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  • Impact damage identification in composite laminates using vibration testing

     Pérez Martínez, Marco Antonio; Gil Espert, Lluis; Oller Martinez, Sergio Horacio
    Composite structures
    Date of publication: 2014-02
    Journal article

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    Due to the problems arising from impact damage in composite laminates, there is a need to develop fast, accurate, cost-effective and non-destructive testing methods to identify this type of damage at an early stage and thus enhance the service life of composite structures. This paper presents the results of an extensive experimental campaign conducted to investigate the feasibility of using vibration-based meth- ods to identify damages sustained by composite laminates due to low-velocity impacts. The experimental programme included an evaluation of impact damage resistance and tolerance according to ASTM test methods, characterisation of induced damage by ultrasonic testing and quantification of the effects on the vibration response. The damage identification involved the detection, localisation, quantification and estimation of the remaining bearing capacity. Four damage indicators based on modal parameters were assessed by comparing pristine and damaged states. The results allowed for conclusions to be drawn regarding the capability and suitability of each damage indicator, including its ability to detect impact-induced damage, its precision in determining the location of damage, its sensitivity regarding damage extent and pertinent correlations with residual bearing capacity.

  • Delamination in laminated plates using the 4-noded quadrilateral QLRZ plate element based on the refined zigzag theory

     Eijo, Ariel; Oñate Ibáñez de Navarra, Eugenio; Oller Martinez, Sergio Horacio
    Composite structures
    Date of publication: 2014-02
    Journal article

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    A numerical method based on the Refined Zigzag Theory (RZT) to model delamination in composite laminated plate/shell structures is presented. The originality of this method is the use of 4-noded quadrilateral plate finite elements whit only seven variables per node to discretize the plate/shell geometry. The ability to capture the relative displacement between consecutive layers in fracture mode II and III is the more important advantage of this element, denoted QLRZ. A continuum isotropic damage model is used to model the mechanical behavior of the plies. The material non-lineal problem is solved with the modified Newton¿Raphson method. The RZT plate theory, the QLRZ finite element and the isotropic damage model are described in this work. Also, the implicit integration algorithm is presented. The performance of the numerical model is analyzed by studying the delamination in a rectangular plate for two different laminates, using the 3D analysis as the reference solution.

  • Comparative experimental analysis of the effect caused by artificial and real induced damage in composite laminates

     Pérez Martínez, Marco Antonio; Gil Espert, Lluis; Sanchez Romero, Montserrat; Oller Martinez, Sergio Horacio
    Composite structures
    Date of publication: 2014-06-02
    Journal article

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    This paper presents the results of an extensive experimental campaign aimed to examine the effect upon the vibration response and on the residual load-bearing capacity caused by both: isolated artificially induced interlaminar damage and low-velocity impact induced damage in composite laminates. The experimental programme included modal testing, drop-weight impact testing, ultrasonic inspection, transverse quasi-static loading testing and compression testing conducted on a set of 72 carbon fibre-reinforced composite laminated coupons. Both types of damage caused measurable changes in laminate performance, however marked divergent trends were observed. The results allowed for conclusions to be drawn regarding the adequacy of the artificial damage approach and highlighted the importance and role of other forms of degradation upon damage tolerance of laminated composites containing damage.

  • A four-noded quadrilateral element for composite laminated plates/shells using the refined zigzag theory

     Eijo, Ariel; Oñate Ibáñez de Navarra, Eugenio; Oller Martinez, Sergio Horacio
    International journal for numerical methods in engineering
    Date of publication: 2013-05-20
    Journal article

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    A new bilinear four-noded quadrilateral element (called quadrilateral linear refined zigzag) for the analysis of composite laminated and sandwich plates/shells based on the refined zigzag theory is presented. The element has seven kinematic variables per node. Shear locking is avoided by introducing an assumed linear shear strain field. The performance of the element is studied in several examples where the reference solution is the 3D finite element analysis using 20-noded hexahedral elements.

    A new bilinear four-noded quadrilateral element (called quadrilateral linear refined zigzag) for the analysis of composite laminated and sandwich plates/shells based on the refined zigzag theory is presented. The element has seven kinematic variables per node. Shear locking is avoided by introducing an assumed linear shear strain field. The performance of the element is studied in several examples where the reference solution is the 3D finite element analysis using 20-noded hexahedral elements.

  • A numerical model of delamination in composite laminated beams using the LRZ beam element based on the refined zigzag theory

     Eijo, Ariel; Oñate Ibáñez de Navarra, Eugenio; Oller Martinez, Sergio Horacio
    Composite structures
    Date of publication: 2013-10
    Journal article

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    A method based on the Refined Zigzag Theory (RZT) to model delamination in composite laminated beam structures is presented. The novelty of this method is the use of one-dimensional finite elements to discretize the geometry of the beam. The key property of this beam element, named LRZ [1], is the possibility to capture the relative displacement between consecutive layers which occurs during delamination. The fracture mode that the LRZ element is capable to predict is mode II. In order to capture the relative displacement using the LRZ element it is necessary to adapt the RZT theory as presented in this paper. The mechanical properties of the layers are modeled using a continuum isotropic damage model [2]. The modified Newton-Raphson method is used for solving the non-linear problem.The RZT theory, the LRZ finite element and the isotropic damage model are described in the paper. Also, the implicit integrations algorithm is presented. The performance of the LRZ element is analyzed by studying the delamination in a beam for two different laminates, using the plane stress solution as a reference.

    A method based on the Refined Zigzag Theory (RZT) to model delamination in composite laminated beam structures is presented. The novelty of this method is the use of one-dimensional finite elements to discretize the geometry of the beam. The key property of this beam element, named LRZ [1], is the possibility to capture the relative displacement between consecutive layers which occurs during delamination. The fracture mode that the LRZ element is capable to predict is mode II. In order to capture the relative displacement using the LRZ element it is necessary to adapt the RZT theory as presented in this paper. The mechanical properties of the layers are modeled using a continuum isotropic damage model [2]. The modified Newton–Raphson method is used for solving the non-linear problem. The RZT theory, the LRZ finite element and the isotropic damage model are described in the paper. Also, the implicit integrations algorithm is presented. The performance of the LRZ element is analyzed by studying the delamination in a beam for two different laminates, using the plane stress solution as a reference.

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    Impact damage prediction in carbon fiber-reinforced laminated composite using the matrix-reinforced mixing theory  Open access

     Pérez Martínez, Marco Antonio; Martinez Garcia, Xavier; Oller Martinez, Sergio Horacio; Gil Espert, Lluis; Rastellini Canela, Fernando Gabriel; Flores, Fernando
    Composite structures
    Date of publication: 2013-10-01
    Journal article

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    The impact damage tolerance of fiber-reinforced laminated composite materials is a source of concern, mainly due to internal induced damage which causes large reductions on the strength and stability of the structure. This paper presents a procedure based on a finite element formulation that can be used to perform numerical predictions of the impact induced internal damage in composite laminates. The procedure is based on simulating the composite performance using a micro-mechanical approach named matrix-reinforced mixing theory, a simplified version of the serial/parallel mixing theory that does not require neither the iterative procedure nor the calculation of the tangent stiffness tensor. The numerical formulation uses continuum mechanics to simulate the phenomenon of initiation and propagation of interlaminar damage with no need to formulate interface elements, resulting in a computationally less demanding formulation. To demonstrate the capability of numerical procedure when applied to a low-velocity impact problem, numerical results are compared with the experimental ones obtained in a test campaign performed on 44 laminates specimens subjected to an out-of-plane and concentrated impact event, according to ASTM test method. Results are in good agreement with experimental data in terms of delamination onset and the internal spatial distribution of induced damage.

    The impact damage tolerance of fiber-reinforced laminated composite materials is a source of concern, mainly due to internal induced damage which causes large reductions on the strength and stability of the structure. This paper presents a procedure based on a finite element formulation that can be used to perform numerical predictions of the impact induced internal damage in composite laminates. The procedure is based on simulating the composite performance using a micro-mechanical approach named matrix-reinforced mixing theory, a simplified version of the serial/parallel mixing theory that does not require neither the iterative procedure nor the calculation of the tangent stiffness tensor. The numerical formulation uses continuum mechanics to simulate the phenomenon of initiation and propagation of interlaminar damage with no need to formulate interface elements, resulting in a computationally less demanding formulation. To demonstrate the capability of numerical procedure when applied to a low-velocity impact problem, numerical results are compared with the experimental ones obtained in a test campaign performed on 44 laminates specimens subjected to an out-of-plane and concentrated impact event, according to ASTM test method. Results are in good agreement with experimental data in terms of delamination onset and the internal spatial distribution of induced damage.

  • Robust design optimisation of advance hybrid (fiber¿metal) composite structures

     Lee, Dong Seop; Morillo Carbonell, Carlos; Oller Martinez, Sergio Horacio; Bugeda Castelltort, Gabriel; Oñate Ibáñez de Navarra, Eugenio
    Composite structures
    Date of publication: 2013-05
    Journal article

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  • An integrated procedure for the structural design of a composite rotor-hydrofoil of a Water Current Turbine (WCT)

     Oller Aramayo, Sergio A.; Nallim, Liz G.; Oller Martinez, Sergio Horacio
    Applied composite materials
    Date of publication: 2013-12
    Journal article

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    This paper shows an integrated structural design optimization of a composite rotor-hydrofoil of a water current turbine by means the finite elements method (FEM), using a Serial/Parallel mixing theory (Rastellini et al. Comput. Struct. 86:879¿896, 2008, Martinez et al., 2007, Martinez and Oller Arch. Comput. Methods. 16(4):357¿397, 2009, Martinez et al. Compos. Part B Eng. 42(2011):134¿144, 2010) coupled with a fluid-dynamic formulation and multi-objective optimization algorithm (Gen and Cheng 1997, Lee et al. Compos. Struct. 99:181¿192, 2013, Lee et al. Compos. Struct. 94(3):1087¿1096, 2012). The composite hydrofoil of the turbine rotor has been design using a reinforced laminate composites, taking into account the optimization of the carbon fiber orientation to obtain the maximum strength and lower rotational-inertia. Also, these results have been compared with a steel hydrofoil remarking the different performance on both structures. The mechanical and geometrical parameters involved in the design of this fiber-reinforced composite material are the fiber orientation, number of layers, stacking sequence and laminate thickness. Water pressure in the rotor of the turbine is obtained from a coupled fluid-dynamic simulation (CFD), whose detail can be found in the reference Oller et al. (2012). The main purpose of this paper is to achieve a very low inertia rotor minimizing the start-stop effect, because it is applied in axial water flow turbine currently in design by the authors, in which is important to take the maximum advantage of the kinetic energy. The FEM simulation codes are engineered by CIMNE (International Center for Numerical Method in Engineering, Barcelona, Spain), COMPack for the solids problem application, KRATOS for fluid dynamic application and RMOP for the structural optimization. To validate the procedure here presented, many turbine rotors made of composite materials are analyzed and three of them are compared with the steel one.

  • Characterization of mechanical properties of biological tissue: application to the FEM analysis of the urinary bladder

     Oñate Ibáñez de Navarra, Eugenio; Bellomo, F.J; Monteiro, Virginia; Oller Martinez, Sergio Horacio; Nallim, Liz G.
    Date of publication: 2013
    Book chapter

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    This text presents an approach for the mechanical behavior of soft biological tissue using the finite elements method and a general constitutive model. Specifically we analyze the mechanical behavior of a urinary bladder starting from a procedure for obtaining the mechanical characterization of the biological tissue. Bladder tissue is modelled as a composite material formed by soft matrix reinforced with preferentially oriented fibres. In the first part of the text we present a procedure for identifying the mechanical properties of biological tissue main constituents by an inverse method. Then this information is used for the numerical simulation of the mechanical behavior of the bladder within the FEM. The formulation can be applied to various types of biological tissues, both in the field of material characterization, and in the numerical simulation of the tissue biomechanical behavior.

  • Analysis of ultra low cycle fatigue problems with the Barcelona plastic damage model

     Martinez Garcia, Xavier; Oller Martinez, Sergio Horacio; Barbu, Lucia Gratiela; Barbat Barbat, Horia Alejandro
    International Conference on Computational Plasticity Fundamentals and Applications
    Presentation's date: 2013-09-05
    Presentation of work at congresses

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  • Analytical and numerical analysis of human dental occlusal contact

     Bastos de Souza, FLAVIA; Casas, Estevam; Oller Martinez, Sergio Horacio
    Computer methods in biomechanics and biomedical engineering
    Date of publication: 2013
    Journal article

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    The knowledge of contact forces in teeth surfaces during mastication or para-functional movements can help to understand processes related to friction and wear of human dental enamel. The development of a numerical model for analysis of the occlusal contact between two antagonistic teeth is proposed, which includes three basic steps: the characterisation of the surface roughness, its homogenisation using an assumed distribution function and the numerical determination of the resulting forces. Finite element strain results for the main different asperities are statistically combined, deriving the predicted macroscopic behaviour of the interface. Axisymmetric and 3D numerical models with an elasto-plastic constitutive law are used to simulate micro-indentations and micro-contacts, respectively. The contact is allowed to occur locally in planes not necessarily parallel to the surface's mean plane, a problem for which there is no analytical solution. The three identified parameters, homogenised surface hardness (3.68 GPa), surface yield stress (3.08 GPa) and static friction coefficient (0.23), agree with the experimental values reported in the literature.

    The knowledge of contact forces in teeth surfaces during mastication or para-functional movements can help to understand processes related to friction and wear of human dental enamel. The development of a numerical model for analysis of the occlusal contact between two antagonistic teeth is proposed, which includes three basic steps: the characterisation of the surface roughness, its homogenisation using an assumed distribution function and the numerical determination of the resulting forces. Finite element strain results for the main different asperities are statistically combined, deriving the predicted macroscopic behaviour of the interface. Axisymmetric and 3D numerical models with an elasto-plastic constitutive law are used to simulate micro-indentations and micro-contacts, respectively. The contact is allowed to occur locally in planes not necessarily parallel to the surface's mean plane, a problem for which there is no analytical solution. The three identified parameters, homogenised surface hardness (3.68 GPa), surface yield stress (3.08 GPa) and static friction coefficient (0.23), agree with the experimental values reported in the literature.

  • Modelización numérica del comportamiento constitutivo del daño local y global y su correlación con la evolución de las frecuencias naturales en estructuras de hormigón reforzado  Open access

     Paredes Lopez, Jairo Andrés
    Defense's date: 2013-11-11
    Universitat Politècnica de Catalunya
    Theses

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    Esta tesis surge de la necesidad de establecer un método racional y práctico para identificar, a través del cambio de las frecuencias naturales , el daño en estructuras de hormigón armado. Por lo que se desarrolla un nuevo modelo constitutivo de daño diferenciado con dos variables internas de daño utilizando una única superficie de discontinuidad para tracción, compresión y cortante; asimismo, se plantea un procedimiento numérico para el cálculo de las frecuencias naturales con sus formas modales de vibración para diferentes estados de daño de estructuras de hormigón armado; se presenta una metodología para correlacionar daño global y frecuencias naturales . Esta tesis se desarrollará en el marco del análisis de estructuras y la mecánica computacional, particularmente, del análisis de materiales compuestos utilizando la teoría de mezclas serie/paralelo. Los resultados numéricos obtenidos se han comparado con resultados experimentales y numéricos obtenidos de la literatura, se ha observado buenos ajustes de los resultados

    This thesis emerges from the need to establish a rational and smart method to estimate the global damage through changes on eigenfrequency of concrete structures. Therefore, a new constitutive damage model is developed with two scalar inner variables and using only one surface of discontinuity to traction, compression and shear. Additionally, a numerical process has been proposed to calculate eigenfrequencies and eigenvectors of concrete structure with different levels of damage. A methodology to correlation ship between global damage and changes on e1genfrequencies has been proposed. This thesis is based on structural analysis and computational mechanics and, particularly, on analysis of composite materials using serial/parallel mix theory. The numerical results obtained in this thesis is have been compared with experimental and numerical results obtained of literature, and can be seen a good fit.

  • On the theory of cell migration: durotaxis and chemotaxis  Open access

     Diego Iñiguez, Javier
    Defense's date: 2013-07-03
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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    Cell migration is a fundamental element in a variety of physiological and pathological processes. Alteration of its regulatory mechanisms leads to loss of adhesion and increased motility, critical steps in the initial stages of metastasis. Consequently, cell migration has become the focus of intensive experimental and theoretical studies; however the understanding many of its mechanisms remains elusive. Cell migration is the result of a periodic sequence of protrusion, adhesion remodeling and contraction stages that leads to directed movement towards external stimuli. The spatio-temporal coordination of these processes depends on the activation of the signaling networks that regulate them at specific subcellular locations. Particularly, the family of small RhoGTPases plays a central role in regulating cell polarization, the formation of adhesion sites and the generation of the forces that drive motion. Theoretical models based on an independent description of these processes have a limited capacity to predict cellular behavior observed in vitro, since their functionality depends on the cross-regulation between their signaling pathways. This thesis presents a model of cell migration that integrates a description of force generation and cell deformation, adhesion site dynamics and RhoGTPases activation. The cell is modeled as a viscoelastic body capable of developing traction and protrusion forces. The forces are determined by the activation level of the RhoGTPases, whose distribution in the cell is described by a set of reaction-diffusion equations. Adhesion sites are modeled as punctual clusters of transmembrane receptors that dynamically bind and unbind the extracellular matrix depending on the force transimtted to them and the distance with ligands coating the substrate. On the theoretical level, the major findings relate the topology of a Crosstalk Scheme and the properties inherited by the associated reaction network as a gradient sensing and regulatory system: reversible polarization, adaptation to uniform stimulus, multi-stimuli response and amplification. Models formulated according to these principles remain functional against the biological diversity associated to different cell types and match the observed behavior in Chemotaxis essays: the capacity of cells to detect shallow gradients, polarize without featuring Turing patterns of activation, and switch the direction of migration after the stimulus source is changed. The biological implications challenge a long held view on the mechanisms of RhoGTPase crosstalk and suggests that the role of GDIs, GEFs and GAPs has to be revised, as supported by recent experimental evidence. In addition, the model recapitulates a continuous transition from the tear-like shape adopted by neutrophiles to the fan-like shape of keratocytes during migration by varying the magnitudes of protrusion and contraction forces or, alternatively, the strength of RhoGTPase Crosstalk. The second mechanism represents a novel explanation of the different morphologies observed in migrating cells. On cell mechanosensing, a new hypothesis is proposed to explain how cells sense the mechanical properties of the ECM. The hypothesis provides a unifying explanation to apparently conflicting observations on force development and growth in real time at cell Focal adhesions, previously attributed to differences in experimental set-ups or cell types studied. An interpretation for the observed relationships between polarization time, migration speed, mechano-sensing limits and substrate rigidity follows from this hypothesis. Further, the theory directly suggests the currently unknown mechanisms that could explain the universal preference of cells (bar neurons) to migrate along stiffness gradients, and for the first time, a plausible biological function for the existence of this phenomenon. It is known as Durotaxis, and its abnormal regulation has been associated to the malignant behaviour of cancer cells.

  • Computational Model of the Human Urinary Bladder  Open access

     Silva Araujo monteiro, Virginia
    Defense's date: 2013-06-10
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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    La propuesta de una vejiga artificial es un obstáculo a trasponer. El cáncer de vejiga está entre los casos más frecuentes de enfermedades oncológicas en Estados Unidos y Europa. Ese cáncer es considerado un problema médico importante una vez que esa enfermedad presenta altas tasas de re-ocurrencia, muchas veces llevando a la remoción del órgano. La solución más sofisticada para remplazar ese órgano es la vejiga ileal, que consiste en una neovejiga hecha de tejido intestinal del enfermo. Desafortunadamente, esa solución presenta no solo problemas mecánicos funcionales, descritos en la literatura como problemas de vaciado y fuga, peo también problemas de orden biológica (como ejemplo pérdida ósea, debido a la absorción por el intestino de substancias que necesitan ser eliminadas del organismo). A través de la solicitación de la comunidad urológica del Hospital Clínico de Barcelona y con su experiencia en modelos numéricos para estructuras biomédicas, el Centro de Métodos Numéricos en Ingeniería (CIMNE) ha tenido la iniciativa de proporcionar actividad investigadora de la mecánica de la vejiga urinaria y de la simulación de interacción fluidoestructura para reproducir el llenado y vaciado de ese órgano con la orina. La simulación de la vejiga humana por el Método de los Elementos Finitos (FEM) y un completo entendimiento de la mecánica de ese órgano y de su interacción con la orina dará la posibilidad de proponer mejora en la geometría y de analizar materiales para la solución artificial en caso de remplazamiento de la vejiga. Para lograr ese objetivo, primeramente procedemos a una revisión bibliográfica de los modelos matemáticos del aparato urinario y un estudio comprehensivo de la fisiología y dinámica de la vejiga. Presentamos una revisión de las principales estructuras urológicas, riñón, uréter y uretra. Las estructuras anexas también son consideradas para entender las condiciones de contorno del problema estudiado. Posteriormente, proponemos el modelo constitutivo para estudiar la vejiga urinaria humana. El comportamiento del musculo detrusor durante llenado y vaciado de la vejiga con orina, su habilidad de retención de orina a baja presión debe ser correctamente representada por medio de la implementación de un modelo constitutivo no-lineal. El modelo matemático necesita representar las variables mecánicas que gobiernan ese órgano, y también las propiedades de la orina. El comportamiento no-lineal de tejidos vivos es implementado y validado con ejemplos de la literatura. La propiedad quasi-incompressible de la orina y las ecuaciones Navier-Stokes son consideradas para análisis del fluido. Para representar la geometría de la vejiga, implementamos un modelo computacional 3D a partir de imágenes de tomografía computadorizada de un cadáver adulto. Los datos son tratados para considerar las condiciones de contorno. Hemos construido dos modelos de malla: un mallado con tetrahedos de cuatro nodos y otro mallado con elementos de membrana de tres nodos. El esquema utilizado para calcular la interacción fluido-estructura debe ser adecuado para materiales de densidad muy parecidas. El análisis numérico de llenado y vaciado de la vejiga humana es validada con tests urodinámicos estandarizados. La parte final de la tesis, presentamos una simulación de una neo-vejiga, siendo el primer paso para representar numéricamente materiales artificiales para remplazamiento de la vejiga.

    The proposal of an artificial bladder is still a challenge to overcome. Bladder cancer is among the most frequent cases of oncologic diseases in United States and Europe. It is considered a major medical problem once this disease has high rates of reoccurrence, often leading to the extirpation of this organ. The most refined solution to replace this organ is the ileal bladder, which consists of a neobladder made of the patient’s intestinal tissue. Unfortunately this solution presents not only functional mechanical problems, described on the literature as voiding and leaking problems, but also biological ones (i.e. bone loss, given the absorption by the intestine of substances that should be eliminated from the organism). Urged by the urological community of the Hospital Clinic de Barcelona and backgrounded by its experience in the numerical simulation of biomedical structures, the Center of Numerical Methods in Engineering (CIMNE) had the initiative to provide the research of the mechanics of the urinary bladder and the simulation of fluid structure interaction (FSI) to account for the filling and voiding of this organ with urine. The Finite Element Method (FEM) simulation of the real bladder and the comprehensive understanding of the mechanics of this organ and its interaction with urine will give the possibility to propose geometrical improvements and study suitable materials for an artificial solution to address the cases on which the bladder needs to be removed. To reach this goal, first we proceeded to the bibliographic review of mathematical models of the urinary apparatus and to a comprehensive study of the physiology and dynamics of the bladder. A review of the major urological structures, kidney, ureter and urethra, takes place. To consider boundary conditions other surrounding structures to the urinary system are also studied. In the second part of the thesis, we propose the numerical model to study the human urinary bladder. The behavior of the detrusor muscle during filling and voiding of the bladder with urine and its ability to promote the storage of urine under low pressure need to be accurately represented, requiring the implementation of a non-linear constitutive model. The mathematical model needs to be capable to simulate the mechanical variables that govern this organ and the properties of the urine. The nonlinear behavior of living tissues is implemented and validated with examples from the literature. The quasi-incompressibility property of urine and the navierstokes equations for the fluid are taken into account. The geometry of the bladder needs to be taken into account, and the implementation of a 3D computational model obtained from the computerized tomography of a cadaver male adult is considered. The data has been treated to consider boundary conditions. Two models have been conceived: one meshed with four nodes tetrahedral and another meshed with shell elements. FSI must work for the simulation of filling and voiding of the bladder. Due to the close densities of the materials the scheme used to solve fluid-structure needs to be carefully selected. The proposed numerical model and the filling and voiding analysis are finally validated with standardized urodynamic tests. The final part of the thesis, the simulation of a neobladder is presented, being the first step to simulate numerically artificial materials for bladder replacement.

  • El barrido de deformaciones como método para evaluar el comportamiento a fatiga de los materiales bituminosos  Open access

     Botella Nieto, Ramon
    Defense's date: 2013-05-27
    Universitat Politècnica de Catalunya
    Theses

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    El comportamiento a fatiga de las mezclas bituminosas es de vital importancia para el diseño de pavimentos. Como componente que cohesiona la mezcla, el ligante hidrocarbonado influye fuertemente en esta propiedad de la mezcla. Por ello es necesario estudiar el comportamiento a fatiga de este material. Clásicamente las propiedades de los ligantes en condiciones de fatiga han sido estudiadas mediante la obtención de la ley de fatiga del material. Para ello son necesarios numerosos ensayos cíclicos a amplitud de deformación o carga constante, denominados barridos de tiempo. Este tipo de ensayos son largos y costosos. Esta tesis doctoral presenta un nuevo procedimiento de ensayo que pretende caracterizar el comportamiento a fatiga de ligantes con menor número de ensayos y de menor duración. El nuevo procedimiento se basa en aplicar ensayos cíclicos de amplitud de deformación ascendente. Este tipo de ensayos son conocidos como barridos de deformaciones y por ello, el nuevo procedimientos se nombró EBADE (Ensayo de BArrido de DEformaciones). Previo a un procedimiento inicial de calibración el EBADE ha sido empleado para caracterizar el comportamiento a fatiga de varios ligantes y se ha demostrado que los resultados obtenidos mediante este ensayo están relacionados con la ley de fatiga del material, cuya obtención implica un consumo de tiempo muy superior. Así mismo, se ha demostrado que la pérdida de módulo experimentada por el ligante durante los ensayos cíclicos es, en su mayor parte, debida a la tixotropía del material y no a un proceso de daño irreversible. Como aportación final, se propone el empleo de dos parámetros obtenidos en los ensayos EBADE, el módulo inicial y la deformación de fallo, como parámetros característicos del comportamiento de los ligantes bituminosos a fatiga, en substitución de las leyes de fatiga obtenidas por medio de los ensayos de barrido de tiempo.

    The fatigue behavior of bituminous mixtures plays a major role in pavement design. The asphalt binder has a strong influence in that property and because of that it is necessary to study its fatigue behavior. Typically the fatigue properties of binders have been studied by obtaining the fatigue law of the material. This requires several cyclic tests at constant strain or stress amplitude, called time sweep tests. Such tests are long and expensive. This thesis presents a new test procedure that aims to characterize the fatigue behavior of binders with fewer and shorter tests. The new procedure is based on applying increasing strain amplitude cyclic tests. The test was called EBADE, standing for strain sweep tests in Spanish. Prior to an initial calibration procedure, the EBADE has been used to characterize the fatigue behavior of several binders and demonstrated that the results obtained by this test are related to the fatigue law of the material, which calculation involves several time consuming tests. In addition, it has been proven that modulus loss experienced by the binder during cyclic testing is mostly due to the thixotropy of the material and not to a process of irreversible damage. As a final remark, two parameters obtained in the EBADE tests, initial modulus and failure strain, are proposed to characterize the fatigue behavior of asphalt binders, instead of the fatigue laws obtained by means of the time sweep tests.

  • Multifield-based modeling of material failure in high performance reinforced cementitious composites  Open access

     Mora Mendez, Diego Fernando
    Defense's date: 2013-04-26
    Universitat Politècnica de Catalunya
    Theses

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    Cementitious materials such as mortar or concrete are brittle and have an inherent weakness in resisting tensile stresses. The addition of discontinuous fibers to such matrices leads to a dramatic improvement in their toughness and remedies their deficiencies. It is generally agreed that the fibers contribute primarily to the post-cracking response of the composite by bridging the cracks and providing resistance to crack opening. On the other hand, the multifield theory is a mathematical tool able to describe materials which contain a complex substructure. This substructure is endowed with its own properties and it interacts with the macrostructure and influences drastically its behavior. Under this mathematical framework, materials such as cement composites can be seen as a continuum with a microstructure. Therefore, the whole continuum damage mechanics theory, incorporating a new microstructure, is still applicable. A formulation, initially based on the theory of continua with microstructure Capriz, has been developed to model the mechanical behavior of the high perfor-mance fiber cement composites with arbitrarily oriented fibers. This formulation approaches a continuum with microstructure, in which the microstructure takes into account the fiber-matrix interface bond/slip processes, which have been recognized for several authors as the principal mechanism increasing the ductility of the quasi-brittle cement response. In fact, the interfaces between the fiber and the matrix become a limiting factor in improving mechanical properties such as the tensile strength. Particularly, in short fiber composites is desired to have a strong interface to transfer effectively load from the matrix to the fiber. However, a strong interface will make difficult to relieve fiber stress concentration in front of the approaching crack. According to Naaman, in order to develop a better mechanical bond between the fiber and the matrix, the fiber should be modified along its length by roughening its surface or by inducing mechanical defor-mations. Thus, the premise of the model is to take into account this process considering a micro field that represents the slipping fiber-cement displacement. The conjugate generalized stress to the gradient of this micro-field verifies a balance equation and has a physical meaning. This contribution includes the computational modeling aspects of the high fiber rein-forced cement composites (HFRCC) model. To simulate the composite material, a finite element discretization is used to solve the set of equations given by the multifield approach for this particular case. A two field discretization: the standard macroscopic and the micro-scopic displacements, is proposed through a mixed finite element methodology. Furthermore, a splitting procedure for uncoupling both fields is proposed, which provides a more convenient numerical treatment of the discrete equation system. The initiation of failure in HPFRCC at the constitutive level identified as the onset of strain localization depends on the mechanical properties of the all compounds and not only on the matrix ones. As localization criteria is considered the bifurcation analysis in combination with the localized strain injection technique presented by Oliver et al. It consists of injecting a specific localization mode during the localization stage, via mixed finite element formulations, to the path of elements that are going to capture the cracks, and, in this way, the spurious mesh orientation dependence is removed. Model validation was performed using a selected set of experiments that proves the via-bility of this approach. The numerical examples of the proposed formulation illustrated two relevant aspects, namely: 1) the role of the bonding mechanism in the strain hardening be-havior after cracking in the HPFRCC and 2) the role that plays the finite element formulation in capturing the displacement localization in the localization stage.

  • Análisis dinámico de placas laminadas mediante macro-elementos usando teoría trigonométrica de deformación por corte

     Rango, Rita; Nallim, Liz G.; Oller Martinez, Sergio Horacio
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-14
    Presentation of work at congresses

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    En este trabajo se incorpora la cinemática correspondiente a una Teoría Trigonométrica de Deformación por Corte (TSDT) a macro-elementos enriquecidos, los cuales fueron formulados originalmente por los autores para el análisis de placas delgadas. En esta teoría se emplean funciones trigonométricas, definidas en la variable del espesor y dependientes del mismo, que incorporan una variación sinusoidal de los desplazamientos a lo largo del espesor y verifican la condición de tensiones de corte nulas en las superficies superior e inferior del laminado y la compatibilidad de desplazamientos en el plano en las superficies interlaminares. Por otro lado, se evita el bloqueo por cortante y no se requiere de un factor de corrección por corte. Las relaciones constitutivas entre tensiones y deformaciones cortantes se satisfacen en cada lámina o capa. El macro-elemento formulado constituye un elemento finito jerárquicamente enriquecido con polinomios de Gram-Schmidt que, a su vez, es mapeado utilizando el concepto de isoparametría en coordenadas naturales. Así, la superficie de una placa se discretizaría con una malla de macro-elementos que a diferencia de elementos finitos clásicos, no requieren densificar el mallado. La formulación obtenida se emplea en este trabajo para el análisis dinámico de placas gruesas laminadas, considerando diferentes condiciones de contorno. Los resultados numéricos obtenidos con la presente formulación muestran una buena correlación con resultados publicados por otros autores. Además, se pueden realizar con facilidad estudios paramétricos y llevar a cabo procesos de optimización.

  • Regularización de la energía de fractura utilizando un modelo de fisura distribuida de Hillerborg y daño isotrópico mediante un método sin malla de puntos finitos

     Pérez Pozo, Luis; Valdivia, Fernado; Oller Martinez, Sergio Horacio
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-15
    Presentation of work at congresses

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    El fenómeno de fractura en materiales heterogéneos cuasi-frágiles ha sido estudiado extensivamente con modelación numérica basada en el Método de Elementos Finitos (MEF), mostrando dependencias en la orientación y tamaño de la malla en los resultados. Es por eso que el uso de técnicas sin malla junto con modelos de fisura distribuida surgen como alternativas a estas dependencias. En este trabajo, un modelo de fisura distribuida aplicado al fenómeno de daño isotrópico mediante un método sin malla de Puntos Finitos es presentado. Se propone el uso de una longitud geométrica dada por la dirección de los esfuerzos principales en reemplazo de una longitud característica, la cual se basa en el modelo de Fisura Ficticia de Hillerborg. El criterio de fluencia de Rankine y un esquema iterativo incremental de Newton-Raphson son usados para predecir el comportamiento no-lineal en la zona fracturada del material.

  • Modelo de degradación diferenciada para materiales compuestos

     Quinteros, Ricardo D.; Oller Martinez, Sergio Horacio; Nallim, Liz G.
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-16
    Presentation of work at congresses

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    En este trabajo se presenta la formulación y validación de un modelo de degradación elástica diferenciado. El modelo propuesto permite degradar en forma diferenciada la parte volumétrica y desviadora del comportamiento constitutivo del material y está basado en una generalización del modelo de degradación simple de Kachanov. De esta manera, el modelo resulta especialmente apropiado para simular el comportamiento de materiales friccionales. En particular, este modelo se aplica a los materiales componentes de una mampostería de ladrillos homogeneizada mediante una técnica particular ad hoc¸ que permite su tratamiento como material compuesto anisótropo, partiendo de las características geométricas y mecánicas de los materiales constituyentes. Para el tratamiento de la anisotropía, se usa la técnica de mapeo de espacios a través del concepto de espacio isótropo ficticio. La metodología planteada brinda una gran potencia de simulación pudiéndose también utilizar, si fuera necesario, otras formulaciones constitutivas diferentes para el comportamiento de los materiales componentes de la mampostería. El modelo constitutivo propuesto, junto con la técnica de homogeneización se implementa en un programa general de elementos finitos. Finalmente, se muestran los resultados obtenidos en ejemplos simples de validación del modelo y luego se presentan ejemplos de aplicación que muestran la respuesta no lineal del compuesto y su comparación con resultados publicados por otros autores.

  • Simulación numérica del crecimiento hipertrófico de tejidos biológicos

     Bellomo, Facundo; Oller Martinez, Sergio Horacio; Nallim, Liz G.
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-15
    Presentation of work at congresses

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    El crecimiento de los tejidos biológicos blandos tiene un papel fundamental en una serie de procesos normales y patológicos de gran incidencia en la salud humana. Algunos de los factores que regulan estos procesos en los tejidos biológicos son fundamentalmente mecánicos, de allí la importancia del estudio de estos fenómenos en el marco de una formulación que permita estimar los campos de tensión y deformación, así como la interrelación con los fenómenos de crecimiento. En general, los modelos de crecimiento publicados hasta la fecha consideran el estímulo mecánico para el crecimiento a partir de las tensiones instantáneas a que está sometido el tejido en un determinado instante. Estos modelos, sin embargo, no resultan adecuados para el caso del crecimiento hipertrófico de tejidos biológicos blandos. El proceso de hipertrofia consiste en un incremento en el volumen del tejido, que se genera cuando las fibras dañadas del mismo son reparadas y por lo tanto esta asociado al daño que sufren las mismas. Este proceso es el responsable del crecimiento del tejido muscular, de allí la importancia de su estudio con miras a su aplicación en el estudio de metodologías de entrenamiento de alto rendimiento y patologías relacionadas con la hipertrofia muscular, como por ejemplo la hipertrofia cardiaca. Para la modelación del tejido se propone el empleo de un modelo constitutivo basado en la teoría de mezclas generalizada considerando crecimiento isótropo. En el modelo el crecimiento es controlado por una variable interna de daño reversible que constituye el estímulo mecánico para la reparación de las fibras del tejido. La capacidad de reparación del tejido y el crecimiento hipertrófico asociado dependerán, a su vez, de la capacidad del metabolismo para llevar adelante estos procesos. Esto se tiene en cuanta en el modelo a través de una variable de disponibilidad biológica. Finalmente, se presentan ejemplos de validación que permiten demostrar que el modelo resulta adecuado para el estudio del crecimiento hipertrófico, considerando la acción simultánea de los campos biológico y mecánico.

  • Simulación numérica del flujo de un tramo fluvial con una turbina de paso situada en su lecho y los efectos en el comportamiento de su lámina libre

     Oller Aramayo, Sergio A.; Bateman Pinzon, Allen; Nallim, Liz G.; Oller Martinez, Sergio Horacio
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-14
    Presentation of work at congresses

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    Las turbinas de paso o WCT (Water Current Turbines) abren nuevas y mayores posibilidades, con grandes ventajas en cuanto al aprovechamiento hidroeléctrico. Éstas evitan las construcciones de costosas presas y reducen considerablemente el impacto medioambiental que producen otras soluciones de aprovechamiento energético. Pero a su vez esta solución plantea una serie de nuevos retos tecnológicos, debido a sus diseños sin todavía ningún tipo de estandarización y pocos antecedentes en su utilización. Además, a todo esto hay que agregar la complejidad que añade el estudio del comportamiento del flujo del río donde deben instalarse. El modelo que se analiza en el presente trabajo representa un tramo fluvial con una turbina de paso axial situada en su lecho. Dicho modelo se analiza mediante la técnica de simulación numérica CFD (Computational Fluid Dynamics) y la turbina de paso se representa introduciendo en el cauce del río un bloque constituido por un medio continuo poroso para representar las perturbaciones que esta introduce y sus intercambios energéticos con el flujo del cauce. En este artículo se analizan las velocidades alcanzadas en las distintas zonas del río, presiones, líneas de corriente, y el comportamiento de la lámina libre que resulte de la presencia de la turbina representada a través de un bloque de medio poroso. Con este trabajo se estudia un eslabón importante en la cadena de desarrollos necesarios para el diseño y factibilidad de aplicación y puesta en marcha de una turbina de paso que está actualmente en desarrollo por los autores. (Oller et al., Revista Iberoamericana de Ingeniería Mecánica, Vol.16 Nº1 pp. 03-16 (2012)).

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    Modelling viscoelastic behaviour of carbón nanotube-reinforced thermo-plastics  Open access

     Otero Gruer, Fermin Enrique; Oller Martinez, Sergio Horacio; Martinez Garcia, Xavier; Salomon, Ramon Omar
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-15
    Presentation of work at congresses

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    Carbon nanotubes (CNTs), since their discovery by Lijima (S. Lijima, Nature, 354:56-58 (1991)), are considered a new generation of reinforcement. Their "nano" size structure makes them potentially free of defects, which provides them with excellent physical properties. There are two main nanotube types: single wall nanotubes (SWCNTs), which are made of a single wall tube; and multiwall nanotubes (MWCNTs), which consist in several concentric walls, one inside the other. A key factor for the reinforcement efficiency in a composite it is the interface bonding between the CNTs and the matrix. This work presents a new constitutive model to predict the mechanical performance of composites made of a thermo-plastic matrix reinforced with CNTs. The model takes into account explicitly the mechanical contribution of the interface between the matrix and the CNTs (F. Otero et. al., Comp Structures, 94:2920-2930 (2012)). The constitutive model is based in the mixing theory, which obtains the composite performance from the response of each constituent component, each one simulated with its own constitutive law. The model has been implemented into an in-house FEM code: PLCd. As an application example, this code is used to predict the mechanical properties of a straight beam with different material configurations. In this case, a viscoelastic constitutive model is proposed for the polymeric matrix. The viscous response within the elastic range of the materials is studied. This response shows a high capacity of energy dissipation in composites reinforced with MWCNTs.

  • Numerical modeling of a cervical spine discectomy

     Comellas Sanfeliu, Ester; Oller Martinez, Sergio Horacio; Poblete, José; Berenguer, Joan; Prats-Galino, Alberto
    Congreso Argentino de Mecánica Computacional
    Presentation's date: 2012-11-16
    Presentation of work at congresses

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    Cervical spine discectomy is a relatively common medical procedure which entails the surgical removal of a herniated intervertebral disc which is then replaced with an adequate prosthesis. Surgeons rely on their expertise to minimize the damage induced on the adjacent vertebrae and discs during this invasive procedure in order to reduce the patient's postsurgical distress. A typical cervical spine discectomy has been modelled and tested using the Finite Element Method (FEM) with the intention of contributing to better elucidate its immediate physical consequences on the vertebrae and intervertebral discs. Internal stresses, strains and damage levels can be obtained through the use of FE models, which can prove useful in improving surgical procedures or tailoring them to the need of particular patients. To this aim, a model of four cervical vertebrae with their corresponding discs has been built. Spinal ligaments, zygapophyseal joints and uncovertebral joints have also been included. The vertebrae have been modelled using a damage model whilst the intervertebral discs and ligaments have been treated as separate hyperelastic materials. Also, the nucleus and the annulus of the discs have been di erentiated. The problem has been solved following non-linear large deformation theory and considering prestress in the ligaments. The model's accuracy has been assessed through comparison of previously published results for di erent spinal movements (N. Kallemeyn et al., Med Eng Phys, 32(5):482-489 (2010)). Then, the model has been numerically tested for a load case representative of the discectomy procedure.

  • The Quadrants Method: a procedure to evaluate the seismic performance of existing buildings

     Vielma Pérez, Juan Carlos; Martínez, Y; Barbat Barbat, Horia Alejandro; Oller Martinez, Sergio Horacio
    World Conference on Earthquake Engineering
    Presentation's date: 2012-09-25
    Presentation of work at congresses

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    In this work a new simple procedure for the evaluation of the seismic performance is formulated. It combines the results of non-linear static analysis with a specific Limit State defined by inter-storey drift-based damage threshold and the seismic demand obtained from the inelastic design spectrum. With both values it is possible to define four zones in the capacity curve which characterizes four different seismic performances. The procedure is used to evaluate two reinforced concrete framed buildings, the first one consist in a typical lowrise building designed according to current Venezuelan codes; the second one is a four-storey building designed according to older code, whose had been damaged by fire. Results shown both cases do not meet the Quadrants Method criterion then they need to be redesigned. Redesigned buildings were submitted to a non-linear dynamic analysis, representative of three hazard levels associated with three Limit States. These redesigned buildings met all the objectives associated with the hazard levels, showing the efficiency of the method for the rapid evaluation of the seismic performance of existing buildings.

  • An inverse approach for the mechanical characterisation of vascular tissues via a generalised rule of mixtures

     Bellomo, Facundo; Oller Martinez, Sergio Horacio; Nallim, Liz G.
    Computer methods in biomechanics and biomedical engineering
    Date of publication: 2012-12
    Journal article

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    Mechanical factors such as stresses and strains play a major role in the growth and remodelling of soft biological tissues. The main constituents of tissue undergo different processes reacting to mechanical stimulus. Thereby, the characterisation of growth and remodelling requires an accurate estimation of the stresses and strains of their main components. Many soft tissues can be considered as composite materials and can be analysed using an appropriate rule of mixtures. Particularly, arterial tissue can be modelled as an isotropic soft matrix reinforced with preferentially oriented collagen fibres. An inverse approach to obtain the mechanical characterisation of each main component is proposed in this work. The procedure is based on a rule of mixtures raised in a finite deformation framework and generalised to include kinematics and compatibility equations for serial–parallel behaviour. This methodology allows obtaining the stress–strain relationship of the components fitting experimental data.

  • Nonlinear homogenization techniques to solve masonry structures problems

     Quinteros, Ricardo D.; Oller Martinez, Sergio Horacio; Nallim, Liz G.
    Composite structures
    Date of publication: 2012-01
    Journal article

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  • A constitutive model for tissue adaptation: necrosis and stress driven growth

     Bellomo, Facundo; Armero, Francisco; Nallim, Liz G.; Oller Martinez, Sergio Horacio
    Mechanics research communications
    Date of publication: 2012-06
    Journal article

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  • Estudio numérico-experimental de la interfaz hormigón-epoxi-FRP para una estructura reforzada sometida a doble corte

     Molina, Maritzabel; Cruz Solis, Juan Jose; Oller Martinez, Sergio Horacio; Barbat Barbat, Horia Alejandro; Gil Espert, Lluis
    Revista internacional de métodos numéricos para cálculo y diseño en ingeniería
    Date of publication: 2012-02
    Journal article

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  • Study and prediction of the mechanical performance of a nanotube-reinforced composite

     Otero, Fermin; Martinez Garcia, Xavier; Oller Martinez, Sergio Horacio; Salomon, Ramon Omar
    Composite structures
    Date of publication: 2012-09
    Journal article

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  • Numerical-experimental study of the FRP-epoxy-concrete interface for reinforced structures under double shear

     Molina Herrera, Maritzabel; Cruz Solis, Juan Jose; Oller Martinez, Sergio Horacio; Barbat Barbat, Horia Alejandro; Gil Espert, Lluis
    Revista internacional de métodos numéricos para cálculo y diseño en ingeniería
    Date of publication: 2012
    Journal article

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  • Dimensionado sísmico de edificios porticados de hormigón armado mediante factores de amplificación con base en energía

     Vielma Pérez, Juan Carlos; Barbat Barbat, Horia Alejandro; Oller Martinez, Sergio Horacio
    Hormigon y acero
    Date of publication: 2012
    Journal article

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    El proyecto sismorresistente de estructuras actual se realiza mediante una serie de prescripciones normativas que persiguen garantizar su adecuado comportamiento. Los procedimientos actuales de análisis se realizan utilizando el comportamiento elástico, aplicando los factores de reducción de respuesta para obtener fuerzas sísmicas equivalentes, representativas del comportamiento inelástico que pueden alcanzar las estructuras al ser sometidas a la acción de sismos fuertes. A partir de los resultados del análisis elástico se obtienen los desplazamientos inelásticos, amplificados mediante factores que son función de los factores de reducción; sin embargo, se ha observado una amplia variación en los valores de los factores de amplificación prescritos en las normas a nivel mundial, por lo que se justifica profundizar en el estudio de dichos factores. En este trabajo se presenta un procedimiento de dimensionado sísmico con base en la respuesta no lineal. El procedimiento es aplicado a un conjunto de edificios regulares de hormigón armado de diferentes alturas, proyectados para niveles altos de amenaza sísmica y es evaluado aplicando análisis dinámico no lineal para tres Estados Límite, correlacionado con tres umbrales de daño que se definen a través de las derivas de planta. Los resultados demuestran que el procedimiento es adecuado para realizar el dimensionado sísmico de edificios, proporcionando al proyectista criterios de evaluación más cónsonos con las características de los edificios y de la amenaza sísmica.

  • Simulación numérica del comportamiento mecánico en vivo de tejidos biológicos blandos

     Bellomo, Facundo Javier
    Defense's date: 2012-03-30
    Universidad Nacional de Salta
    Theses

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  • Formulación de elementos finitos para vigas de sección abierta en laminados compuestos

     Vargas Mendoza, Pablo Enrique
    Defense's date: 2012-01-27
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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  • Caracterización estadística de los modelos de confinamiento y determinación de coeficientes parciales de seguridad para el refuerzo de pilas de puentes de hormigón mediante materiales compuestos (CFRP)  Open access

     Chambi Chuquichambi, José Luis
    Defense's date: 2012-11-30
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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    El continuo deterioro y/o daños causados en las estructuras de hormigón durante su vida útil, son fenómenos inevitables que conlleva el paso del tiempo y viene a ser un problema importante que ha conducido al desarrollo de nuevos materiales compuestos para el refuerzo estructural. En el caso de puentes, las situaciones en las que se hacen necesarias intervenciones de refuerzo pueden ser: estructuras afectadas por patologías inherentes a problemas de durabilidad, estructuras mal proyectadas y/o mal ejecutadas, estructuras que han sido dañadas por causas externas (impacto de vehículos, incendio, sismo, desastres naturales, etc.), estructuras afectadas por un cambio de uso o por un aumento de la carga de proyecto, o para soportar futuros sismos. Frente a ello, los métodos de refuerzo tradicional tales como el recrecido de hormigón, postensado exterior y refuerzo con chapas de acero, frecuentemente tienen desventajas inherentes como el incremento del peso de la estructura, la corrosión del acero y su elevado costo de manipulación y colocación. Ante estos inconvenientes, y más concretamente refiriéndonos al refuerzo de pilas de puentes, en años recientes se ha incrementado el uso de polímeros reforzados con fibra (FRP) como elemento de confinamiento de elementos de hormigón sometidos a compresión, debido a sus excelentes propiedades mecánicas y químicas, incrementando principalmente la resistencia y ductilidad de las pilas de hormigón. Sin embargo, su aplicación presenta aún lagunas que exigen seguir investigando aspectos como la falta de códigos y normas, la falta de experiencia en el comportamiento a largo plazo, la susceptibilidad al fuego, etc. Existen incertidumbres en las bases de cálculo durante el dimensionamiento de este refuerzo, una de ellas está precisamente en los coeficientes parciales de seguridad para el hormigón confinado que se deben adoptar en el diseño. Por ello, la presente tesis doctoral pretende aportar conocimientos en este campo, y más concretamente, definiendo dichos coeficientes parciales de seguridad para el hormigón confinado para el diseño del refuerzo de pilas de puentes de hormigón armado con polímeros reforzados con fibras de carbono (CFRP), mediante proceso de calibración basado en fiabilidad estructural. En ese sentido, primeramente, se ha recopilado toda la información bibliográfica disponible referida principalmente a dos aspectos: descripción de modelos de confinamiento para hormigón confinado con CFRP y datos de ensayos experimentales para la elaboración de una base de datos. Los modelos de confinamiento considerados en este estudio para elementos de hormigón de sección circular son de: Mander et al. 1988, Pilakoutas y Mortazavi 1997, Toutanji 1999, Spolestra y Monti 1999, Eid y Paultre 2008, Teng et al. 2009, Wu y Zhou 2010. Aplicando las expresiones de predicción de estos modelos a los datos experimentales recogidos en la base de datos, obtenemos como resultado tensiones y deformaciones últimas, y con ellos, mediante un análisis estadístico de valores teóricos respecto a valores experimentales para cada modelo de confinamiento, y, tras un análisis comparativo entre los diferentes modelos definimos el modelo que mejor ajuste presenta respecto a los datos experimentales (modelo de Teng et al. 2009). Para dicho modelo, se ha realizado la caracterización estadística. Puesto que es poco probable esperar que un formato de norma de diseño estructural pueda representar todas las situaciones de diseño, los sistemas estructurales y tipologías de pilas de puentes para los cuales se ha realizado la calibración en este estudio, son los puentes de tramo recto de varios vanos con pilas de hormigón armado tipo columnas circulares. Las pilas pueden estar formadas por un fuste único o bien por dos fustes. Esta tipología de tablero y de pila cubre la mayor parte de los puentes existentes en las actuales vías de comunicación. Para definir los coeficientes parciales de seguridad para el hormigón confinado γcc, se ha realizado la calibración basado en fiabilidad estructural, para un rango posible de soluciones de refuerzo con CFRP en las tipologías de pilas de puentes de los sistemas estructurales estudiados. Para ello, se tienen en cuenta las incertidumbres de las variables que intervienen, y mediante simulación se caracterizan estadísticamente las variables de resistencia y solicitación. Teniendo caracterizadas estas dos variables, según los niveles requeridos de seguridad estructural se determina la probabilidad de fallo o índice de fiabilidad para diferentes valores de γcc. Los coeficientes parciales de seguridad quedarán definidos para aquellas situaciones, tales que, los índices de fiabilidad calculados en las pilas reforzadas dimensionadas con dichos coeficientes, den valores iguales o superiores al índice de fiabilidad objetivo, que en este estudio se ha fijado en . Finalmente, la propuesta de coeficientes parciales de seguridad a utilizar es la siguiente: γcc = 1,15 para tmín ≤ t ≤ 2 mm γcc = 1,30 para t > 2mm

    The continuous deterioration and/or damages caused in the concrete structures during their lifetime, are unavoidable phenomena which the passage of time entails and gets to be an important problem that it has led to the development of new composite materials used for the structural reinforcement. The methods of traditional reinforcement such as concrete increased, prestressed exterior and the reinforcement with steel sheets, frequently they have inherent disadvantages like the increase of the weight of the structure, the corrosion of the steel and its high cost of manipulation and positioning. Given these disadvantages, and more specifically referring us to the reinforcement of pillars of bridges, in recent years the use of fibre reinforced polymers (FRP) has increased as an element of confinement. However, its use still still shows some gaps which demand to continue investigating some of their aspects such as the lack of codes and rules, the lack of experience in the long term behaviour, the susceptibility to the fire, etc. Uncertainties exist in the calculation bases along the dimensioning of this reinforcement, one of them precisely lays in the partial coefficients of safety for the confined concrete that must be to adopted in the design. For this purpose, the present doctoral thesis tries to contribute knowledge in this field, and more specifically, defining these partial coefficients of safety of the confined concrete for the design of the reinforcement of bridge pillars of reinforced concrete with carbon fibre reinforced polymers (CFRP), by means of calibration process based on structural reliability. In this sense, firstly, has been compiled all the bibliographical information available mainly referred to two aspects: description of confinement models for confined concrete with CFRP and data obtained from experimental tests for the elaboration of a data base. Applying the prediction expressions of confinement models to the experimental data collected in the data base, we obtain as a result the stresses and strains ultimate, and from them, by means of a statistical analysis of theoretical values in respect to experimental values for each single model of confinement, and, after a comparative analysis between the different models we define the model that beste adjustment presents with respect to the experimental data (model of Teng et al.). For this model, the statistical characterization has been performed. Since it is unlikely to expect that a structural design frame rule may represent all the situations of design, the structural systems and typologies of bridge pillars for which the calibration has been performed in this study, are the bridges of straight section with several spans with reinforced concrete pillars of the type of circular columns. The pillars can be formed by a only fust or by two fusts. This typology of roadway and pillar covers most of the existing bridges in the current communication routes. In order to define the partial coefficients of safety for the confined concrete ¿cc, the calibration has been performed based on structural reliability, for a possible range of solutions of reinforcement with CFRP in the typologies of bridges pillars of the studied structural systems. For such a purpose, the uncertainties related of the variables which intervene have been taken into consideration, and by means of simulation the variables of strength and stress have been statistically characterized. Once these two variables have been characterized, according to the required levels of structural safety the probability of failure or reliability index for different values of ¿cc has been determined. The partial coefficients of safety will be defined for those situations, such that, the reliability indexes calculated in the reinforced pillars dimensioned with such coefficients, give results equal or higher than the objective reliability index, which has been established in ß=3,5 in this study.

    La contínua deterioració i/o danys causats en les estructures de formigó durant la seva vida útil, són fenòmens inevitables que comporta el pas del temps i es convertix en un problema important que ha conduït al desenvolupament de nous materials composts per al reforç estructural. En el cas de ponts, les situacions en les quals es fan necessàries intervencions de reforç poden ser: estructures afectades per patologies inherents a problemes de durabilitat, estructures mal projectades i/o mal executades, estructures que han estat danyades per causes externes (impacte de vehicles, incendi, sisme, desastres naturals, etc.), estructures afectades per un canvi d'ús o per un augment de la càrrega de projecte, o per suportar futurs sismes. Davant tot això, els mètodes de reforç tradicional tals com el recrescut de formigó, post tensat exterior i reforç amb xapes d'acer, freqüentment tenen desavantatges inherents com ara el increment del pes de l'estructura, la corrosió de l'acer i el seu elevat cost de manipulació i col·locació. Davant aquests inconvenients, i més concretament fent esment al reforç de pilars de ponts, en anys recents s'ha incrementat l'ús de polímers reforçats amb fibra (FRP) com a element de confinament d'elements de formigó sotmesos a compressió, a causa de les seves excel·lents propietats mecàniques i químiques, incrementant principalment la resistència i ductilitat de les pilars de formigó. No obstant això, la seva aplicació presenta encara punts foscos que exigeixen seguir investigant aspectes com la falta de codis i normes, la falta d'experiència en el comportament a llarg termini, la susceptibilitat al foc, etc. Existeixen incerteses a les bases de càlcul durant el dimensionament d'aquest reforç, una d'elles està precisament en els coeficients parcials de seguretat per al formigó confinat que s'han d'adoptar en el disseny. Per això, la present tesi doctoral pretén aportar coneixements en aquest camp, i més concretament, definint aquests coeficients parcials de seguretat per al formigó confinat per al disseny del reforç de pilars de ponts de formigó armat amb polímers reforçats amb fibres de carboni (CFRP), mitjançant procés de calibratge basat en fiabilitat estructural. En aquest sentit, primerament, s'han recopilat tota la informació bibliogràfica disponible referida principalment a dos aspectes: descripció de models de confinament per a formigó confinat amb CFRP i dades d'assajos experimentals per a l'elaboració d'una base de dades. Els models de confinament considerats en aquest estudi per a elements de formigó de secció circular són de: Mander et al. 1988, Pilakoutas i Mortazavi 1997, Toutanji 1999, Spolestra i Monti 1999, Eid i Paultre 2008, Teng et al. 2009, Wu i Zhou 2010. Aplicant les expressions de predicció d'aquests models a les dades experimentals recollides en la base de dades, obtenim com resultat tensions i deformacions últimes, i amb ells, mitjançant una anàlisi estadística de valors teòrics respecte a valors experimentals per a cada model de confinament, i, després d'una anàlisi comparativa entre els diferents models definim el model que millor ajust presenta respecte a les dades experimentals (model de Teng et al. 2009). Per a aquest model, s'ha realitzat la caracterització estadística. Ates que és poc probable esperar que un format de norma de disseny estructural pugui representar totes les situacions de disseny, els sistemes estructurals i tipologies de pilars de ponts pels quals s'ha realitzat el calibratge en aquest estudi, són els ponts de tram recte de diverses obertures amb pilars de formigó armat tipus columnes circulars. Les pilars poden estar formades per un fust únic o bé per dos fustos. Aquesta tipologia de tauler i de pilar cobreix la major part dels ponts existents en les actuals vies de comunicació. Per tal de definir els coeficients parcials de seguretat per al formigó confinat γcc, s'ha realitzat el calibratge basat en fiabilitat estructural, per a un rang possible de solucions de reforç amb CFRP en les tipologies de pilars de ponts dels sistemes estructurals estudiats. Per a això, es tenen en compte les incerteses de les variables que intervenen, i mitjançant simulació es caracteritzen estadísticament les variables de resistència i sol·licitació. Tenint caracteritzades aquestes dues variables, segons els nivells requerits de seguretat estructural es determina la probabilitat de fallada o índex de fiabilitat per a diferents valors de γcc. Els coeficients parcials de seguretat quedaran definits, per a aquelles situacions, tals que, els índexs de fiabilitat calculats en les pilars reforçades dimensionadas amb aquests coeficients donin valors iguals o superiors a l'índex de fiabilitat objectiu, que en aquest estudi s'ha fixat en β = 3,5. Finalment, la proposta de coeficients parcials de seguretat a utilitzar és la següent: γcc = 1,15 per a tmín ≤ t ≤ 2 mm

  • Análisis numérico de presas de hormigón bajo acciones expansivas  Open access

     Campos de Moura, Andre
    Defense's date: 2012-11-23
    Department of Construction Engineering, Universitat Politècnica de Catalunya
    Theses

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    The main objective of this thesis is to study the mechanical behavior of a dam concrete under internal expansive phenomena in both the macro-structural and the meso-structural level. An important contribution of this work is the diffusion-reaction model proposed for the internal sulfate attack (ISA), which simulates the oxygen diffusion into the concrete and its consumption in the iron sulfides oxidation in the aggregates. This model allows obtaining the volume increments over time caused by the chemical reactions involved in the process. The mechanical analysis was conducted in the macro-structural and the meso-structural levels. The latter takes into account the expansions due to delayed ettringite formation in conventional concrete considering the explicit representation of the first level internal structure or meso-structure. The numerical results obtained were validated with experimental data from the literature. Then, the effects of the ISA in conventional concrete and in a dam concrete were analyzed, yielding a prediction of the young modulus and the tensile strength degradation depending on the degree of expansion of concrete. Furthermore, this dissertation shows the evolution of cracking and fracture energy dissipation, both of which were correlated with the degree of expansion and the variation of mechanical parameters. The results obtained illustrate the potential of the meso-structural model for analyzing different expansive processes in concrete. In the macro-structural level, real cases of dams affected by expansive problems were analyzed. The dams studied were Mequinenza, Paso Nuevo and Graus. The case of Mequinenza focused on the behavior of block 13-14 that presented pronounced non-recoverable displacements. A 2D model was used, including horizontal joints with non-linear behavior located where cracks had been observed. The numerical results show how the formation, evolution and opening of a series of horizontal cracks cause much of the displacements measured, thus realistically explaining the behavior of the dam. For the case of Paso Nuevo (an arch gravity dam affected by internal expansive problems), a 3D model was used to adjust the calculated displacement with the measured one. A hyperbolic strength criterion was used to assess the global behavior of the dam. Then, the left abutment zone was analyzed using a continuum non-linear damage model. Finally, the case of Graus was studied with linear models (2D and 3D). Then the analysis of block 5 was performed using a 2D model that included non-linear joints located where cracks and casting joints were present. The results obtained throughout this study reproduce satisfactorily the evolution of the expansive process in the macro-structural and meso-structural levels. They explain with good accuracy the anomalous behavior observed in the real cases studied and show the potential of the tools and methodology used in this work.

    El objetivo principal de esta tesis es el estudio del comportamiento mecánico del hormigón de presas bajo la acción de fenómenos expansivos de origen interno, tanto en el nivel mesoestructural como en el macroestructural. Un importante aporte de este trabajo es el modelo de difusión-reacción propuesto para el ataque sulfático interno (ASI), que permite simular la difusión de oxígeno hacia el interior del hormigón y su consumo en la oxidación de los sulfuros de hierro presentes en los áridos, y con ello obtener los incrementos de volumen a lo largo del tiempo que pueden producir las reacciones involucradas en el proceso. El análisis mecánico se ha abordado en dos niveles: el mesoestructural y el macroestructural. En el análisis mesoestructural (en que se representa explícitamente la estructura interna de primer nivel o meso-estructura) se analizan expansiones debidas a formación tardía de etringita en hormigón convencional contrastando los resultados numéricos obtenidos con experimentales existentes en la bibliografía. Luego, se analizan los efectos del ASI, en primer lugar, en un hormigón convencional y posteriormente, en un hormigón de presa. Se han obtenido curvas de la degradación del módulo de elasticidad y de la resistencia a tracción en función del grado de expansión. También se muestran aspectos de la fisuración y de la energía disipada, que se correlacionan con el nivel de expansión y la evolución de los parámetros mecánicos. En el análisis macroestructural, se estudian casos reales de presas afectadas por problemas expansivos. Las presas estudiadas son: Mequinenza, Paso Nuevo y Graus. En el caso de Mequinenza se analiza el comportamiento del bloque 13-14 que presenta unos elevados desplazamientos no recuperables. Para ello se utiliza una modelización 2D en el que se incluyen juntas horizontales (con un comportamiento no-lineal) donde se ubican fisuras observadas en la estructura. Los resultados numéricos obtenidos muestran cómo la formación, evolución y apertura de una serie de fisuras horizontales aportan gran parte de los desplazamientos observados en la estructura y permiten explicar de forma consistente el comportamiento diferenciado detectado en la presa. En el caso de la presa de Paso Nuevo (de tipo arco gravedad), afectada por problemas expansivos internos, se utiliza una modelización 3D con la que se ajustan los desplazamientos calculados con los medidos, y se evalúa el comportamiento global de la presa empleando un criterio hiperbólico de resistencia. Posteriormente, se realiza un análisis no-lineal de la zona cercana al estribo izquierdo utilizando para ello un modelo de daño no-lineal en el medio continuo. Finalmente, se estudia el comportamiento de la presa de Graus, inicialmente mediante modelos lineales (2D y 3D) y posteriormente se analiza la sección transversal del bloque 5 mediante una modelización 2D, en la que se incluyen juntas no-lineales donde se ubican las juntas de hormigonado y algunas fisuras observadas en la presa.

  • Numerial modelling based on the multiscale homogenization theory. Application in composite materials and structures  Open access

     Badillo Almaraz, Hiram
    Defense's date: 2012-04-16
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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    A multi-domain homogenization method is proposed and developed in this thesis based on a two-scale technique. The method is capable of analyzing composite structures with several periodic distributions by partitioning the entire domain of the composite into substructures making use of the classical homogenization theory following a first-order standard continuum mechanics formulation. The need to develop the multi-domain homogenization method arose because current homogenization methods are based on the assumption that the entire domain of the composite is represented by one periodic or quasi-periodic distribution. However, in some cases the structure or composite may be formed by more than one type of periodic domain distribution, making the existing homogenization techniques not suitable to analyze this type of cases in which more than one recurrent configuration appears. The theoretical principles used in the multi-domain homogenization method were applied to assemble a computational tool based on two nested boundary value problems represented by a finite element code in two scales: a) one global scale, which treats the composite as an homogeneous material and deals with the boundary conditions, the loads applied and the different periodic (or quasi-periodic) subdomains that may exist in the composite; and b) one local scale, which obtains the homogenized response of the representative volume element or unit cell, that deals with the geometry distribution and with the material properties of the constituents. The method is based on the local periodicity hypothesis arising from the periodicity of the internal structure of the composite. The numerical implementation of the restrictions on the displacements and forces corresponding to the degrees of freedom of the domain's boundary derived from the periodicity was performed by means of the Lagrange multipliers method. The formulation included a method to compute the homogenized non-linear tangent constitutive tensor once the threshold of nonlinearity of any of the unit cells has been surpassed. The procedure is based in performing a numerical derivation applying a perturbation technique. The tangent constitutive tensor is computed for each load increment and for each iteration of the analysis once the structure has entered in the non-linear range. The perturbation method was applied at the global and local scales in order to analyze the performance of the method at both scales. A simple average method of the constitutive tensors of the elements of the cell was also explored for comparison purposes. A parallelization process was implemented on the multi-domain homogenization method in order to speed-up the computational process due to the huge computational cost that the nested incremental-iterative solution embraces. The effect of softening in two-scale homogenization was investigated following a smeared cracked approach. Mesh objectivity was discussed first within the classical one-scale FE formulation and then the concepts exposed were extrapolated into the two-scale homogenization framework. The importance of the element characteristic length in a multi-scale analysis was highlighted in the computation of the specific dissipated energy when strain-softening occurs. Various examples were presented to evaluate and explore the capabilities of the computational approach developed in this research. Several aspects were studied, such as analyzing different composite arrangements that include different types of materials, composites that present softening after the yield point is reached (e.g. damage and plasticity) and composites with zones that present high strain gradients. The examples were carried out in composites with one and with several periodic domains using different unit cell configurations. The examples are compared to benchmark solutions obtained with the classical one-scale FE method.

    En esta tesis se propone y desarrolla un método de homogeneización multi-dominio basado en una técnica en dos escalas. El método es capaz de analizar estructuras de materiales compuestos con varias distribuciones periódicas dentro de un mismo continuo mediante la partición de todo el dominio del material compuesto en subestructuras utilizando la teoría clásica de homogeneización a través de una formulación estándar de mecánica de medios continuos de primer orden. La necesidad de desarrollar este método multi-dominio surgió porque los métodos actuales de homogeneización se basan en el supuesto de que todo el dominio del material está representado por solo una distribución periódica o cuasi-periódica. Sin embargo, en algunos casos, la estructura puede estar formada por más de un tipo de distribución de dominio periódico. Los principios teóricos desarrollados en el método de homogeneización multi-dominio se aplicaron para ensamblar una herramienta computacional basada en dos problemas de valores de contorno anidados, los cuales son representados por un código de elementos finitos (FE) en dos escalas: a) una escala global, que trata el material compuesto como un material homogéneo. Esta escala se ocupa de las condiciones de contorno, las cargas aplicadas y los diferentes subdominios periódicos (o cuasi-periódicos) que puedan existir en el material compuesto; y b) una escala local, que obtiene la respuesta homogenizada de un volumen representativo o celda unitaria. Esta escala se ocupa de la geometría, y de la distribución espacial de los constituyentes del compuesto así como de sus propiedades constitutivas. El método se basa en la hipótesis de periodicidad local derivada de la periodicidad de la estructura interna del material. La implementación numérica de las restricciones de los desplazamientos y las fuerzas derivadas de la periodicidad se realizaron por medio del método de multiplicadores de Lagrange. La formulación incluye un método para calcular el tensor constitutivo tangente no-lineal homogeneizado una vez que el umbral de la no-linealidad de cualquiera de las celdas unitarias ha sido superado. El procedimiento se basa en llevar a cabo una derivación numérica aplicando una técnica de perturbación. El tensor constitutivo tangente se calcula para cada incremento de carga y para cada iteración del análisis una vez que la estructura ha entrado en el rango no-lineal. El método de perturbación se aplicó tanto en la escala global como en la local con el fin de analizar la efectividad del método en ambas escalas. Se lleva a cabo un proceso de paralelización en el método con el fin de acelerar el proceso de cómputo debido al enorme coste computacional que requiere la solución iterativa incremental anidada. Se investiga el efecto de ablandamiento por deformación en el material usando el método de homogeneización en dos escalas a través de un enfoque de fractura discreta. Se estudió la objetividad en el mallado dentro de la formulación clásica de FE en una escala y luego los conceptos expuestos se extrapolaron en el marco de la homogeneización de dos escalas. Se enfatiza la importancia de la longitud característica del elemento en un análisis multi-escala en el cálculo de la energía específica disipada cuando se produce el efecto de ablandamiento. Se presentan varios ejemplos para evaluar la propuesta computacional desarrollada en esta investigación. Se estudiaron diferentes configuraciones de compuestos que incluyen diferentes tipos de materiales, así como compuestos que presentan ablandamiento después de que el punto de fluencia del material se alcanza (usando daño y plasticidad) y compuestos con zonas que presentan altos gradientes de deformación. Los ejemplos se llevaron a cabo en materiales compuestos con uno y con varios dominios periódicos utilizando diferentes configuraciones de células unitarias. Los ejemplos se comparan con soluciones de referencia obtenidas con el método clásico de elementos finitos en una escala.

  • Simulación de los materiales compuestos como refuerzo en estructuras de hormigón armado

     Molina Herrera, Maritzabel
    Defense's date: 2012-01-12
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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  • Evaluación del daño por impacto en laminados de material compuesto mediante la respuesta dinámica

     Pérez Martínez, Marco Antonio
    Defense's date: 2012-02-06
    Department of Strength of Materials and Structural Engineering, Universitat Politècnica de Catalunya
    Theses

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  • Evaluación del daño por impacto en laminados de material compuesto mediante la respuesta dinámica

     Pérez Martínez, Marco Antonio; Gil Espert, Lluis; Oller Martinez, Sergio Horacio
    Date of publication: 2012-04-23
    Book

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  • Multilayered composite structure design optimisation using distributed/parallel multi-objective evolutionary algorithms

     Lee, Dong Seop; Morillo Carbonell, Carlos; Bugeda Castelltort, Gabriel; Oller Martinez, Sergio Horacio; Oñate Ibáñez de Navarra, Eugenio
    Composite structures
    Date of publication: 2012-02
    Journal article

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  • Simple and accurate two-noded beam element for composite laminated beams using a refined zigzag theory

     Oñate Ibáñez de Navarra, Eugenio; Eijo, A.; Oller Martinez, Sergio Horacio
    Computer methods in applied mechanics and engineering
    Date of publication: 2012-03
    Journal article

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  • Dynamical analysis of long fiber-reinforced laminated plates with elastically restrained edges

     Bellomo, Facundo; Quinteros, Ricardo D.; Oller Martinez, Sergio Horacio; Nallim, Liz G.
    Advances in Acoustics and Vibration
    Date of publication: 2012
    Journal article

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  • Seismic response of RC framed buildings designed according to Eurocodes

     Vielma Pérez, Juan Carlos; Barbat Barbat, Horia Alejandro; Oller Martinez, Sergio Horacio
    Date of publication: 2011-01-15
    Book chapter

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    This book provides an insight in advanced methods and concepts for structural analysis and design against seismic loading. The book consists of 25 chapters dealing with a wide range of timely issues in contemporary Earthquake Engineering. In brief, the topics covered are: collapse assessment, record selection, effect of soil conditions, problems in seismic design, protection of monuments, earth dam structures and liquid containers, numerical methods, lifetime assessment, post-earthquake measures. A common ground of understanding is provided between the communities of Earth Sciences and Computational Mechanics towards mitigating seismic risk. The topic is of great social and scientific interest, due to the large number of scientists and practicing engineers currently working in the field and due to the great social and economic consequences of earthquakes.

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    Numerical modelling of behaviour of carbon nanotube-reinforced composites  Open access

     Otero-Gruer, Fermín; Oller Martinez, Sergio Horacio; Martinez Garcia, Xavier; Salomon, Ramon Omar
    International Conference on Computational Plasticity Fundamentals and Applications
    Presentation's date: 2011-09-08
    Presentation of work at congresses

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    Since their discovery by Lijima in 1991[1], carbon nanotubes (CNTs), are considered a new generation of reinforcement [2]. Their "nano" size structure makes them potentially free of defects, which provides them with excellent physical properties [3,4]. There are two main nanotube types: single wall nanotubes (SWNT) and multi wall nanotubes (MWNT). These last ones consist in several concentric walls, one inside the other. In a composite, one the most important factors that condition their mechanical performance is the interfacial tension between matrix and reinforcement. In general, the loads in a composite structure are introduced to the matrix and then are transferred to the reinforcement through the interface [5]. Therefore, the interface can be defined as the region, surrounding the reinforcement, where this stress transfer takes place. The properties of the composite depend on the properties of this region and its ability to transfer the load efficiently. This work proposes a new formulation to predict the mechanical properties and mechanical behaviour of nanotube-reinforced composites. The formulation is based on the mixing theory [6]. It obtains the behaviour of the composite from the mechanical performance of its constitutive materials: matrix, carbon-nanotube and the interface that bonds both of them.

  • Modelling the elastic behaviour of carbon nanotube-reinforced composites

     Otero-Gruer, Fermín; Oller Martinez, Sergio Horacio; Martinez Garcia, Xavier; Salomon, Ramon Omar
    ECCOMAS Thematic Conference on Mechanical Response of Composites
    Presentation's date: 2011-09-22
    Presentation of work at congresses

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    Carbon nanotubes (CNTs), since their discovery by Lij ima in 1991 [1], are considered a new generation of reinforcement [2]. Their "nano" size structure makes them potentially free of defects, which provides them with excellent physical properties [3,4]. There are two main nanotube types: single wall nanotubes (SWNT), which are made of a single wall tube; and multiwall nanotubes (MWNT), which consist in several concentric walls, one inside the other. In a composite, one the most important factor is the interfacial tension between matrix and reinforcement. In general, the loads in a composite structure are introduced through the matrix and then are transferred to the reinforcement through the interface [5]. Therefore, the interface can be defined as the region surrounding the reinforcement where this stress transfer takes place. The properties of the composite depend on the properties of this regíon and its ability to transfer the load efficiently. This work proposes a new formulation to predict the mechanical properties of nanotube-reinforced composites. The formulation is based on the mixing theory [6]. It obtains the properties of the composite from the mechanical performance of its constitutive materials: matrix, carbon-nanotube and the interface that bonds both of them.

  • Modelling of behaviour of carbon nanotube-reinforced composites

     Otero, Fermin; Oller Martinez, Sergio Horacio; Martinez Garcia, Xavier; Salomon, Ramon Omar
    Congreso Nacional de Materiales Compuestos
    Presentation's date: 2011-07-06
    Presentation of work at congresses

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  • Non-destructive testing evaluation of low velocity impact damage in carbon fiber-reinforced laminated composites

     Pérez Martínez, Marco Antonio; Gil Espert, Lluis; Oller Martinez, Sergio Horacio
    NDT&E of Composite Materials
    Presentation's date: 2011-05-16
    Presentation of work at congresses

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  • Dimensionado sísmico de estructuras porticadas de hormigón armado con base en la respuesta no lineal

     Vielma Pérez, Juan Carlos; Barbat Barbat, Horia Alejandro; Oller Martinez, Sergio Horacio
    Congreso Nacional de Ingeniería Sísmica
    Presentation's date: 2011-05-20
    Presentation of work at congresses

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    Nowadays, the seismic-resistant structural design is conducted using a set of code prescriptions that allows to reach a consistent behavior. Analysis procedures are carried out considering elastic behavior, applying behavior factors in order to obtain equivalent seismic forces. From the displacements of the elastic analysis it is possible to calculate the inelastic ones by means of amplification factors usually based on the behavior factors; however, it can be observed a high variability of these factors that the seismic codes prescribe. In this work, a new procedure of seismic sizing based on non linear analysis is presented. The procedure is applied to design a set of regular RC framed buildings with different heights, for high seismic hazard level. The seismic capacity of the buildings is assessed through a set of Limit States. Results shows that the new procedure is suitably in order to do a seismic sizing of structures, providing the designer assessment criteria in accordance with the buildings characteristics and the seismic hazard.

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     Pérez Martínez, Marco Antonio; Gil Espert, Lluis; Oller Martinez, Sergio Horacio
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    Date of publication: 2011-04-29
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