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• Methodology for the analysis of post-tensioned structures using a constitutive serial-parallel rule of mixtures: large scale non-linear analysis

  L. Barbu, A. Cornejo Velázquez, X. Martinez, S. Oller, A. Barbat

  Composite Structures (2019). Vol. 216, pp. 315-330

  
  

  Abstract

  The main purpose of this paper is to develop a reliable method based on a three-dimensional (3D) finite-element (FE) model to simulate the constitutive behaviour of reinforced concrete structures strengthened with post-tensioned or pre-stressed tendons well beyond the elastic domain. The post-tensioned concrete is modelled as a composite material whose behaviour is described with the serial-parallel rule of mixtures (S-P RoM) (Rastellini et al., 2008; Martinez et al., 2008; Martinez et al., 2014 [3]) and the nonlinear behaviour of each component is accounted for by means of plasticity and damage models. 3D FE models were used, where the nonlinear material behaviour and geometrical analysis based on incremental-iterative load methods were adopted. Several examples are shown where the capabilities of the method on large scale structures are exhibited taking into account the self-weight, the post-tension load and different pressure loads. A new metric for computing the crack opening displacement inside a finite element is proposed.

  Abstract
  The main purpose of this paper is to develop a reliable method based on a three-dimensional (3D) finite-element (FE) model to simulate the constitutive behaviour of reinforced [...]

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• A FE 2 multi-scale implementation for modeling composite materials on distributed architectures

  G. Giuntoli, J. Aguilar, M. Vázquez, S. Oller, G. Houzeaux

  Coupled Systems Mechanics (2019). Vol. 8, pp. 99-109

  
  

  Abstract

  This work investigates the accuracy and performance of a FE2 multi-scale implementation used to predict the behavior of composite materials. The equations are formulated assuming the small deformations solid mechanics approach in non-linear material models with hardening plasticity. The uniform strain boundary conditions are applied for the macro-to-micro transitions. A parallel algorithm was implemented in order to solve large engineering problems. The scheme proposed takes advantage of the domain decomposition method at the macro-scale and the coupling between each subdomain with a micro-scale model. The precision of the method is validated with a composite material problem and scalability tests are performed for showing the efficiency.

  Abstract
  This work investigates the accuracy and performance of a FE2 multi-scale implementation used to predict the behavior of composite materials. The equations are formulated assuming [...]

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• Damage assessment using modeling of large-scale confined masonry building

  C. Escudero, S. Oller, X. Martinez, A. Barbat

  Journal of Engineering Mechanics (2018). Vol. 144 (12), pp. 04018107-1/04018107-17

  
  

  Abstract

  The seismic behavior of a representative medium-rise building of Mexico City is evaluated using the capacity spectrum method. This method is widely used in the seismic assessment of buildings because it allows obtaining fragility curves which permit evaluating the ability of a building to resist earthquakes. A real full-height multistory model is proposed to test the capabilities of the algorithm exhibited. The model is outlined through structural drawings sized and structured in accordance with building code regulations for masonry structures in Mexico City. Computational requirements for the analysis of large structures are indicated in addition to improvements to a nonlinear computing code for better performance in terms of memory management and execution times. Finally, a comparison of obtained results and building code regulations is carried out, highlighting differences in the obtained results. The need to handle meshes with a high number of finite elements (FEs) led to development of a new layered finite element that can reproduce the nonlinear behavior of its constituent materials when there are out-of-plane stresses without having to introduce additional degrees of freedom. The proposed FE is compared with the standard FE, presenting different kinematics, and excellent results are obtained. This work emerges from the need to combine and improve existing technologies in the field of finite-element analysis. One such technology is the numerical simulation of the behavior of composite materials. Therefore, it was also necessary to develop a computing program capable of reading both finite-element meshes and patterns of fibers to calculate the information of the composite materials, such as volumetric participation and fiber’s orientation.

  Abstract
  The seismic behavior of a representative medium-rise building of Mexico City is evaluated using the capacity spectrum method. This method is widely used in the seismic assessment [...]

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• On the analysis of non-homogeneous laminates using the refined zigzag theory

  F. Flores, S. Oller, L. Nallim

  Composite Structures (2018). Vol. 204, pp. 791-802

  
  

  Abstract

  This work shows possibilities and limitations of the refined zigzag theory (RZT) that has been used in different structural (beam, plate and shell) finite elements. The refined zigzag theory can deal with composite laminates, adding only one nodal degree of freedom per spatial dimension of the laminate, obtaining very good accuracy. It assumes that the in-plane displacements have a piece-wise linear shape across the thickness depending on the shear stiffness of each composite layer. This paper presents the main aspects of a beam/shell of revolution element used for the numerical simulations. The details of the refined zigzag theory are given also in order to discuss some limitations that occur when dealing with the non-linear phenomenon of delamination. Two examples are presented and discussed, including different inhomogeneities that show the limitations of the RZT for the treatment of partially delaminated beams

  Abstract
  This work shows possibilities and limitations of the refined zigzag theory (RZT) that has been used in different structural (beam, plate and shell) finite elements. The refined [...]

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• Methodology for the analysis of post-tensioned structures using a constitutive serial-parallel rule of mixtures

  A. Cornejo Velázquez, L. Barbu, C. Escudero, X. Martinez, S. Oller, A. Barbat

  Computers and Structures (2018). Vol. 200, pp. 480-497

  
  

  Abstract

  The main purpose of this paper is to develop a reliable method based on a three-dimensional (3D) finite-element (FE) model to simulate the constitutive behaviour of reinforced concrete structures strengthened with post-tensioned tendons taking into account the reduction of the pre-stressing stress due to the steel relaxation. The post-tensioned concrete is modelled as a composite material whose behaviour is described with the serial-parallel rule of mixtures (S/P RoM) (Rastellini et al, 2008; Martinez et al., 2008, 2014) whereas the stress relaxation of the steel is simulated using a viscoelastic model called Generalized Maxwell. A 3D FE model was used, where the nonlinear material behaviour and geometrical analysis based on incremental–iterative load methods were adopted. Validation by comparison with the analytic solution will be done for the case of a concrete beam with a linear steel tendon and for a parabolic pre-tensioned steel tendon embedded. Some viscoelastic cases are presented in order to perceive the behaviour of the Generalized Maxwell model. Several examples are shown where the capabilities of the method on large scale structures are exhibited.

  Abstract
  The main purpose of this paper is to develop a reliable method based on a three-dimensional (3D) finite-element (FE) model to simulate the constitutive behaviour of reinforced [...]

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• Outlook on ecologically improved composites for aviation interior and secondary structures

  J. Bachmann, X. Yi, H. Gong, X. Martinez, G. Bugeda, S. Oller, K. Tserpes, E. Ramon, C. Paris, Z. Fang, Y. Li, Y. Liu, X. Liu, G. Xian, J. Tong, J. Wei, X. Zhang, J. Zhu

  CEAS Aeronautical Journal (2018). Vol. 9, pp. 533-543

  
  

  Abstract

  Today, mainly man-made materials such as carbon and glass fibres are used to produce composite parts in aviation. Renewable materials such as natural fibres or bio-sourced resin systems have not found their way into aviation, yet. The project ECO-COMPASS aims to evaluate the potential applications of ecologically improved composite materials in the aviation sector in an international collaboration of Chinese and European partners. Natural fibres such as flax and ramie will be used for different types of reinforcements and sandwich cores. Furthermore, the bio-based epoxy resins to substitute bisphenol-A based epoxy resins in secondary structures are under investigation. Adapted material protection technologies to reduce environmental influence and to improve fire resistance are needed to fulfil the demanding safety requirements in aviation. Modelling and simulation of chosen eco-composites aims for an optimized use of materials while a life cycle assessment aims to prove the ecological advantages compared to synthetic state-of-the-art materials. In this paper, the status of selected ecologically improved materials will be presented with an outlook for potential application in interior and secondary structures.

  Abstract
  Today, mainly man-made materials such as carbon and glass fibres are used to produce composite parts in aviation. Renewable materials such as natural fibres or bio-sourced [...]

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• Numerical study on the relevance of columns hidden failure modes in the seismic capacity of non-ductile RC Frames

  B. Alfarah, F. López-Almansa, S. Oller

  Journal of eathquake engineering (2018). (preprint) p. 1-18

  
  

  Abstract

  In simplified seismic structural analyses, not all the deterioration modes are adequately considered. This work discusses the relation among the hidden failure modes of columns of non-ductile reinforced concrete building frames and their global collapse mechanism. With this aim, a numerically efficient model is developed and implemented in OpenSEES. Two benchmark problems are analyzed with this model: the well-known Van Nuys Hotel and a prototype building designed for gravity loads only; in this last case, the results are compared with experiments on a one-third scale model. The obtained results confirm that simplified models grossly overestimate the building capacity.

  Abstract
  In simplified seismic structural analyses, not all the deterioration modes are adequately considered. This work discusses the relation among the hidden failure modes of columns [...]

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• Multiscale computational homogenization: review and proposal of a new enhanced-first-order method

  F. Otero, S. Oller, X. Martinez

  Archives of Comp. Meths. Engng. (2018). Vol. 25 (2), pp. 479-505

  
  

  Abstract

  The continuous increase of computational capacity has encouraged the extensive use of multiscale techniques to simulate the material behaviour on several fields of knowledge. In solid mechanics, the multiscale approaches which consider the macro-scale deformation gradient to obtain the homogenized material behaviour from the micro-scale are called first-order computational homogenization. Following this idea, the second-order FE2 methods incorporate high-order gradients to improve the simulation accuracy. However, to capture the full advantages of these high-order framework the classical boundary value problem (BVP) at the macro-scale must be upgraded to high-order level, which complicates their numerical solution. With the purpose of obtaining the best of both methods i.e. first-order and second-order, in this work an enhanced-first-order computational homogenization is presented. The proposed approach preserves a classical BVP at the macro-scale level but taking into account the high-order gradient of the macro-scale in the micro-scale solution. The developed numerical examples show how the proposed method obtains the expected stress distribution at the micro-scale for states of structural bending loads. Nevertheless, the macro-scale results achieved are the same than the ones obtained with a first-order framework because both approaches share the same macro-scale BVP. This is a copy of the author 's final draft version of an article published in the Archives of computational methods in engineering.

  Abstract
  The continuous increase of computational capacity has encouraged the extensive use of multiscale techniques to simulate the material behaviour on several fields of knowledge. [...]

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• On the feasibility of the computational modelling of the endoluminal vacuum-assisted closure of an oesophageal anastomotic leakage

  E. Comellas, F. Bellomo, I. Rosales, L. Castilo, R. Sánchez, P. Turon, S. Oller

  Royal Society Open Science (2018). Vol. 5 (3), pp. 1-23

  
  

  Abstract

  Endoluminal vacuum-assisted closure (E-VAC) is a promising therapy to treat anastomotic leakages of the oesophagus and bowel which are associated with high morbidity and mortality rates. An open-pore polyurethane foam is introduced into the leakage cavity and connected to a device that applies a suction pressure to accelerate the closure of the defect. Computational analysis of this healing process can advance our understanding of the biomechanical mechanisms at play. To this aim, we use a dual-stage finite-element analysis in which (i) the structural problem addresses the cavity reduction caused by the suction and (ii) a new constitutive formulation models tissue healing via permanent deformations coupled to a stiffness increase. The numerical implementation in an in-house code is described and a qualitative example illustrates the basic characteristics of the model. The computational model successfully reproduces the generic closure of an anastomotic leakage cavity, supporting the hypothesis that suction pressure promotes healing by means of the aforementioned mechanisms. However, the current framework needs to be enriched with empirical data to help advance device designs and treatment guidelines. Nonetheless, this conceptual study confirms that computational analysis can reproduce E-VAC of anastomotic leakages and establishes the bases for better understanding the mechanobiology of anastomotic defect healing.

  Abstract
  Endoluminal vacuum-assisted closure (E-VAC) is a promising therapy to treat anastomotic leakages of the oesophagus and bowel which are associated with high morbidity and mortality [...]

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• RC structures cyclic behavior simulation with a model integrating plasticity, damage, and bond-slip

  B. Alfarah, J. Murcia-Delso, F. Almansa, S. Oller

  Earthquake engineering and structural dynamics (2018). (preprint) Vol. 47 (2), pp. 460-478

  
  

  Abstract

  The behavior of reinforced concrete structures under severe demands, as strong ground motions, is highly complex; this is mainly due to the complexity of concrete behavior and to the strong interaction between concrete and steel, with several coupled failure modes. On the other hand, given the increasing awareness and concern on the worldwide seismic risk, new developments have arisen in earthquake engineering; nonetheless, some developments are mainly based on simple analytical tools that are widely used, given their moderate computational cost. This research aims to provide a solid basis for validation and calibration of such developments by using computationally efficient continuum mechanics-based tools. Within this context, this paper presents a model for 3D simulation of cyclic behavior of RC structures. The model integrates a bond-slip model developed by one of the authors and the damage variable evolution methodology for concrete damage plastic model developed by some authors. In the integrated model, a new technique is derived for efficient 3D analysis of bond-slip of 2 or more crossing reinforcing bars in beam-column joints, slabs, footings, pile caps, and other similar members. The analysis is performed by implementing the bond-slip model in a user element subroutine of Abaqus and the damage variable evolution methodology in the original concrete damage plastic model in the package. Two laboratory experiments consisting of a column and a frame subjected to cyclic displacements up to failure are simulated with the proposed formulation.

  Abstract
  The behavior of reinforced concrete structures under severe demands, as strong ground motions, is highly complex; this is mainly due to the complexity of concrete behavior [...]

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