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Fusao Oka - One of the best experts on this subject based on the ideXlab platform.
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dynamic analysis of Strain Localization in water saturated elasto viscoplastic material
International Workshop on Bifurcation and Degradation in Geomaterials, 2014Co-Authors: Fusao Oka, Shahbodagh B Khan, M Mirjalili, Sayuri KimotoAbstract:Although Strain Localization has been studied for the last four decades, there are still many problems, in particular for the dynamic Localization problem. In the present study, we have numerically analyzed a dynamic Strain Localization problem for the water-saturated elasto-viscoplastic constitutive material. For the material model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil skeleton are considered. In order to numerically simulate the large deformation phenomenon in Strain Localization analysis, the dynamic finite element formulation for a two-phase mixture using the updated Lagrangian method is adopted. The shear band development is discussed through the distributions of the viscoplastic shear Strain in a material.
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Effects of sample shape on the Strain Localization of water‐saturated clay
International Journal for Numerical and Analytical Methods in Geomechanics, 2007Co-Authors: Takeshi Kodaka, Yosuke Higo, Sayuri Kimoto, Fusao OkaAbstract:Strain Localization is an important geotechnical problem related to large deformations and the onset of failure, such as slope failure. It is necessary, therefore, to clarify the mechanisms of the Strain Localization of geomaterials in order to predict large deformations of the ground. For the last two decades, many researchers have studied the Strain Localization of geomaterials through both experimental and numerical works. Most of the works, however, particularly the numerical studies, have been treated as two-dimensional plane Strain problems for the sake of simplicity, even though the actual phenomena are generally three dimensional. In order to understand the deformation, the failure, and the Strain Localization of clay under three-dimensional conditions, triaxial tests on clay and their numerical simulation are performed in the present study. In particular, focus is mainly placed on the effects of sample shape on the Localization behaviour of normally consolidated and overconsolidated clays. A series of undrained triaxial compression tests, using rectangular clay specimens with different shapes and Strain rates, is conducted. Localized shear Strain distributions are successfully observed with an image analysis of digital photographs. It is seen that the shape of a specimen affects the various bifurcation phenomena of clay, e.g. the formation and the progress of various three-dimensional shear bands, failure with buckling, etc. The numerical simulation using the finite element method, with an elasto-viscoplastic model and considering structural changes, can reproduce the generation and the growing process of shear bands well. A comparison between the results of the experiment and the simulation offers new findings regarding the Strain Localization of clay under three-dimensional conditions.
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Instability of gradient-dependent elastoviscoplastic model for clay and Strain Localization analysis
Computer Methods in Applied Mechanics and Engineering, 2000Co-Authors: Fusao Oka, Atsushi Yashima, Kazuhide Sawada, Elias C. AifantisAbstract:The instability of the Strain gradient-dependent elastoviscoplastic constitutive model for water saturated clay is studied in relation to the Strain Localization phenomena and pattern formation during deformation. The second-order gradient of volumetric viscoplastic Strain is introduced into the constitutive equations to account for the non-local effects associated with the motion of microstructures. Boundary value problems by using finite element are formulated and Strain Localization analyses are carried out.
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A Strain Localization analysis using a viscoplastic softening model for clay
International Journal of Plasticity, 1995Co-Authors: Fusao Oka, Toshihisa Adachi, Atsushi YashimaAbstract:Strain Localization has become an attractive subject in geomechanics during the past decade. Shear bands are well known to develop in clay specimens during the Straining process. Strain Localization is closely related to plastic instability. In the present paper, a non-linear instability condition for the viscoplastic Strain softening model during the creep process is firstly obtained. It is found that the proposed viscoplastic model is capable of describing plastic instability. Secondly, a two-dimensional linear instability analysis is performed and the preferred orientation for the growth of fluctuation and the instability condition are derived. It is worth noting that the two instability conditions are equivalent. Finally, the behavior of the clay is numerically analyzed in undrained plane-Strain compression tests by the finite element method, considering a transport of pore water in the material at a quasi-static Strain rate. The numerical results show that the model can predict Strain Localization phenomena, such as shear banding. From the numerical calculations, the effects of Strain rate and permeability are discussed.
Mao Song Huang - One of the best experts on this subject based on the ideXlab platform.
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Study on the Initiation of Strain Localization in Soils by 3D Non-coaxial Plasticity
Springer Series in Geomechanics and Geoengineering, 2014Co-Authors: Jianju Qian, Mao Song HuangAbstract:The Strain Localization in soil under plane Strain and true triaxial conditions were predicted by bifurcation analysis on the non-coaxial plasticity. By using an elliptical shape function to modify the Mohr-Coulomb failure criterion, a 3D non-coaxial non-associated elasto-plasticity model was proposed by adding a non-coaxial plasticity flow rule. Comparing to the experiments, bifurcation analysis on the non-coaxial model gives out better prediction than the coaxial models. Strain Localization occurs at the hardening regime of the soil under plane Strain condition, the influence of confining stress on the Strain Localization could be described by non-coaxial model rightly. The prediction of true triaxial tests showed the non-coaxial model reflects the influence of the intermediate stress ratio on Strain Localization well.
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Prediction of plane Strain undrained diffuse instability and Strain Localization with non-coaxial plasticity
Soils and Foundations, 2014Co-Authors: Mao Song Huang, Jiangu QianAbstract:Abstract In this paper, undrained diffuse instability and Strain Localization of frictional materials under plane Strain conditions were studied. Based on a 3D non-associated Mohr–Coulomb hardening model, the theoretical criteria for diffuse instability and Strain Localization were proposed by the second-order work theorem and the vanishing of the determinant of the acoustic tensor. The criteria were used to predict the instability characteristics of soil specimen in isotropically and anisotropically consolidated plane Strain tests. The studies showed that, when the soil specimen was loaded under Strain-controlled loading mode, the soil becomes potentially unstable slightly before the shear stress reaches its peak value. The initiation of diffuse instability accompanies with the peak of shear stress, and Strain softening occurs with further loading. Strain Localization was predicted by the vanishing of the determinant of the acoustic tensor, and it is shown to occur after the diffuse instability. The non-coaxial plasticity flow rule was adopted to improve the prediction the onset of Strain Localization, while the inclusion of non-coaxial plasticity flow rule shows no influence on diffuse instability. Both diffuse instability and Strain Localization occur at the hardening stage of the soil and result in the reduction of the deviatoric shear stress.
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The Critical State Constitutive Model and Strain Localization for Hostun Sand
Applied Mechanics and Materials, 2014Co-Authors: Mao Song HuangAbstract:Aiming at the limitation of traditional constitutive theories which can not explain the decrease of inclination angle of shear bands correctly when confining pressure increases, a state-dependent critical state model is employed to analyze Strain Localization of Hostun sands. Firstly, the critical state formulation and model parameters are calibrated through available drained triaxial test results of Hostun sand. Then an analysis of Strain Localization is performed on the drained plane Strain tests. The results show that the state-dependent critical state model is capable of simulating the effect of initial effective confining pressure on inclination angle of shear bands.
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Strain Localization of Sand Based on a State-Dependent Critical State Model
Advanced Materials Research, 2011Co-Authors: Mao Song Huang, Hui Lai QinAbstract:A state-dependent critical state model is employed to analyze Strain Localization of sand in this paper. The sand model is developed based on the concept of the state-dependent critical state for sands, and an analysis of Strain Localization is performed on the drained plane Strain tests of Hostun sand. The results show that the state-dependent critical state model is capable of simulating the effect of initial state on Strain Localization compatibly.
Yong-qiang Zhang - One of the best experts on this subject based on the ideXlab platform.
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Analysis of Strain Localization for ductile materials with effect of void growth
International Journal of Mechanical Sciences, 2004Co-Authors: Yong-qiang Zhang, Gui-rong Liu, Xiao HanAbstract:In the framework of thermodynamics and damage mechanics, an elastoplastic constitutive relation of ductile materials is developed. Based on this constitutive relation, the properties of Strain Localization for ductile materials are investigated with the effect of void growth included. The influences of void growth on Strain Localization are analyzed based on the unified strength criterion. Finally, the effects of yield criteria on the properties of Strain Localization are discussed.
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Investigation of Strain Localization in elastoplastic materials with transversely isotropic elasticity
International Journal of Mechanical Sciences, 2003Co-Authors: Yong-qiang ZhangAbstract:Transversely isotropic materials are of primary interest in many engineering applications. In this paper, general description of the properties of Strain Localization is deduced for the elastoplastic materials with transversely isotropic elasticity which follow the non-associated plasticity and are subjected to tri-axial stress states. Then the explicit expressions for the direction of localized band and the corresponding hardening modulus at the onset of Strain Localization are obtained for plane Strain condition. Furthermore, the effect of deviation from isotropic elasticity in the formulation of Strain Localization is discussed based on those yield criteria such as Tresca criterion and twin-shear criterion. In the end, the influences of yield criteria on the properties of Strain Localization are elucidated.
Atsushi Yashima - One of the best experts on this subject based on the ideXlab platform.
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Instability of gradient-dependent elastoviscoplastic model for clay and Strain Localization analysis
Computer Methods in Applied Mechanics and Engineering, 2000Co-Authors: Fusao Oka, Atsushi Yashima, Kazuhide Sawada, Elias C. AifantisAbstract:The instability of the Strain gradient-dependent elastoviscoplastic constitutive model for water saturated clay is studied in relation to the Strain Localization phenomena and pattern formation during deformation. The second-order gradient of volumetric viscoplastic Strain is introduced into the constitutive equations to account for the non-local effects associated with the motion of microstructures. Boundary value problems by using finite element are formulated and Strain Localization analyses are carried out.
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A Strain Localization analysis using a viscoplastic softening model for clay
International Journal of Plasticity, 1995Co-Authors: Fusao Oka, Toshihisa Adachi, Atsushi YashimaAbstract:Strain Localization has become an attractive subject in geomechanics during the past decade. Shear bands are well known to develop in clay specimens during the Straining process. Strain Localization is closely related to plastic instability. In the present paper, a non-linear instability condition for the viscoplastic Strain softening model during the creep process is firstly obtained. It is found that the proposed viscoplastic model is capable of describing plastic instability. Secondly, a two-dimensional linear instability analysis is performed and the preferred orientation for the growth of fluctuation and the instability condition are derived. It is worth noting that the two instability conditions are equivalent. Finally, the behavior of the clay is numerically analyzed in undrained plane-Strain compression tests by the finite element method, considering a transport of pore water in the material at a quasi-static Strain rate. The numerical results show that the model can predict Strain Localization phenomena, such as shear banding. From the numerical calculations, the effects of Strain rate and permeability are discussed.
Claudio Marchesi - One of the best experts on this subject based on the ideXlab platform.
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Fluid-assisted Strain Localization in the shallow subcontinental lithospheric mantle
Lithos, 2016Co-Authors: Károly Hidas, Andréa Tommasi, Carlos J. Garrido, José Alberto Padrón-navarta, David Mainprice, Alain Vauchez, Fabrice Barou, Claudio MarchesiAbstract:We report microstructural evidence for fluid-assisted ductile Strain Localization in a ≤ 50 m-wide mylonitic–ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain Localization occurred at relatively low pressure (< 0.8 GPa) and moderate temperature (750–1000 °C). Initial deformation by dislocation creep resulted in formation of mylonites. Focusing of aqueous fluids in the shear zone favored the activation of dissolution–precipitation creep, resulting in further Strain Localization. This process is recorded by two generations of ultramylonitic bands composed of fine-grained, well-mixed olivine–orthopyroxene aggregates. Microstructural observations in the ultramylonites suggest alternating dissolution and precipitation of olivine and orthopyroxene, which may be explained by local changes in silica molality of the percolating fluid (disequilibrium and mass transfer at scales > mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution–precipitation. In the present study, dissolution–precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between Strain Localization and fluid transport.
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Fluid-assisted Strain Localization in the shallow subcontinental lithospheric mantle
Lithos, 2016Co-Authors: Károly Hidas, Andréa Tommasi, Carlos J. Garrido, José Alberto Padrón-navarta, David Mainprice, Alain Vauchez, Fabrice Barou, Claudio MarchesiAbstract:We report microstructural evidence for fluid-assisted ductile Strain Localization in a ≤ 50 m-wide mylonitic–ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain Localization occurred at relatively low pressure ( mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution–precipitation. In the present study, dissolution–precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between Strain Localization and fluid transport.
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Strain Localization in Pyroxenite by Reaction-Enhanced Softening in the Shallow Subcontinental Lithospheric Mantle
Journal of Petrology, 2013Co-Authors: Károly Hidas, Andréa Tommasi, Carlos J. Garrido, José Alberto Padrón-navarta, Marcel Thielmann, Zoltan Konc, Erwin Frets, Claudio MarchesiAbstract:We report structural evidence of ductile Strain Localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain Localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction ('dehydroxylation') from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further Strain Localization in these pyroxenite bands. The Strain Localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1*0 to 0*5 GPa.
