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Formulation and verification of a concrete model with strong coupling between isotropic damage and elastoplasticity and comparison to a weak coupling model / Ziad N. Taqieddin in Journal of engineering mechanics, Vol. 138 N° 5 (Mai 2012) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 138 N° 5 (Mai 2012) . - pp.530-541 Titre : Formulation and verification of a concrete model with strong coupling between isotropic damage and elastoplasticity and comparison to a weak coupling model Type de document : texte imprimé Auteurs : Ziad N. Taqieddin, Auteur ; Voyiadjis, George Z., Auteur ; Amin H. Almasri, Auteur Année de publication : 2012 Article en page(s) : pp.530-541 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Nonlinear  finite  element  analysis  (FEA)  Fracture  energy  based  continuum  damage  mechanics  Elastic-plastic-damage  coupling  Concrete  material  model Résumé : In this work, a new concrete model that strongly couples continuum-damage-mechanics to elastoplasticity is presented. The model incorporates a plasticity yield criterion written in terms of the nominal/damaged, rather than effective, stress space. Tensile and compressive behaviors are modeled through the damage-affected-multi-hardening nature of the plasticity yield criterion and the introduction of two (tensile and compressive) plasticity-affected isotropic damage initiation/growth criteria, where plasticity variables are added to the definition of the tensile and compressive growth functions of damage. A nonassociative plastic flow rule is used to control inelastic dilatancy. Specific expressions of the elastic/damage and plastic/damage components of the Helmholtz free energy function are presented. The constitutive equations of the model are consistently derived within a sound framework of irreversible thermodynamics. These free energy expressions are used to define three plasticity and damage interconnected dissipation mechanisms, one plasticity mechanism and two damage mechanisms. The elastic-plastic-damage, elastic-plastic, and elastic-damage tangent operators are also derived. A computational algorithm for the numerical integration of the constitutive equations is proposed on the basis of the doubly passive predictor–plastic-damage corrector approach. The model is implemented in the Finite Element (FE) analysis software ABAQUS through a user defined material subroutine (UMAT). Two- and three-dimensional verification examples are carried out to demonstrate the effectiveness and robustness of the proposed model in producing results that depict experimentally observed concrete behaviors. The results of the proposed model are compared with those of a weak coupling concrete model recently presented by the authors, in which damage was incorporated in the elastic formulation whereas plasticity remained in the effective stress space. The comparison shows the effect of strong-coupling on increasing/accelerating the degradation of concrete when the same material parameters of the weak coupling model are used. The results of the proposed model are also compared with those of well known strong-coupling models available in contemporary literature. To reduce the sensitivity of the nonlinear FE analysis of concrete structures to the refinement of the FE meshes, the damage density parameters are defined to include embedded dimensionless coefficients that are related to concrete’s fracture energies (in tension and compression) and to the geometrical characteristic length provided by ABAQUS software during the analysis. The conclusions drawn from this work clearly indicate the adequacy of the model to describe the different stages of concrete behavior and stiffness and strength degradations attributable to elastoplasticity-affected damage growth. They also show that the model can be further improved and equipped with nonlocal measures to enhance its future performance. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29EM.1943-7889.0000344 [article] Formulation and verification of a concrete model with strong coupling between isotropic damage and elastoplasticity and comparison to a weak coupling model [texte imprimé] / Ziad N. Taqieddin, Auteur ; Voyiadjis, George Z., Auteur ; Amin H. Almasri, Auteur . - 2012 . - pp.530-541. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 138 N° 5 (Mai 2012) . - pp.530-541 Mots-clés : Nonlinear  finite  element  analysis  (FEA)  Fracture  energy  based  continuum  damage  mechanics  Elastic-plastic-damage  coupling  Concrete  material  model Résumé : In this work, a new concrete model that strongly couples continuum-damage-mechanics to elastoplasticity is presented. The model incorporates a plasticity yield criterion written in terms of the nominal/damaged, rather than effective, stress space. Tensile and compressive behaviors are modeled through the damage-affected-multi-hardening nature of the plasticity yield criterion and the introduction of two (tensile and compressive) plasticity-affected isotropic damage initiation/growth criteria, where plasticity variables are added to the definition of the tensile and compressive growth functions of damage. A nonassociative plastic flow rule is used to control inelastic dilatancy. Specific expressions of the elastic/damage and plastic/damage components of the Helmholtz free energy function are presented. The constitutive equations of the model are consistently derived within a sound framework of irreversible thermodynamics. These free energy expressions are used to define three plasticity and damage interconnected dissipation mechanisms, one plasticity mechanism and two damage mechanisms. The elastic-plastic-damage, elastic-plastic, and elastic-damage tangent operators are also derived. A computational algorithm for the numerical integration of the constitutive equations is proposed on the basis of the doubly passive predictor–plastic-damage corrector approach. The model is implemented in the Finite Element (FE) analysis software ABAQUS through a user defined material subroutine (UMAT). Two- and three-dimensional verification examples are carried out to demonstrate the effectiveness and robustness of the proposed model in producing results that depict experimentally observed concrete behaviors. The results of the proposed model are compared with those of a weak coupling concrete model recently presented by the authors, in which damage was incorporated in the elastic formulation whereas plasticity remained in the effective stress space. The comparison shows the effect of strong-coupling on increasing/accelerating the degradation of concrete when the same material parameters of the weak coupling model are used. The results of the proposed model are also compared with those of well known strong-coupling models available in contemporary literature. To reduce the sensitivity of the nonlinear FE analysis of concrete structures to the refinement of the FE meshes, the damage density parameters are defined to include embedded dimensionless coefficients that are related to concrete’s fracture energies (in tension and compression) and to the geometrical characteristic length provided by ABAQUS software during the analysis. The conclusions drawn from this work clearly indicate the adequacy of the model to describe the different stages of concrete behavior and stiffness and strength degradations attributable to elastoplasticity-affected damage growth. They also show that the model can be further improved and equipped with nonlocal measures to enhance its future performance. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29EM.1943-7889.0000344

Variable material length scale associated with nanoindentation experiments / Voyiadjis, George Z. in Journal of engineering mechanics, Vol. 135 N°3 (Mars 2009) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 135 N°3 (Mars 2009) . - pp. 139-148 Titre : Variable material length scale associated with nanoindentation experiments Type de document : texte imprimé Auteurs : Voyiadjis, George Z., Auteur ; Amin H. Almasri, Auteur Article en page(s) : pp. 139-148 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Metals  Material  tests  Experimentation  Length. Résumé : Material length scale that can be used in nonlocal gradient theories is obtained in this work based on experimental observations for two metals using nanoindentation experiments. The materials are cold rolled 1018 steel and oxygen free high conductivity copper. A fixed value of the material length scale is not always realistic and different problems under various conditions could require different values. Therefore, two models are proposed for a dynamic length scale that depends on strain rates and temperature. First the model is physically based, with parameters related to dislocation densities. This model introduces strain rate and temperature dependency in a coupled form. The second model is a phenomenological one that is based on hardness tests. Both models show that length scale decreases with increasing equivalent strain rate. The temperature effects are not studied in this work. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JENMDT000 [...] [article] Variable material length scale associated with nanoindentation experiments [texte imprimé] / Voyiadjis, George Z., Auteur ; Amin H. Almasri, Auteur . - pp. 139-148. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 135 N°3 (Mars 2009) . - pp. 139-148 Mots-clés : Metals  Material  tests  Experimentation  Length. Résumé : Material length scale that can be used in nonlocal gradient theories is obtained in this work based on experimental observations for two metals using nanoindentation experiments. The materials are cold rolled 1018 steel and oxygen free high conductivity copper. A fixed value of the material length scale is not always realistic and different problems under various conditions could require different values. Therefore, two models are proposed for a dynamic length scale that depends on strain rates and temperature. First the model is physically based, with parameters related to dislocation densities. This model introduces strain rate and temperature dependency in a coupled form. The second model is a phenomenological one that is based on hardness tests. Both models show that length scale decreases with increasing equivalent strain rate. The temperature effects are not studied in this work. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JENMDT000 [...]