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A R Khoei - One of the best experts on this subject based on the ideXlab platform.
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x fem modeling of dynamic ductile fracture problems with a nonlocal damage viscoplasticity model
Finite Elements in Analysis and Design, 2015Co-Authors: P Broumand, A R KhoeiAbstract:In this paper, the dynamic large deformation X-FEM method is presented for modeling the full process of dynamic ductile fracture based on a nonlocal damage visco-plasticity model. The effect of inertia is modeled using an explicit central difference scheme which is enhanced through the use of mass lumping, reduced integration with hourglass control, and numerical damping. The material nonlinearity and the flow stress dependency on strain rate, hardening and temperature are modeled with the Johnson-Cook visco-plastic model. The micro-void nucleation, growth and coalescence are modeled macroscopically with an isotropic damage model. The Localization Phenomenon due to the damage and thermal softening is suppressed by using the visco-plastic regularization in combination with the nonlocal visco-plastic model. The large deformation and large strain formulations are implemented within the X-FEM framework to model the macro crack discontinuities using an updated Lagrangian approach. Crack propagation and crack direction criteria are presented and the issues relating to the combination of X-FEM with the damage visco-plasticity model are addressed. Finally, the robustness and accuracy of the proposed method are verified through several numerical examples. HighlightsA dynamic large X-FEM deformation is presented for modeling dynamic ductile fracture.An explicit algorithm is used with mass lumping, reduced integration and numerical damping.The material nonlinearity is modeled with the Johnson-Cook visco-plastic model.The micro void nucleation, growth and coalescence is modeled with an isotropic damage model.Localization of damage and thermal softening is captured with a nonlocal visco-plastic model.
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an enriched fem model for simulation of Localization Phenomenon in cosserat continuum theory
Computational Materials Science, 2008Co-Authors: A R Khoei, Kamran KarimiAbstract:Abstract The standard finite element models, i.e. the finite element methods that use the classical continuum models, suffer from the excessive mesh dependence when a strain-softening model is used. It cannot converge to a meaningful solution and the governing differential equation loses the ellipticity. This paper presents an enriched finite element algorithm for simulation of Localization Phenomenon using a higher order continuum model based on the Cosserat continuum theory. The governing equations are regularized by adding the rotational degrees-of-freedom to the conventional degrees-of-freedom and including the internal length parameter in the model. The extended finite element method (X-FEM) is employed, in which the discontinuity interfaces are represented independent of element boundaries and the process is accomplished by partitioning the domain with some triangular sub-elements whose Gauss points are used for integration of the domain of elements. Finally, several numerical examples are analyzed to demonstrate the efficiency of the mixed XFEM – Cosserat continuum model in shear band Localization.
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numerical analysis of strain Localization in metal powder forming processes
International Journal for Numerical Methods in Engineering, 2001Co-Authors: R W Lewis, A R KhoeiAbstract:It is well known that strain Localization and indeed displacement discontinuity can arise in materials exhibiting plastic behaviour. Indeed such Localization is almost certain to occur if strain softening or non-associated behaviour exists, though it can be triggered even when ideal plasticity is assumed. This study is concerned mainly with the manner in which the numerical discretization process has to be devised so as to capture the Localization Phenomenon. In this paper, a method is presented for applying the mixed formulation to study the prediction of Localization Phenomenon in powder-forming processes. This study is focused on the performance of mixed u−π triangular elements to study their efficiency in indicating Localization for various mesh refinements. An adaptive analysis using element elongation is applied in the modelling of strain Localization. The numerical results are obtained for a Von Mises yield criterion applied to a multi-level component, at the final stage of compaction. It is shown that if a correct approximation is used then both the uniform and non-uniform mesh refinements will converge to the correct answer and clearly indicate the Localization Phenomenon. It is also observed that an adaptive analysis using element elongation can be effective in the modelling of such phenomena. Copyright © 2001 John Wiley & Sons, Ltd.
Wei Zhang - One of the best experts on this subject based on the ideXlab platform.
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type iv failure in weldment of creep resistant ferritic alloys i micromechanical origin of creep strain Localization in the heat affected zone
Journal of The Mechanics and Physics of Solids, 2020Co-Authors: Wei Zhang, Xue Wang, Yiyu Wang, Yanfei Gao, Zhili FengAbstract:Abstract Creep strength enhanced ferritic (CSEF) steels containing 9-12wt% chromium have been extensively used in fossil-fuel-fired power plants. Despite their excellent creep resistance at high temperatures, premature failures (especially Type IV cracking) are often found in the fine-grained heat affected zone (HAZ) or intercritical HAZ of the welded components. This failure mode is preceded by the strain Localization in the HAZ, as measured by the Digital Image Correlation (DIC) technique. The present work aims to develop a finite-element based computational method to determine the micromechanical and microstructural origin of the strain Localization Phenomenon. We construct a two-dimensional digital microstructure based on the actual microstructure of ferritic steel weldments by using the Voronoi-tessellation method, to account for the effects of its large grain-size gradients. A mechanism-based finite element method is developed for modeling the high temperature deformation resulting from a synergy of thermally activated dislocation movements, diffusional flow and grain boundary sliding. The numerical results agree well with the strain measurements by our DIC technique, particularly revealing the effect of pre-welding tempering on the evolution of strain Localization in HAZ of creep resistant steel weldments. It is found that the diffusional creep with dependence on grain sizes, dislocation creep with dependence on material strength, and more importantly, grain boundary sliding, contribute synergistically to the creep strain accumulation in the HAZ, and their relative degree of significance is quantified. The creep rupture life will be investigated in the companion paper.
Hadrien Rattez - One of the best experts on this subject based on the ideXlab platform.
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Numerical Analysis of Strain Localization in Rocks with Thermo-hydro-mechanical Couplings Using Cosserat Continuum
Rock Mechanics and Rock Engineering, 2018Co-Authors: Hadrien Rattez, Ioannis Stefanou, Jean Sulem, Manolis Veveakis, Thomas PouletAbstract:A numerical model for thermo-hydro-mechanical strong couplings in an elasto-plastic Cosserat continuum is developed to explore the influence of frictional heating and thermal pore fluid pressurization on the strain Localization Phenomenon. This model allows specifically to study the complete stress–strain response of a rock specimen, as well as the size of the strain Localization zone for a rock taking into account its microstructure. The numerical implementation in a finite element code is presented, matching adequately analytical solutions or results from other simulations found in the literature. Two different applications of the numerical model are also presented to highlight its capabilities. The first one is a biaxial test on a saturated weak sandstone, for which the influence on the stress–strain response of the characteristic size of the microstructure and of thermal pressurization is investigated. The second one is the rapid shearing of a mature fault zone in the brittle part of the lithosphere. In this example, the evolution of the thickness of the localized zone and the influence of the permeability change on the stress–strain response are studied.
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the importance of thermo hydro mechanical couplings and microstructure to strain Localization in 3d continua with application to seismic faults part i theory and linear stability analysis
Journal of The Mechanics and Physics of Solids, 2018Co-Authors: Hadrien RattezAbstract:Abstract A Thermo-Hydro-Mechanical (THM) model for Cosserat continua is developed to explore the influence of frictional heating and thermal pore fluid pressurization on the strain Localization Phenomenon. A general framework is presented to conduct a bifurcation analysis for elasto-plastic Cosserat continua with THM couplings and predict the onset of instability. The presence of internal lengths in Cosserat continua enables to estimate the thickness of the Localization zone. This is done by performing a linear stability analysis of the system and looking for the selected wavelength corresponding to the instability mode with fastest finite growth coefficient. These concepts are applied to the study of fault zones under fast shearing. For doing so, we consider a model of a sheared saturated infinite granular layer. The influence of THM couplings on the bifurcation state and the shear band width is investigated. Taking representative parameters for a centroidal fault gouge, the evolution of the thickness of the localized zone under continuous shear is studied. Furthermore, the effect of grain crushing inside the shear band is explored by varying the internal length of the constitutive law.
Yue-sheng Wang - One of the best experts on this subject based on the ideXlab platform.
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the propagation and Localization of rayleigh waves in disordered piezoelectric phononic crystals
Journal of The Mechanics and Physics of Solids, 2008Co-Authors: Yize Wang, Wenhu Huang, Yue-sheng WangAbstract:Abstract In this paper, the propagation and Localization of Rayleigh waves in disordered piezoelectric phononic crystals with material 6 mm are studied taking the electromechanical coupling into account. The electric field is approximated as quasi-static. The analytical solutions of Rayleigh waves are obtained. The 6×6 transfer matrix between two consecutive unit cells is obtained by means of the mechanical and electrical continuity conditions. The expression of the Localization factor in disordered periodic structures is presented by regarding the variables of the mechanical and electrical fields as the elements of the state vector. The numerical results for a piezoelectric phononic crystal—PVDF-PZT-2 piezocomposite—are presented and analyzed. From the results we can see that the Localization is strengthened with the increase of the disorder degree. The characteristics of the passbands and stopbands are influenced by different ratios of the thickness of the polymers to that of the piezoelectric ceramics. Disorder in elastic constant c11 of PZT-2 can also result in the Localization Phenomenon. The propagation and Localization of Rayleigh waves in piezoelectric phononic crystals may be controlled by properly designing some structural parameters.
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Study on band structures and Localization Phenomenon of twodimensional phononic crystals with one-dimensional quasi-periodicity
2008 IEEE Ultrasonics Symposium, 2008Co-Authors: A-li Chen, Yue-sheng WangAbstract:By viewing the quasi-periodicity as the deviation from the periodicity in a particular way, the quasi phononic crystal that has quasi-periodicity (Fibonacci sequence) in one direction and translational symmetry in the other direction is considered. The band structures are characterized by Localization factors which are calculated by using the plane-wave-based transfer-matrix method. The results show that the Localization factor is an effective parameter in characterizing the band gaps of two-dimensional phononic crystals with one-dimensional quasi-periodicity. The quasi phononic crystals exhibit more band gaps with narrower width than the ordered and randomly disordered systems.
Zhili Feng - One of the best experts on this subject based on the ideXlab platform.
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type iv failure in weldment of creep resistant ferritic alloys i micromechanical origin of creep strain Localization in the heat affected zone
Journal of The Mechanics and Physics of Solids, 2020Co-Authors: Wei Zhang, Xue Wang, Yiyu Wang, Yanfei Gao, Zhili FengAbstract:Abstract Creep strength enhanced ferritic (CSEF) steels containing 9-12wt% chromium have been extensively used in fossil-fuel-fired power plants. Despite their excellent creep resistance at high temperatures, premature failures (especially Type IV cracking) are often found in the fine-grained heat affected zone (HAZ) or intercritical HAZ of the welded components. This failure mode is preceded by the strain Localization in the HAZ, as measured by the Digital Image Correlation (DIC) technique. The present work aims to develop a finite-element based computational method to determine the micromechanical and microstructural origin of the strain Localization Phenomenon. We construct a two-dimensional digital microstructure based on the actual microstructure of ferritic steel weldments by using the Voronoi-tessellation method, to account for the effects of its large grain-size gradients. A mechanism-based finite element method is developed for modeling the high temperature deformation resulting from a synergy of thermally activated dislocation movements, diffusional flow and grain boundary sliding. The numerical results agree well with the strain measurements by our DIC technique, particularly revealing the effect of pre-welding tempering on the evolution of strain Localization in HAZ of creep resistant steel weldments. It is found that the diffusional creep with dependence on grain sizes, dislocation creep with dependence on material strength, and more importantly, grain boundary sliding, contribute synergistically to the creep strain accumulation in the HAZ, and their relative degree of significance is quantified. The creep rupture life will be investigated in the companion paper.
