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• Numerical modeling of the electron beam welding and its experimental validation

  M. Chiumenti, M. Cervera, N. Dialami, B. Wu, L. Jinwei, C. Agelet de Saracibar

  Finite Elements in Analysis and Design (2016). (preprint) Vol. 121, pp. 118–133

  
  

  Abstract

  Electron Beam Welding (EBW) is a highly efficient and precise welding method increasingly used within the manufacturing chain and of growing importance in different industrial environments such as the aeronautical and aerospace sectors. This is because, compared to other welding processes, EBW induces lower distortions and residual stresses due to the lower and more focused heat input along the welding line. This work describes the formulation adopted for the numerical simulation of the EBW process as well as the experimental work carried out to calibrate and validate it. The numerical simulation of EBW involves the interaction of thermal, mechanical and metallurgical phenomena. For this reason, in this work the numerical framework couples the heat transfer process to the stress analysis to maximize accuracy. An in-house multi-physics FE software is used to deal with the numerical simulation. The definition of an ad hoc moving heat source is proposed to simulate the EB power surface distribution and the corresponding absorption within the work-piece thickness. Both heat conduction and heat radiation models are considered to dissipate the heat through the boundaries of the component. The material behavior is characterized by an apropos thermo-elasto-viscoplastic constitutive model. Titanium-alloy Ti6A14V is the target material of this work. From the experimental side, the EB welding machine, the vacuum chamber characteristics and the corresponding operative setting are detailed. Finally, the available facilities to record the temperature evolution at different thermo-couple locations as well as to measure both distortions and residual stresses are described. Numerical results are compared with the experimental evidence.

  Abstract
  Electron Beam Welding (EBW) is a highly efficient and precise welding method increasingly used within the manufacturing chain and of growing importance in different industrial [...]

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• Evaluación de la vulnerabilidad sísmica de estructuras de acero residenciales del Ecuador

  J. Vielma, M. Cando

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2015). MIS70, ISBN: 978-84-943928-1-8

  
  

  Abstract

  Nowadays, there are a large number works focused on determining the seismic vulnerability of structures. This feature is not accidental, but rather the product of concern to the scientific, professional and organizations responsible for risk management community to reduce the destructive effects caused by earthquakes in the infrastructure and society. Globally destructive events that have occurred during the previous and present decades have contributed to this initiative; among them, the Sichuan, China (2008), Port au Prince, Haiti (2010), Maule, Chile (2010) or Tohoku, Japan (2011) earthquakes can be mentioned, which had as consequences many dead, injured and homeless people. It is well known that an earthquake can cause humanitarian disasters not only by the number of casualties and injured people but also for the destruction of cities where thousands of families live and work. All the above mentioned reasons reveal the need of revision and improvement of the seismic design codes, construction practices and policies that allow buildings to have a better performance against the strong ground motions. This is the context in which this work was developed. It contains results of a research entitled "Evaluation of seismic vulnerability of residential steel structures designed according to current Ecuadorian codes and the AISC seismic provisions for structural steel buildings" that has been developed in Ecuador with the support of Prometeo Project, sponsored by the Ministry of Science, Technology and Innovation (SENESCYT) of that Country.

  Abstract
  Nowadays, there are a large number works focused on determining the seismic vulnerability of structures. This feature is not accidental, but rather the product of concern [...]

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• Explicit mixed strain-displacement finite elements for compressible and quasi-incompressible elasticity and plasticity

  M. Cervera, N. Lafontaine, R. Rossi, M. Chiumenti

  Computational Mechanics (2016). (Preprint) Vol. 58 (3), pp. 511–532

  
  

  Abstract

  This paper presents an explicit mixed finite element formulation to address compressible and quasi-incompressible problems in elasticity and plasticity. This implies that the numerical solution only involves diagonal systems of equations. The formulation uses independent and equal interpolation of displacements and strains, stabilized by variational subscales. A displacement sub-scale is introduced in order to stabilize the mean-stress field. Compared to the standard irreducible formulation, the proposed mixed formulation yields improved strain and stress fields. The paper investigates the effect of this enhancement on the accuracy in problems involving strain softening and localization leading to failure, using low order finite elements with linear continuous strain and displacement fields (P1P1 triangles in 2D and tetrahedra in 3D) in conjunction with associative frictional Mohr–Coulomb and Drucker–Prager plastic models. The performance of the strain/displacement formulation under compressible and nearly incompressible deformation patterns is assessed and compared to analytical solutions for plane stress and plane strain situations. Benchmark numerical examples show the capacity of the mixed formulation to predict correctly failure mechanisms with localized patterns of strain, virtually free from any dependence of the mesh directional bias. No auxiliary crack tracking technique is necessary.

  Abstract
  This paper presents an explicit mixed finite element formulation to address compressible and quasi-incompressible problems in elasticity and plasticity. This implies that [...]

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• Contribuciones a la evaluación de la vulnerabilidad sísmica de edificios

  J. Vielma

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2014). MIS68, ISBN: 978-84-941686-9-7

  
  

  Abstract

  Una de las principales preocupaciones de los especialistas relacionados con el medio construido tiene que ver con la capacidad de las estructuras para soportar acciones excepcionales. Entre éstas está el efecto de los fenómenos ambientales y naturales que imponen cargas extraordinarias que, de forma inesperada, pueden afectar a los edificios, provocando una respuesta que puede conducir al daño de componentes estructurales y no estructurales o, incluso, al colapso de la estructura. Entre las acciones extraordinarias que pueden afectar las estructuras dentro de la ventana de vida útil se encuentran las producidas por el viento y los terremotos, entre los más importantes. Tanto el daño parcial como el colapso de los edificios pueden afectar a las personas que habitan o realizan actividades en estas, produciéndo heridos o incluso fallecidos y enormes pérdidas económicas. Dentro de este contexto se han venido desarrollando trabajos de investigación que han tenido como objetivo determinar la vulnerabilidad sísmica de los edificios, con el fin de conocer cuál sería su comportamiento frente a la acción hipotética de un sismo determinado. En esta monografía se recogen contribuciones en este campo que se organizan en nueve capítulos.

  Abstract
  Una de las principales preocupaciones de los especialistas relacionados con el medio construido tiene que ver con la capacidad de las estructuras para soportar acciones excepcionales. [...]

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• High-fidelity prediction of crack formation in 2D and 3D pullout tests

  L. Benedetti, M. Cervera, M. Chiumenti

  Computers and Structures (2016). (preprint) Vol. 172, pp. 93–109

  
  

  Abstract

  This paper presents the 2D and 3D numerical analysis of pullout tests on steel anchorages in concrete blocks using standard and mixed finite elements. A novel (stabilized) mixed formulation in the variables of total strain ε and displacements u is introduced to overcome the intrinsic deficiencies of the standard displacement-based one in the context of localization of strains, such as mesh dependency. The quasi-brittle behavior of concrete is described through an elastoplastic constitutive law with a local Rankine yielding criterion. The proposed formulation is shown to be a reliable and accurate tool, sensitive to the physical parameters of the pullout tests, but objective with respect to the adopted FE mesh. Furthermore, the mixed ε/u finite element is able to capture the correct failure mechanism with relatively coarse discretizations. At the same time, the spurious behavior of the standard formulation is not alleviated by mesh-refinement.

  Abstract
  This paper presents the 2D and 3D numerical analysis of pullout tests on steel anchorages in concrete blocks using standard and mixed finite elements. A novel (stabilized) [...]

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• Analysis of the effect of provisional ties in the construction and current deformation of Mallorca cathedral

  L. Pelà, J. Bourgeois, P. Roca, M. Cervera, M. Chiumenti

  International Journal of Architectural Heritage (2016). (preprint) Vol. 10 (4), pp. 418–437

  
  

  Abstract

  This article presents the analysis of the structure of Mallorca Cathedral taking into account the influence on the structural behavior of auxiliary iron ties used during the construction process. Recent studies (Roca et al. 2012, 2013) presented some hypotheses about the construction process of the cathedral. This article complements the previous results by considering the use of auxiliary ties as temporary stabilizing device during the construction. Evidence of the use of ties during the construction has been recognized after a comprehensive survey. The study of the role of such ties and the effect of their later removal are studied by a FE analysis carried out on a representative bay of the structure. The study includes a time-dependent FE analysis after the removal of the ties to assess the long-term structural behavior. The results of the numerical analysis are compared with the deformation trends identified by means of a recent monitoring campaign.

  Abstract
  This article presents the analysis of the structure of Mallorca Cathedral taking into account the influence on the structural behavior of auxiliary iron ties used during the [...]

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• A thermodynamically consistent plastic-damage framework for localized failure in quasi-brittle solids: Material model and strain localization analysis

  J. Wu, M. Cervera

  Int. J. Solids and Structures (2016). (preprint) Vol. 88–89, pp. 227–247

  
  

  Abstract

  Aiming for the modeling of localized failure in quasi-brittle solids, this paper addresses a thermodynamically consistent plastic-damage framework and the corresponding strain localization analysis. A unified elastoplastic damage model is first presented based on two alternative kinematic decompositions assuming infinitesimal deformations, with the evolution laws of involved internal variables characterized by a dissipative flow tensor. For the strong (or regularized) discontinuity to form in such inelastic quasi-brittle solids and to evolve eventually into a fully softened one, a novel strain localization analysis is then suggested. A kinematic constraint more demanding than the classical discontinuous bifurcation condition is derived by accounting for the traction continuity and the loading/unloading states consistent with the kinematics of a strong (or regularized) discontinuity. More specifically, the strain jumps characterized by Maxwell’s kinematic condition have to be completely inelastic (energy dissipative). Reproduction of this kinematics implies vanishing of the aforesaid dissipative flow tensorial components in the directions orthogonal to the discontinuity orientation. This property allows naturally developing a localized plastic-damage model for the discontinuity (band), with its orientation and the traction-based failure criterion consistently determined a posteriori from the given stress-based counterpart. The general results are then particularized to the 2D conditions of plane stress and plane strain. It is found that in the case of plane stress, strain localization into a strong (or regularized) discontinuity can occur at the onset of strain softening. Contrariwise, owing to an extra kinematic constraint, in the condition of plane strain some continuous inelastic deformations and substantial re-orientation of principal strain directions in general have to take place in the softening regime prior to strain localization. The classical Rankine, Mohr–Coulomb, von Mises (J2) and Drucker–Prager criteria are analyzed as illustrative examples. In particular, both the closed-form solutions for the discontinuity angles validated by numerical simulations and the corresponding traction-based failure criteria are obtained.

  Abstract
  Aiming for the modeling of localized failure in quasi-brittle solids, this paper addresses a thermodynamically consistent plastic-damage framework and the corresponding strain [...]

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• Challenges in thermo-mechanical analysis of Friction Stir Welding processes

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

  Archives of Comp. Meths. Engng. (2017). (preprint) Vol. 24 (1), pp. 198-225

  
  

  Abstract

  This paper deals with the numerical simulation of friction stir welding (FSW) processes. FSW techniques are used in many industrial applications and particularly in the aeronautic and aerospace industries, where the quality of the joining is of essential importance. The analysis is focused either at global level, considering the full component to be jointed, or locally, studying more in detail the heat affected zone (HAZ). The analysis at global (structural component) level is performed defining the problem in the Lagrangian setting while, at local level, an apropos kinematic framework which makes use of an efficient combination of Lagrangian (pin), Eulerian (metal sheet) and ALE (stirring zone) descriptions for the different computational sub-domains is introduced for the numerical modeling. As a result, the analysis can deal with complex (non-cylindrical) pin-shapes and the extremely large deformation of the material at the HAZ without requiring any remeshing or remapping tools. A fully coupled thermo-mechanical framework is proposed for the computational modeling of the FSW processes proposed both at local and global level. A staggered algorithm based on an isothermal fractional step method is introduced. To account for the isochoric behavior of the material when the temperature range is close to the melting point or due to the predominant deviatoric deformations induced by the visco-plastic response, a mixed finite element technology is introduced. The Variational Multi Scale method is used to circumvent the LBB stability condition allowing the use of linear/linear P1/P1 interpolations for displacement (or velocity, ALE/Eulerian formulation) and pressure fields, respectively. The same stabilization strategy is adopted to tackle the instabilities of the temperature field, inherent characteristic of convective dominated problems (thermal analysis in ALE/Eulerian kinematic framework). At global level, the material behavior is characterized by a thermo–elasto–viscoplastic constitutive model. The analysis at local level is characterized by a rigid thermo–visco-plastic constitutive model. Different thermally coupled (non-Newtonian) fluid-like models as Norton–Hoff, Carreau or Sheppard–Wright, among others are tested. To better understand the material flow pattern in the stirring zone, a (Lagrangian based) particle tracing is carried out while post-processing FSW results. 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.

  Abstract
  This paper deals with the numerical simulation of friction stir welding (FSW) processes. FSW techniques are used in many industrial applications and particularly in the aeronautic [...]

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• Modelling of Bingham and Herschel–Bulkley flows with mixed P1/P1 finite elements stabilized with orthogonal subgrid scale

  E. Moreno, A. LARESE, M. Cervera

  Journal of Non-Newtonian Fluid Mechanics (2016). (preprint) Vol. 228, pp. 1-16

  
  

  Abstract

  This paper presents the application of a stabilized mixed pressure/velocity finite element formulation to the solution of viscoplastic non-Newtonian flows. Both Bingham and Herschel–Bulkley models are considered. The detail of the discretization procedure is presented and the Orthogonal Subgrid Scale (OSS) stabilization technique is introduced to allow for the use of equal order interpolations in a consistent way. The matrix form of the problem is given. A series of examples is presented to assess the accuracy of the method by comparison with the results obtained by other authors. The extrusion in a Bingham fluid and the movement of a moving and rotating cylinder are analyzed in detail. The evolution of the streamlines, the yielded and unyielded regions, the drag and lift forces are presented. These benchmark examples show the capacity of the mixed OSS formulation to reproduce the behavior of a Bingham and Herschel–Bulkley flows with the required accuracy.

  Abstract
  This paper presents the application of a stabilized mixed pressure/velocity finite element formulation to the solution of viscoplastic non-Newtonian flows. Both Bingham and [...]

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• Respuesta dinámica de un edificio considerando el efecto de interacción suelo-estructura

  F. Diaz-Guzman, F. Espinoza, R. Sanchez, C. Huerta

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2012). MIS67, ISBN: 978-84-939640-1-6

  
  

  Abstract

  En esta monografía se presenta el análisis de un edificio de 12 niveles de elementos prefabricados en la ciudad de Ensenada donde se determina la respuesta dinámica considerando al edificio apoyado en base rígida y teniendo en cuenta el efecto de interacción suelo-estructura, en el cual se considera a la estructura apoyada en suelo blando. Se lleva a cabo un análisis lineal para evaluar la respuesta dinámica de la estructura bajo condiciones de cargas gravitacionales establecidas en un reglamento mexicano, específicamente, en el Capítulo 5 y 6 de las Normas Técnicas Complementarias sobre Criterios y Acciones para el Diseño Estructural de las Edificaciones del DF (2004). Se realiza la obtención y distribución de las fuerzas sísmicas laterales de la estructura apoyada en base rígida utilizándose los criterios de diseño y análisis estructural del Capítulo 9 de las Normas Técnicas Complementarias para Diseño por Sismo del DF (2004), que corresponden al análisis dinámico modal. La respuesta del edificio producida por el efecto de interacción suelo-estructura se determina aplicando el método mencionado en el apéndice A de estas mismas normas. Se evalúa la respuesta de sitio donde se recurre al uso de la vibración ambiental y posteriormente, mediante el método de Nakamura, se obtiene el periodo predominante del suelo necesario para estimar la relevancia del efecto de interacción suelo-estructura. En el transcurso de esta monografía, se presentan estudios de algunos autores sobre la consideración y consecuencias del efecto de interacción suelo-estructura en el análisis sísmico de edificios. Además se muestran los procedimientos a seguir para determinar la respuesta dinámica de la estructura en estudio con base rígida y considerando el efecto de interacción suelo-estructura. También se presentan las distorsiones de entrepiso, que se determinan a partir de los desplazamientos relativos de cada nivel, para la estructura con base rígida y considerando el efecto de interacción suelo-estructura. Por último se hizo la comparación de ambas distorsiones con los valores máximos permitidos de las Normas Técnicas Complementarias para Diseño por Sismo del DF (2004) establecidos en la sección 1.8 y apéndice A.4.

  Abstract
  En esta monografía se presenta el análisis de un edificio de 12 niveles de elementos prefabricados en la ciudad de Ensenada donde se determina la respuesta [...]

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