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George Z. Voyiadjis - One of the best experts on this subject based on the ideXlab platform.
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studying the effect of a hydrostatic stress strain reduction factor on Damage mechanics of concrete materials
Journal of the mechanical behavior of materials, 2013Co-Authors: Ziad N. Taqieddin, George Z. VoyiadjisAbstract:In the Nonlinear Finite Element Analysis (NFEA) of concrete materials, Continuum Damage Mechanics (CDM) provide a powerful framework for the derivation of constitutive Models capable of describing the mechanical behavior of such materials. The internal state variables of CDM can be introduced to the elastic analysis of concrete to form elastic-Damage Models (no inelastic strains), or to the elastic-Plastic analysis in order to form coupled/uncoupled elastic-Plastic-Damage Models. Experimental evidence that is well documented in literature shows that concrete’s susceptibility to Damage and failure is distinguished under deviatoric loading from that corresponding to hydrostatic loading. A reduction factor is usually introduced into a CDM Model to reduce the susceptibility of concrete to hydrostatic stresses/strains. In this work, the effect of a hydrostatic stress reduction factor on the performances of two NFEA concrete Models will be studied. These (independently published) Models did not provide any results showing such effect. One of these two Models is an elastic-Damage Model while the other is an uncoupled elastic-Plastic-Damage Model. Comparisons are carried out between the performances of the two Models under tensile and compressive loadings, clearly showing the effect of the reduction factor on the numerically depicted behaviors of concrete materials. In order to have rational comparisons, the hydrostatic stress reduction factor applied to each Model is chosen to be a function of the internal state variables common to both Models. Therefore, once the two Models are calibrated to simulate the experimental behaviors, their corresponding reduction factors are readily available at every increment of the iterative NFEA procedures.
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a finite strain Plastic Damage Model for high velocity impacts using combined viscosity and gradient localization limiters part ii numerical aspects and simulations
International Journal of Damage Mechanics, 2006Co-Authors: George Z. Voyiadjis, Rashid Abu K AlrubAbstract:In this companion article, we present within the finite element context the numerical algorithms for the integration of the thermodynamically consistent formulation of geometrically nonlinear gradient-enhanced viscoinelasticity derived in the first part of the article. The proposed unified integration algorithms are extensions of the classical rate-independent return mapping algorithms to the rate- dependent problems. An operator split structure is used consisting of a trial state followed by the return map by imposing the generalized viscoPlastic and visco- Damage consistency conditions simultaneously. Furthermore, a trivially incremen- tally objective integration scheme is established for the rate constitutive relations. The proposed finite deformation scheme is based on hypoelastic stress-strain representations and the proposed elastic predictor and coupled viscoPlastic- viscoDamage corrector algorithm allows for the total uncoupling of geometrical and material nonlinearities. A simple and direct computational algorithm is also used for calculation of the higher-order gradients. This algorithm can be implemented in the existing finite element codes without numerous modifications as compared to the current numerical approaches for integrating gradient-dependent Models. The nonlinear algebraic system of equations is solved by consistent linearization and the Newton-Raphson iteration. The proposed Model is implemented in the explicit finite element code ABAQUS via the user subroutine VUMAT. Model capabilities are preliminarily illustrated for the dynamic localization of inelastic flow in adiabatic shear bands and the perforation of a 12 mm thick Weldox 460E steel plates by deformable blunt projectiles at various impact speeds. The simulated shear band results well illustrated the potential of the proposed Model in dealing with the well- known mesh sensitivity problem. Consequently, the introduced implicit and explicit
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a finite strain Plastic Damage Model for high velocity impact using combined viscosity and gradient localization limiters part i theoretical formulation
International Journal of Damage Mechanics, 2006Co-Authors: Rashid Abu K Alrub, George Z. VoyiadjisAbstract:During dynamic loading processes, large inelastic deformation associated with high strain rates leads, for a broad class of ductile metals, to degradation and failure by strain localization. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element computations are considerably affected by the mesh size and alignment. This gives rise to a non-physical description of the localized regions. This article presents a theoretical framework to solve this problem with the aid of nonlocal gradient-enhanced theory coupled to viscoinelasticity. Constitutive equations for anisotropic thermoviscoDamage (rate-dependent Damage) mechanism coupled with thermo-hypoelasto-viscoPlastic deformation are developed in this work within the framework of thermodynamic laws, nonlinear continuum mechanics, and nonlocal continua. Explicit and implicit microstructural length-scale measures, which preserve the well-posedness of the differential equati...
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a finite strain Plastic Damage Model for high velocity impact using combined viscosity and gradient localization limiters part i theoretical formulation
International Journal of Damage Mechanics, 2006Co-Authors: Rashid Abu K Alrub, George Z. VoyiadjisAbstract:During dynamic loading processes, large inelastic deformation associated with high strain rates leads, for a broad class of ductile metals, to degradation and failure by strain localization. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element computations are considerably affected by the mesh size and alignment. This gives rise to a non-physical description of the localized regions. This article presents a theoretical framework to solve this problem with the aid of nonlocal gradient-enhanced theory coupled to viscoinelasticity. Constitutive equations for anisotropic thermoviscoDamage (rate-dependent Damage) mechanism coupled with thermo-hypoelasto-viscoPlastic deformation are developed in this work within the framework of thermodynamic laws, nonlinear continuum mechanics, and nonlocal continua. Explicit and implicit microstructural length-scale measures, which preserve the well-posedness of the differential equations, are introduced through the use of the viscosity and gradient localization limiters. The gradient- enhanced theory that incorporates macroscale interstate variables and their high- order gradients is developed here to describe the change in the internal structure and to investigate the size effect of statistical inhomogeneity of the evolution related Plasticity and Damage. The gradients are introduced in the hardening internal state variables and are considered dependent on their local counterparts. The derived microDamage constitutive Model is destined to be applied in the context of high velocity impact and penetration Damage mechanics. The theoretical framework presented in this article can be considered as a feasible thermodynamic approach that enables to derive various gradient (visco) Plasticity/(visco) Damage theories
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a finite strain Plastic Damage Model for high velocity impacts using combined viscosity and gradient localization limiters part ii numerical aspects and simulations
International Journal of Damage Mechanics, 2006Co-Authors: George Z. Voyiadjis, Rashid Abu K AlrubAbstract:In this companion article, we present within the finite element context the numerical algorithms for the integration of the thermodynamically consistent formulation of geometrically nonlinear gradi...
Jian-fu Shao - One of the best experts on this subject based on the ideXlab platform.
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a micro mechanics based Plastic Damage Model for quasi brittle materials under a large range of compressive stress
International Journal of Plasticity, 2018Co-Authors: Jian-fu Shao, Qi-zhi Zhu, Lunyang ZhaoAbstract:Abstract In this paper, a new micro-mechanics based Plastic Damage Model is proposed for quasi-brittle materials under a large range of compressive stress. The Damage is due to initiation and propagation of micro-cracks while the Plastic deformation is directly related to frictional sliding along micro-cracks. The two dissipation processes are then physically coupled. With the Mori-Tanaka homogenization procedure and thermodynamics framework, the macroscopic state equations are deduced and the local driving forces of Damage and Plasticity are defined. New specific criteria are proposed for the description of Damage evolution and Plastic flow. These criteria take into account the variation of material resistance to Damage with confining pressure and the degradation of surface asperity of micro-cracks during the frictional sliding. An analytical analysis of macroscopic peak strength and volumetric compressibility-dilatancy transition is provided. A specific calibration procedure is further proposed for the determination of all Model's parameters from conventional triaxial compression tests. The efficiency of the proposed Model is verified against experimental data on three different materials and for a very large range of stress. All main features of mechanical behaviors of materials are well captured by the proposed Model.
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a discrete thermodynamic approach for anisotropic Plastic Damage Modeling of cohesive frictional geomaterials
International Journal for Numerical and Analytical Methods in Geomechanics, 2010Co-Authors: Qi-zhi Zhu, Jian-fu Shao, Chuangbing Zhou, Djimedo KondoAbstract:A discrete Plastic–Damage Model is developed for cohesive-frictional geomaterials subjected to compression-dominated stresses. Macroscopic Plastic strains of material are physically generated by frictional sliding along weakness planes. The evolution of Damage is related to the evolution of weakness planes physically in connection with the propagation of microcracks. A discrete approach is used to account for anisotropic Plastic flow and Damage evolution, by introducing two stress invariants and one Plastic hardening variable for each family of sliding weakness planes. Plastic flow in each family is coupled with Damage evolution. The proposed Model is applied to typical geomaterials and comparisons between numerical predictions and experimental data are presented.
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a discrete approach for anisotropic Plasticity and Damage in semi brittle rocks
Computers and Geotechnics, 2010Co-Authors: Qi-zhi Zhu, Hui Zhou, Jian-fu ShaoAbstract:Abstract In this paper, we propose an anisotropic Plastic Damage Model for semi-brittle geomaterials based on a discrete thermodynamic approach. The macroscopic Plastic deformation is generated by frictional sliding of weakness planes. The evolution of Damage is related to growth of such weakness planes. The local frictional sliding in each family of weakness planes is described by a non-associated Plastic Model taking into account material softening and volumetric dilatancy. The Damage evolution is coupled with Plastic deformation and Modelled by an isotropic Damage criterion. The proposed Model is applied to Modelling mechanical responses of typical sandstone under different loading paths. There is good agreement between numerical predictions and experimental data. Further, the anisotropic distributions of Plastic deformation and induced Damage are analysed and discussed.
Sergio Oller - One of the best experts on this subject based on the ideXlab platform.
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new methodology for calculating Damage variables evolution in Plastic Damage Model for rc structures
Engineering Structures, 2017Co-Authors: Bashar Alfarah, F Lopezalmansa, Sergio OllerAbstract:The behavior of reinforced concrete (RC) structures under severe demands, as strong ground motions, is highly complex; this is mainly due to joint operation of concrete and steel, with several coupled failure modes. Furthermore, given the increasing awareness and concern for the important seismic worldwide risk, new developments have arisen in earthquake engineering. Nonetheless, simplified numerical Models are widely used (given their moderate computational cost), and many developments rely mainly on them. The authors have started a long-term research whose final objective is to provide, by using advanced numerical Models, solid basis for these developments. Those Models are based on continuum mechanics, and consider Plastic Damage Model to simulate concrete behavior. Within this context, this paper presents a new methodology to calculate Damage variables evolution; the proposed approach is based in the Lubliner/Lee/Fenves formulation and provides closed-form expressions of the compressive and tensile Damage variables in terms of the corresponding strains. This methodology does not require calibration with experimental results and incorporates a strategy to avoid mesh-sensitivity. A particular algorithm, suitable for implementation in Abaqus, is described. Mesh-insensitivity is validated in a simple tension example. Accuracy and reliability are verified by simulating a cyclic experiment on a plain concrete specimen. Two laboratory experiments consisting in pushing until failure two 2-D RC frames are simulated with the proposed approach to investigate its ability to reproduce actual monotonic behavior of RC structures; the obtained results are also compared with the aforementioned simplified Models that are commonly employed in earthquake engineering.
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validation on large scale tests of a new hardening softening law for the barcelona Plastic Damage Model
International Journal of Fatigue, 2015Co-Authors: Lucia Gratiela Barbu, Xavier Martinez, Sergio Oller, Alex H BarbatAbstract:Abstract This paper presents the results of finite element simulations made on a bent pipe subjected to an in-plane variable cyclic displacement combined with internal pressure. Special emphasis is put on the capacity of the Model to illustrate different failure modes depending on the internal pressure applied on the pipe. The results of the numerical analyses will be compared to experimental ones. The constitutive Model used for the simulation of Ultra Low Cycle Fatigue (ULCF) loading and the hardening–softening law used are only briefly touched upon. The monotonic behavior of a large diameter pipe, as obtained from the constitutive Model proposed, is also shown and compared to experimental results under two different loading conditions. The total axial load at failure for this case resulted in less than 10% error as compared to the experiments. Regarding the ULCF in-plane bending simulations conducted on a 16-in. 90° elbow, the results were in good agreement with the experimental test in terms of force–displacement hysteresis loops and total fatigue life of the specimen. An analysis of the dependence of the failure mode to the internal pressure applied has been conducted, showing that the formulation is capable of obtaining both habitual failure types.
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analysis of ultra low cycle fatigue problems with the barcelona Plastic Damage Model and a new isotropic hardening law
International Journal of Fatigue, 2015Co-Authors: Xavier Martinez, Sergio Oller, Lucia Gratiela Barbu, Alex H Barbat, A M P De JesusAbstract:Abstract This paper presents a Plastic-Damage formulation and a new isotropic hardening law, based on the Barcelona Plastic Damage Model initially proposed by Lubliner et al. (1989) [1], which is capable of predicting steel failure due to Ultra Low Cycle Fatigue (ULCF). This failure mechanism is obtained when the material is subjected to cyclic loads and breaks after applying a very low number of cycles, usually less than hundreds. The failure is driven by the Plastic response of the material, and it is often predicted based on the Plastic strains applied to it. The Model proposed in this work has been formulated with the objective of predicting accurately the Plastic behavior of the material, as well as its failure due to ULCF. This is achieved taking into account the fracture energy dissipated during the whole loading process. This approach allows the simulation of ULCF when it takes place due to regular cyclic loads or non-regular cyclic loads, as it is the case of seismic loads. Several simulations are conducted in order to show the capabilities of the formulation to reproduce the mechanical response of steel when it is subjected to regular and non-regular cyclic loads. The formulation is validated comparing the numerical results with several experimental tests made on X52 steel specimens. The agreement between the numerical and experimental results asses the validity of the proposed Model to predict the Plastic behavior of steel and its failure due to Ultra Low Cycle Fatigue.
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a general framework for continuum Damage Models ii integration algorithms with applications to the numerical simulation of porous metals
International Journal of Solids and Structures, 2000Co-Authors: F Armero, Sergio OllerAbstract:In this paper, we develop numerical algorithms for the integration of the continuum Plastic Damage Models formulated in the general framework identified in Part I of this work. More specifically, we focus our attention on a particular Plastic Damage Model of porous metals, involving a classical von Mises yield criterion coupled with a pressure dependent Damage surface to Model the nucleation and growth of voids in the metallic matrix. Unilateral Damage leading to a sudden change of stiffness in the material's response due to the closing/opening of these voids is also incorporated through the imposition of the unilateral constraint of a positive void fraction, thus, illustrating the clear physical significance added by this framework in the resulting constitutive Models. The proposed integration algorithms fully use the modularity of the identified framework, leading in this way to independent integration algorithms for the elastoPlastic part and each Damage mechanism. Remarkably, all these individual integration schemes share the same formal structure as the classical return mapping algorithms employed in the numerical integration of elastoPlastic Models, namely an operator split structure consisting of a trial state and the return map imposing the Plastic and Damage consistency, respectively. A Newton iterative scheme imposes the equilibrium (equal stresses) among the different mechanisms of the response of the material. This modular structure allows to obtain the closed-form consistent linearization, involving in a simple form the algorithmic consistent tangents corresponding to each independent mechanism, thus resulting in a very modular and efficient computational implementation. The performance of the proposed algorithms is illustrated in several representative numerical simulations.
Lunyang Zhao - One of the best experts on this subject based on the ideXlab platform.
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a micro mechanics based Plastic Damage Model for quasi brittle materials under a large range of compressive stress
International Journal of Plasticity, 2018Co-Authors: Jian-fu Shao, Qi-zhi Zhu, Lunyang ZhaoAbstract:Abstract In this paper, a new micro-mechanics based Plastic Damage Model is proposed for quasi-brittle materials under a large range of compressive stress. The Damage is due to initiation and propagation of micro-cracks while the Plastic deformation is directly related to frictional sliding along micro-cracks. The two dissipation processes are then physically coupled. With the Mori-Tanaka homogenization procedure and thermodynamics framework, the macroscopic state equations are deduced and the local driving forces of Damage and Plasticity are defined. New specific criteria are proposed for the description of Damage evolution and Plastic flow. These criteria take into account the variation of material resistance to Damage with confining pressure and the degradation of surface asperity of micro-cracks during the frictional sliding. An analytical analysis of macroscopic peak strength and volumetric compressibility-dilatancy transition is provided. A specific calibration procedure is further proposed for the determination of all Model's parameters from conventional triaxial compression tests. The efficiency of the proposed Model is verified against experimental data on three different materials and for a very large range of stress. All main features of mechanical behaviors of materials are well captured by the proposed Model.
A Venturagouveia - One of the best experts on this subject based on the ideXlab platform.
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Plastic Damage smeared crack Model to simulate the behaviour of structures made by cement based materials
International Journal of Solids and Structures, 2015Co-Authors: Edalat A Behbahani, Joaquim A O Barros, A VenturagouveiaAbstract:Abstract This work proposes a constitutive Model to simulate nonlinear behaviour of cement based materials subjected to different loading paths. The Model incorporates a multidirectional fixed smeared crack approach to simulate crack initiation and propagation, whereas the inelastic behaviour of material between cracks is treated by a numerical strategy that combines Plasticity and Damage theories. For capturing more realistically the shear stress transfer between the crack surfaces, a softening diagram is assumed for Modelling the crack shear stress versus crack shear strain. The Plastic-Damage Model is based on the yield function, flow rule and evolution law for hardening variable, and includes an explicit isotropic Damage law to simulate the stiffness degradation and the softening behaviour of cement based materials in compression. This Model was implemented into the FEMIX computer program, and experimental tests at material scale were simulated to appraise the predictive performance of this constitutive Model. The applicability of the Model for simulating the behaviour of reinforced concrete shear wall panels submitted to biaxial loading conditions, and RC beams failing in shear is investigated.