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• Modelización numérica del comportamiento estructural de barras de pandeo restringido

  J. Castro, F. López-Almansa, S. Oller

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2011). MIS66, ISBN: 978-84-95999-12-2

  
  

  Abstract

  Los disipadores de energía son elementos pasivos que se incorporan a edificios y a otras construcciones que, pueden estar sometidas a acciones dinámicas, especialmente movimientos sísmicos. Su finalidad es absorber la mayor parte de la energía introducida en la estructura por la excitación, protegiendo de esta manera la estructura principal. Estos dispositivos son relativamente ajenos a la estructura, en el sentido que no participan en la resistencia a cargas verticales, y, por tanto, son fácilmente reemplazadas después de sufrir daños importantes. Se conectan a la estructura a proteger de forma que cuando ésta sufre la acción del terremoto, se generan deformaciones importantes en los disipadores produciéndose de esta manera la absorción de energía. En estructuras de edificación, los disipadores se colocan en pórticos; habitualmente en barras de arriostramiento concéntrico entre plantas, (generalmente diagonales o en V invertida) ya que, al producirse desplazamientos relativos entre plantas (“interstory drift”), aparecen deformaciones apreciables en estos elementos. Se han propuesto distintos tipos de disipadores para estructuras de edificación. Los basados en plastificación de metales, conocidos habitualmente como histeréticos, destacan por su sencillez, economía y robustez; de entre éstos las denominadas barras de pandeo restringido han conocido un notable desarrollo por poseer ventajas relevantes. Básicamente son barras concéntricas de arriostramiento, constituidas por un núcleo delgado de acero que se rodea de un revestimiento, generalmente de mortero y/o acero. Es de vital importancia que exista un interfaz, entre el núcleo y el revestimiento, que permita el deslizamiento entre ambos para evitar que parte de las estensiones del núcleo se plastifica, ya que el revestimiento impide su pandeo; estos ciclos de plastificación por tracción y compresión cosntituyen los lazos de histéresis a través de los cuales se disipa energía. A pesar de la notable experiencia, tanto a nivel de investigación como de aplicaciones prácticas, que existe sobre las barras de pandeo restringido, aún quedan numerosos interrogantes abiertos. En particular, no han sido propuesto ningún modelo numérico que simule con exactitud y fiabilidad su comportamiento estructural, ya que, éste es notablemente complejo, debido al trabo conjunto y multiaxial de distintos materiales (acero del núcleo,mortero y acero del revestimiento). Esta ausencia impide comprender a fondo los fenómenos complejos que suceden durante la operación de estos elementos, y dificulta el desarrollo de soluciones innovadora, ya que no es posible conocer con exactitud su comportamiento de barras de pandeo restringido formadas por un núcleo de acero, un interfaz deslizante y un revestimiento de mortero eventualmente rodeado por una funda exterior de acero. En el modelo numérico propuesto, el acero tiene un comportamiento elastoplástico con endurecimiento cinemático isótropo. Para el mortero que le rodea se han desarrollado u modelo isótropo de daño. La parte de acero del revestimiento (funda) se homogeneiza al mortero. La interfaz entre el núcleo de acero puede penetrar en el mortero. En el presente trabajo, estos modelos se implementan en subrutinas del programa Abaqus/Explicit, permitiendo comprobar el funcionamiento del modelo que se propone. Los resultados obtenidos, con este modelo numérico, se compararon con los resultados experimentales obtenidos en la Universidad de Girona y en la Universidad de California.

  Abstract
  Los disipadores de energía son elementos pasivos que se incorporan a edificios y a otras construcciones que, pueden estar sometidas a acciones dinámicas, especialmente [...]

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• Theoretical and experimental analysis of dissipative buckling restrained barces

  G. Palazzo, F. López-Almansa, X. Cahís, F. Crisafulli

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2011). MIS64, ISBN: 978-84-96736-98-6

  
  

  Abstract

  Buckling restrained braces are passive energy dissipators used for seismic protection of building frames; such devices consist of slender steel bars connected usually to the frame to be protected either like conventional (concentric) diagonal braces or like chevron braces. Under horizontal seismic motions, the interstory drifts generate axial strains in the steel bars beyond their yielding points; such tension-compression cycles constitute the hysteresis loops. The buckling of the steel bars (core) is prevented by embedding them in a stockiest encasing; it consists usually of a steel tube filled with mortar. A crucial issue is to allow sliding between the core and the encasing to prevent relevant shear stress transfer. This work aims to contribute to a better understanding of the behavior of buckling restrained braces; the final objective is to foster its mass use in developing countries (in earthquake prone regions), particularly for reinforced concrete building frames. The research approach consists of designing, producing and testing (in Argentina) five reduced scale dissipators (about 400 mm long) and of taking profit of the gained experience to design, to produce and to test (in Spain) four full size (near 3000 mm long) prototype devices. All these tests are individual, i.e. no subassemblies (accounting for the building frames) are considered. The main conclusion is that it is possible to obtain a reasonably cheap (about 1000 US$ per unit; this amount corresponds to production in Spain (summer 2006) without optimizing the fabrication process), efficient, robust, low maintenance and durable prototype device requiring only a low-tech production process (suitable for developing countries). Moreover, the results show that the fatigue life of buckling restrained braces, even highly uncertain, can be significantly bigger than expected (according to some previously published results); it might allow extending the life of these devices after a number of strong seismic inputs. A numerical analysis of the buckling behavior of these devices is performed; it allows formulating some design recommendations. Further research needs are identified.

  Abstract
  Buckling restrained braces are passive energy dissipators used for seismic protection of building frames; such devices consist of slender steel bars connected usually to [...]

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• Reliability problems in eartquake engineering

  J. Hurtado

  Monograph Series in Earthquake Engineering, editor A. H. Barbat (2010). MIS63, ISBN: 978-84-96736-86-3

  
  

  Abstract

  This monograph deals with the problem of reliability analysis in the field of Earthquake Engineering. Chapter 1 is devoted to a summary of the most widely used reliability methods, with emphasis on Monte Carlo and solver surrogate techniques used in the subsequent chapters. Chapter 2 presents a discussion of the Monte Carlo from the viewpoint of Information Theory. Then, a discussion is made in Chapter 3 on the selection of random variables in Earthquake Engineering. Next, some practical methods for computing failure probabilities under seismic loads are reported in Chapter 4. Finally, a method for reliability-based design optimization under seismic loads is presented in Chapter 5.

  Abstract
  This monograph deals with the problem of reliability analysis in the field of Earthquake Engineering. Chapter 1 is devoted to a summary of the most widely used reliability [...]

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• 3D numerical models of FSW processes with non-cylindrical pin

  P. Bussetta, N. Dialami, M. Chiumenti, C. Agelet de Saracibar, M. Cervera, J. Ponthot

  Advances in Materials and Processing Technologies (2016). (preprint) Vol. 1 (3-4), pp. 275-287

  
  

  Abstract

  Friction stir welding (FSW) process is a relatively recent welding process (patented in 1991). FSW is a solid-state joining process during which materials to be joined are not melted. During the FSW process, the behaviour of the material is at the interface between solid mechanics and fluid mechanics. In this paper, a 3D numerical model of the FSW process with a non-cylindrical tool based on a solid formulation is compared to another one based on a fluid formulation. Both models use advanced numerical techniques such as the arbitrary Lagrangian–Eulerian formulation, remeshing or the orthogonal sub-grid scale method. It is shown that these two formulations essentially deliver the same results.

  Abstract
  Friction stir welding (FSW) process is a relatively recent welding process (patented in 1991). FSW is a solid-state joining process during which materials to be joined are [...]

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• A crack-tracking technique for localized cohesive-frictional damage

  S. Saloustros, L. Pelà, M. Cervera

  Engineering Fracture Mechanics (2015). (preprint) Vol. 150, pp. 96-114

  
  

  Abstract

  This paper extends the use of crack-tracking techniques within the smeared crack approach for the numerical simulation of cohesive–frictional damage on quasi-brittle materials. The mechanical behaviour is described by an isotropic damage model with a Mohr–Coulomb failure surface. The correct crack propagation among the two alternative fracture planes proposed by the Mohr–Coulomb theory is selected with the use of an energy criterion based on the total elastic strain energy. The simulation of three benchmark problems of mixed-mode fracture in concrete demonstrates that the proposed methodology can reproduce the material’s frictional characteristics, showing robustness, as well as mesh-size and mesh-bias independence.

  Abstract
  This paper extends the use of crack-tracking techniques within the smeared crack approach for the numerical simulation of cohesive–frictional damage on quasi-brittle [...]

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• 3D numerical models of FSW processes with non-cylindrical pin

  P. Bussetta, N. Dialami, M. Chiumenti, C. Agelet de Saracibar, M. Cervera, R. Boman, J. Ponthot

  Advanced Modeling and Simulation in Engineering Sciences (2015). (preprint) Vol. 2 (27), pp. 1-19

  
  

  Abstract

  Friction stir welding process is a relatively recent welding process (patented in 1991). FSW is a solid-state joining process during which materials to be joined are not melted. During the FSW process, the behaviour of the material is at the interface between solid mechanics and fluid mechanics. In this paper, a 3D numerical model of the FSW process with a non-cylindrical tool based on a solid formulation is compared to another one based on a fluid formulation. Both models use advanced numerical techniques such as the Arbitrary Lagrangian Eulerian formulation, remeshing or the Orthogonal Sub-Grid Scale method. It is shown that these two formulations essentially deliver the same results.

  Abstract
  Friction stir welding process is a relatively recent welding process (patented in 1991). FSW is a solid-state joining process during which materials to be joined are not melted. [...]

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• On the equivalence between traction- and stress-based approaches for the modeling of localized failure in solids

  J. Wu, M. Cervera

  Journal of the Mechanics and Physics of Solids (2015). (preprint) Vol. 82, pp. 137-163

  
  

  Abstract

  This work investigates systematically traction- and stress-based approaches for the modeling of strong and regularized discontinuities induced by localized failure in solids. Two complementary methodologies, i.e., discontinuities localized in an elastic solid and strain localization of an inelastic softening solid, are addressed. In the former it is assumed a priori that the discontinuity forms with a continuous stress field and along the known orientation. A traction-based failure criterion is introduced to characterize the discontinuity and the orientation is determined from Mohr's maximization postulate. If the displacement jumps are retained as independent variables, the strong/regularized discontinuity approaches follow, requiring constitutive models for both the bulk and discontinuity. Elimination of the displacement jumps at the material point level results in the embedded/smeared discontinuity approaches in which an overall inelastic constitutive model fulfilling the static constraint suffices. The second methodology is then adopted to check whether the assumed strain localization can occur and identify its consequences on the resulting approaches. The kinematic constraint guaranteeing stress boundedness and continuity upon strain localization is established for general inelastic softening solids. Application to a unified stress-based elastoplastic damage model naturally yields all the ingredients of a localized model for the discontinuity (band), justifying the first methodology. Two dual but not necessarily equivalent approaches, i.e., the traction-based elastoplastic damage model and the stress-based projected discontinuity model, are identified. The former is equivalent to the embedded and smeared discontinuity approaches, whereas in the later the discontinuity orientation and associated failure criterion are determined consistently from the kinematic constraint rather than given a priori. The bi-directional connections and equivalence conditions between the traction- and stress-based approaches are classified. Closed-form results under plane stress condition are also given. A generic failure criterion of either elliptic, parabolic or hyperbolic type is analyzed in a unified manner, with the classical von Mises (J2), Drucker–Prager, Mohr–Coulomb and many other frequently employed criteria recovered as its particular cases.

  Abstract
  This work investigates systematically traction- and stress-based approaches for the modeling of strong and regularized discontinuities induced by localized failure in [...]

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• On the conformity of strong, regularized, embedded and smeared discontinuity approaches for the modeling of localized failure in solids

  M. Cervera, J. Wu

  Int. J. Solids and Structures (2015). (preprint) Vol. 71 (1), pp. 19-38

  
  

  Abstract

  Once strain localization occurs in softening solids, inelastic loading behavior is restricted within a narrow band while the bulk unloads elastically. Accordingly, localized failure in solids can be approached by embedding or smearing a traction-based inelastic discontinuity (band) within an (equivalent) elastic matrix along a specific orientation. In this context, the conformity of the strong/regularized and embedded/smeared discontinuity approaches are investigated, regarding the strategies dealing with the kinematics and statics. On one hand, the traction continuity condition imposed in weak form results in the strong and regularized discontinuity approaches, with respect to the approximation of displacement and strain discontinuities. In addition to the elastic bulk, consistent plastic-damage cohesive models for the discontinuities are established. The conformity between the strong discontinuity approach and its regularized counterpart is shown through the fracture energy analysis. On the other hand, the traction continuity condition can also be enforced point-wisely in strong form so that the standard principle of virtual work applies. In this case, the static constraint resulting from traction continuity can be used to eliminate the kinematic variable associated with the discontinuity (band) at the material level. This strategy leads to embedded and smeared discontinuity models for the overall weakened solid which can also be cast into the elastoplastic degradation framework with a different kinematic decomposition. Being equivalent to the kinematic constraint guaranteeing stress continuity upon strain localization, Mohr’s maximization postulate is adopted for the determination of the discontinuity orientation. Closed-form results are presented in plane stress conditions, with the classical Rankine, Mohr–Coulomb, von Mises and Drucker–Prager criteria as illustrative examples. The orientation of the discontinuity (band) and the stress-based failure criteria consistent with the given traction-based counterparts are derived. Finally, a generic failure criterion of either elliptic, parabolic or hyperbolic type, appropriate for the modeling of mixed-mode failure, is analyzed in a unified manner. Furthermore, a novel method is proposed to calibrate the involved mesoscopic parameters from available macroscopic test data, which is then validated against Willam’s numerical test.

  Abstract
  Once strain localization occurs in softening solids, inelastic loading behavior is restricted within a narrow band while the bulk unloads elastically. Accordingly, localized [...]

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• Material flow visualization in Friction Stir Welding via particle tracing

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

  Int. J. of Material Forming (2015). (preprint) Vol. 8 (2), pp. 167-181

  
  

  Abstract

  This work deals with the modeling of the material flow in Friction Stir Welding (FSW) processes using particle tracing method. For the computation of particle trajectories, three accurate and computationally efficient integration methods are implemented within a FE model for FSW process: the Backward Euler with Sub-stepping (BES), the 4-th order Runge–Kutta (RK4) and the Back and Forth Error Compensation and Correction (BFECC) methods. Firstly, their performance is compared by solving the Zalesak’s disk benchmark. Later, the developed methodology is applied to some FSW problems providing a quantitative 2D and 3D view of the material transport in the process area. The material flow pattern is compared to the experimental evidence.

  Abstract
  This work deals with the modeling of the material flow in Friction Stir Welding (FSW) processes using particle tracing method. For the computation of particle trajectories, [...]

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• Explicit mixed strain-displacement finite element for dynamic geometrically non-linear solid mechanics

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

  Computational Mechanics (2015). (preprint) Vol. 55 (3), pp. 543-559

  
  

  Abstract

  Low-order finite elements face inherent limitations related to their poor convergence properties. Such difficulties typically manifest as mesh-dependent or excessively stiff behaviour when dealing with complex problems. A recent proposal to address such limitations is the adoption of mixed displacement-strain technologies which were shown to satisfactorily address both problems. Unfortunately, although appealing, the use of such element technology puts a large burden on the linear algebra, as the solution of larger linear systems is needed. In this paper, the use of an explicit time integration scheme for the solution of the mixed strain-displacement problem is explored as an alternative. An algorithm is devised to allow the effective time integration of the mixed problem. The developed method retains second order accuracy in time and is competitive in terms of computational cost with the standard irreducible formulation.

  Abstract
  Low-order finite elements face inherent limitations related to their poor convergence properties. Such difficulties typically manifest as mesh-dependent or excessively stiff [...]

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