CONTINUUM MECHANICS

Author: PEÑUELAS SÁNCHEZ INÉS.
Year: 2004.
University: OVIEDO.
Place of defense: ESCUELA POLITÉCNICA SUPERIOR DE INGENIERÍA DE GIJÓN.
Place of preparation: ESCUELA POLITÉCNICA SUPERIOR DE INGENIERÍA DE GIJÓN.

Summary: In this thesis deals with the influence of design and geometrical factors in the fracture behavior of welded joints of steel. Especially considering the influence of the difference between the mechanical properties of different materials forming the union welded, and the geometry of the test tube on which the tests are conducted fracture. We consider two types of joints, according to the molten material input has a greater or lesser strength than the base metal. The theoretical study was initially based on the numerical modeling of the union welded, consisting of two or three as deemed material or not the effect of ductile damage. In all cases, quantifies the change in construction that produce these differences in the fields of mechanical stress. It also tries to tie this variation in the fields with the pressure caused by constriction geometric (due to the geometry of the sample), thus establishing a methodology to quantify the total constriction and extrapolate the results for a given sample of a fracture union welded to another with different geometry and different material contribution to base. It has also programmed a model of ductile damage based on the model of Gurson Tvergaard Needleman and in the view of coalescencia Thomason and has been incorporated into the analysis by finite element from user subroutines to study the effect of welding on the spread ductile. Finally, the numerical results have been verified through testing experimental fracture, which also allowed know micromecanismos fracture in welded joints involved, thus enabling its numerical modeling.

Author: COMINO MATEOS LUCÍA.
Year: 2004.
University: GRANADA.
Place of defense: ESCUELA TÉCNICA SUPERIOR DE INGENIEROS DE CAMINOS, CANALES Y PUERTOS.
Place of preparation: ESCUELA TÉCNICA SUPERIOR DE INGENIEROS DE CAMINOS, CANALES Y PUERTOS.

Summary: From a theoretical standpoint, obtaining parameters of a system (material properties, internal voids, boundary conditions not accessible, and so on.) From the response excitation known is what is called, from the point of view mathematical Problem Inverso (IP). Due to the complexity of the formulation of these problems in materials anisótropos, from the existing literature is scarce. Therefore it is increasingly evident the need to develop and implement digital tools for solving these problems inverse anisótropos. Specifically, this paper have developed equations and algorithms applicable to two-dimensional solid materials elastic anisótropos, subjected to static excitation to solve the following problems inverse: * IP Cauchy or reconstruction of the boundary conditions. * IP Identification of the elastic properties of the material. * IP Identification of defects such as cracks, cavities or inclusions. For them have been analyzed in depth as crucial aspects computational calculating derivative or sensitivities, having used various methods like direct derivation method of the variable attached or derived topological. Another important aspect of the resolution is the choice of minimization algorithm. They are considering zero-order algorithms, such as Genetic Algorithms, and others like BFGS, Levenberg-Marquardt.

Author: ALFARO RUIZ ICÍAR.
Year: 2004.
University: ZARAGOZA.
Place of defense: CENTRO POLITÉCNICO SUPERIOR.
Place of preparation: CENTRO POLITÉCNICO SUPERIOR.

Author: LAZARO FERNANDEZ CARLOS.
Year: 2005.
University: POLITÉCNICA DE VALENCIA.
Place of defense: TEORÍA DE ESTRUCTURAS.
Place of preparation: UNIVERSIDAD POLITÉCNICA DE VALENCIA.

Summary: SUMMARY: An analysis of the response of mechanical parts elongated experiencing large displacements and rotations is a field in which there has been significant progress in the last three decades, both from the viewpoint of the development of physical models and the search numerical solutions. The scope of such models is separated perhaps in the range of problems which traditionally has dealt civil engineering, approaching more situations in other disciplines, such as aeronautics, robotics or biomechanics. Within the range of one-dimensional models developed for analyzing nonlinear pieces enlarged, as proposed by Simó extension of the work of Reissner is capable of reproducing arbitrarily large rotations of the cross sections. His conceptual simplicity and power of the numerical solutions based on it have become obligatory reference and starting point of many recent investigations. Simó introduced himself the title of emph (geometrically accurate model of elongated pieces) to refer to it. However, the model Reissner -- Simó is not without difficulties. These stemmed mainly from treatment exact rotations, which requires working in a space configurations nonlinear or commutative. This thesis examines the fundamentals of geometrically accurate model and its connection to the non-linear theory of elasticity. The relationship between variables materials and space through the transformation defined by the rotation of each section sets out the method of analysis. In a first step has been developed entirely kinematics model and the field equations in its two facets -- material and space -- which has helped systematize the deductive process and bring some new theoretical results. The analysis of the problem from the point of view variational has revealed the formal connections between the equations of static parts elongated and the equations of the dynamics of rigid bodies, and led to update and extend the analogy kinetics using Kirchhoff the modern language of classical mechanics. As a prelude to the development of numerical solutions has been deducted expression space operator tangent from the linearization of the equation consisting of virtual work. When this process takes place before the discretization gives rise to additional terms in the new operator, but its influence on the numerical results has proved to be negligible. It has also deducted the material form of the operator, not yet published. The fact that expressions of operator tangent are independent parameterization chosen for the description of the rotation is an important aspect of the formulation used. The final part of the thesis focuses on the numerical solution of the model. In a first stage has been developed finite element based on the spatial form of the operator, which is essentially the one proposed by Simó and Vu -- Quoc with some modifications introduced by Ibrahimbegovi '(c) and Taylor. The analysis shows several examples of the power of the model, but also shows some numerical problems. Lastly has developed a new operator using the finite element tangent in material form, which incorporates the interpolation spherical proposed by Crisfield and Jeleni '(c). After prosecuting cases concludes that the material element provides the same accuracy in the solution space, but its speed 8 Rate 2e5 ergencia is considerably smaller.

Author: Fabregat Tomàs Alexandre.
Year: 2006.
University: ROVIRA I VIRGILI.
Place of defense: Escola Técnica Superior d'Enginyeria Química.
Place of preparation: Escuela Técnica Superior de Ingeniería Química.

Summary: The main objective of this work is to study the turbulent heat transfer in a fully developed flow in a channel through the Direct Numerical Simulation (DNS). In these simulations are solved explicitly all scales exist in the turbulent flow so that, even for moderate Reynolds numbers, the discretization meshes must be fine enough to capture the smaller existing structures in the flow and, consequently, computational requirements of the DNS are often considerable. The flow, forced by a pressure gradient constant average, is confined between two thin walls, a separate and parallel infinite distance 2d. The turbulent flow of heat has been studied in three different channel configurations: A case: boom generated by a scalar introduced from a source in a linear horizontal channel event B: mixed convection in the main flow direction (vertical channel) C event: with flotation boom generated by a source in a linear horizontal channel in the event the B heat flux is generated due to the temperature difference between the two walls of the channel. The vector of gravity is aligned with the direction of the main flow and the numbers of Grashof, Reynolds and Prandtl are Gr = 9.6  106, Ret = 150 and Pr = 0.71 respectively. Close to the wall hot, flotation and the pressure gradient acting jointly half causing an increase in speed compared with the case of pure forced convection. In contrast, close to the wall cold flotation opposes the flow and consequently the speed decreases. Cases A and C are similar in that both a warm fluid is introduced in cold flow through a linear source centered vertically at the entrance to the channel. The height of the source is 0.054d. The fluid is introduced dispersed forming a pen that is transported downstream between the two walls adiabáticas channel. The difference between A and C cases lies in the fact that the first shipments of movement and amount of heat and temperature are decoupled serving as a scalar. The advection and diffusion phenomena are the only ones involved in the evolution of the pen. In the case C, however, the flotation coupled transport of heat and amount of movement, and thus the pen suffers a deflection into the upper wall due to its lower density. Given that the current direction is not homogeneous in this case and to ensure the flow conditions fully developed at the entrance of the channel, it has been necessary to create an auxiliary domain docked at the entrance to the domain of computing. In this region auxiliary equations are solved only carriers amount of movement for a fully developed flow mesh with a resolution equal to the primary domain. The results of the cases A and B have been used to validate the code 3DINAMICS compared with data available in various literature. This computer code is written in FORTRAN 90 and parallelised interface using the Message Passing (bookstores MPI-CH). It has implemented a scheme of the second order rate Crank-Nicholson for numerically integrating the equations of transport discretizadas using finite volumes with a scheme centered second order. The analysis of statistics and the different contributions of the relevant terms of the transport equations is used to analyze the effect of floating on the turbulent transport quantity of movement and heat. Finally, following a configuration similar to that used in A, a reagent is introduced through a linear source reacting with another reagent p 8 resente 82f in the flow channel by following a chemical reaction of second order with a number of Damkh ¨ smell equal to 1. Preliminary results for the turbulent transport of various chemical species have also been included in this work. Because of the non-linear equations of transport, special attention has been given to the discretization of the terms advectivos to avoid senseless physical values for the variables transported. We have implemented convective boundary conditions at the exit of the canal to ensure the preservation of the amounts when they are transported outside the computational domain in the direction not homogeneous. For homogeneous directions have been implemented periodic boundary conditions. The resolutions mesh used in the simulations of this work (until 8 million items for the event including the C domain assistant) consume large computational resources. It has replaced the solver conjugate gradient type hitherto used in the computer code for calculating the pressure for a multigrid solver parallel. The computational efficiency of multigrid method has been compared to two other algorithms type conjugate gradient proving to be the best choice in terms of CPU time. The current version of multigrid algorithm, however, scalability minor compared to the other methods.

Author: FALEIRO DE FREITAS JEOVAN.
Year: 2006.
University: POLITÉCNICA DE CATALUÑA.
Place of defense: SALA 002 MODUL C1. CAMPUS NORD.
Place of preparation: EDIFICI C1 Campus NORD.

Summary: The objective of this thesis is to develop an improved analytical model for predicting the plastic-damage response of multi-storey reinforced concrete frames, in accordance with the classic theories of Continuum Damage Mechanics and of classic Theories of Plasticity. What distinguishes this work from others is the fact the complete plastic-damage constitutive model, as well as the global damage, is here implemented into a frame analysis algorithm, where the frame is described by elastic beams and columns with two inelastic hinges at its ends. The behaviour of the reinforced concrete is described throughout by means of continuum constitutive equations rates. We assume that the reinforced concrete presents two distinguished phases: the cracking of the concrete and the yielding of the reinforcement. The concrete cracking phase is described by means of Continuum Damage Mechanics, while the yielding of steel is described by means of the Plasticity Theory. Both, damage and plasticity, are solved simultaneously by means of an uncoupled plastic-damage model proposed for framed structures. With this model, we can describe adequately the behaviour of the reinforced concrete elements. The elastoplastic behaviour of the frame is give by means of plastic hinges in agreement with the classical Plastic Analysis Theory. The evolution of the plastic hinges is obtained by yield functions for beams and columns. The damage in the hinges is obtained by means of the concentrated damage concepts, based on isotropic strain damage. To obtain the damage of a frame member, a new evaluation method is developed, based on a member damage index, which also leads to a meaningful global damage index of the whole structure. Those parameters are based on continuum mechanics principles in which the label âmember damage❠is applied only to damage indices describing the state of frame member while the âglobal' damage index refers to the state of the whole structure. Both damages indices are independent of the chosen constitutive models for the structural material. A numerical procedure for predicting the damage indices of the structures using matrix structural analysis, Plastic Theory and Continuum Damage model is also developed. The method is adequate for the prediction of the failure mechanisms. In order to illustrate the effectiveness of the method, several numerical analyses of framed structures are presented in which various kinds of materials and structural shapes are considered. Several loading conditions are studied: pushover loads, loading-unloading cyclic and dynamic loads. The agreement with experimental data and finite element analysis is also discussed. The obtained results endorse the proposed plastic-damage model as an effective tool for numerical simulation of the collapse of frames. Its implementation under a matrix analysis program gives an efficient tool, computationally economic, which it is a valuable alternative to other types of analysis, such as those based on multi-layer models, when these appear to be too expensive or impractical due the size and complexity of the structure. According to the results obtained, it can be observed that the proposed plastic-damage model shows a good precision in comparison with results obtained by means of finite element models. When comparing with the results obtained by means of experimental tests, the results with the proposed plastic-damage model are better than those obtained by means of finite element models. The proposed model it has been demonstrated that under cyclic or dynamic loads can represent with sufficient accuracy the real behaviour of reinforced concrete. Finally, the global damage index, together with the member and the concentrated damage indexes, provide accurate quantitative measure for evaluating the state of any component of a damaged structure a 8 nd of th 2a7 e overall structural behaviour.

Author: LINERO SEGRERA DORIAN LUIS.
Year: 2006.
University: POLITÉCNICA DE CATALUÑA.
Place of defense: CAMPUS NORD.
Place of preparation: EDIFICI C1 Campus NORD.

Summary: In this work a two-dimensional formulation, that describe the fracture process in rein-forced concrete, is developed, implemented and validated. The cracks in the material are captured by means of the continuum strong discontinuity approach (CSDA) (Oliver 1996) and the constitutive model of composite material is defined through the mixing theory (Trusdell & Toupin 1960). The material is constituted by one or two groups of long fibers (steel bars) embed-ded within a matrix of concrete. Likewise, each component is characterized by a con-stitutive model. The concrete is described by a damage model with degradation in ten-sion and compression (Oliver, Cervera et al. 1990). A uniaxial plasticity model (Simó & Hughes 1998) is used for the steel. Also, phenomenon like bond-slip fiber-matrix and dowel action are included and represented by additional models. The initiation and propagation of cracks are understood as a strain localization proc-ess and then are described by means of CSDA. A bifurcation analysis of composite material is proposed to establish the bifurcation time and direction of the crack. The model has been implemented in a two-dimensional analysis program using the finite element method (FEM), where it is assumed material non-linearity and infinitesi-mal strains. An implicit-explicit integration scheme for the constitutive equation (Oliver, Huespe et al. 2004; Oliver, Huespe et al. 2006) ensures a positive defined stiff-ness matrix of the problem and increases the robustness and stability of the solution. On the another hand, a strategy to tracking discontinuity paths (Samaniego 2002; Oliver & Huespe 2004), allows that discontinuity paths match among elements. According to the proposed formulation, on each point of solid, the deformation and tension fields of composite material like reinforced concrete are described. This has the following advantages: 1) the model facilitates the implementation on the finite ele-ment method, since many ingredients of standard numerical process remain, 2) the mac-roscopic scale of analysis avoids the discretization of each component material and in-teraction effects, consequently the computational cost is reduced. The model can reproduce two difference stages of cracking in the reinforced con-crete. Initially, the capacity of steel and interface bond produce a stable stage of dis-tributed cracking, where appear many cracks with constant separation and opening. Afterwards, a localization cracking stage is characterized by few cracks while the struc-tural capacity is decreasing. Reinforced concrete members submit to tension, bending and shear are simulated. The numerical results, mainly the structural response and the cracks path, are compared with experimental test (Leonhardt 1965; Collins, Vecchio et al. 1985; Ouyang & Shah 1994; Ruiz, Elices et al. 1998). The correlation between numerical results using the propose formulation and actual results is quantitative and qualitatively satisfactory

Author: BLANCO IBAÑEZ SERGIO.
Year: 2006.
University: POLITÉCNICA DE CATALUÑA.
Place of defense: Sala 002 del Módulo C1 (planta baja).
Place of preparation: EDIFICI C1 Campus NORD.

Summary: This work presents a reformulation of the Continuum Strong Discontinuity Approach applied to the numerical simulation of material failure in structures. The goal pursued has been the improvement of the robustness of this kind of numerical analysis. It has also provided a set of tools that guarantees the accuracy of the results. The Continuum Strong Discontinuity Approach reproduces the strain localization phenomenon due to the material softening. As opposed to the discrete approaches, this methodology uses a continuum stress-strain format to describe the whole process of the material exhaustion. Thanks to the regularization (reinterpretation) of the kinematics of the problem and the softening modulus it is guaranteed that the dissipation of the material model is given by the superficial density of fracture energy. Additionally, the use of a thermal-like algorithm to capture and manage the strain localization surfaces makes possible to deal with the numerical study of multi-fissure problems. The improvement of the robustness in the numerical analysis has been achieved by adopting the symmetric kinematically consistent formulation and formulating a new integration scheme, called IMPL-EX, that assures that the algorithmic tangent operators in the problem remain positive definite. The accuracy of the numerical result has been accomplish by an error-control algorithm and by a new arc-length method. This two algorithms have been developed specifically for the integration scheme IMPL-EX. This formulation has been applied to the study of two characteristic phenomena in fracture mechanics: the study of the size effect in geometrically comparable structures and the study/measurement of the fracture processing length. Finally, a series of numerical examples of material failures in three-dimensional structures is presented. These examples have been divided in three groups: examples in which the failure mode is predominantly in mode I, examples in which the failure mode is predominantly a sliding (analysis of slopes) and examples in which the failure mode involves three-dimensional resistant mechanisms (analysis of the arch effect in double or simple curvature dams).