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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Near-coincident doubly-symmetric branching systems : elastic post-buckling behaviour and imperfection sensitivity

Johnson, Barbara Helen January 1990 (has links)
No description available.
2

Studies in the mixed-mode analysis of partially prismatic thin-walled structures

Mansour-Tehrani, F. January 1989 (has links)
No description available.
3

An experimental investigation into the lateral buckling strength of plate girders

Oheachteirn, P. January 1983 (has links)
No description available.
4

Buckling Analysis of Composite Stiffened Panels and Shells in Aerospace Structure

Beji, Faycel Ben Hedi 08 January 2018 (has links)
Stiffeners attached to composite panels and shells may significantly increase the overall buckling load of the resultant stiffened structure. Initially, an extensive literature review was conducted over the past ten years of published work wherein research was conducted on grid stiffened composite structures and stiffened panels, due to their applications in weight sensitive structures. Failure modes identified in the literature had been addressed and divided into a few categories including: buckling of the skin between stiffeners, stiffener crippling and overall buckling. Different methods have been used to predict those failures. These different methods can be divided into two main categories, the smeared stiffener method and the discrete stiffener method. Both of these methods were used and compared in this thesis. First, a buckling analysis was conducted for the case of a grid stiffened composite pressure vessel. Second, a buckling analysis was conducted under the compressive load on the composite stiffened panels for the case of one, two and three longitudinal stiffeners and then, using different parameters, stiffened panels under combined compressive and shear load for the case of one longitudinal centric stiffener and one longitudinal eccentric stiffener, two stiffeners and three stiffeners. / Master of Science / Aircraft in flight is subjected to different loads due to maneuvers and gust, external forces cause internal loads, which depends on the location of the panel in the aircraft, those internal loads, may result in the buckling of the panel. There is an imminent need for structural efficiency, strong and lightweight material. Stiffened composite panels is a promising technology capable of addressing those needs. Composite stiffened panels have many advantages including but not limited to, small manufacturing cost, high stability, great energy absorption, superior damage tolerance etc. The main failure modes for stiffened composite panels is buckling. Buckling failure modes could be of a global nature, local skin buckling or stiffener/rib crippling, predicting those failure is of high practical importance and a predominant design criterion. An extensive literature review on buckling of stiffened composite panels was conducted in this thesis. Buckling analysis as well as a parametric study of grid stiffened composite cylindrical shell for a pressure vessel was conducted, an analytical solution was derived and verified using ABAQUS, a Finite Element Software. Buckling analysis as well as a parametric study of stiffened panels with longitudinal stiffeners, under different structural situations, was also conducted and results verified.
5

脊柱特発性側彎症の成因に関する非線形座屈解析

青山, 大樹, AOYAMA, Taiki, 畔上, 秀幸, AZEGAMI, Hideyuki 11 1900 (has links)
No description available.
6

Buckling Analysis of Sandwich Pipes Under External Pressure

Hashemian, Rouzbeh January 2014 (has links)
A general eigen-value buckling solution is developed for the buckling of long thick pipes subjected to internal and external hydrostatic pressure. The principle of stationary potential energy is used to formulate the conditions of equilibrium, neutral stability conditions, and associated boundary conditions using polar coordinates. The formulation accounts for shear deformation effects and is suited for composite pipe systems with thick cores. It involves destabilizing terms: one is due to the external hydrostatic pressure and incorporates the follower effects, and the other, is due to the pre-bucking stresses undergoing the nonlinear components of strains. The formulation adopts a work conjugate triplet consisting the Cauchy stress tensor, the Green Lagrange strain tensor, and constant constitutive relations. A Fourier series expansion of the displacement fields is adopted to transform the 2D problem into a series of independent 1D problems, thus keeping the computational effort to a minimum while preserving the accuracy of the solution. Two numerical solutions were developed and implemented under MATLAB; the first one is based on the finite difference technique and the second one is based on the finite element solution. Both solutions were shown to converge to the same solution, the finite difference from below, while the finite element converges from above. The finite element solution is then applied to predict the buckling capacity of sandwich pipes consisting of two steel pipes with a soft core. A comprehensive verification study is conducted and the validity of the formulation was established through comparison with other solutions. A parametric study is then conducted to investigate the effect of hydrostatic internal pressure, core material, core thickness, and internal and external pipe thicknesses, on the external buckling pressure of sandwich pipes.
7

Mechanical and thermal buckling analysis of laminated composite plates

Paremmal, Aswin, Kuruthukulangara Varghese, Roopak January 2023 (has links)
Composite laminated structures have many application fields such as aerospace, civil, marine, bio medical, transportation, and mechanical engineering because of their convenience in handling, and very good mechanical properties. Buckling behavior of laminated composite plates subjected to plane loads is indeed an important consideration in the primary design of aircraft and launch vehicle components. In addition, these elements may expose to high-temperature fields (while launching or reentry) which may result in failure due to thermal buckling. Composite laminated plates with holes and other openings are used as structural members in aerospace industry. The buckling behavior of such plates has always received much attention.In this study buckling analysis has been carried out of a laminar composite plate with an elliptical cut to shape. In the analysis, finite element method (FEM) was applied to perform parametric studies on various plates based on the shape and position and orientation of the hole.ANSYS software has been used as a platform for buckling analysis.
8

Reduced Order Modeling for Efficient Stability Analysis in Structural Optimization

Sanmugadas, Varakini 15 October 2024 (has links)
Design optimization involving complex structures can be a very resource-intensive task. Convex optimization problems could be solved using gradient-based approaches, whereas non-convex problems require heuristic methods. Over the past few decades, many optimization techniques have been presented in the literature to improve the efficiency of both these approaches. The present work focuses on the non-convex optimization problem involving eigenvalues that arises in structural design optimization. Parametric Model Order Reduction (PMOR) was identified as a potential tool for improving the efficiency of the optimization process. Its suitability was investigated by applying it to different eigenvalue optimization techniques. First, a truss topology optimization study was conducted that reformulated the weight minimization problem with a non-convex lower-bound constraint on the fundamental frequency into the standard convex optimization form of semidefinite programming. Applying PMOR to this, it was found the reduced system was able to converge to the correct final designs, given a reduced basis vector of suitable size was chosen. At the same time, it was shown that preserving the sparse nature of the mass and stiffness matrices was crucial to achieving reduced solution times. In addition, the reformulation to convex optimization form, while possible with the discretized form of vibrational governing equations, is not straightforward with the buckling problem. This is due to the non-linear dependence of the geometric stiffness matrix on the design variables. Hence, we turned to a metaheuristic approach as an alternative and explored the applicability of PMOR in improving its performance. A two-step optimization procedure was developed. In the first step, a set of projection vectors that can be used to project the solutions of the governing higher-order partial differential equations to a lower manifold was assembled. Invariant components of the system matrices that do not depend on the design variables were identified and reduced using the projection vectors. In the second (online) step, the buckling analysis problem was assembled and solved directly in the reduced form. This approach was applied to the design of variable angle tow (VAT) fiber composite structures. Affine matrix decompositions were derived for the linear and geometric stiffness matrices of VAT composites. The resulting optimization framework can rapidly assemble the reduced order matrices related to new designs encountered by the optimizer, perform the physics analysis efficiently in the reduced space, evaluate heuristics related to the objective function, and determine the search direction and convergence based on these evaluations. It was shown that the design space can be traversed efficiently by the developed PMOR-based approach by ensuring a uniform error distribution in objective values throughout the design space. / Doctor of Philosophy / When designing complex structures, designers often have specific performance criteria based on which they improve their preliminary conceptual designs. This could be done by varying some features of the initial designs in a way that these performance criteria are improved. However, it is not always intuitive or efficient to do this manually. Design optimization techniques provide efficient mathematical algorithms that can extract useful information from the governing partial differential equations of the structure and use it to identify the optimal combination of values for a certain set of features, called the design variables, to achieve the optimal performance criteria, referred to as the objective function. As the complexity and size of the structural design problem further increases, typical optimization techniques become slow and resource-intensive. In this work, we propose an optimization framework that uses parametric model order reduction (PMOR) to address this bottleneck. In essence, PMOR filters the large order matrices that arise in these structural analysis problems and provides the optimizer with smaller order matrices that retain the most important features of the original system. This was applied to a truss topology optimization and fiber-composite plate optimization study, both conducted with different types of optimization solvers. It was shown that PMOR resulted in significant efficiency improvements in the design optimization process when paired with an appropriate optimization algorithm.
9

Analysis Of Sinusoidal And Helical Buckling Of Drill String In Horizontal Wells Using Finite Element Method

Arpaci, Erdogan 01 August 2009 (has links) (PDF)
The number of horizontal wells is increasing rapidly in all over the world with the growth of new technological developments. During horizontal well drilling, much more complex problems occur when compared with vertical well drilling, such as decrease in load transfer to the bit, tubular failure, tubular fatigue and tubular lock-up. This makes selection of appropriate tubular and making the right drill string design more important. As the total compression load on the horizontal section increases, the behavior of the tubular changes from straight to sinusoidal buckling, and if the total compression load continues to increase the behavior of the tubular changes to helical buckling. Determination of critical buckling loads with finite element method (FEM) in horizontal wells is the main objective of this study. Initially, a computer program (ANSYS) that uses FEM is employed to simulate different tubular and well conditions. Four different pipe sizes, four different wellbore sizes and three different torque values are used to model the cases. Critical buckling load values corresponding to significant variables are collected from these simulated cases. The results are classified into different buckling modes according to the applied weight on bit values and the main properties of the simulated model, such as modulus of elasticity, moment of inertia of tubular cross section, weight per unit length of tubular and radial clearance between the wellbore and the tubular. Then, the boundary equations between the buckling modes are obtained. The equations developed in this thesis by simulating the cases for the specific tubular sizes are used to make a comparison between the critical buckling load values from the models in the literature and this work. It is observed that the results of this work fit with literature models as the tubular size increases. The influence of torque on critical buckling load values is investigated. It is observed that torque has a slight effect on critical buckling load values. Also the applicability of ANSYS for buckling problems was revealed by comparing the ANSYS results with the literature models&amp / #8217 / results and the experimental study in the literature.
10

Buckling analysis of laminated composite beams by using an improved first order formulation

Ayala, Shammely, Vallejos, Augusto, Arciniega, Roman 01 January 2021 (has links)
El texto completo de este trabajo no está disponible en el Repositorio Académico UPC por restricciones de la casa editorial donde ha sido publicado. / In this work, a finite element model based on an improved first-order formulation (IFSDT) is developed to analyze buckling phenomenon in laminated composite beams. The formulation has five independent variables and takes into account thickness stretching. Threedimensional constitutive equations are employed to define the material properties. The Trefftz criterion is used for the stability analysis. The finite element model is derived from the principle of virtual work with high-order Lagrange polynomials to interpolate the field variables and to prevent shear locking. Numerical results are compared and validated with those available in literature. Furthermore, a parametric study is presented.

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