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• An energy-equivalent d+/d− damage model with enhanced microcrack closure-reopening capabilities for cohesive-frictional materials

  M. Cervera, C. Tesei

  Materials (2017). (preprint) Vol. 10 (4), pp. 433

  
  

  Abstract

  In this paper, an energy-equivalent orthotropic d+/d− damage model for cohesive-frictional materials is formulated. Two essential mechanical features are addressed, the damage-induced anisotropy and the microcrack closure-reopening (MCR) effects, in order to provide an enhancement of the original d+/d−model proposed by Faria et al. 1998, while keeping its high algorithmic efficiency unaltered. First, in order to ensure the symmetry and positive definiteness of the secant operator, the new formulation is developed in an energy-equivalence framework. This proves thermodynamic consistency and allows one to describe a fundamental feature of the orthotropic damage models, i.e., the reduction of the Poisson’s ratio throughout the damage process. Secondly, a “multidirectional” damage procedure is presented to extend the MCR capabilities of the original model. The fundamental aspects of this approach, devised for generic cyclic conditions, lie in maintaining only two scalar damage variables in the constitutive law, while preserving memory of the degradation directionality. The enhanced unilateral capabilities are explored with reference to the problem of a panel subjected to in-plane cyclic shear, with or without vertical pre-compression; depending on the ratio between shear and pre-compression, an absent, a partial or a complete stiffness recovery is simulated with the new multidirectional procedure.

  Abstract
  In this paper, an energy-equivalent orthotropic d+/d− damage model for cohesive-frictional materials is formulated. Two essential mechanical features are addressed, [...]

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• 3D numerical modelling of twisting cracks under bending and torsion of skew notched beams

  L. Benedetti, M. Cervera, M. Chiumenti

  Engineering Fracture Mechanics (2017). (preprint) Vol. 176, pp. 235–256

  
  

  Abstract

  The testing of mode III and mixed mode failure is every so often encountered in the dedicated literature of mechanical characterization of brittle and quasi-brittle materials. In this work, the application of the mixed strain displacement ε-u finite element formulation to three examples involving skew notched beams is presented. The use of this FE technology is effective in problems involving localization of strains in softening materials. The objectives of the paper are: (i) to test the mixed formulation in mode III and mixed mode failure and (ii) to present an enhancement in terms of computational time given by the kinematic compatibility between irreducible displacement-based and the mixed strain-displacement elements. Three tests of skew-notched beams are presented: firstly, a three point bending test of a PolyMethyl MethaAcrylate beam; secondly, a torsion test of a plain concrete prismatic beam with square base; finally, a torsion test of a cylindrical beam made of plain concrete as well. To describe the mechanical behavior of the material in the inelastic range, Rankine and Drucker-Prager failure criteria are used in both plasticity and isotropic continuum damage formats. The proposed mixed formulation is capable of yielding results close to the experimental ones in terms of fracture surface, peak load and global loss of carrying capability. In addition, the symmetric secant formulation and the compatibility condition between the standard irreducible method and the strain-displacement one is exploited, resulting in a significant speedup of the computational procedure.

  Abstract
  The testing of mode III and mixed mode failure is every so often encountered in the dedicated literature of mechanical characterization of brittle and quasi-brittle [...]

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• A fast and accurate two-stage strategy to evaluate the effect of the pin tool profile on metal flow, torque and forces during friction stir welding

  N. Dialami, M. Cervera, M. Chiumenti, C. Agelet de Saracibar

  Int. J. Mechanical Sciences (2017). (preprint) Vol. 122, pp. 215–227

  
  

  Abstract

  Pin geometry is a fundamental consideration in friction stir welding (FSW). It influences the thermal behaviour, material flow and forces during the weld and reflects on the joint quality. This work studies four pin tools with circular, triflute, trivex, and triangular profiles adopting a validated model of FSW process developed by the authors. The effect of the rotating tool geometry on the flow behaviour and process outcomes is analysed. Additionally, longitudinal and transversal forces and torque are numerically calculated and compared for the different pin shapes. The study is carried out for slip and stick limiting friction cases between pin and workpiece. The main novelties of the paper are a “speed-up” two-stage simulation methodology and a piecewise linear version of the constitutive model, both of them conceived for the use in real case industrial applications, where the achievement of accuracy with affordable simulation times is of importance. The Norton-Hoff constitutive model is adopted to characterize the material behaviour during the weld. The piecewise linear version of the model developed by the authors greatly facilitates the convergence of the numerical solution ensuring both computational efficiency and accuracy. A two-stage computational procedure is applied. In the first stage, a forced transient is carried out; in the second one, the magnitudes of interest are computed. The study shows that the proposed modelling approach can be used to predict and interpret the FSW behaviour for a specific pin geometry. Moreover, the reduction of the simulation time using the two-stage strategy can be up to 90%, compared to a standard single stage strategy.

  Abstract
  Pin geometry is a fundamental consideration in friction stir welding (FSW). It influences the thermal behaviour, material flow and forces during the weld and [...]

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• Finite element modelling of internal and multiple localized cracks

  S. Salaustros, L. Pelà, M. Cervera, P. Roca

  Computational Mechanics (2017). (preprint) Vol. 59 (2), pp. 299–316

  
  

  Abstract

  Tracking algorithms constitute an efficient numerical technique for modelling fracture in quasi-brittle materials. They succeed in representing localized cracks in the numerical model without mesh-induced directional bias. Currently available tracking algorithms have an important limitation: cracking originates either from the boundary of the discretized domain or from predefined “crack-root” elements and then propagates along one orientation. This paper aims to circumvent this drawback by proposing a novel tracking algorithm that can simulate cracking starting at any point of the mesh and propagating along one or two orientations. This enhancement allows the simulation of structural case-studies experiencing multiple cracking. The proposed approach is validated through the simulation of a benchmark example and an experimentally tested structural frame under in-plane loading. Mesh-bias independency of the numerical solution, computational cost and predicted collapse mechanisms with and without the tracking algorithm are discussed.

  Abstract
  Tracking algorithms constitute an efficient numerical technique for modelling fracture in quasi-brittle materials. They succeed in representing localized cracks in the numerical [...]

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• Numerical simulation and experimental calibration of Additive Manufacturing by blown powder technology. Part I: thermal analysis

  M. Chiumenti, X. Lin, M. Cervera, W. Lei, Y. Zheng, W. Huang

  Rapid Prototyping Journal (2017). (preprint) Vol. 23 (2), pp. 448-463

  
  

  Abstract

  In this work the numerical simulation of Additive Manufacturing (AM) processes is addressed. The numerical results are compared with the experimental campaign carried out at SKLSP laboratories, where a Laser Solid Forming (LSF) machine, also referred to as Laser Engineered Net Shaping (LENS), is used to fabricate metal parts directly from CAD models. Ti-6Al-4V metal powder is injected into the molten pool created by a focused, high-energy laser beam and a layer of added material is sinterized according to the laser scanning pattern speciÖed by the user. The objectives of this paper are twofold: Örstly, this work is intended to calibrate the software for the numerical simulation of the AM process, to achieve high accuracy. Secondly, the sensitivity of the numerical model to the process parameters and modelling data is analysed. The numerical model adopts an apropos FE activation technology, which reproduces the same scanning pattern set for the numerical control system of the AM machine. This consists of a complex sequence of polylines, used to deÖne the contour of the component, and hatches patterns to Öll the inner section. The full sequence is given through the Common Layer Interface (CLI) format, a standard format for di§erent manufacturing processes such as Rapid Prototyping (RP), Shape Metal Deposition (SMD) or machining processes, among others. The result is a layer-by-layer metal deposition which can be used to build-up complex structures for components such as turbine blades, aircraft sti§eners, cooling systems, or medical implants, among others.

  Abstract
  In this work the numerical simulation of Additive Manufacturing (AM) processes is addressed. The numerical results are compared with the experimental campaign carried out [...]

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• Análisis sísmico de estructuras con disipadores de energía ADAS o TADAS

  R. Aguiar, M. Rodríguez, D. Mora

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2016). MIS75, ISBN: 978-84-945077-5-5

  
  

  Abstract

  Existe abundante bibliografía sobre disipadores de energía sísmicos, ya que con estos dispositivos se puede reforzar estructuras o contemplarles en el diseño para controlar los desplazamientos y fuerzas debidos a un sismo severo. Pero la mayor parte de esta bibliografía está dirigida a investigadores o profesionales que vienen trabajando en el tema, de tal manera que para un estudiante de pre grado o post grado, que se inicia en esta temática le va a ser bastante difícil entender a ciencia cierta la forma de análisis y diseño sísmico de estructuras con disipadores de energía. Por este motivo se ha escrito esta publicación que trata fundamentalmente sobre el análisis sísmico espacial, de estructuras con disipadores de energía ADAS (Added damping and stiffnes) y TADAS (Triangular plate added damping and stiffness). Se ha desarrollado el estudio para el caso de que estos disipadores se encuentren sobre un contraviento Chevrón, el mismo que ha sido modelado de dos formas, la primera la simplificada como una diagonal equivalente y la segunda como se verá en el apartado 7 modelando al disipador como un elemento más de la estructura. Se considera un modelo de losa rígida para el análisis sísmico, considerando tres grados de libertad por planta y en el apartado 6 se presenta el método de superposición lineal. Sin embargo y pensando en la aplicación del método del espectro de capacidad en el apartado 7 se presentan dos formas de cálculo de la matriz de rigidez del elemento aislador, la primera en forma analítica exacta y la segunda mediante el método de las dovelas; luego se presentan dos modelos para el análisis sísmico de la estructura con disipadores que difieren en la forma de numeración de los grados de libertad. Finalmente, en el apartado 8 se analiza el conjunto diagonales-disipador; conformado por tres elementos para lo cual al disipador se lo considera como un elemento corto cuya matriz de rigidez en coordenadas globales se halla a partir del análisis de una viga de sección variable de forma en X, para los disipadores ADAS y triangular para los TADAS. El marco teórico, que se presenta en los diferentes apartados, está complementado con el análisis sísmico de una estructura de cuatro pisos de hormigón armado, simétrica, que presenta derivas de piso, superiores a las permitidas por las normativas sísmicas, por lo que ha sido reforzada con disipadores de energía ADAS/TADAS. En los anexos se detalla el procedimiento de cálculo de la estructura con disipadores. Para facilitar el análisis sísmico de estructuras con disipadores de energía ADAS/TADAS se presentan programas del sistema de computación CEINCI-LAB que ayudan notablemente al cálculo y el aprendizaje con la lectura detenida de los mismos.

  Abstract
  Existe abundante bibliografía sobre disipadores de energía sísmicos, ya que con estos dispositivos se puede reforzar estructuras o contemplarles en el [...]

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• Modelización probabilista de efectos de sitio en ciudades y su aplicación en Bogotá

  G. Bernal, O. Cardona

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2016). MIS74, ISBN: 978-84-945077-6-2

  
  

  Abstract

  A methodology for assessing site effects in cities is presented, which is based on the geometry of the prevailing geological formations and their relation to soft soil deposits. Based on a geological interpretation for the area of the city, the elevation (above sea level) of geological contacts is defined, and a three-dimensional geometric model of the subsoil of the city is obtained. The geotechnical information available is used to map soil types to the geometric model. The static and dynamic characteristics of the soil are defined as random variables and their probability moments are calculated using statistics on the geotechnical data available. A computational rectangular grid must be defined over the city and for each node synthetic stratigraphies are constructed with geotechnical parameters obtained following their probability distributions. The dynamic response is calculated for each node of the grid, by a 1D nonlinear analysis (linear equivalent) using a set of Fourier amplitude spectra generated at bedrock level using a source spectrum model, for different combinations of moment magnitude and rupture distance. Site-specific attenuation functions are generated and used to calculate seismic hazard at surface level. Uniform hazard spectra for 475 years return period are obtained. From these spectra harmonized design spectra to the NSR 10 are generated by random search of Fa and Fv parameters to ensure optimum fit. Following this methodology a site effects model for the city of Bogotá, Colombia, is constructed. This is used to exemplify trhee main direct application: i) obtaining transfer functions of the response spectrum for risk assessment, ii) generating Shake- Maps for emergency response purposes, and iii) the seismic microzonation of the city and obtaining elastic seismic-resistant design spectra.

  Abstract
  A methodology for assessing site effects in cities is presented, which is based on the geometry of the prevailing geological formations and their relation to soft soil deposits. [...]

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• Caracterización de propiedades dinámicas de un puente de autopista en el noroeste de México

  M. Calvillo, C. Huerta, F. Espinoza

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2016). MIS73, ISBN: 978-84-945077-4-8

  
  

  Abstract

  Una predicción precisa de la respuesta de estructuras sujetas a sismos, requiere información de sus propiedades dinámicas en sitio, tanto de la estructura como de terreno en su vecindad, incluyendo frecuencias naturales y relaciones de amortiguamiento. En el área de ingeniería civil, hay un creciente interés en procedimientos de análisis modal utilizando señales de salida. Las experiencias reportadas han demostrado que una estimación rápida y confiable de las formas de los modos y frecuencias puede obtenerse, inclusive con cantidades pequeñas de datos. En el presente trabajo se describe el estudio realizado sobre el puente Manantiales ubicado en la carretera de cuota Tijuana-Ensenada, México. En la zona en la que se localiza la estructura se han identificado deslizamientos importantes del suelo, y se han detectado deformaciones en los apoyos, por lo que se hizo necesaria la estimación de las propiedades dinámicas del puente. Por medio de estudios de vibración ambiental se estimó el amortiguamiento y la frecuencia fundamental de la estructura y se hizo una comparación con valores obtenidos teóricamente. Además se obtuvo el patrón de comportamiento de las deflexiones experimentales que se presentaron al centro de cada uno de los claros del puente y se comparó con el estimado teóricamente. Debido a la importancia de conocer las propiedades dinámicas del suelo en el cual se desplanta la estructura, se estimó la función de transferencia de cuatro puntos cercanos al puente Manantiales mediante cocientes espectrales, y se obtuvo el modelo numérico del suelo mediante el método de propagación de ondas en una dimensión, el cual está basado en matrices de rigidez.

  Abstract
  Una predicción precisa de la respuesta de estructuras sujetas a sismos, requiere información de sus propiedades dinámicas en sitio, tanto de la estructura [...]

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• Local–global strategy for the prediction of residual stresses in FSW processes

  N. Dialami, M. Cervera, M. Chiumenti, C. Agelet de Saracibar

  International Journal of Advanced Manufacturing Technology (2017). (preprint) Vol. 88 (9), pp. 3099–3111

  
  

  Abstract

  This work describes the local–global strategy proposed for the computation of residual stresses in friction stir welding (FSW) processes. A coupling strategy between the analysis of the process zone nearby the pin tool (local level analysis) and the simulation carried out for the entire structure to be welded (global level analysis) is implemented to accurately predict the temperature histories and, thereby, the residual stresses in FSW. As a first step, the local problem solves the material stirring as well as the heat generation induced by the pin and shoulder rotation at the heat affected zone. The Arbitrary Lagrangian Eulerian (ALE) formulation is adopted to deal with the rotation of complex pin shapes. A thermo-rigid-viscoplastic constitutive law is employed to characterize the viscous flow of the material, driven by the high-strain rates induced by the FSW process. A mixed temperature–velocity–pressure finite element technology is used to deal with the isochoric nature of the strains. The output of this local analysis is the heat generated either by plastic dissipation or by friction, and it is used as the power input for the welding analysis at structural (global) level. The global problem is tackled within the Lagrangian framework together with a thermo-elasto-viscoplastic constitutive model. In addition, in this case, the mixed temperature–displacement–pressure format is introduced to deal with the deviatoric nature of the plastic strains. The outcomes of this analysis are the distortions and the residual stresses after welding. The material used in this work is stainless steel 304 L; however, the methodology presented is applicable to a wide range of materials. The proposed numerical strategy is validated by the experimental evidence.

  Abstract
  This work describes the local–global strategy proposed for the computation of residual stresses in friction stir welding (FSW) processes. A coupling strategy between [...]

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• Espectros de control para ciudad de Quito-Ecuador

  R. Aguiar

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2015). MIS72, ISBN: 978-84-944244-3-4

  
  

  Abstract

  En esta publicación se presenta, en primer lugar, las fallas ciegas inversas de Quito, denominadas de sur a norte: Puengasí; Ilumbisí-La Bota (ILB); Carcelén el Inca (CI); Bellavista Catequilla (BC) y Tangahuilla. Luego se determina para varios rangos de magnitud, el período de recurrencia de cada segmento de falla empleando el modelo de Gutenberg y Richter modificado y truncado desde una magnitud mínima a una magnitud máxima. Después se indica una zonificación de los suelos de Quito, basada en la velocidad de la onda de corte en los treinta primeros metros. Para el efecto se consideró como base la clasificación de suelos realizada por la Politécnica Nacional en el 2002 y estudios de sísmica de refracción realizados por ERN para la microzonificación sísmica de Quito en el 2011; estudios para el Metro de Quito y estudios realizados por el autor de ésta publicación para reforzar Centros de Educación Municipales. Posteriormente se presentan los modelos de movimientos fuertes desarrollados por Abrahamson et al. (2014); Campbell y Borzognia (2014); Chiou y Youngs (2014), que sirven para encontrar espectros de respuesta elástica y leyes de atenuación del movimiento del suelo de sismos que se registran en fallas corticales.

  Abstract
  En esta publicación se presenta, en primer lugar, las fallas ciegas inversas de Quito, denominadas de sur a norte: Puengasí; Ilumbisí-La Bota (ILB); Carcelén [...]

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