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An?lise n?o-linear de treli?as pelo m?todo dos elementos finitos posicional / Nonlinear analysis of trusses using the positional finite element methodLacerda, Est?fane George Macedo de 28 February 2014 (has links)
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Previous issue date: 2014-02-28 / This work presents the positional nonlinear geometric formulation for trusses using different
strain measures. The positional formulation presents an alternative approach for nonlinear problems.
This formulation considers nodal positions as variables of the nonlinear system instead
of displacements (widely found in literature). The work also describes the arc-length method
used for tracing equilibrium paths with snap-through and snap-back. Numerical applications for
trusses already established in the literature and comparisons with other studies are provided
to prove the accuracy of the proposed formulation / Este trabalho apresenta a formula??o posicional n?o linear geom?trica para treli?as usando
diferentes medidas de deforma??o. A formula??o posicional ? uma abordagem alternativa
para problemas n?o lineares. Essa formula??o considera as posi??es nodais como vari?veis
do sistema n?o linear em vez dos deslocamentos (que ? largamente utilizado na literatura). O
trabalho tamb?m descreve o m?todo do comprimento de arco, usado para tra?ar caminhos de
equil?brio com snap-through e snap-back. Aplica??es num?ricas com treli?as j? consagradas
na literatura e compara??es com outros trabalhos s?o fornecidos para provar a acur?cia da
formula??o proposta
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Étude des instabilités dans les membranes minces sous chargements thermomécaniques / Instabilities in thin memebranes under thermomechanical loadingAttipou, Kodjo 04 December 2015 (has links)
Le plissement est généralement observé dans les structures minces ayant un comportement de type membrane. Ces structures minces ne supportent pas d'effort de flexion et sont donc sollicitées en traction. Dans cette thèse, nous avons développé une technique de réduction de modèle pour la modélisation du plissement des membranes minces. Cette technique, basée sur les séries de Fourier à double échelle, permet de déduire d'un modèle complet de membrane, un modèle réduit capable de prendre en compte les instabilités globales et locales. Les valeurs critiques de charge et longueur d'onde sont déterminées analytiquement puis numériquement. Des exemples numériques nous ont permis de valider le modèle numérique par rapport au modèle analytique. Les modèles numériques étudiés prennent en compte le modèle complet et le modèle réduit de la membrane. Le modèle complet est simulé dans Abaqus et résolu numériquement à l'aide de la méthode de la longueur d'arc et le modèle réduit est implémenté dans Matlab et résolu numériquement à l'aide de la méthode asymptotique numérique. Nous avons étudié le comportement de la membrane sous sollicitation mécanique, thermique et thermo-mécanique. Les résultats obtenus montrent que le modèle réduit est capable de se substituer au modèle complet dans la détermination des contraintes critiques et longueurs d'onde correspondantes. Le gain en temps de calcul obtenu est important, ceci grâce à la très faible densité de maillage requis par le modèle réduit. Le modèle réduit est très sensible aux conditions aux bords de la membrane et requiert d'avoir une longueur d'onde des plis quasiment constante dans la largeur de la membrane / Wrinkling is an instability phenomenon generally observed in thin structures with membrane's behavior. Those thin structures have no rigidity to flexion and are therefore used in traction. In this thesis, we developed a reduction model's technique for the modeling of wrinkling phenomenon in thin membranes. This technique, based on the double scale Fourier series, allow us to deduce from a full membrane model, a reduced membrane model that is able to take into account the global and local instability of the structure. The critical load and critical wavelength are determined analytically on one side, then numerically on the other side. Numerical exemples are conducted to validate the numerical model towards the analytical one. Numerical models studied take into account both full and reduce membrane models. The full model is simulated in Abaqus and solved numerically using the arc length method and the reduced model is implemented in Matlab and solved numerically using the asymptotic numerical method. We studied the membrane behavior under mechanical, thermal and thermo-mechanical loading. The results obtained show that the full membrane model can be replaced by the reduced one in determining critical loads and corresponding wavelengths. The gain in computation time obtained is important, due to the coarse mesh required by the reduced model. The reduced model is very sensitive to membrane's boundaries conditions and requires to have a quasi constant wavelength along the membrane width
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Nonlinear dynamics of one-way clutches and dry friction tensioners in belt-pulley systemsZhu, Farong 25 September 2006 (has links)
No description available.
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Global and Local Buckling Analysis of Stiffened and Sandwich Panels Using Mechanics of Structure GenomeNing Liu (6411908) 10 June 2019 (has links)
Mechanics of structure genome (MSG) is a unified homogenization theory that
provides constitutive modeling of three-dimensional (3D) continua, beams and plates.
In present work, the author extends the MSG to study the buckling of structures such
as stiffened and sandwich panels. Such structures are usually slender or flat and easily
buckle under compressive loads or bending moments which may result in catastrophic
failure.<div><br><div>Buckling studies of stiffened and sandwich panels are found to be scattered. Most
of the existed theories employ unnecessary assumptions or only apply to certain types
of structures. There are few unified approaches that are capable of studying the
buckling of different kinds of structures altogether. The main improvements of current
approach compared with other methods in the literature are avoiding unnecessary
assumptions, the capability of predicting all possible buckling modes including the
global and local buckling modes, and the potential in studying the buckling of various
types of structures.<br></div><div><br></div><div>For global buckling that features small local rotations, MSG mathematically decouples
the 3D geometrical nonlinear problem into a linear constitutive modeling using
structure genome (SG) and a geometrical nonlinear problem defined in a macroscopic
structure. As a result, the original structures are simplified as macroscopic structures
such as beams, plates or continua with effective properties, and the global buckling
modes are predicted on macroscopic structures. For local buckling that features
finite local rotations, Green strain is introduced into the MSG theory to achieve geometrically nonlinear constitutive modeling. Newton’s method is used to solve
the nonlinear equilibrium equations for fluctuating functions. To find the bifurcated
fluctuating functions, the fluctuating functions are then perturbed under the Bloch-periodic
boundary conditions. The bifurcation is found when the tangent stiffness
associated with the perturbed fluctuating functions becomes singular. Moreover, the
arc-length method is introduced to solve the nonlinear equilibrium equations for post-local-buckling
predictions because of its robustness. The imperfection is included in
the form of geometrical imperfection by superimposing the scaled buckling modes in
linear perturbation analysis on mesh.<br></div><div><br></div><div>Extensive validation case studies are carried out to assess the accuracy of the
MSG theory in global buckling analysis and post-global-buckling analysis, and assess
the accuracy of the extended MSG theory in local buckling and post-local-buckling
analysis. Results using MSG theory and extended MSG theory in buckling analysis
are compared with direct numerical solutions such as 3D FEA results and results in
literature. Parametric studies are performed to reveal the relative influence of selective
geometric parameters on buckling behaviors. The extended MSG theory is also
compared with representative volume element (RVE) analysis with Bloch-periodic
boundary conditions using commercial finite element packages such as Abaqus to
assess the efficiency and accuracy of the present approach.<br></div></div>
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Geometric approach to multi-scale 3D gesture comparisonOchoa Mayorga, Victor Manuel 11 1900 (has links)
The present dissertation develops an invariant framework for 3D gesture comparison studies. 3D gesture comparison without Lagrangian models is challenging not only because of the lack of prediction provided by physics, but
also because of a dual geometry representation, spatial dimensionality and non-linearity associated to 3D-kinematics.
In 3D spaces, it is difficult to compare curves without an alignment operator since it is likely that discrete curves are not synchronized and do not share a common point in space. One has to assume that each and every single trajectory in the space is unique. The common answer is to assert the similitude between two or more trajectories as estimating an average distance error from the aligned curves, provided that the alignment operator is found.
In order to avoid the alignment problem, the method uses differential geometry for position and orientation curves. Differential geometry not only reduces the spatial dimensionality but also achieves view invariance. However,
the nonlinear signatures may be unbounded or singular. Yet, it is shown that pattern recognition between intrinsic signatures using correlations is robust for position and orientation alike.
A new mapping for orientation sequences is introduced in order to treat quaternion and Euclidean intrinsic signatures alike. The new mapping projects a 4D-hyper-sphere for orientations onto a 3D-Euclidean volume. The projection uses the quaternion invariant distance to map rotation sequences into 3D-Euclidean curves. However, quaternion spaces are sectional discrete spaces.
The significance is that continuous rotation functions can be only approximated for small angles. Rotation sequences with large angle variations can only be interpolated in discrete sections.
The current dissertation introduces two multi-scale approaches that improve numerical stability and bound the signal energy content of the intrinsic signatures. The first is a multilevel least squares curve fitting method similar to Haar wavelet. The second is a geodesic distance anisotropic kernel filter.
The methodology testing is carried out on 3D-gestures for obstetrics training. The study quantitatively assess the process of skill acquisition and transfer of manipulating obstetric forceps gestures. The results show that the multi-scale correlations with intrinsic signatures track and evaluate gesture differences between experts and trainees.
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Geometric approach to multi-scale 3D gesture comparisonOchoa Mayorga, Victor Manuel Unknown Date
No description available.
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