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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

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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.

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.

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.

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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