridges and valleys forming a helical pattern due to the nature of loading. The amplitudes of the deformations are dependent on cross-sectional geometry. Some elliptical cylinders develop wave-like deformations prior to buckling. Instabilities in the postbuckling range result in shape changes and loss of torque capacity. Material failure occurs on ridges and in valleys. Cylinder size and cross-sectional geometry influence the initiation and progression of failure.Ph. D.
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Saizar, Pedro. "Multiwavelength studies of classical nova shells /." The Ohio State University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487780865408208.
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Keene, Frank W. "Thermal stresses in closed spherical shells /." Online version of thesis, 1991. http://hdl.handle.net/1850/11039.
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Trevor, Simon. "Dimensional accuracy of investment casting shells /." [St. Lucia, Qld.], 2000. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe16943.pdf.
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Hu, Bin. "Stability analysis of linear thin shells." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7360/.
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Shell structure is widely used in engineering area. The purpose of this dissertation is to show the behavior of a thin shell under external load, especially for long cylindrical shell under compressive load, I analyzed not only for linear elastic problem and also for buckling problem, and by using finite element analysis it shows that the imperfection of a cylinder could affect the critical load which means the buckling capability of this cylinder.
For linear elastic problem, I compared the theoretical results with the results got from Straus7 and Abaqus, and the results are really close. For the buckling problem I did the same: compared the theoretical and Abaqus results, the error is less than 1%, but in reality, it’s not possible to reach the theoretical buckling capability due to the imperfection of the cylinder, so I put different imperfection for the cylinder in Abaqus, and found out that with the increasing of the percentage of imperfection, the buckling capability decreases, for example 10% imperfection could decrease 40% of the buckling capability, and the outcome meet the buckling behavior in reality.
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Rémillard, Olivier. "Embedded thin shells for wrinkle simulation." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119548.
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We present a new technique for simulating high resolution surface wrinkling deformations of composite objects consisting of a soft interior and a harder skin. We combine high resolution thin shells with coarse finite element lattices and define frequency based constraints that allow the formation of wrinkles with properties matching those predicted by the physical parameters of the composite object. Our two-way coupled model produces the expected wrinkling behavior without the computational expense of a large number of volumetric elements to model deformations under the surface. We use C1 quadratic shape functions for the interior deformations, allowing very coarse resolutions to model the overall global deformation efficiently, while avoiding visual artifacts of wrinkling at discretization boundaries. We demonstrate that our model produces wrinkle wavelengths that match both theoretical predictions and high resolution volumetric simulations. We also show example applications in simulating wrinkles on passive objects, such as furniture, and for wrinkles on faces in character animation.Nous presentons une nouvelle technique pour la simulation des deformation de haute resolution du plissement en surface d'objets composites constitues d'un interieur mou et d'une peau plus rigide. Nous combinons des plaques minces haute resolution avec une structure d'elements nis plus approximative, et denissons les contraintes d'accouplement qui permettent la formation de rides qui possedent des proprietes correspondant celles predites par les parametres physiques de l'objet composite. Le couplage de ces deux modeles permet notre procede de produire le comportement attendu sans la charge de calcul d'un grand nombre d'elements volumiques du aux deformations sous la surface du modele. Nous utilisons des fonctions de forme quadratiques B-splines pour les deformations interieures, ce qui permet des resolutions tres grossieres pour modeliser les deformations globales ecacement, tout en evitant les artefacts visuels de ridules au niveau des limites de discretisation. Nous demontrons que ce modele produit des longueurs d'onde de rides qui correspondent aux predictions theoriques et aux simulations volumetriques de haute resolution. Nous presentons aussi des exemples d'applications en simulant le plissement d'objets passifs, tels que des meubles, et aussi de la formation de rides sur le visage de personnages en animation.
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ALEMAN, ROMAN AUGUSTO ARCINIEGA. "STABILITY ANALYSIS OF LAMINATED COMPOSITE CYLINDRICAL SHELLS AND PANELSSTABILITY ANALYSIS OF LAMINATED COMPOSITE CYLINDRICAL SHELLS AND PANELS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1997. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=1946@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOREste trabalho apresenta um estudo do comportamento não- linear e instabilidade de cascas e painéis cilíndricos laminados de materiais compósitos. Com esta finalidade é desenvolvida uma formulação de alta ordem de deformação cisalhante que leva en conta estes efeitos nas relações deformação-deslocamento. O comportamento da casca é descrito por uma consistente teoria não-linear para cascas laminadas que considera pequenas deformações e rotações moderadas e incorpora automaticamente o efeito das deformações cisalhantes. O modelo de bifurcação clássico é utilizado para estudar a estabilidade da casca compósita. O comportamento pós-crítico é examinado a partir de uma solução modal obtida com técnicas de perturbação. Em ambos os casos aplica-se o método de Rayleigh-Ritz para discretizar o sistema de equações diferenciais de equilíbrio em um sistema de equações algébricas. O método de Newton-Raphson é empregado na resolução das equações não- lineares de equilíbrio do caminho pós-crítico e na obtenção do caminho fundamental da estrutura imperfeita. A implementação numérica (em álgebra simbólica) é feita utilizando a linguagem de programação Maple V release 3. É então desenvolvido um estudo paramétrico com o objetivo de determinar a influência dos parâmetros geométricos e das características próprias do laminado (número de lâminas e orientação das fibras) na resposta crítica e pós-crítica da casca compósita para dois tipos de carregamento, a saber: pressão lateral e compressão axial. É analisado, também, o grau de sensibilidade às imperfeições geométricas destas estruturas. São apresentadas comparações dos resultados teóricos aqui obtidos com outros existentes na literatura com o objetivo de demonstrar a confiabilidade da formulação e metodologia numérica aqui desenvolvidas.
The purpose of the present work is to study the non-linear behaviour and instability of laminated composite cylindrical shells and panels under axial and pressure loading. The analysis is performed within a refined non- linear theory for composite laminated shells incorporating the effects of transverse shear and the geometric imperfections. The classical bifurcation theory is used to analyze the critical behavior of the shell. To examine the post-critical behavior of the shell, a modal solution based on the basic ideas of Koiter`s theory is deduced and the Rayleigh-Ritz method together with the Newton-Raphson strategy are used to solve the non-linear equilibrium problem and plot either the post-critical path or the non- linear equilibrium path of the imperfect shell. The analytical and numerical procedures were performed by the use of the symbolic algebra package Maple V release 3. The influence played by the geometrical parametrs of the shell and physical parameters of the composite laminate, such as stacking sequences and fiber orientation in each lamina, on the critical and post-critical behavior of the shell is examined and a series of conclusions are outlined. The imperfection sensitivity of these shells is also analyzed. Comparisons of the present results with those obtained by other theories and experiments are found to be satisfactory and show the validity of the present methodology.
Este trabajo presenta un estudio de la inestabilidad y comportamiento no lineal y la inestabilidad de cortezas y paneles cilíndricos laminados de materiales compuestos. Con esta finalidad se desarrolla una formulación de alta orden de deformación cisallante que considera estos hechos en las relaciones deformación desplazamiento. EL comportamiento de la corteza se describe a través de una consistente teoría no lineal para cascas laminadas. Esta teoría considera pequeñas deformaciones y rotaciones moderadas e incorpora automáticamente las deformaciones cisallantes. El modelo de bifurcación clásico se utiliza para estudiar la estabilidad de la corteza. El comportamiento poscrítico se examina a partir de una solución modal obtenida con técnicas de perturbación. En ambos casos se aplica el método de Rayleigh Ritz para discretizar el sistema de ecuaciones diferenciales de equilibrio en un sistema de ecuaciones algébraicas. El método de Newton Raphson es utilizado en la resolución de las ecuaciones no lineares de equilibrio del camino postcrítico y en la obtención del camino fundamental de la extructura imperfecta. La implementación numérica (en álgebra simbólica) se realiza utilizando el lenguaje de programación Maple V release 3. Con el objetivo de determinar la influencia de los parámetros geométricos y de las características proprias del laminado en la respuesta crítica y postcrítica de la casca compósita, se realiza un estudio paramétrico para para dos tipos de carga: presión lateral y compresión axial. Se analiza también, el grado de sensibilidad a las imperfeiciones geométricas de estas extructuras. Finalmente, y con el objetivo de demostrar la confiabilidad de la formulación y la metodología numérica aqui desarrolladas, se comparan los resultados teóricos obtenidos con los reportados en la literatura.
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Xu, Bo. "Vibration and stability analysis of toroidal shells." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0006/MQ45257.pdf.
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Kwok, Raymond Moon Keung. "Mechanics of damaged thin-walled cylindrical shells." Thesis, University of Surrey, 1991. http://epubs.surrey.ac.uk/993/.
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Mustafa, B. A. J. "Free vibration analysis of stiffened circular shells." Thesis, Loughborough University, 1986. https://dspace.lboro.ac.uk/2134/14141.
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This thesis is concerned with the free vibration analysis of stiffened circular cylindrical and conical shells. The study was carried out with a view to improving the accuracy and efficieny of various modelling techniques for the prediction of the stiffened shells natural frequencies and mode shapes. A review of the existing literature covering various aspects of the shell vibrations problem and modelling techniques has been given at the beginning of each chapter with a critical appraisal of the assumptions made and results obtained. Initially the continuum energy approach was used for the analysis. The developed method allows for discrete consideration of stiffeners having arbitrary location and properties. Energy of bending in two planes and rotary inertia have been included in the analysis. Various types of stiffened shells with shear diaphragm ends condition were analysed using this method for the prediction of their natural frequencies. The predicted results have been compared with published experimental results. The finite element technique was also used for the prediction of natural frequencies and mode shapes of stiffened shells. Axisymmetric shell of revolution element, Facet shell element, semi-100f element and various types of beam elements, which are available within the PAFEC 75 suite of programs, were used for these studies. An efficient modelling technique has been introduced for ring stiffened cylindrical and conical shells by the use of the shell of revolution element. For improved accuracy of the predicted frequencies and a more economical model, shell symmetry was utilized for the free vibration analysis of unstiffened and stiffened shells. Half, quarter and one eighth models were studied by developing the appropriate boundary conditions and are discussed here. The free vibration characteristics of orthogonally stiffened cylindrical shells and an orthogonally stiffened cylindrical panel have been studied. An eight noded stiffened super shell element was developed for this study. This element was used for modelling various other types of stiffened shells and the predicted natural frequencies have been compared with known published experimental and theoretical results. Experimental verification of theoretical predictions of natural frequencies and mode shapes of an orthogonally stiffened cylindrical panel was carried out and is reported here.
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Lemos, Jose Pizarro de Sande. "Shells, discs and homology in general relativity." Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.254240.
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JUNIOR, JOAO BATISTA MARQUES DE SOUSA. "SELF-ADAPTIVITY AND SHAPE OPTIMIZATION OF SHELLS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2000. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=2050@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOREste trabalho consiste no desenvolvimento e implementação de um sistema computacional integrado para Modelagem Geo- étrica, Geração de Malhas, Análise Numérica, Auto- Adaptação do tipo h e Otimização de Forma e Espessura em Cascas. O Modelo Geométrico adotado consiste em representar a superfície por meio de B-Splines Racionais Recortadas, admitindo variação de espessura segundo as mesmas funções que descrevem a superfície. Para a utilização nos módulos de Auto-Adaptação e Otimização, Geradores de Malhas Não-Estruturadas sobre superfícies paramétricas foram empregados. Com base em um gerador de malhas triangulares que utiliza a técnica de avanço de Fronteira, dois geradores de malhas quadrilaterais foram desenvolvidos. Os elementos finitos empregados são baseados nas hipóteses de Reissner-Mindlin e no conceito de degeneração de sólidos. São considerados os elementos tradicionais, baseados puramente em deslocamentos, bem como elementos de formulação mais robusta, com base em campos assumidos de deformação. Um procedimento para Auto-Adaptação de Malhas do tipo foi desenvolvido para o Modelo Geométrico e para os elementos considerados. As malhas obtidas a partir de níveis de erro prescritos permitem aos modelos a obtenção de suas taxas de convergência mesmo em problemas com singularidades e efeitos de fronteira. A Análise de Sensibilidade, ferramenta essencial nos procedimentos de Otimização, é feita com a utilização do Método Semi-Analítico, considerando as características especiais dos elementos de casca. Foi desenvolvida para elementos degenerados de casca uma versão do Método Semi- Analítico que elimina sua imprecisão eventual, através da diferenciação exata das componentes de deslocamento de corpo rígido. Para os elementos baseados puramente em deslocamentos, o Método Analítico também foi desenvolvido. Os módulos de Otimização de Forma e Espessura trabalham sobre diversas possibilidades de definição das variáveis de projeto e com diferentes algoritmos de Programação Matemá tica. Permitem também a Otimização de Forma com consideração de Auto-Adaptatividade para obter as malhas durante o processo de mudança de forma. Devido à interconexão entre os diversos módulos e com o objetivo de facilitar a comunicação e a implementação dos mesmos, o sistema computacional foi completamente desenvolvido utilizando técnicas de Programação Orientada para Objetos.
The purpose of this thesis is the development of an integrated computational system for Geometric Modelling, Unstructured Mesh Generation, Numerical Analysis, Adaptivity and Shape Optimization of Shells.The Geometric Model is composed of Non-Uniform Rational B-Spline Surfaces (NURBS), further modified by trimming loops described in the parametric plane. Smooth thickness variations may be modelled by means of the same functions that describe the surface geometry.For Unstructured Mesh Generation, two algorithms were implemented for quadrilateral elements and one for triangles. The triangular Mesh Generator is based on the Advancing Front Technique applied to parametric surfaces. The quadrilateral Mesh Generators employ the indirect approach for converting the triangular meshes to pure quadrilateral ones. The Finite Element formulation is based on the degenerated isoparametric approach. Pure displacement based elements, as well as assumed strain robust shell elements, are employed in the Analysis, Adaptivity and Optimization modules. A procedure for h-Adaptive Mesh Refinement was developed for the shell models. For this purpose an Error Estimator scheme, based upon a Zienkiewicz-Zhu Patch Recovery Technique, adapted to handle curved shell surfaces, was employed. The adaptive procedure allows the convergence rates of the Finite Element Model to be maintained even in the presence of singularities and boundary layers.For the Sensitivity Evaluation, the well- known Semi-Analytical Method is employed and adapted for the degenerated solid shell element approach. In order to solve the inaccuracy problems inherent to the application of the method for certain types of structures, the recent Refined Semi- Analytical Method, is extended for degenerated shell elements. For the pure displacement-based elements, the Analytical Method is also developed. The Shape and Thickness Optimization modules work with a wide variety of design variable descriptions, different mathematical Programming algorithms, Sensitivity schemes and Finite Element Models. The possibility of h-Adaptive Mesh refinement in conjunction with Shape Optimization is also considered in this stage. In order to ease up code expansion, communication and data exchange between the modules,the computational system was fully developed employing Object-Oriented Programming techniques.
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Lawrie, Jane Barbara. "Interaction of sound with cylindrical elastic shells." Thesis, Imperial College London, 1988. http://hdl.handle.net/10044/1/47151.
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Hiller, Jean-François 1974. "Finite element analysis of shells with layers." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/29255.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002.Includes bibliographical references (leaves 160-165).
It is well established that thin shell structures frequently feature narrow bands of strain concentration and localized displacement irregularities referred to as boundary and internal layers. It is crucial to capture these layers properly as they can be sources of structural failures. Unfortunately, while the absence of analytical solutions to most shell problems of practical interest has spawned the development of a variety of finite element formulations over the years, largely speaking these schemes were proposed without a rigorous and comprehensive testing procedure available. We are now faced with a wealth of existing formulations and little way to assess their ability to model boundary and internal layers. Most of the difficulties in assessing the performance of shell finite elements stem from the use of mixed formulations. These are necessary to alleviate the locking phenomenon present in bending-dominated problems when displacement-based formulations are used. We develop a new error measure approach that is physically-based and can be used to assess the performance of mixed-interpolated shell finite element formulations. We apply this approach to the MITC (Mixed Interpolation of Tensorial Components) family of shell elements, a widely-used mixed formulation. We focus in particular on the performance of these elements when employed to analyze problems featuring layers, and specifically we assess the effect of mesh refinement in the regions where layers are present.
(cont.) We demonstrate that the MITC elements are consistent with the basic shell model and find that local mesh refinement allows us to obtain optima order convergence of the MITC solution to the solution of the mathematical model even in the presence of layers. The proposed error measure can be easily extended to other mixed-formulated finite elements used for problems such as incompressible materials and fluids, beams or plates.
by Jean-François N. Hiller.
Ph.D.
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Al-Rashed, Rashed Abdulazeez. "Buckling patterns on pre-stretched bilayer shells." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/113759.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 59-63).
We introduce a new experimental system to study the effects of pre-stretch on the buckling patterns that emerge from the biaxial compression of elastomeric bilayer shells. After fabricating our samples, releasing the pre-stretch in the substrate by deflation places the outer film in a state of biaxial compression and yields a variety of buckling patterns. We systematically explore the parameter space by varying the pre-stretch of the substrate and the ratio of film stiffness to substrate stiffness. The phase diagram of the system exhibits a variety of buckling patterns: from the classic periodic wrinkle to creases, folds, and high aspect ratio ridges. Our system is capable of easily transitioning between these buckling patterns, a first for biaxial systems. We focus on the wrinkle to ridge transition, where we find that pre-stretch plays an essential role in ridge formation, and that the ridge geometry (width, height) remains approximately constant throughout the process. For the localized ridge patterns, we find that the propagation of the ridge tip depends strongly on both strain and stiffness ratio, in a way that is akin to hierarchical fracture of brittle coatings.
by Rashed Abdulazeez Al-Rashed.
S.M.
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Chang, Jin-Der. "Theory of thick, laminated composite shallow shells /." The Ohio State University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487779914824542.
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Raouf, Raouf A. "Nonlinear forced response of circular cylindrical shells." Thesis, Virginia Polytechnic Institute and State University, 1985. http://hdl.handle.net/10919/101255.
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A combination of the Galerkin procedure and the method of multiple scales is used to analyze the nonlinear forced response of circular cylindrical shells in the presence of internal (autoparametric) resonances. If ωf and af denote the frequency and amplitude of a flexural mode and ωb and ab denote the frequency and amplitude of the breathing mode, the steady-state response exhibits a saturation phenomenon when ωb ≈ 2wf if the shell is excited by a harmonic load having a frequency Ω near ωb. As the amplitude f of the excitation increases from zero, ab increases linearly whereas af remains zero until a threshold is reached. This threshold is a function of the damping coefficients and ωb -2wf. Beyond this threshold, ab remains constant (i.e., the breathing mode saturates) and the extra energy spills over into the flexural mode. In other words, although the breathing mode is directly excited by the load, it absorbs a small amount of the input energy (responds with a small amplitude) and passes the rest of the input energy into the flexural mode (responds with a large amplitude).M.S.