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• A finite thickness band method for ductile fracture analysis

  A. Huespe, A. Needlman, J. Oliver, P. Sánchez

  International Journal of Plasticity (2009). Vol. 25, pp. 2349-2365

  
  

  Abstract

  We present a finite element method with a finite thickness embedded weak discontinuity to analyze ductile fracture problems. The formulation is restricted to small geometry changes. The material response is characterized by a constitutive relation for a progressively cavitating elastic–plastic solid. As voids nucleate, grow and coalesce, the stiffness of the material degrades. An embedded weak discontinuity is introduced when the condition for loss of ellipticity is met. The resulting localized deformation band is given a specified thickness which introduces a length scale thus providing a regularization of the post-localization response. Also since the constitutive relation for a progressively cavitation solid is used inside the band in the post-localization regime, the traction-opening relation across the band depends on the stress triaxiality. The methodology is illustrated through several example problems including mode I crack growth and localization and failure in notched bars. Various finite element meshes and values of the thickness of the localization band are used in the calculations to illustrate the convergence with mesh refinement and the dependence on the value chosen for the localization band thickness

  Abstract
  We present a finite element method with a finite thickness embedded weak discontinuity to analyze ductile fracture problems. The formulation is restricted to small geometry [...]

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• A contact domain method for large deformation frictional contact problems. Part 1: Theoretical basis

  J. Oliver, S. Hartmann, J. Cante, R. Weyler, J. Hernández

  Computer Methods in Applied Mechanics and Engineering (2009). Vol. 198, pp. 2591-2606

  
  

  Abstract

  In the first part of this work, the theoretical basis of a frictional contact domain method for two-dimensional large deformation problems is presented. Most of the existing contact formulations impose the contact constraints on the boundary of one of the contacting bodies, which necessitates the projection of certain quantities from one contacting surface onto the other. In this work, the contact constraints are formulated on a so-called contact domain, which has the same dimension as the contacting bodies. This contact domain can be interpreted as a fictive intermediate region connecting the potential contact surfaces of the deformable bodies. The introduced contact domain is subdivided into a non-overlapping set of patches and is endowed with a displacement field, interpolated from the displacements at the contact surfaces. This leads to a contact formulation that is based on dimensionless, strain-like measures for the normal and tangential gaps and that exactly passes the contact patch test. In addition, the contact constraints are enforced using a stabilized Lagrange multiplier formulation based on an interior penalty method (Nitsche method). This allows the condensation of the introduced Lagrange multipliers and leads to a purely displacement driven problem. An active set strategy, based on the concept of effective gaps as entities suitable for smooth extrapolation, is used for determining the active normal stick and slip patches of the contact domain.

  Abstract
  In the first part of this work, the theoretical basis of a frictional contact domain method for two-dimensional large deformation problems is presented. Most of the [...]

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• A mixture theory based method for three-dimensional modeling of reinforced concrete members with embedded crack finite elements

  O. Manzoli, J. Oliver, A. Huespe, G. Diaz

  Computers and Concrete (2008). Vol. 5 (4), pp. 401-416

  
  

  Abstract

  The paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuum Strong Discontinuous Approach (CSDA). Mixture theory concepts are used to model reinforced concrete as a 3D composite material constituted of concrete with long fibers (rebars) bundles oriented in different directions embedded in it. The effects of the rebars are modeled by phenomenological constitutive models devised to reproduce the axial non-linear behavior, as well as the bond-slip and dowel action. The paper presents the constitutive models assumed for the components and the compatibility conditions chosen to constitute the composite. Numerical analyses of existing experimental reinforced concrete members are presented, illustrating the applicability of the proposed methodology.

  Abstract
  The paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuum Strong Discontinuous [...]

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• On some topics for the numerical simulation of ductile fracture

  P. Sánchez, A. Huespe, J. Oliver

  International Journal of Plasticity (2008). Vol. 24 (6), pp. 1008-1038

  
  

  Abstract

  In this work, we analyze some aspects of the macroscopic Gurson–Tvergaard–Needleman (GTN) constitutive model when it is addressed to solve ductile fracture problems by means of numerical simulations: (i) The analytical solutions of the material discontinuous bifurcation problem is performed. Closed and exact formulas are obtained. The so determined critical conditions and the developed strain localization mode are afterward studied and compared in crack growth problems. Even when this methodology of analysis is rather standard at the present, the conclusions drawn from this study differ significantly from that obtained with a similar analysis in quasi-brittle fracture problems. (ii) A new very robust numerical integration method for the GTN model, namely the Impl–Ex Method, is proposed. It is a low computational cost algorithm, equivalent to a linear problem per each integration step, with a reasonable precision for engineering purposes. Its accuracy and convergence rate is assessed by means of an error study applied to a ductile fracture test simulation. (iii) A detailed analysis of a plane strain ductile crack growth problem is performed in a material containing two size-scale of voids. In the analysis, particular attention is given to the mesh size dependence and to the coalescence of the larger void

  Abstract
  In this work, we analyze some aspects of the macroscopic Gurson–Tvergaard–Needleman (GTN) constitutive model when it is addressed to solve ductile fracture problems [...]

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• An implicit/explicit integration scheme to increase computability of non-linear material and contact/friction problems

  J. Oliver, A. Huespe, J. Cante

  Computer Methods in Applied Mechanics and Engineering (2008). Vol. 197 (21-24), pp. 1865-1889

  
  

  Abstract

  An implicit/explicit integration scheme for non-linear constitutive models is presented. It aims at providing additional computability to those solid mechanics problems were robustness is an important issue, i.e. material failure models equipped with strain softening, soft materials, contact-friction models, etc., although it can also provide important advantages, in terms of computational cost, with respect to purely implicit integration schemes. The proposed scheme is presented based on general families of constitutive models (continuum damage and elasto-plasticity) and its properties, in terms of robustness and accuracy, are analytically derived and computationally assessed by means of numerical simulations. An adaptive time stepping algorithm, based on a priori control of the committed error and the application of the proposed scheme to contact/friction interfaces are also presented.

  Abstract
  An implicit/explicit integration scheme for non-linear constitutive models is presented. It aims at providing additional computability to those solid mechanics problems were [...]

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• Three-dimensional analysis of reinforced concrete members via embedded discontinuity finite elements

  O. Manzoli, J. Oliver, A. Huespe

  IBRACON structures and materials journal (2008). Vol. 1 (1), pp. 85-83

  
  

  Abstract

  This paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuous Strong Discontinuous Approach (CSDA). Mixture theory concepts are used to model reinforced concrete as a 3D composite material constituted of concrete with long fiber bundles (rebars) oriented in different directions embedded in it. The effects of the rebars are provided by phenomenological constitutive models designed to reproduce the axial non-linear behavior, as well as bond-slip and dowel action. This paper is focused on the constitutive models assumed for the components and the compatibility conditions chosen to constitute the composite. Numerical analyses of existing experimental reinforced concrete members are presented, illustrating the applicability of the methodology.

  Abstract
  This paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuous Strong Discontinuous [...]

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• Two-dimensional modelling of material failure in reinforced concrete by means of a continuum strong discontinuity approach

  J. Oliver, D. Linero, A. Huespe, O. Manzoli

  Computer Methods in Applied Mechanics and Engineering (2008). Vol. 197 (5), pp. 332-348

  
  

  Abstract

  The paper presents a new methodology to model material failure, in two-dimensional reinforced concrete members, using the Continuum Strong Discontinuity Approach (CSDA). The mixture theory is used as the methodological approach to model reinforced concrete as a composite material, constituted by a plain concrete matrix reinforced with two embedded orthogonal long fiber bundles (rebars). Matrix failure is modeled on the basis of a continuum damage model, equipped with strain softening, whereas the rebars effects are modeled by means of phenomenological constitutive models devised to reproduce the axial non-linear behavior, as well as the bond–slip and dowel effects. The proposed methodology extends the fundamental ingredients of the standard Strong Discontinuity Approach, and the embedded discontinuity finite element formulations, in homogeneous materials, to matrix/fiber composite materials, as reinforced concrete. The specific aspects of the material failure modeling for those composites are also addressed. A number of available experimental tests are reproduced in order to illustrate the feasibility of the proposed methodology

  Abstract
  The paper presents a new methodology to model material failure, in two-dimensional reinforced concrete members, using the Continuum Strong Discontinuity Approach (CSDA). The [...]

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• Vulnerability analysis of large concrete dams using the continuum strong discontinuity approach and neural networks

  M. Papadrakakis, V. Papadopoulos, N. Lagaros, J. Oliver, A. Huespe, P. Sánchez

  Structural safety (2007). Vol. 30 (3), pp. 217-235

  
  

  Abstract

  Probabilistic analysis is an emerging field of structural engineering which is very significant in structures of great importance like dams, nuclear reactors etc. In this work a Neural Networks (NN) based Monte Carlo Simulation (MCS) procedure is proposed for the vulnerability analysis of large concrete dams, in conjunction with a non-linear finite element analysis for the prediction of the bearing capacity of the Dam using the Continuum Strong Discontinuity Approach. The use of NN was motivated by the approximate concepts inherent in vulnerability analysis and the time consuming repeated analyses required for MCS. The Rprop algorithm is implemented for training the NN utilizing available information generated from selected non-linear analyses. The trained NN is then used in the context of a MCS procedure to compute the peak load of the structure due to different sets of basic random variables leading to close prediction of the probability of failure. This way it is made possible to obtain rigorous estimates of the probability of failure and the fragility curves for the Scalere (Italy) dam for various predefined damage levels and various flood scenarios. The uncertain properties (modeled as random variables) considered, for both test examples, are the Young’s modulus, the Poisson’s ratio, the tensile strength and the specific fracture energy of the concrete.

  Abstract
  Probabilistic analysis is an emerging field of structural engineering which is very significant in structures of great importance like dams, nuclear reactors etc. In this [...]

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• Stabilized mixed finite elements with embedded discontinuities for shear bans modelling

  P. Sánchez, V. Sonzogni, A. Huespe, J. Oliver

  Journal of Applied Mechanics. Transactions of the ASME. (2006). Vol. 73 (12), pp. 995-1004

  
  

  Abstract

  A stabilized mixed finite element with elemental embedded strong discontinuities for shear band modeling is presented. The discrete constitutive model, representing the cohesive forces acting across the shear band, is derived from a rate-independent J(2) plastic continuum material model with strain softening, by using a projection-type procedure determined by the Continuum-Strong Discontinuity Approach. The numerical examples emphasize the increase of the numerical solution accuracy obtained with the present strategy as compared with alternative procedures using linear triangles

  Abstract
  A stabilized mixed finite element with elemental embedded strong discontinuities for shear band modeling is presented. The discrete constitutive model, representing the cohesive [...]

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• Stability and robustness issues in numerical modeling of material failure with the strong discontinuity approach

  J. Oliver, A. Huespe, S. Blanco, D. Linero

  Computer Methods in Applied Mechanics and Engineering (2006). Vol. 195 (92), pp. 7093-7114

  
  

  Abstract

  Robustness and stability of the Continuum Strong Discontinuity Approach (CSDA) to material failure are addressed. After identification of lack of symmetry of the finite element formulation and material softening in the constitutive model as possible causes of loss of robustness, two remedies are proposed: (1) the use of an specific symmetric version of the elementary enriched (E-FEM) finite element with embedded discontinuities and (2) a new implicit–explicit integration of the internal variable, in the constitutive model, which renders the tangent constitutive algorithmic operator positive definite and constant. The combination of both developments leads to finite element formulations with constant, in the time step, and non-singular tangent structural stiffness, allowing dramatic improvements in terms of robustness and computational costs. After assessing the convergence and stability properties of the new strategies, three-dimensional numerical simulations of failure problems illustrate the performance of the proposed procedures

  Abstract
  Robustness and stability of the Continuum Strong Discontinuity Approach (CSDA) to material failure are addressed. After identification of lack of symmetry of the finite element [...]

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