Scipedia: Documents published in 2020

Formulation of enriched macro elements using trigonometric shear deformation theory for free vibration analysis of symmetric laminated composite plate assemblies

María Jesús Samper — Thu, 06 Feb 2020 15:25:27 +0100

The formulation of an enriched macro element suitable to analyze the free vibration response of composite plate assemblies is presented in this article. Based on the Trigonometric Shear Deformation Theory (TSDT) and the use of Gram–Schmidt orthogonal polynomials as enrichment functions a finite macro element is developed. In the TSDT framework, shear stresses are vanished at the top and bottom surfaces of the plates and shear correction factors are no longer required. The Principle of Virtual Work is applied to derive the governing equations of motion. A special connectivity matrix is obtained; so that hierarchically enriched global stiffness matrix and mass matrix of general laminated plate structures are derived, allowing to study generally coplanar plate assemblies by combining two or more macro elements. This procedure gives a matrix-eigenvalue problem that can be solved with optimum efficiency. Results of free vibration analysis for symmetric laminated plates of different thickness ratios, geometrical planform shapes and boundary conditions are presented. The accuracy of the formulation is ensured by comparing some numerical examples with those available in the literature.

A finite points method approach for strain localization using the gradient plasticity formulation

María Jesús Samper — Thu, 06 Feb 2020 15:14:01 +0100

The softening elastoplastic models present an unsuitable behavior after reaching the yield strength: unbounded strain localization. Because of the material instability, which is reflected in the loss of ellipticity of the governing partial differential equations, the solution depends on the discretization. The present work proposes to solve this dependency using the meshless Finite Points Method. This meshfree spatial discretization technique allows enriching the governing equations using gradient’s plasticity and introducing an internal length scale parameter at the material model in order to objectify the solution.

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

María Jesús Samper — Thu, 06 Feb 2020 14:31:46 +0100

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.

Impact damage identification in composite laminates using vibration testing

María Jesús Samper — Thu, 06 Feb 2020 10:57:51 +0100

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 methods 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.

An Integrated Procedure for the Structural Design of a Composite Rotor-Hydrofoil of a Water Current Turbine (WCT)

María Jesús Samper — Thu, 06 Feb 2020 10:46:03 +0100

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.

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

María Jesús Samper — Thu, 06 Feb 2020 10:30:30 +0100

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.

Analytical and numerical analysis of human dental occlusal contact

María Jesús Samper — Wed, 05 Feb 2020 10:36:16 +0100

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.

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

María Jesús Samper — Wed, 05 Feb 2020 10:22:57 +0100

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.

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

María Jesús Samper — Wed, 05 Feb 2020 09:53:10 +0100

Carbon nanotubes (CNTs) have been regarded as ideal reinforcements for high-performance composites. A key factor for the reinforcement efficiency is the interface bonding between the CNTs and the matrix. This paper presents a new constitutive model to predict the mechanical performance of composites made with CNTs. The model takes into account explicitly the performance of the interface between the matrix and the CNTs. The formulation developed is based in the mixing theory. It divides the composite in matrix and in a new material result of coupling the CNTs with the interface. The relation defined between interface and CNTs assumes that the load is transferred to the nanotubes along their ends and that in the central part the CNTs can develop their full strength. The composite non-linear behavior results from the non-linearities of its constituents. In case of interface damage, it also becomes non-linear the law defined to couple the interface with the CNTs. After describing the formulation, it is validated studying the elastic response of several composites made with different types of CNTs reinforcements. The non-linear behavior provided by the formulation is also studied. In both cases the numerical results are compared with experimental data showing good agreement.

A constitutive model for tissue adaptation: Necrosis and stress driven growth

María Jesús Samper — Wed, 05 Feb 2020 09:34:44 +0100

This paper presents a general constitutive model for tissue stress driven growth and necrotic remodeling. The general theoretical framework is given by a kinematics formulation in large strain. The model allows the analysis of tissue behaviour considering the coupling of biological and mechanical fields, through the implementation of an internal variable that represents the bioavailability, together with the growth induced by mechanical stimuli. To take into account the tissue necrosis phenomenon a reinterpretation of the classical theory of mixtures is proposed. The proposed formulation is based on the consideration of two components, normal tissue and necrotic tissue, each one with its own volumetric participation, which is treated in an evolutive way, allowing simulating the phenomena of ischemia. This article also presents a brief description of the numerical implementation of the model as well as the obtained results, which shows the process of necrosis of part of the tissue, its influence on the surrounding healthy tissue, and the interaction between the nutrients availability and the stress driven growth and necrotic remodeling process.

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

María Jesús Samper — Wed, 05 Feb 2020 09:27:47 +0100

This paper presents a variational formulation for the free vibration analysis of unsymmetrically laminated composite plates with elastically restrained edges. The study includes a micromechanics approach that allows starting the study considering each layer as constituted by long unidirectional fibers in a continuous matrix. The Mori-Tanaka method is used to predict the mechanical properties of each lamina as a function of the elastic properties of the components and of the fiber volume fraction. The resulting mechanical properties for each lamina are included in a general Ritz formulation developed to analyze the free vibration response of thick laminated anisotropic plates resting on elastic supports. Comprehensive numerical examples are computed to validate the present method, and the effects of the different mechanical and geometrical parameters on the dynamical behavior of different laminated plates are shown. New results for general unsymmetrical laminates with elastically restrained edges are also presented. The analytical approximate solution obtained in this paper can also be useful as a basis to deal with optimization problems under, for instance, frequency constraints.

Nonlinear homogenization techniques to solve masonry structures problems

María Jesús Samper — Tue, 04 Feb 2020 16:46:34 +0100

The behaviour of masonry material subjected to different in-plane loading combination is studied in this work. The masonry is considered as a periodic composite material composed by a regular distribution of brick and mortar and it is analyzed using a homogenization technique. The mechanical properties of the masonry, as an orthotropic homogeneous material, depend on the geometrical and mechanical properties of the components based on the study of the equilibrium and compatibility of a basic cell. The masonry is a frictional material and its behaviour depends on the loading direction, for these reasons, a unilateral damage model is chosen for the analysis. This model describes the behaviour of brittle materials subjected to tension–compression cyclic loads based on the introduction of two damage variables and it assumes that the damage is due to the beginning and growth of cracks only in the mortar joints. It is considered that the bricks have a linear elastic constitutive relationship. Numerical applications are performed with a nonlinear finite element code in order to test the proposed procedure by comparing the results with those available in the literature and also with experimental data.

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

María Jesús Samper — Tue, 04 Feb 2020 15:39:19 +0100

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.

Comportamiento de la interfaz concreto-epoxi-FRP utilizando la teoría de mezclas serie/paralelo

María Jesús Samper — Tue, 04 Feb 2020 15:28:14 +0100

Diversos estudios experimentales han mostrado que uno de los factores que afectan el comportamiento de estructuras de concreto externamente reforzadas con Fiberglass Reinforced Plastic (FRP, por sus siglas en inglés) es la adherencia entre el concreto y el FRP. En complemento a la experimentación, en este artículo se propone el uso de la teoría de mezclas serie/paralelo en la simulación numérica como una herramienta para estudiar el comportamiento de la interfaz concreto-epoxi-FRP. Además, se presenta un ejemplo en el que se analizan los resultados de la simulación de un ensayo de adherencia por flexión y se contrastan con los obtenidos experimentalmente.

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

María Jesús Samper — Tue, 04 Feb 2020 15:10:50 +0100

Fiber-reinforced laminated composite materials are widely used in aircraft, modern vehicles and light-weight structures. With their high elastic modulus, high strength, and capability of being tailored for a several applications, these materials offer definite advantages compared to more traditional structural materials. However, their behavior under impact is of concern since those events may occur during manufacturing, normal operation or maintenance. The situation is critical for impacts that induce significant internal damage undetectable by visual inspection, which causes large reductions on the strength and stability of the structure. New standardized tests to assess the impact tolerance (ASTM D7136 and D7137) lose some of its appeal by its destructive nature as well as costs involved. Non-destructive techniques such as ultrasonic phased array or experimental modal analysis can be complementary in the evaluation of damage and structural integrity. The results of both destructive and non-destructive experiments carried out on forty-eight composite specimens are presented. A good correlation between incident impact energy and delaminated area as well as variation of eigenfrequencies is noticed. This study does not only provide a better understanding of the impact phenomenon but can also help in design and implementation of new test procedures for structural assessment.

A general constitutive model for vascular tissue considering stress driven growth and biological availability

María Jesús Samper — Tue, 04 Feb 2020 14:47:33 +0100

Some of the key factors that regulate growth and remodeling of tissues are fundamentally mechanical. However, it is important to take into account the role of biological availability to generate new tissue together with the stresses and strains in the processes of natural or pathological growth. In this sense, the model presented in this work is oriented to describe growth of vascular tissue under "stress driven growth" considering biological availability of the organism. The general theoretical framework is given by a kinematic formulation in large strain combined with the thermodynamic basis of open systems. The formulation uses a multiplicative decomposition of deformation gradient, splitting it in a growth part and visco-elastic part. The strains due to growth are incompatible and are controlled by unbalanced stresses related to a homeostatic state. Growth implies a volume change with an increase of mass maintaining constant the density. One of the most interesting features of the proposed model is the generation of new tissue taking into account the contribution of mass to the system controlled through the biological availability. Because soft biological tissues in general have a hierarchical structure with several components (usually a soft matrix reinforced with collagen fibers), the developed growth model is suitable for the growth characterization of each component. This allows considering a different behavior for each of them in the context of a generalized theory of mixtures.

Numerical Simulation of Matrix Reinforced Composite Materials Subjected to Compression Loads

María Jesús Samper — Tue, 04 Feb 2020 14:14:34 +0100

This paper reviews the most common formulations to obtain the compression strength of long fiber composites due to fiber buckling. This failure mode was first studied by Rosen (Fibre Composite Materials, pp. 37–45, 1965) who defined two different fiber buckling modes, extensional and transverse. Further studies improved the first model proposed by Rosen by defining with more accuracy the mechanics of the problem. Although each formulation use a different approach to solve the problem, all of them agree in the dependence of fiber buckling on three main parameters: matrix shear strength, fiber initial misalignments and volumetric participation of the fibers in the composite.

Once having described the different approaches used, and the parameters on which they depend, this paper describes a new formulation capable of obtaining the compression strength of composites taking into account the fiber buckling phenomenon. This formulation uses the serial/parallel mixing theory developed by Rastellini et al. (Comput. Struct. 86(9):879–896, 2008) to simulate the composite, and takes advantage of knowing the mechanical performance of the composite constituents to simulate the fiber buckling phenomenon. This is done with an homogenization procedure. It consists in introducing the interaction between fibers and matrix into their respective constitutive equations. The interaction between fiber and matrix takes into account fiber initial misalignments, its volumetric participation and the mechanical properties of both constituents.

The new formulation proposed is implemented in a finite element code, taking into account that fibers can have different misalignment levels, and that the composite behaves differently if it is under tensile or compression forces. The mechanical performance of the formulation proposed is studied with several finite element simulations of compressed composites. Finally, the correctness of the formulation is proved by comparing the numerical results with the experimental tests provided by Barbero and Tomblin (Int. J. Solids Struct. 33(29):4379–4393, 1996), Tomblin et al. (Int. J. Solids Struct. 34(13):1667–1679, 1997).

Determinación de umbrales de daño sísmico en edificios porticados de hormigón armado proyectados conforme al EC-2 y EC-8

María Jesús Samper — Tue, 04 Feb 2020 13:53:50 +0100

El Proyecto por Prestaciones de edificios contempla una serie de Estados Límite que el proyectista debe aplicar a los edificios emplazados en zonas sísmicas. Sin embargo, actualmente existe la dificultad de poder incorporar estos Estados Límite en el proyecto convencional de estructuras. En este trabajo se estudia la determinación objetiva de los umbrales de daño correspondientes a unos Estados Límite específicos, aplicando criterios con base en desplomes relativos de niveles, calculados a partir de la respuesta no lineal de los edificios porticados de hormigón armado, proyectados según las disposiciones normativas del EC-2 y EC-8. Los resultados muestran que los valores de los umbrales de daño son prácticamente independientes de la altura del edificio y que se pueden incorporar al proceso de proyecto de forma racional, correlacionándolos además con los índices de daño sísmico, para facilitar la evaluación rápida de la respuesta sísmica de los edificios de hormigón armado.

Estudio de estructuras de hormigón reforzadas con FRP mediante la teoría de mezclas serie/paralelo

María Jesús Samper — Tue, 04 Feb 2020 13:38:08 +0100

La simulación numérica del comportamiento de los materiales compuestos es un campo en desarrollo cuya aplicación a estructuras ha experimentado avances importantes que han conducido a mejores en el refuerzo de las mismas. En relación con este tema, el artículo parte de la teoría de mezclas serie/paralelo que permite deducir el comportamiento de los compuestos a partir de las ecuaciones constitutivas de los materiales componentes. Con el fin de destacar las ventajas que tiene esta herramienta en el análisis y el diseño de estructuras reparadas o rehabilitadas con polímeros reforzados con fibras largas (FRP), en el artículo se realizan análisis numéricos mediante modelos de elementos finitos de un conjunto de pórticos con distintas configuraciones de refuerzo. Los resultados obtenidos muestran que el uso esta teoría en los análisis de las estructuras reforzadas con FRP contribuyente a mejorar el diseño y el comportamiento de las estructuras.

Estudio de la interfaz hormigón-epoxi-frp de un ensayo de doble cortante por medio de la teoría de mezclas serie/paralelo

María Jesús Samper — Tue, 04 Feb 2020 13:10:09 +0100

El uso de los compuestos para la rehabilitación y la reparación de estructuras de hormigón y de acero se ha ido imcrementando en las ultimas décadas. Frente a la necesidad de entender de una forma más apropiada el comportamiento de las estructuras reforzadas, la simulación numérica de los materiales compuestos ha surgido como una herramienta; sin embargo, es un campo que aún está en desarrollo. En este trabajo se desarrolla la teoría de mezclas serie/paralelo como una alternativa eficiente para el análisis de estructuras reforzados con fibras largas (FRP) y se estudia con detalle el comportamiento de la interfaz hormigón-adhesivo-FRP. Utilizando el programa de elementos finitos no lineal PLCDYN, se hace una simulación tridimensional de un ensayo de doble cortante y se compara con los resultados obtenidos en el laboratorio. Se han podido evidenciar las ventajas de esta teoría como herramienta para el análisis numérico de los materiales compuestos.

Discrete-continuum hybrid modelling of flowing and static regimes

María Jesús Samper — Tue, 04 Feb 2020 09:34:50 +0100

Bulk handling, transport and processing of granular materials and powders are fundamental
operations in a wide range of industrial processes and geophysical phenomena. Particulate materials,
which can be found in nature, are usually characterized by a grain size which can range across several
scales: from nanometre to the order of metre. Depending on the volume fraction and on the shear strain
conditions, granular materials can have different behaviours and often can be expressed as a new state
of matter with properties of solids, liquids and gases. For the above reasons, both the experimental
and the numerical analysis of granular media is still a difficult task and the prediction of their dynamic
behaviour still represents, nowadays, an important challenge. The main goal of the current monograph
is the development of a numerical strategy with the objective of studying the macroscopic behaviour
of dry granular flows in quasi-static and dense flow regime. The problem is defined in a continuum
mechanics framework and the balance laws, which govern the behaviour of a solid body, are solved by
using a Lagrangian formalism. The Material Point Method (MPM), a particle-based method, is chosen
due to its features which make it very suitable for the solution of large deformation problems involving
complex history-dependent constitutive laws. An irreducible formulation using a Mohr-Coulomb
constitutive law, which takes into account geometric non-linearities, is implemented within the MPM
framework. The numerical strategy is verified and validated against several benchmark tests and
experimental results, available in the literature. Further, a mixed formulation is implemented for the
solution of granular flows that undergo undrained conditions. Finally, the developed MPM strategy is
used and tested against the experimental study performed for the characterization of the flowability of
several types of sucrose. The capabilities and limitations of this numerical strategy are observed and
discussed and the bases for future research are outlined.

Comportamiento sísmico de edificios de hormigón armado de ductilidad limitada

María Jesús Samper — Fri, 31 Jan 2020 13:19:20 +0100

En el proyecto sismorresistente de edificios es necesario conocer previamente el valor de la ductilidad que éstos podrán llegar a alcanzar al ser sometidos a movimientos sísmicos fuertes. Los valores de ductilidad de referencia están incluidos en las normas de proyecto sismorresistente. Su estimación también puede hacerse bajo criterio de expertos o de la observación de la respuesta que los edificios han tenido ante determinados terremotos. Sin embargo, en el pasado, el problema se ha enfocado generalmente en la respuesta de edificios dúctiles, como son los edificios porticados con vigas de canto, sin que existan muchas referencias que permitan conocer la respuesta de los edificios de ductilidad limitada, entre los que se encuentran los edificios con forjados reticulares, que es la tipología más usual en España. En este trabajo se estudia la respuesta no lineal de edificios de ductilidad limitada, proyectados conforme a los requisitos de la instrucción española (EHE) y la norma sismorresistente NCSE-02, y se obtienen los valores de ductilidad a partir de dicha respuesta, lo que permiten verificar los valores de los factores de reducción aplicados en la determinación de las fuerzas sísmicas y los valores de reserva de resistencia. La evaluación de los edificios se complementa mediante la determinación de las curvas de fragilidad y de las matrices de probabilidad de daño, que permiten conocer la probabilidad de excedencia de Estados Límite previamente establecidos.

An analytical–numerical approach to simulate the dynamic behaviour of arbitrarily laminated composite plates

María Jesús Samper — Fri, 31 Jan 2020 11:51:32 +0100

A general analytical–numerical approach developed for the dynamical analysis of unsymmetrically laminated plates of general quadrilateral planforms is presented in this work. An arbitrary quadrilateral thin flat laminate is mapped onto a square basic one, so that a unique macro-element is constructed for the whole plate. The Ritz method is applied to evaluate the governing equation in which the coupling effects of bending and stretching are contained. All possible transverse boundary conditions combining with the different in-plane constraints are considered in the analysis. The resulting algorithm possesses great flexibility, it is easy to program and it needs minimal input information. For these reasons, the proposed methodology results convenient for large scale structural design and analysis where repeated calculations are often required.

Reserva de resistencia de edificios porticados de concreto armado diseñados conforme al ACI-318/IBC-2006

María Jesús Samper — Fri, 31 Jan 2020 11:45:40 +0100

La adecuada capacidad demostrada por los edificios sometidos a la acción de terremotos fuertes ha sido atribuida a la reserva de resistencia de la estructura. Sin embargo, la gran cantidad de factores que contribuyen en la reserva de resistencia, son un obstáculo para que esta haya sido incorporada de forma explícita y transparente en las normas de proyecto sismorresistente. En este artículo se aborda el tema de la reserva de resistencia, para lo que se estudia un conjunto de edificios regulares porticados de concreto armado que han sido proyectados conforme a las prescripciones del ACI318, con las cargas sísmicas prescritas en el IBC-2006. Los resultados muestran que la reserva de resistencia es independiente de la redundancia de los edificios y que los valores recomendados en el IBC-2006 subestiman la capacidad de los edificios con períodos intermedios y largos.

Confinamiento y ductilidad de los edificios de hormigón armado

María Jesús Samper — Fri, 31 Jan 2020 11:31:54 +0100

Seismic behaviour of limited ductility buildings

María Jesús Samper — Thu, 30 Jan 2020 16:32:41 +0100

The most important aspects of the seismic design and behaviour of reinforced concrete buildings with limited ductility, like the buildings with waffled slabs or flat beams, are examined in this work. The structures with these typologies are the most used in Spain for new buildings and many seismic codes do not recommend their use in seismic areas. The expected seismic performance of these structures is studied herein by means of incremental non linear structural analysis (pushover analysis) which provides capacity curves. Their behaviour is compared with that of buildings with moment resisting frames designed according to the Spanish EHE and NCSE-02 codes and also to the ACI-318 (2005) and IBC-2003. The most important results of the study show that only the moment-resisting framed buildings exhibit sufficient ductility and overstrength to guarantee a stable seismic behaviour. The behaviour of limited ductility buildings is strongly influenced by the structural type; even if they are reinforced with ductile steel or if their confinement if improved, they exhibit slightly higher ductility

Un índice de daño sísmico objetivo para la evaluación de los edificios de hormigón armado

María Jesús Samper — Thu, 30 Jan 2020 16:27:10 +0100

En las normas que regulan el proyecto sismorresistente de edificios porticados se considera que los elementos estructurales (pilares y vigas) tienen un comportamiento no lineal durante la acción de un terremoto similar al de proyecto. Esto implica que dichos elementos se dañan y para el proyectista es de mucho interés poder estimar el daño global esperado en la estructura y relacionarlo tanto con la ductilidad de proyecto como con la demanda de ductilidad. Contrariamente a lo que ocurre con los edificios dúctiles, en el caso de los edificios proyectados para bajas ductilidades los índices de daño calculados a partir de un análisis por el método de elementos finitos, tienen valores que no reflejan adecuadamente el deterioro. Por esta razón, en este artículo se realiza una propuesta de un índice objetivo de daño formulado en función de la ductilidad y del valor de la rigidez elástica y la correspondiente al colapso del edificio y que es independiente de la tipología estructural utilizada. El procedimiento se ilustra mediante la aplicación del índice de daño propuesto a tres edificios dos de los cuales han sido proyectados para bajas ductilidades (edificio con forjados reticulares y edificio porticado con vigas planas) y el tercero, un edificio porticado con vigas de canto, proyectado para alta ductilidad. A los tres edificios se les ha determinado la respuesta estática no lineal mediante un procedimiento con control de fuerzas, así como el punto de capacidad por demanda. Los resultados obtenidos demuestran que el índice de daño objetivo propuesto proporciona unos valores que caracterizan adecuadamente el daño sufrido por los tres edificios.

Advanced composite material simulation

María Jesús Samper — Thu, 30 Jan 2020 13:42:48 +0100

A computational methodology is presented for modeling the non-linear mechanical behavior of composite structures made of FRP (Fiber-Reinforced Polymers) laminates. The model is based on the appropriate combination of the constitutive models of compounding materials, considered to behave as isolated continua, together with additional “closure equations” that characterize the micro-mechanics of the composite from a morphological point of view. To this end, any appropriate constitutive model may be selected for each phase. Each component is modeled separately and the global response is obtained by assembling all contributions taking into account the interactions between components in a general phenomenological way. To model the behavior of a single uni-directional (UD) composite laminated, a Serial-Parallel continuum approach has been developed assuming that components behave as parallel materials in the fibers alignment direction and as serial materials in orthogonal directions. Taking into account the internal morphology of the composite material, it is devised a strategy for decoupling and coupling component phases. This methodology [Rastellini 2006], named "compounding of behavior", allows to take into consideration local non linear phenomenon in the compounding materials, like damage, plasticity, etc. in a coupled manner. It is based on the proper management of homogenous constitutive models, already available for each component. In this way, it is used all developments achieved in constitutive modeling for plain materials, what makes possible the transference of this technology to composites. A laminated theory complemented with the proposed UD model is employed to describe the mechanical behavior of multi-directional laminates. A specific solution strategy for the general non linear case is proposed. It provides quick local and global convergences, what makes the model suitable for large scale structures. The model brings answers on the non-linear behavior of composites, where classical micro-mechanics formulas are restricted to their linear elastic part. The methodology is validated through several numerical analyses and contrasted against experimental data and benchmark tests.

Comparación de los factores de reducción de respuesta de la norma NCSE-02 y del Eurocódigo 8

María Jesús Samper — Thu, 30 Jan 2020 13:35:31 +0100

En los procedimientos de análisis y diseño sismorresistentes de estructuras se hace necesario introducir simplificaciones o aproximaciones que permitan estudiar la equivalencia entre el análisis elástico a partir del cual se efectúan los cálculos y el comportamiento elásto-plástico, que es el comportamiento real de las estructuras cuando son sometidas a la acción de terremotos fuertes. Una de estas simplificaciones, consiste en proyectar las estructuras de manera que dispongan de una resistencia lateral inferior a la que resulta de un dimensionado al límite elástico, lo que se consigue aplicando cargas laterales calculadas a partir de los espectros de diseño, que son espectros elásticos, afectados por factores de reducción de respuesta, conocidos como factores R. En este artículo se realiza una revisión exhaustiva del estado del arte de los factores de reducción de respuesta, partiendo de su definición, pasando por su implementación en las normas de diseño sismorresistente, hasta llegar a las nuevas propuestas de racionalización de su aplicación. Para poder alcanzar una mejor comprensión del tema, se realiza una comparación entre la aplicación de los factores de reducción de respuesta propuestos en la norma sismorresistente española NCSE-02 y en el Eurocódigo 8.

Aspectes del disseny conceptual dels edificis de formigó armat

María Jesús Samper — Thu, 30 Jan 2020 09:34:42 +0100

A-posteriori error estimation for the finite point method with applications to compressible flow (preprint)

Roberto Flores — Thu, 30 Jan 2020 03:24:02 +0100

An a-posteriori error estimate with application to inviscid compressible flow problems is presented. The estimate is a surrogate measure of the discretization error, obtained from an approximation to the truncation terms of the governing equations. This approximation is calculated from the discrete nodal differential residuals using a reconstructed solution field on a modified stencil of points. Both the error estimation methodology and the flow solution scheme are implemented using the Finite Point Method, a meshless technique enabling higher-order approximations and reconstruction procedures on general unstructured discretizations. The performance of the proposed error indicator is studied and applications to adaptive grid refinement are presented.

Frequency and damping identification in flutter flight testing using singular value decomposition and QR factorization

Roberto Flores — Thu, 30 Jan 2020 02:57:01 +0100

A new method, based on singular value decomposition and QR factorization, has been developed and applied to the analysis of F-18 flutter flight test data. The method is capable of identifying the frequency and damping of the critical aircraft modes, those responsible for the flutter phenomenon. The procedure relies on the capability of singular value decomposition for the analysis, modeling, and prediction of data series with periodic features and also on its power to identify matrix rank. The analysis of simulated and real flutter flight test data demonstrates the effectiveness, robustness, noise-immunity, and the capability for automation of the method proposed under specific conditions.

Aeroelastic analysis of parachute deceleration systems with empirical aerodynamics

Roberto Flores — Thu, 30 Jan 2020 02:35:03 +0100

A technique for the aeroelastic solution of parachute decelerators is presented in this work. The methodology uses empirical aerodynamics, based on a filling-time inflation model and Ludtke’s area law, coupled to two explicit structural solution approaches. A mass-spring-damper technique allows solving the deployment of the system (when the grid is highly distorted) efficiently, and a finite element model is used for the accurate calculation of the structural loads and stresses during parachute opening and steady flight. The coupling strategy is staggered and the models share the same mesh. The methodology is intended for practical calculations of deceleration systems, and provides useful performance and structural data minimizing model complexity and computational cost. The suitability of the proposed technique is assessed by comparisons with reference test drop data.

On the solution of Lambert’s problem by regularization

Roberto Flores — Thu, 30 Jan 2020 02:17:02 +0100

Lambert’s problem is the two-point boundary-value problem resulting from a two-body orbital transfer between two position vectors in a given time. It lies at the very heart of several fundamental astrodynamics and space engineering problems and, as such, it has attracted the interest of scientists over centuries. In this work, we revisit the solution of Lambert’s problem based on Levi-Civita regularization developed by Carles Simó in 1973. We offer an exhaustive derivation of the theory, including proofs of all the results and the formulae employed, and we extend the algorithm to deal with multi-revolution transfers. Then, after investigating a range of initial guess search techniques and testing different numerical methods to approximate the solution, we propose a procedure in which the initial guess is assigned very efficiently by querying a pre-defined interpolating table. Then, in order to achieve both speed and robustness, we combine Newton-Raphson with safety checks to avoid out of boundary deviations to approximate the solution. We validate the method through several tests and applications, and we assess its convergence and performance. The algorithm presents no singularities, converges in all realistic scenarios and its computational cost is comparable with state of the art algorithms.

Geosynchronous inclined orbits for high-latitude communications

Roberto Flores — Thu, 30 Jan 2020 02:03:02 +0100

We present and discuss a solution to the growing demand for satellite telecommunication coverage in the high-latitude geographical regions (beyond 55◦N), where the signal from geostationary satellites is limited or unavailable. We focus on the dynamical issues associated to the design, the coverage, the maintenance and the disposal of a set of orbits selected for the purpose. Specifically, we identify a group of highly inclined, moderately eccentric geosynchronous orbits derived from the Tundra orbit (geosynchronous, eccentric and critically inclined). Continuous coverage can be guaranteed by a constellation of three satellites in equally spaced planes and suitably phased. By means of a highprecision model of the terrestrial gravity field and the relevant environmental perturbations, we study the evolution of these orbits. The effects of the different perturbations on the ground track (which is more important for coverage than the orbital elements themselves) are isolated and analyzed. The physical model and the numerical setup are optimized with respect to computing time and accuracy. We show that, in order to maintain the ground track unchanged, the key parameters are the orbital period and the argument of perigee. Furthermore, corrections to the right ascension of the ascending node are needed in order to preserve the relative orientation of the orbital planes. A station-keeping strategy that minimizes propellant consumption is then devised, and comparisons are made between the cost of a solution based on impulsive maneuvers and one with continuous thrust. Finally, the issue of end-of-life disposal is discussed.

Numerical simulation of the seismic behavior of building structures equipped with friction energy dissipators

María Jesús Samper — Wed, 29 Jan 2020 15:27:59 +0100

This paper presents a new algorithm to simulate the seismic response of N-story building frames incorporating friction energy dissipators; a device per floor is considered. The frames with the dissipators are described by 2D lumped masses models with two degrees of freedom per floor, namely the horizontal displacements of the main structure and of the dissipators. The proposed algorithm consists of a modification of the linear acceleration method; the main innovation consists of checking at each calculation instant the sliding or sticking condition at each floor, hence, the number of “active” degrees of freedom changes continuously, ranging in between N (there is sticking condition at every dissipator) and 2N (there is sliding condition at every dissipator). Some results given by this algorithm are compared to experimental results from ad-hoc testing and to numerical results obtained with the ADINA software package. In both cases, agreement is satisfactory while the proposed method is more computationally efficient.

Respuesta no lineal de edificios de concreto armado diseñados para baja ductilidad

María Jesús Samper — Wed, 29 Jan 2020 15:03:10 +0100

En el diseño sismorresistente de edificios es necesario conocer previamente el valor de la ductilidad que éstos podrán llegar a alcanzar al ser sometidos a movimientos sísmicos fuertes. Los valores de ductilidad de referencia están incluidos en las normas de diseño sismorresistente. Su estimación también puede hacerse bajo criterio de expertos o de la observación de la respuesta que los edificios han tenido ante determinados terremotos. Sin embargo, generalmente el problema estudiado se ha centrado en la respuesta de edificios con respuesta dúctil, como son los edificios aporticados resistentes a momentos, sin que existan muchos datos que avalen la respuesta de los edificios de ductilidad limitada, entre los que se encuentran los edificios aporticados con vigas planas y los edificios con forjados reticulares. En este trabajo se estudia la respuesta no lineal de edificios de ductilidad limitada, proyectados conforme a los requisitos de la instrucción española (EHE) y la norma NCSE-02, y se obtienen los valores de ductilidad a partir de dicha respuesta, lo que permiten verificar los valores de los factores de reducción aplicados en la determinación de las fuerzas sísmicas y los valores de sobrerresistencia. De manera adicional, se estudia la respuesta de los edificios proyectados con aceros con diferentes características de ductilidad y tensión de plastificación. Finalmente, se comparan las respuestas de los edificios de ductilidad limitada con la respuesta de un edificio aporticado, verificándose el cumplimiento de las hipótesis consideradas para el proceso de análisis elástico de las mismas. Los resultados se comparan con los obtenidos de la respuesta no lineal de edificios con pórticos resistentes a momentos, diseñados según las normas españolas y la norma americana ACI-318.

Procedimiento de evaluación de edificios de concreto armado mediante un índice de daño sísmico objetivo

María Jesús Samper — Wed, 29 Jan 2020 14:20:49 +0100

En este trabajo se hace una revisión de los índices que permiten evaluar el daño sísmico de edificios, encontrando la necesidad de formular un índice objetivo que refleje adecuadamente el daño en los edificios de concreto armado, independientemente de la tipología estructural. Se expone un procedimiento de análisis no lineal de estructuras de concreto armado, aplicando control de fuerzas en el que se aplica como criterio de convergencia un índice de daño basado en elementos finitos. El procedimiento es validado mediante la comparación de resultados de dos pórticos de concreto armado, uno de los cuales es ensayado bajo la acción de cargas pseudo estáticas y el otro mediante la aplicación de cargas dinámicas. Los resultados muestran una buena aproximación de los resultados obtenidos mediante simulación numérica. Finalmente, el procedimiento se aplica en el cálculo de la curva de capacidad de tres edificios proyectados para diferentes niveles de ductilidad, evaluando el desempeño de los tres edificios mediante la aplicación del índice de daño sísmico objetivo.

Numerical tool to study RC structures under seismic loads using fiber reinforced polymers

María Jesús Samper — Wed, 29 Jan 2020 13:49:50 +0100

Strengthening or retrofitting existing structures in order to increase their ductility and improve their seismic response has traditionally been accomplished using conventional materials and construction techniques. Composite materials of a polymeric matrix reinforced with long fibers (FRP) have emerged as an alternative to these methods.
To view the performance of these reinforcements when seismic loads are applied this work studies the structural response of a frame joint when a horizontal load is applied to it. Under a seismic load, joints are one of the weakest parts of these structures. The response of a plain concrete frame joint is compared with the response obtained when it is reinforced using FRP. Different configurations of FRP reinforcements are considered to compare their behavior.
The structural response of all structures considered is obtained with a numerical simulation. This is done using the finite element method. Composites are treated using the mixing theory, which obtains the composite behavior by means of the composition of each component material properties. Each component is simulated with its own constitutive equation. The anisotropy usually found in composite
components is treated using a mapped space theory. The debounding effects found in composite materials are treated using the formulation developed by E. Car and S. Oller [Car, 2000].
The results obtained validate the use of FRP reinforcements to improve frames seismic response. The structure load capacity is increased in a 20% when lateral reinforcements are applied to the joint. This can avoid the structure to collapse in a seismic case. They result also show a good performance of the numerical tool developed.

Factores de reducción de respuesta: estado del arte y estudio comparativo entre códigos

María Jesús Samper — Wed, 29 Jan 2020 13:26:39 +0100

En los procedimientos de análisis y diseño sismorresistentes de estructuras se hace necesario introducir simplificadores o aproximaciones que permitan estudiar la equivalencia entre el análisis elástico a partir del cual se efectúan los cálculos, con el comportamiento elástico plástico, que es el comportamiento real de las estructuras cuando son sometidas a la acción de terremotos fuertes. Una de estas simplificaciones consiste en proyectar las estructuras de manera que estas dispongan de una resistencia lateral inferior a la resistencia elástica, lo cual se consigue mediante la aplicación de cargas laterales calculadas a partir de espectros de diseño inelásticos, que no son más que los conocidos como factores R. En este artículo se realiza una revisión exhaustiva del estado del arte de los factores de reducción de respuesta, que comprende desde su propuesta original, pasando por su implementación en los códigos de diseño sismorresistente, hasta llegar a las nuevas propuestas de racionalización de su aplicación. Para poder alcanzar una mejor comprensión sobre el tema, es necesario realizar una comparativa entre aplicación de los factores de reducción de respuesta en los diferentes códigos de diseño sismorresistente a nivel mundial.

Epoch-by-epoch phase difference method to evaluate GNSS single-frequency phase data quality

Xin Liu — Wed, 29 Jan 2020 12:36:11 +0100

The quality of Global Navigation Satellite SystemsGlobal Navigation Satellite Systems Global Navigation Satellite Systems (Global Navigation Satellite Systems Global Navigation Satellite Systems Global Navigation Satellite Systems GNSS) data is one important factor for precise navigation and positioning by using GNSS technique. We proposed a new method to monitor GNSS single-frequency phase data quality by using high-rate epoch-by-epoch phase difference (EEPD). We analyzed the influence of ephemeris, carrier frequency (L1, L2 and L3) and latitude (high latitude, middle latitude and low latitude) on the phase data quality evaluation. It is concluded that the use of broadcast and final orbit has little effect on the phase data quality evaluation with EEPD. The broadcast ephemeris can be used while analyzing GNSS single-frequency data quality in real time. The results obtained by using L1, L2 and L3 observations are almost the same and the GNSS phase quality evaluation can be analyzed by using single-frequency data. The latitudes of stations have a great influence on the quality evaluation of phase data using EEPD. At the same time, Bernese software is used to analyze the feasibility and reliability of EEPD. The results obtained with EEPD are compared with those obtained by Bernese. The results by different methods show good consistency, which shows that EEPD can be reliably used to evaluate the GNSS data quality.

Epoch-by-epoch Phase Difference Method to Evaluate GNSS Single-frequency Phase Data Quality[[#fn-2|²]]

Xin Liu — Wed, 29 Jan 2020 12:26:14 +0100

1 Corresponding author: Xin Liu; E-mail: [mailto:xinliu1969@126.com xinliu1969@126.com]

Epoch-by-epoch Phase Difference Method to Evaluate GNSS Single-frequency Phase Data Quality

Xin Liu — Wed, 29 Jan 2020 12:02:21 +0100

Vulnerability and ductility concepts in the seismic design of buildings

María Jesús Samper — Wed, 29 Jan 2020 11:47:44 +0100

In spite of the every time better understanding and knowledge of the factors that influence in the seismic behaviour of the buildings, there is a great difference at the moment between the theoretical developments that are carried out continually in the field of earthquake engineering and the concepts that are applied in the seismic design codes of many countries. This fact is reflected clearly in the very irregular seismic behaviour of the buildings during the big earthquakes occurred in the world in the last years. The earthquake design codes make exhaustive recommendations concerning the seismic loads, the simplified methods of structural analysis to be applied, to the details that should incorporate in the seismic design and construction of the structural elements and, in lesser measure, to the conceptual design of the buildings. Therefore, special attention is paid in this work to the structural ductility and its influence on the seismic vulnerability of the buildings and on their seismic damage and risk.

A Lagrangian nodal integration method for free-surface fluid flows

María Jesús Samper — Tue, 28 Jan 2020 16:16:17 +0100

We present a Lagrangian nodal integration method for the simulation of Newtonian and non-Newtonian free-surface fluid flows. The proposed nodal Lagrangian method uses a finite element mesh to discretize the computational domain and to define the (linear) shape functions for the unknown nodal variables, as in the standard Particle Finite Element Method (PFEM). In this approach, however, the integrals are performed over nodal patches and not over elements, and strains/stresses are defined at nodes and not at Gauss points. This allows to limit the drawbacks associated with the remeshing and leads to a more accurate stress computation than in the classical elemental PFEM. Several numerical tests, in 2D and in 3D, are presented to validate the proposed nodal PFEM. In all cases, the method has shown a very good agreement with analytical solutions and with experimental and numerical results from the literature. A thorough comparison between nodal and elemental PFEMs is also presented, focusing on crucial issues, such as solution accuracy, convergence, mass conservation and sensitivity to mesh distortion.

Statical and dynamical behaviour of thin fibre reinforced composite laminates with different shapes

María Jesús Samper — Tue, 28 Jan 2020 12:11:28 +0100

Based on the classical laminated plate theory, a variational approach for the study of the statical and dynamical behaviour of arbitrary quadrilateral anisotropic plates with various boundary conditions is developed. The analytical formulation uses the Ritz method in conjunction with natural coordinates to express the geometry of general plates in a simple form. The deflection of the plate is approximated by a set of beam characteristic orthogonal polynomials generated using the Gram–Schmidt procedure. The algorithm developed is quite general and can be used to study fibre reinforced composite laminates with symmetric lay-ups, which may have general anisotropy and any combinations of clamped, simply supported and free edge support conditions. Various numerical applications are presented and some results are compared with existing values in the literature to demonstrate the accuracy and flexibility of the present method. New results were also determined for plates with different geometrical shapes, combinations of boundary conditions, several stacking sequences and various angles of fibre orientation.

Estudio numérico del comportamiento de los hormigones reforzados con fibras cortas

María Jesús Samper — Tue, 28 Jan 2020 11:48:49 +0100

En este trabajo se presenta un acercamiento al estudio del comportamiento de los hormigones reforzados con fibras cortas. Se establece una formulación basada en una modificación de la teoría de mezclas clásica para determinar sus parámetros mecánicos a partir de las características de los materiales componentes (hormigón fibras cortas). También se muestran resultados numéricos obtenidos mediante el método de los elementos finitos y se comentan las posibilidades de un método analítico simplificado para el estudio de su comportamiento. El desarrollo del trabajo trata sobre el comportamiento del hormigón reforzado con fibras y las características que justifican su utilización. Se estudia el equilibrio interno y mecanismo de transmisión de tensiones entre el hormigón y las fibras cortas y la formulación general del problema e inserción dentro de la técnica de los elementos finitos. Se hace un breve comentario sobre el tratamiento simplificado del comportamiento de los hormigones reforzados con fibras cortas y se presenta un ejemplo de comprobación.

Definition of a general implicit orthotropic yield criterion

María Jesús Samper — Tue, 28 Jan 2020 11:01:01 +0100

The definition of an orthotropic yield criterion presents a serious challenge in the formulation of constitutive models based on such theories as elastoplasticity, viscoplasticity, damage, etc. The need to model the behavior of a real orthotropic material requires the formulation of orthotropic yield criteria, and these may be difficult to obtain. For metals, orthotropic yield functions have been formulated by Hill [Proc. Roy. Soc. Lon. Ser. A 193 (1948) 281; J. Mech. Phys. Solids 38(3) (1990) 200], Barlat [Int. J. Plasticity 5 (1989) 51; 7 (1991) 693], Chu [NUMISHEET 93 (1993) 199] and Dutko et al. [Comput. Methods Appl. Mech. Engrg. 109 (1993) 73], but in many cases these functions do not describe the true behavior of the metal. The situation is worse when one attempts to represent a nonmetal such as a polymer, ceramic or composite.

In this paper, we present a general definition of an explicit orthotropic yield criterion together with a general method for defining implicit orthotropic yield functions. The latter formulation is based on the transformed-tensor method, whose principal advantage lies in the possibility of adjusting an arbitrary isotropic yield criterion to the behavior of an anisotropic material. As example we choose the adjustment to the Hill, Hoffman [J. Comp. Materials 1 (1967) 200] and Tsai–Wu [J. Comp. Materials 5 (1971) 58] criteria, but these particular cases serve to establish the methodology for achieving the desired function adjustment for any other well-known criterion or experimental set of data obtained from laboratory.

Life prediction of structures. Numerical study and treatment of structural durability, a brief report

María Jesús Samper — Tue, 28 Jan 2020 10:39:00 +0100

This work is a brief report about the problem of life -or, durability study- of structural materials, undergoing mechanical, thermal and chemical actions. This document is approached from numerical techniques and, it highlights the potential of these types of tools un the study of structures undergoing very complex and coupled phenomena.

A general framework for continuum damage models. II. Integration algorithms, with applications to the numerical simulation of porous metals

María Jesús Samper — Tue, 28 Jan 2020 10:19:40 +0100

In this paper, we develop numerical algorithms for the integration of the continuum plastic damage models formulated in the general framework identified in Part I of this work. More specifically, we focus our attention on a particular plastic damage model of porous metals, involving a classical von Mises yield criterion coupled with a pressure dependent damage surface to model the nucleation and growth of voids in the metallic matrix. Unilateral damage leading to a sudden change of stiffness in the material's response due to the closing/opening of these voids is also incorporated through the imposition of the unilateral constraint of a positive void fraction, thus, illustrating the clear physical significance added by this framework in the resulting constitutive models. The proposed integration algorithms fully use the modularity of the identified framework, leading in this way to independent integration algorithms for the elastoplastic part and each damage mechanism. Remarkably, all these individual integration schemes share the same formal structure as the classical return mapping algorithms employed in the numerical integration of elastoplastic models, namely an operator split structure consisting of a trial state and the return map imposing the plastic and damage consistency, respectively. A Newton iterative scheme imposes the equilibrium (equal stresses) among the different mechanisms of the response of the material. This modular structure allows to obtain the closed-form consistent linearization, involving in a simple form the algorithmic consistent tangents corresponding to each independent mechanism, thus resulting in a very modular and efficient computational implementation. The performance of the proposed algorithms is illustrated in several representative numerical simulations.

A general framework for continuum damage models. I. Infinitesimal plastic damage models in stress space

María Jesús Samper — Tue, 28 Jan 2020 10:12:39 +0100

We identify in this paper a general framework for the development of continuum damage models in their fully coupled plastic damage form. The focus of this paper is directed to the general formulation of infinitesimal models defined by yield and damage surfaces in stress space. The main feature of the proposed formulation is the direct and independent consideration of the damage mechanisms (isotropic damage, cracking, etc.) degrading the stiffness of the material, thus allowing for a complete physical characterization of these effects. This modular structure is accomplished by a kinematic decomposition of the strains in an elastic, plastic and multiple damage parts, as belonging to each activated damage mechanism. An additive decomposition in the infinitesimal range of interest is considered. Based on this decomposition, the constitutive characterization alluded to above for each damage mechanism is carried out in a complete thermodynamically consistent framework. One of the virtues of the considered framework is the fact that it includes many of the diverse damage models existing in the literature as particular cases. In this way, the developments presented herein furnish a unified framework for the formulation of continuum damage models, including isotropic damage, compliance based formulations, effective stress anisotropic models, smeared crack models and the related formulations of cracking and damage based on strong discontinuities. Besides the clear physical significance added to these existing formulations, the proposed framework also defines a very convenient context for the efficient numerical integration of the resulting models. This aspect is explored in Part II of this work, as it is the application of the framework proposed herein to the numerical simulation of porous metals.

The use of particle methods for the simulations of additive manufacturing processes

María Jesús Samper — Mon, 27 Jan 2020 16:56:02 +0100

Massively parallel methods for immersed boundary simulations of particles in viscoelastic fluids

María Jesús Samper — Mon, 27 Jan 2020 16:53:50 +0100

Particle scale modelling in soil mechanics: Stiffness, strength, and seepage

María Jesús Samper — Mon, 27 Jan 2020 16:48:26 +0100

Coupled plastic-damaged model

María Jesús Samper — Mon, 27 Jan 2020 16:42:03 +0100

A constitutive model that couples plasticity and damage is presented. The model is thermodynamically consistent and comes from a generalization of classical plasticity theory and isotropic damage theory of Kachanov. Coupling between plasticity and damage is achieved through a simultaneous solution of the plastic and the damage problem. After a description of the model, a numerical algorithm for the integration of the resulting constitutive equations is presented. It is an Euler Backward type of algorithm that is particularly suitable to solve plain stress non-linear problems with a 2D finite element program. The consistent stiffness matrix is also derived. The paper is completed with some application examples that show that the model presented accurately reproduces the behaviour of elastic-plastic-damaged materials.

Is particle and mesh a promissory combination for stable and high order schemes

María Jesús Samper — Mon, 27 Jan 2020 16:40:34 +0100

Discrete element modelling in granular mechanics, Recent breakthroughs

María Jesús Samper — Mon, 27 Jan 2020 16:29:32 +0100

The bonded DEM. Strength and weakness

María Jesús Samper — Mon, 27 Jan 2020 16:25:21 +0100

A plasticity model for simulation of industrial powder compaction processes

María Jesús Samper — Mon, 27 Jan 2020 16:22:53 +0100

A constitutive model, based on large strain plasticity, for simulation of industrial powder compaction processes is presented. The elastic response is stated in terms of a hyperelastic model based on a hookean elastic free energy. Plastic response is defined in terms of a two parameter yield surface that evolves in terms of the relative density. Two different flow rules are considered and tested in front of some available experimental results. Application to the simulation of an actual powder-metallurgy compaction process is also shown.

Granular flows and particle-laden fluids interacting with thin flexible structures

María Jesús Samper — Mon, 27 Jan 2020 16:17:41 +0100

Evaluation of the damage degree in buildings structures subjected to earthquakes

María Jesús Samper — Mon, 27 Jan 2020 14:48:32 +0100

This paper represents an overview of the investigation pursued by the authors in the field of damage evaluation of concrete structures. A procedure to evaluate the damage of reinforced concrete structures subjected to static and dynamic actions has been developed, which is based on a local damage constitutive model using Kachanov’s theory. The constitutive model is used within a finite element frame, a non-linear Newmark-type tangent incremental time integration scheme is employed and finally, an overall global damage index, based on potential energy criteria, is developed and justified.

Analysis of MFS-DRM formulations for nonhomogeneous diffusion problems

DANIELA SANTURIO — Wed, 22 Jan 2020 00:32:08 +0100

This paper presents tests and applications related to the Method of the Fundamental Solutions (MFS) for nonhomogeneous diffusion equations. The dual reciprocity method was applied to approximate the particular solution and different MSF were applied to solve the homogeneous equation. The computational accuracy for three different versions submitted to diverse geometries, initial, boundary and nonhomogeneous conditions is compared. The first version is the MFS, based on the fundamental solutions to the modified Helmholtz operator; the second and third versions are based on transient fundamental solutions proposed directly for the diffusion governing equation. This paper evaluates particularities for all methods proposed as the solution advances in time until steady-state solutions are reached. The developed computer codes are applied to several examples with the intention of demonstrating the methods’ effectiveness and applicability.

Optimización geométrica de aliviaderos en tecla de piano para la mejora de su capacidad hidráulica

Fernando Salazar — Tue, 21 Jan 2020 09:04:02 +0100

En esta comunicación se expone el estudio realizado para la mejora de la capacidad de desagüe de aliviaderos en tecla de piano mediante la optimización de su diseño geométrico en base a modelación numérica validada físicamente. Se muestra para un cierto diseño base de aliviadero en tecla de piano, el patrón de flujo que induce la geometría y su influencia en la capacidad de desagüe. Se observó una reconducción del flujo por los voladizos del aliviadero, lo que provoca zonas de baja velocidad y concentración de líneas de corriente, dificultando el desagüe. Una vez estudiado el patrón flujo se procedió al rediseño de la geometría del aliviadero, en base a la detección en el modelo numérico de las zonas de baja velocidad inducidas por el contorno geométrico. La nueva geometría de aliviadero optimizada se modeló numéricamente, observándose un aumento de la
capacidad de desagüe respecto a la geometría base.

Algoritmo adaptativo para la estimación de pérdidas de agua en redes basado en análisis avanzado de Caudales Mínimos Nocturnos (CMN)

Fernando Salazar — Tue, 21 Jan 2020 08:50:03 +0100

El método del Caudal Mínimo Nocturno (CMN) es un concepto ampliamente utilizado para la estimación de fugas y otros objetivos estratégicos en empresas operadoras de agua. Si bien es un procedimiento de fácil aplicabilidad, existe un gran abanico de submétodos de aplicación, pudiendo introducirse una gran incertidumbre en los cálculos si no se seleccionan correctamente los criterios a utilizar. En este estudio se presenta un algoritmo capaz de asistir en la elección del mejor enfoque para el cálculo de CMN en función de los datos disponibles, las características de cada caso y el objetivo perseguido. Para elaborar dicho algoritmo se han ejecutado tres acciones: (i) revisión exhaustiva de literatura especializada y recopilación de submétodos utilizados en casos reales a nivel global, (ii) elaboración de un algoritmo multi-criterio capaz de seleccionar la mejor opción en cada fase del cálculo y (iii) presentación de una plataforma informática, llamada WatEner, en la que se integrará progresivamente el algoritmo propuesto.

Mejora de la seguridad hidrológica e incremento de la capacidad de embalse de presas de fábrica mediante aliviaderos con cajeros altamente convergentes

Fernando Salazar — Tue, 21 Jan 2020 08:46:04 +0100

El empleo en aliviaderos de presas de fábrica de canales laterales para la recogida de los vertidos en la zona de estribos ha sido una solución utilizada con relativa frecuencia lo largo de la historia en casos en los que la longitud de vertido era sensiblemente superior a la anchura del valle aguas abajo de la presa. Este tipo de aliviaderos en presas existentes puede tener como objetivo el aumento de capacidad de su aliviadero o bien el aumento de los niveles máximos de explotación del embalse manteniendo la capacidad de desagüe con los resguardos exigidos. A pesar de estos antecedentes, no existen estudios metodológicos que proporcionen criterios para su diseño, ni conclusiones sobre el efecto que producen este tipo de aliviaderos en la disipación de energía en cuencos de pie de presa. Ante esta situación, se está desarrollando el proyecto de investigación aplicada CALA, cuyo objeto principal es definir criterios para el diseño de ese tipo de aliviaderos y el desarrollo de un software de cálculo que permita su predimensionamiento y optimización.

Tratamiento y análisis inteligente de datos del comportamiento de estructuras hidráulicas

Fernando Salazar — Tue, 21 Jan 2020 08:35:02 +0100

El rendimiento de los dispositivos de monitorización ha experimentado mejoras permitiendo disponer de más información sobre el comportamiento de las estructuras. Sin embargo, las inversiones realizadas en modernización de sistemas de medida no se recuperan a menos que se complementen con aplicaciones capaces de manejar una información tan amplia y diversa.

En esta contribución, se presenta una herramienta de software para importar, explorar, limpiar y analizar datos de monitorización. Además, permite la generación de modelos de predicción basados en machine learning, así como la interpretación de la respuesta del sistema a las acciones o cargas en funcionamiento.

La metodología y la estructura general se dividen en dos fases: i) carga, depuración, completado y análisis datos, y ii) generación e interpretación de modelos predictivos basados en machine learning. El mismo modelo puede utilizarse para la detección de anomalías comparando las predicciones con el comportamiento registrado.

Teaching structural engineering concepts in a civil engineering course based on a didactic game

Eric Forcael — Wed, 15 Jan 2020 18:23:03 +0100

This research analyzes a methodology of non-traditional teaching, based on a didactic game applied into teaching structural engineering concepts within a Statics course —part of a Civil Engineering program—, in which students performed structural analyses in trusses and their respective support systems, by constructing small structures, to later compare results with structural analyses based on theory. Two groups were conformed, where the control group was taught based on traditional teaching lectures, and the test group, to which a didactic teaching methodology based on the structural game developed in this research was applied. For the analysis of both groups, non-parametrical statistical analyses were conducted, along with a qualitative analysis. The statistical analyses evidenced that the didactic methodology proposed here showed to be more efficient for teaching structural concepts, obtaining a high degree of acceptance from students, according to the qualitative analysis carried out, which would motivate its use in other engineering courses.

Análisis de la percepción de las condiciones ergoambientales de una sala de aprendizaje activo

Diego Yáñez — Wed, 08 Jan 2020 18:16:02 +0100

Contexto: Las Aulas de Aprendizaje Activo son entornos de aprendizaje centrados en el estudiante en los que abunda la tecnología, los cuales favorecen un aprendizaje dinámico y colaborativo. En este sentido, la ergonomía ambiental de estas Aulas, la cual considera factores como el ruido, la iluminación y la temperatura, es fundamental para el logro del bienestar de los individuos en el contexto educativo.

Objetivo: Este estudio busca conocer la percepción que tienen los estudiantes respecto de las condiciones de una Sala de Aprendizaje Activo.

Metodología: Para esto se aplicó una encuesta a 85 estudiantes de Ingeniería Civil Industrial que utilizaron esta sala durante el primer semestre de 2019.

Resultados: Los resultados evidencian una valoración positiva a la iluminación artificial, pero declaran cierta insatisfacción respecto de iluminación natural, asimismo, muestran satisfacción respecto del ruido que afecta a la sala aun cuando manifiestan que la sala carece de un buen sistema de aislación respecto de salas adyacentes. Además, consideran que las condiciones de temperatura y ventilación no son adecuadas. Lo anterior plantea desafíos a futuro en torno a mejorar las condiciones ergoambientales de la sala, además de evaluar el impacto de estas condiciones en el rendimiento del aprendizaje de los estudiantes.

Experiments on reinforced brick masonry vaulted light roofs

María Jesús Samper — Wed, 08 Jan 2020 10:32:44 +0100

This paper describes structural tests of thin vaults made of reinforced brick masonry. The experiments consist of concentrated loading tests of 14 full-scale laboratory vaults. These vaults are designed to include common situations such as short- to midspan length, low-mid-high rise, rigid-flexible-sliding supports, instantaneous-sustained loading, low-high strength mortar, point-line loading, central-eccentric loading, point-line supports, hinged-clamped supports, symmetric-asymmetric shape, double layer versus single layer reinforcement, and uniaxial-biaxial bending, among others. The tests mainly aim to obtain the collapse loads and to characterize the pre- and post-peak response. The results show satisfactory structural performance, both in terms of ductility and strength. Moreover, it is possible to predict the structural response with numerical models developed specifically for this purpose. Flat specimens were also tested to determine the punching shear strength of the vaults. This work is part of a larger research project aimed at promoting innovative semi-prefabrication techniques for reinforced brick masonry vaulted light roofs.

A numerical-experimental method for characterizing recycled asphalt mixtures

María Jesús Samper — Wed, 08 Jan 2020 09:46:02 +0100

Experimental techniques usually employed in the characterization of conventional mixes are not able to clearly describe some specific properties of the recycled mixes, which are determinant in the material's mechanical response like cracking strength or tenacity. These new specific properties demands new methods of testing to know the really conditioning mechanical variables taking part in the process, so that being able to characterize and design new recycled mixes in an optimum way. In this paper, the direct tensile test is analyzed as a suitable method for evaluating the properties of recycled asphalt mixtures for different percentages of reclaimed asphalt pavement. A constitutive formulation has been developed to simulate the experimental results and can be used in conjunction with the experimental analysis of the recycled asphalt properties to provide an accurate characterization of the material.

Limit analysis of reinforced masonry vaults

María Jesús Samper — Tue, 07 Jan 2020 16:47:09 +0100

Reinforced brick masonry has experienced only scarce use as a fully structural material due to, among other reasons, the lack of design criteria and calculation tools allowing a scientific, but also practical, engineering approach to design and assessment. Aiming at contributing to a more widespread use of this material, a simplified method for the ultimate analysis of reinforced masonry arches and cylindrical vaults, based on the lower-bound theorem (or static approach) of plasticity, has been developed. This approach has been satisfactorily validated by comparison with experimental and numerical results obtained by more accurate numerical models.

A viscoplastic constitutive model with strain rate variables for asphalt mixtures—numerical simulation

María Jesús Samper — Tue, 07 Jan 2020 15:36:39 +0100

The study and development of recycling techniques for pavements is an increasing activity in engineering nowadays. This research line demands a more realistic characterization of the material properties with the aim of simulate the asphalt mixture’s response placed into a multilayered system over granular bases, under dynamic loads, considering also temperature variation or strength reduction for cyclic loads.

In order to improve the current formulations, a new viscoplastic model has been developed assuming the strain rate dependency of the material’s response observed in the experimental tests. The strain rate variable affects in a significant way the Young modulus and the viscosity parameter of the model. According to this hypothesis a constitutive equations have been formulated. The mechanical variables involved have been calibrated according to experimental results, developing new expressions for the strain rate dependent parameters. The new viscoplastic model permits us to characterize the material’s response with a few mechanical values, easily obtained from standard laboratory tests. The results obtained show a good approximation to experimental laboratory curves for different rates of loading and temperatures.

The model has been applied to simulate the response of a real flexible pavement structure conformed by two asphalt layers over two granular bases, that’s materials with different constitutive behaviors. Experimental tests in the recycled track have been made obtaining the horizontal strain evolution under dynamic load. Different loading rates and temperatures, as well as cracked and continuum pavement responses have been considered in the study. Strains were measured in the interface between the two asphalt layers and simulated using the here proposed model offering a fairly good approximation of the real response observed in the track, although the degree of variation even in the experimental curves is quite high.

The results of this study represent a proper base for further developments in structural analysis of pavement layers, considering more complex phenomena, determinant in the long term material’s response, to develop a numerical tool for pavements’ design and lifetime prediction.

Composite material behavior using a homogenization double scale method

María Jesús Samper — Tue, 07 Jan 2020 15:06:31 +0100

In this paper, we present a two-scale numerical method in which structures made up of composite materials are simulated. The method proposed lies within the context of homogenization theory and assumes the periodicity of the internal structure of the material. The problem is divided into two scales of different orders of magnitude: A macroscopic scale in which the body and structure of the composite material is simulated, and a microscopic scale in which an elemental volume called a “cell” simulates the material. In this work, the homogenized strain tensor is related to the transformation of the periodicity vectors. The problem of composite materials is posed as a coupled, two-scale problem, in which the constitutive equation of the composite material becomes the solution of the boundary-value problem in the cell domain. Solving various examples found in the bibliography on this subject demonstrates the validity of the method.

Numerical study of the behaviour of concrete reinforced with short fibers

María Jesús Samper — Tue, 07 Jan 2020 14:58:31 +0100

An approach for the short fibers reinforced concrete behavior is presented in this work. A formulation based on a modification of the classical mixing theory to obtain the composite mechanical properties starting from the single compounding characteristics (concrete−short fibers) is developed.

Numerical results obtained by means of the finite element method are also shown and the possibilities of using a simplified analytical method in the study of the short fibers reinforced concrete are commented.

The development of the work studies the behavior of the concrete reinforced with short fibers and the characteristics that justify its use. It studies the internal balance and the stress transmission mechanism between the concrete and the short fibers as well as the general formulation of the problem and its insertion within the frame of the finite element technique.

A brief comment is made on the simplified treatment of the behavior of the concrete reinforced with short fibers and a validation example is presented.

Use of dampers and seismic connectors in bridges

María Jesús Samper — Tue, 07 Jan 2020 14:16:29 +0100

The earthquakes have always represented on of the most dread destructive phenomena, for they affect the last stability of the structures. The modern societies are not prepared any more to accept then as a fatility, and the trend is now to provide, for structures situated in critical seismic zones, specific devices which nullify, or at least reduce the effect of the earthquakes.

In this paper we are going to present the effects that the establishment of two similar seismic devices - the dampers and the seismic connectors - produces in different kinds of bridges.

Plataforma computacional para el análisis y simulación de sistemas rotor-cojinete de múltiples grados de libertad

José Mendoza — Thu, 02 Jan 2020 23:19:05 +0100

En este artículo, se presenta el desarrollo de una plataforma computacional para el análisis y simulación de sistemas rotor-cojinete de múltiples grados de libertad. La plataforma computacional se programó utilizando el software comercial Matlab, y está soportada por la formulación matemática basada en el método del elemento finito para el modelado de sistemas rotodinámicos. Para lo anterior, se consideró un elemento finito tipo viga con cuatro grados de libertad por nodo, efectos de inercia rotacional, momentos giroscópicos, deformaciones por cortante y amortiguamiento externo. El objetivo de la plataforma computacional es resolver la ecuación general de movimiento del sistema rotor-cojinete mediante los métodos de solución: directo y Newmark. La solución mediante el método directo permite conocer información del sistema tal como: frecuencias naturales, diagrama de Campbell, formas modales y la respuesta de vibración del sistema en estado estable. Por otro lado, la respuesta vibratoria en función del tiempo se obtiene de forma numérica mediante el método de Newmark. La validación de la plataforma computacional se realizó comparando los resultados obtenidos con resultados reportados en la literatura, así como con datos experimentales.

¿Descanso para hoy o estrés para mañana?: Beneficios y costos de los paros estudiantiles en ingeniería y ciencias

Juan P, Ross — Thu, 02 Jan 2020 22:33:03 +0100

El objetivo de este trabajo es analizar los beneficios y costos de los paros estudiantiles según la percepción de los estudiantes de la Facultad de Ciencias Físicas y Matemáticas de la Universidad de Chile. Para esto, se realizó un cuestionario sobre el último paro mayor a cinco semanas vivenciado por los estudiantes. La muestra final consistió en 875 estudiantes, representativos de la Facultad. En particular, este trabajo presenta una categorización a las respuestas de texto libre a la pregunta: ¿Qué impactos positivos y negativos en tu rendimiento y en otros aspectos de tu vida académica y/o personal tuvo la paralización? Mediante un análisis cualitativo, se definieron ocho categorías para los impactos positivos y negativos. Entre las categorías se encuentran aspectos académicos, tales como “Rendimiento académico” y “Ritmo de estudio”, y otros aspectos personales como “Estrés”, “Tiempo para compartir con cercanos” y “Actividades recreativas”. Los estudiantes reportan como mayor beneficio que el paro les da tiempo para descansar y reducir significativamente los niveles de estrés. Por otro lado, el principal impacto negativo es el cambio de ritmo asociado a las paralizaciones y la posterior vuelta a clases. Este estudio muestra un pionero acercamiento a uno de los fenómenos que con recurrencia afectan a los estudiantes de la educación superior chilena.

Analytical formulation of the stiffness method for 2D reticular structures using Green functions

Juan Camilo Molina-Villegas — Thu, 02 Jan 2020 16:15:20 +0100

Green functions (F.G.) are defined as the response of a medium to a unit point load and are widely used to solve boundary value problems. Unfortunately, in structural analysis, its use is limited and they are only used indirectly and with another name in the calculation of influence lines and in the formulation of the virtual work method. This article presents the Green functions stiffness method, which is a novel methodology to obtain the analytical or exact response of two dimensional frames, which mixes the stiffnes method and the Green functions, the latter used for the calculation of displacement fields. In particular, the formulation will be carried out for bar elements (subjected to axial force), beam elements (subjected to shear force and bending moment), beam over flexible foundation elements (subjected to shear force and bending moment) and two dimensional frames (subjected to axial force, cutting force and bending moment). This formulation has as its main property that it can be used to compute the analytic reponse for any external load distribution and minimizes the number of elements to be used in discretizations. In addition, the equivalence of this formulation with that obtained by an “exact” implementation of the finite element method is presented.

Survey research on dynamic CTA data analysis for peripheral artery occlusive disease

Zehong LIN — Wed, 25 Dec 2019 01:04:14 +0100

The purpose of this study is to review the research which was mainly about the evaluation the vascular stenosis or muscle ischemia to assist in the diagnosis of peripheral arterial occlusive disease using dynamic CTA data. With the recent rapid evolution of multislice computed tomography scanners, runoff CTA of the peripheral artery has become a widely used diagnostic option in patients with PAOD. Furthermore the dynamic CTA of the lower extremity provides improved arterial contrast enhancement and a higher diagnostic confidence for assessment of stenosis and occlusion than runoff CTA. The method, the data characteristic and the conclusion of all the qualitative and quantitative research were covered in this study. Finally, the future research direction of the dynamic CTA analysis to assist diagnosis was given.

Cálculo de daño en pavimentos mediante un análisis hidro-mecánico de pavimentos flexibles

Carlos Chávez — Fri, 20 Dec 2019 15:24:03 +0100

A hydro-mechanical analysis of a pavement is presented in which the variation of the water content is considered for the estimation of modulus of elasticity of the layers of soil in the pavement. To do this, the Richards differential equation and equilibrium in the axi-symmetric condition are solved simultaneously, considering a non-linear elasticity that depends on the suction and stress state in the soil. With this type of analysis, it is possible to quantify the variation of the damage with respect to time and visualize the deterioration of the pavement structures. It is considered a practical case in which the increase in damage is quantified in function of the variation of environmental conditions and conclusions are obtained.

Design and implementation of prototype for XLPE cable aging test

Sergio Rivera — Thu, 19 Dec 2019 05:49:13 +0100

The current increase in the demand for electric power and the obligation to comply with the regulations associated with reliability and continuity in the supply, have prompted the manufacturers of electrical conductors to focus on designing their products with special care when considering the electrical properties of the materials. The use of XLPE insulated cables in underground transmission lines in 115 kV systems has increased in cities due to urban, environmental and reliability requirements. This is also related to the fact that air networks are exposed to external factors such as lightning, comets, vegetation, animals, etc., which can affect their functioning failures. The fact of using transmission technology with insulated cables gives rise to the need to evaluate the behavior of its components when operating in the long term. To achieve this, it is necessary to implement a procedure that allows analyzing the aging of these elements when operating in load cycles for a certain period of time. Thus, this document presents the development of a prototype that allows conducting the aging test to conductors isolated by XLPE.

Experimental numerical comparision of the pressure drop through an hydrocyclone

Emmanuel Alcocer De Agüero — Wed, 18 Dec 2019 19:57:05 +0100

This paper shows the comparative results of an experimental and numerical study. The numerical simulations were performed with the ANSYS/Fluent software by means of CFD applying RNG k-ε and standard k-ω turbulence models.

Experimental data were recorded of a hydrocyclone mounted on a test bench which operates with a water flow. The values extracted are the inlet pressure, outlet pressure, inlet velocity and outlet velocity. In order to investigate the pressure drop underwent by a hydrocyclone, a pressure assessment was carried out in different areas of the divice geometry.

The results of the numerical study were compared and correlated with the experimental data. The numerical values report a percentage difference of 10.45% for the RNG k-ε model and 1.57% for the standard k-ω model, with respect to the experimentally obtained output pressure. The percentage difference between experimental and numerical pressure drop is 0.04% for standard k-ω model and 0.24% for RNG k-ε model.

By studying finite volumes, the components of axial velocity, tangential velocity and turbulence intensity are calculated. These phenomena are studied simultaneously in this work. The turbulence models used to generate the corresponding analyzes in this work show similarity between both results and flow behavior. The concordance between the computational results and the experimental data is acceptable, so that the numerical simulations applied reach the purpose of this investigation.

Structural optimization of forklift fork based on numerical simulation and mathematical modeling of stress and fatigue

Enlai Zhang — Tue, 17 Dec 2019 04:12:54 +0100

Fork’s related structural design and detail have a significant impact on the overall performance of a forklift. In this paper, the structural stress analysis and strength calculation of the fork were carried out firstly, and the rated load calculations considering dynamic and unbalanced conditions were performed. Also, the restraint conditions were analyzed. Then, a finite element model was established using ANSYS software. In particular, the fork’s structure was optimized by increasing the fillet radius of the fork root and the variation rules between the fillet radius and the maximum structural stress and minimum fatigue life were numerically studied, which was in accordance with the mathematical models of ExpAssoc and Logistic regression, and the corresponding fitting coefficients of R2 was respectively equal to 0.99 and 0.96. Overall, the stress concentration issue of the fork root was improved and the structural strength perquisite was satisfied when using a fillet radius of 75 mm. Both the numerical calculations and theorized mechanical analysis show consistency, where the stress concentration was observed in the fork root’s fillet area, testifying the accuracy of the established finite element model. The numerous methods employed in this work lay an important theoretical basis for the structural optimization of the target fork.

Effect of elbow implant design parameters on loosening: A finite element analysis

Milad Heidari — Fri, 13 Dec 2019 13:35:03 +0100

Loosening of an implant is the main complication after total elbow arthroplasty. The design parameters of the implant are a factor that contributes to the loosening of the implant. In this study, the correlation of loosening and geometry of the elbow implant component was investigated using the finite element method. Image processing software was used to construct elbow bones. Humeral and ulnar components were modeled with various cross-sections and flanges using modeling computer-aided design software. Modeling of all stems was based on conventional cemented fixation. Axial torsion and Anterior-posterior force were applied to the articular surface. Zero-displacement was applied to the proximal humerus and distal ulna. Bone graft was used between the flange and distal humerus bone. The stress shielding of bones and micro-motion were obtained in cement-implant-bone interfaces. Results showed that humeral and ulnar stems with modified diamond cross-section observed less micro-motion in the cement-bone-implant interfaces and as well as the least stress shielding in the distal humerus and proximal ulna compared to others. Furthermore, the modified flange, concave, transferred the least micro-motion to the bone-cement-implant interface and less stress shielding in distal humerus in comparison with trapezium and rectangular ones. This computational analysis represented a step in quantifying the role of interface micro-motion stress shielding of bone in initiating elbow implant loosening. A reduction of micro-motion and stress shielding through design modifications may improve the clinical outcomes.

Numerical implementation for conception of strut and tie models in reinforced concrete structures

Artur Ladeira — Fri, 06 Dec 2019 18:18:54 +0100

O Modelo de Bielas e Tirantes pode ser uma excelente alternativa para o dimensionamento de elementos estruturais em concreto armado submetidos a estado plano de tensão e para regiões que apresentem descontinuidade de ordem geométrica ou estática, substituindo procedimentos empíricos por uma metodologia racional de projeto. Para tornar a concepção do modelo menos dependente da experiência do projetista, o presente artigo tem como objetivo aliar a técnica de otimização topológica ESO (Evolutionary Structural Optimization) ao Método dos Elementos Finitos para geração automática dos modelos de bielas e tirantes. O critério de evolução do método de otimização topológica adotado considera a eliminação de elementos menos solicitados em termos de tensão, a partir de uma análise elástico-linear. Nesse contexto, é possível obter soluções otimizadas de problemas complexos envolvendo o concreto estrutural. São apresentados três exemplos numéricos para comprovação e validação das formulações e técnicas implementadas, cujos modelos de bielas e tirantes obtidos apresentam boa concordância em relação às respostas encontradas em trabalhos científicos precursores sobre o tema.

Guide to multi-objective optimization for the green vehicle routing problem

Júlio César Ferreira — Mon, 02 Dec 2019 20:58:04 +0100

This article aims to present research conducted on the literature regarding Multi-objective Optimization (MOO) for routing problems with environmental considerations (EC), referred to here as Multi-objective Optimization for the Green Vehicle Routing Problem (MOOGVRP). A Brazilian database, CAPES (Coordination for the Improvement of Higher Education Personnel), was used to collect articles of general application, case studies and reviews in English starting from, since 2012. The terms “green vehicle routing problem” (GVRP), “pollution routing problem” (PRP), “vehicle routing problem in reverse logistics” (VRPRL) and “multi-objective” were used in the research protocol. Consequently, this study obtained 1,744 research results that, following the application of the filtering criterion, resulted in a sample of 76 articles from 38 journals, for which a bibliometric data (bibliometric review) survey was conducted. When dealing with the bibliometric data of the sample, it was possible to identify information such as the number of publications per year and types of published works. Information was also identified regarding the most frequently used journals and the countries and institutions that published the most articles on the proposed theme. It was also possible to analyze the frequency of the protocol terms in the title, abstracts and keywords, the relationship between taxonomies, vehicle fleet types, multi-objective procedures, and VRP procedures. Information was also found regarding solution procedures, number of objectives, and programming languages for computational implementation, the frequency of the most used objectives, and the most cited articles in the sample. The originality of this article lies in how the research is presented, highlighting the results and particular details obtained through the survey, which may be considered of great academic importance in the sense of guiding the trends for future research.

Graphical solutions of the two-dimensional Saint-Venant equations by means of channels reticles

Justino Alavez-Ramírez — Wed, 27 Nov 2019 17:38:04 +0100

The knowledge of ancestral peoples is retaken as an important knowledge in their relationship with nature and to face climate change; culture that must be confirmed by science. To give the reason for the mentioned knowledge, when using channel lattices for their plots, it is necessary to model them using flow equations. Applying the equations of shallow water can simulate the hydrodynamic behavior and dissipation of energy by passing a flow in a lattice of channels. To model the grid the geometry of the flow networks used in geotechnics was drawn, the networks are an engineering solution to the Laplace equation. When passing the water through the reticle it was observed, in a 2D simulation with the Iber v2.5 software, that the draft remains constant. It was also noted that the reticle is an efficient way to dissipate energy. In this way it was discovered that the channel lattice is a solution to the equations of a shallow water joint. Through the results obtained, a field of research is intuited when modeling reticles of channels for the use and management of water.

On the vibration behavior of rotatory blades: a numerical approach using the finite element method and the singular value decomposition

María Jesús Samper — Wed, 27 Nov 2019 15:42:39 +0100

The physics lying behind rotordynamics is complex tomodel, so that in many cases numerical
processing is the only feasible approach. Being rotordynamics a field of great interest
in the aerospace industry, the efforts devoted to its understanding are increasing day by
day. Following this tendency, the aim of the present study is to develop a simplified elastodynamic
model for the case of rotating structures such that can be addressed through
numerical tools, built using the finite element method. For the purpose of analysing the
vibration phenomena, modal decomposition and numerical integration have been taken
advantage from. In this context, it has been found that the singular value decomposition
could be applied in structural analysis to extract dominant displacement fluctuations, allowing
the unfolding of global properties of the dynamic response. In the present report,
the singular value decomposition has been applied to cantilever beams undergoing a single
rotation, giving rise to reasonably satisfactory results.

Evaluation of alternatives for parameterization of momentum and water vapor roughness lengths in lakes

Fernando Armani — Tue, 19 Nov 2019 20:13:07 +0100

An assessment of the performance of various parameterization alternatives for roughness lengths of momentum and water vapour was performed. The micrometereological data used in this work was measured in the Itaipu reservoir, Brazil. Four models was tested for momentum roughness length, and four for water vapour. The parameterization for horizontal momentum flux are based on the Charnock equation and generalizations while the water vapour model are based on equations proposed by Brutsaert. The four models for momentum flux produced very similar results, in terms of goodness of fit and errors, and turned out only weakly dependent on friction velocity. Already the parameterizations for water vapor produced more dispersed results, and the best parameterizations found very poorly depend on the Reynolds roughness number, or are independent of it. Both in the case of momentum and water vapour, the optimal parameter values? of each parameterization found for Itaipu are significantly larger than those reported in the literature.

Matching user accounts based on location verification across social networks

Xueyan Zhou — Thu, 31 Oct 2019 11:43:20 +0100

Social network has become the main platform for people to obtain information and connect with each other. Matching user accounts can help us build better users’ profiles and benefit many applications. It has attracted much attention from both industry and academia. At present, cross-platform user identification can be divided into three categories: based on user basic attribute information, user online social structure relationship and user behavior. Research on mobile social networks is a kind of dynamic mixed data analysis. Due to the remarkable heterogeneity of its data across platforms and the incomplete and untrue information caused by users’ concealment behavior, the recognition rate of the algorithm is relatively low. The paper provides a new matching user accounts method based on location verification. First, the self-centered network algorithm is applied to find cross-network edges in the respective networks of the two users to be matched, which is taken as the initial similarity value of the two users. Secondly, the longitude, latitude and time coordinates of a single platform node were used to modify the similarity. Specific 5 time points were selected within 24 hours and the error range of 10min was taken as the calculation method of great circle distance. Thirdly, since the user did not log in a certain social platform in a certain period of time, the convolutional neural network algorithm was adopted to mark the trajectory. Finally, all users in the whole network are identified by iterative operation. Experimental results on artificial random networks and real social networks show that the proposed algorithm has a high readiness rate and recall rate.

Non-linear analyses to assess the seismic performance of RC buildings retrofitted with FRP

Juan-Carlos Vielma-Perez — Sat, 26 Oct 2019 14:07:04 +0200

Post-seismic reinforcement is extensively used to repair structural elements of buildings hit by strong earthquakes, while pre-seismic reinforcement can be very convenient to improve the seismic performance of buildings designed according to less stringent standards than current. One of the most commonly used techniques for both pre and post-seismic retrofit of structures is that of wrapping elements previously identified as weak or damaged with sheets of fiber-reinforced polymers (FRP). This technique may be in fact more advantageous than other strengthening approaches, due to speed of placement, low environmental impact and small load increase. Non-linear methods of analysis can be very helpful for planning the retrofit strategy and assessing in advance its effectiveness. Based on non-linear static and dynamic numerical analyses, the effectiveness of the FRP pre-seismic reinforcement on the global performance of buildings was investigated in this paper with reference to a residential Italian building designed according to obsolete standards. The seismic capacity of the building, before and after the FRP retrofit, was assessed and the efficacy of the seismic rehabilitation was evaluated. The results show that, by improving the local resistance of the most vulnerable elements and the global ductility of the building, the FRP wrapping of the ground-floor columns of the considered building has an important impact on the overall seismic response of the structure, although it cannot entirely avoid undesirable global collapse mechanisms.

Determination of independent damping frequency in civil structures even under very close modes of vibration by applying optimization techniques

LUIS MANUEL VILLA GARCIA — Fri, 18 Oct 2019 18:15:43 +0200

Specific computer programs for identification in modal analysis of structures are not economical, and they may not always be available. This paper presents a technique for simple optimization, getting fast and quality results, in the estimation of the damping ratios for each separate mode, and that can be resolved with a demo version (downloadable for free from the internet) of commercial optimization software. It is suitable for low levels of structural damping (or non-frequency-dependent damping), which are those that appear in large structures. Likewise, it is not necessary for the modes to be separate, or even that the magnitude of the near modes be less than that of those with which we are working; within the algorithm movable residual complements optimize their position according to the frequency and vertical axes (the response magnitude), to take into account the influence of the modes outside the considered range. In addition, it is not very sensitive to the presence of noise in the signal.

Assessment of one equation model for natural convection and the prediction of impingement heat transfer

Mohamed Omri — Sun, 29 Sep 2019 16:07:09 +0200

Spallart and Allmaras [1] is a one-equation turbulence model developed for predicting complex aerodynamic flows. It will be of great interest to extend its application to predict other kind of flows in presence of heat and mass transfer. Specifically we will demonstrate, by considering two applications, that this one equation model may replace the two equation models in several heat transfer applications such as HVAC, electronics cooling and fire propagation in buildings. This will reduce computing time and numerical stiffness in CFD calculations. The first application is the 2D natural turbulent convection in a cavity differentially heated; which was considered as a benchmark case before applying CFD in any HVAC configuration. The exact configuration proposed by Ampofo [2], was then modeled and the radiation effect was studied. The second application proposed by Riera [4] was the numerical prediction of a heat exchanger based on hybrid jet impingement/micro-channel; it was used for managing high heat-flux thermal of power electronics devices. Those applications deal with low and high fluxes transfer respectively and allow demonstrating clearly the capacity of the one equation model to predict confined turbulent natural/ forced convection and jet-impingement heat transfer.

Study of the aerodynamic forces on a simplified shape X-43 aircraft under supersonic conditions, design optimization

Jordi Pons Prats — Fri, 20 Sep 2019 20:19:02 +0200

The paper focuses on the shape optimization, based on the resultant aerodynamic forces, of a simplified X-43 aircraft. The equations of oblique shock waves and Prandtl-Meyer expansion waves, along with the improvement of the aircraft geometry using a genetic algorithm optimisation, will allow to shape and optimize the external dimensions of the bi-dimensional simplified aircraft design. The main purpose of the optimisation is the reduction of the drag force while maintaining a sufficient lift to hold the weight of the aircraft. The optimization is undertaken at four different altitudes, obtaining for each altitude the most favourable aircraft shape to maximize the aircraft efficiency. The study can be extended to various possible aircraft weight configurations.

Pipeline Information Modeling: A New Tool for Pipeline Engineering

Bao Tian — Tue, 27 Aug 2019 09:50:57 +0200

Pipeline Engineering is a comprehensive system project with huge investment, high complexity, strong technical and strict security requirements. The conventional construction and management mode for pipeline project has encountered a variety of bottlenecks in the development, and digital informatization of pipeline is the inevitable trend for the future pipeline industry development. Therefore, combining with the existing technical achievements, this paper puts forward and defines a new concept- “Pipeline Information Modeling (PIM)”, which can use digital model for the whole life-cycle of pipeline management. Its characteristics and architecture are expounded, and the relationship and difference are summarized between PIM and other related theories such as BIM, pipeline integrity management, and digital pipeline. Meanwhile, a case study of several applications in long-distance oil & gas transmission pipeline engineering is analyzed. Finally, the challenges PIM will face in the future development are discussed. PIM is still in its infancy, and theoretical system and technical method still need to be constantly perfected. But its emergence has important significance for improving pipeline engineering scientific and technological level and accelerating transformation of industry informatization, which will have great application foreground. '''Key words:''' Pipeline engineering; Life-cycle; Digital model; Long-distance pipeline;

Time-scale analysis based modal identification using mobile sensors

Mario Marmolejo — Wed, 21 Aug 2019 21:05:08 +0200

'Civil structures, once built and released for use, are expected to fulfil operational standards for extended periods. The fact, a civil structure´s life, is determined by temporary deterioration, harsh ambient, misuse, destruction, and, eventual rehabilitation for current use or conservation for historical reasons. Initially, vibrational analysis with stationary sensors, broad logistics and a large array of sensors to provide densely detailed data were required. The next evolutionary step; the simplification of the while-under-use data-acquisition stage; better, faster, more detailed parameters for diagnosis. With the structure under normal operation, a stationary reference sensor, a mobile sensor able to travel through specific trajectories over the structure, deep computational data analysis provides better, deeper, clearer frequencies, mode shapes and damping ratios. A structural health monitoring system should be able to reliably assess structural wellness under operational conditions. Such are the benefits of a mobile sensor monitoring and diagnosis system. This paper utilizes the time–scale analysis method and the Continuous Wavelet Transform (CWT) through a complex Morlet wavelet to extract modal parameters from the signal responses. The theoretical development is validated by numeric simulations on a simply supported beam and by double experimental confirmation on a pedestrian bridge at Universidad del Valle, in Cali (Colombia), by the technique herein, and by a stationary sensor scheme.

Hybrid numerical scheme for simulation of transient flow in a pluvial water reservoir

Sara Coelho da Silva — Wed, 21 Aug 2019 16:25:29 +0200

This work presents the numeric simulation of flow in a reservoir, using the finite volume method for solver the system of equations that model the two-dimensional flow in shallow water, neglecting the tangential tensions. The solution of the system of linearized equations was obtained by computational implementation of the Gauss-Seidel iterative method. To illustrate the application of the numerical scheme in hydraulic engineering, it was considered the study of flow in an underground reservoir with internal pillars, whose flow is generated by the opening of two outlet gates. The employed code allowed the temporal analysis of the volume and the flow in the reservoir, the graphical interpretation of the investigated physical phenomenon as well as, the calculation of the precision of the model. The results obtained show the expected physical interpretation, good agreement with literature data, good precision,stability and low computational cost.

Evaluation of the optical contamination of open path CO2 gas analyzers in measurements on a freshwater surface

Fernando Armani — Sun, 14 Jul 2019 15:51:56 +0200

Measurements of air-sea CO₂ fluxes with Eddy Covariance (EC) are generally affected by the optical contamination of open path gas analyzers. Since the conditions of saltwater environments may be similar to those of freshwater environments, in this work we evaluate whether this contamination is occurring in the LI-7500 open path CO₂ gas analyzer (''LI-COR Biosciences Ltd''), used in an experiment to measure greenhouse gas fluxes over the reservoir of the Itaipu Hydroelectric Power Plant. We evaluated the optical contamination indicators in the CO₂ measurements and two corrections for this: the numerical method called PKT method, proposed for CO₂ flux correction over ocean, and a multiple linear regression model. We find that the PKT method does not produce physically plausible results and that, contrary to what has been reported in ocean measurements, the measurements of the Itaipu experiment suffer less interference in more humid conditions. CO₂ correction by the regression model preserved the pattern of CO₂ flux daily variation, but it attenuated the magnitude of the fluxes: the average of the uncorrected fluxes was $-24.35$ μgm^-2s^-1, while the average of the fluxes corrected by the statistical model was $-11.73$ μgm^-2s^-1.

A matheuristic approach to solve the fixed charge transportation problem

Crisiane Oliveira — Thu, 11 Jul 2019 19:49:03 +0200

The Fixed Charge Transportation Problem (FCTP) is a Linear Programming (LP) class, whereby the total shipping cost of a product, from a source to a destination, consists of a unit transportation cost, proportional to the amount of sent items and a fixed charge associated with the opening of the route. The FCTP is NP-hard and has a characteristic in which, as far as the difference between the value of the unit cost and the fixed charge increases, the computational time changes, worsening the performance. This paper purpose a matheuristic approach to the FCTP which results were good when compared to other methods available in the Literature.

Numerical approach to determine the capacity curves of clay masonry walls with openings

Jaime Campbell — Mon, 01 Jul 2019 23:49:11 +0200

In this work a numerical model to obtain the pushover curves of clay masonry walls with openings is presented. These curves represent the non-linear structural behavior of masonry walls with openings subjected to loads in their own plane. The openings represent non-structural elements like doors, windows, among others. These walls can be considered as reinforced or confined masonry and the opening must be rectangular and may have different dimensions and a variable position inside the wall. The main result of the model was a pushover curve, which is compared with the curves obtained from laboratory tests (for walls with and without openings). The model was developed using the software ANSYS [1] version 14.5, based on the model proposed by Campbell and Durán [2].