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Iai S.,Kyoto University | Ueda K.,Kyoto University | Tobita T.,Kyoto University | Ozutsumi O.,Meisosha Co.
International Journal for Numerical and Analytical Methods in Geomechanics | Year: 2013

This paper presents the finite strain formulation of a strain space multiple mechanism model for granular materials. Because the strain space multiple mechanism model has an appropriate micromechanical background in which the branch and complementary vectors are defined in the material (or referential) coordinate, the finite strain formulation is carried out by following the change in these vectors, in direction and magnitude, associated with deformation in the material. By applying the methodology for compressible materials established in the finite strain continuum mechanics, decoupled formulation that decomposes the kinematic mechanisms into volumetric and isochoric components is adopted for the strain space multiple mechanism model. Lagrangian (material) description of integrated form is given by a relation between the second Piola-Kirchhoff effective stress tensor and the Green-Lagrange strain tensor; Eulerian (spatial) description by a relation between the Cauchy effective stress tensor and the Euler-Almansi strain tensor. In particular, the volumetric strain is defined as a logarithm of Jacobian determinant. Lagrangian (material) description of incremental form is derived through the material time derivative of the integrated form. The counterpart in the spatial description is derived through the Lie time derivative, given as a relation between the Oldroyd stress rate of Kirchhoff stress and the rate of deformation tensor (sometimes called stretching in the literatures). An example is shown to discuss the applicability of the finite strain formulation. © 2012 John Wiley & Sons, Ltd. Source


Iai S.,Kyoto University | Tobita T.,Kyoto University | Ozutsumi O.,Meisosha Co.
International Journal for Numerical and Analytical Methods in Geomechanics | Year: 2013

SUMMARY: The strain space multiple mechanism model idealizes the behavior of granular materials based on a multitude of virtual simple shear mechanisms oriented in arbitrary directions. Within this modeling framework, the virtual simple shear stress is defined as a quantity that depends on the contact distribution function as well as the normal and tangential components of inter-particle contact forces, which evolve independently during the loading process. In other terms, the virtual simple shear stress is an intermediate quantity in the upscaling process from the microscopic level (characterized by the contact distribution and inter-particle contact forces). The stress space fabric (i.e. the orientation distribution of the virtual simple shear stress) produces macroscopic stress through the tensorial average. Thus, the stress space fabric characterizes the fundamental and higher modes of anisotropy induced in granular materials. Comparing an induced fabric associated with the biaxial shear of plane granular assemblies obtained via a simulation using Discrete Element Method to the strain space multiple mechanism model suggests that the strain space multiple mechanism model has the capability to capture the essential features in the evolution of an induced fabric in granular materials. © 2012 John Wiley & Sons, Ltd. Source


Iai S.,Kyoto University | Tobita T.,Kyoto University | Ozutsumi O.,Meisosha Co.
International Journal for Numerical and Analytical Methods in Geomechanics | Year: 2013

The strain space multiple mechanism model idealizes the behavior of granular materials on the basis of a multitude of virtual simple shear mechanisms oriented in arbitrary directions. Within this modeling framework, the virtual simple shear stress is defined as a quantity dependent on the contact distribution function as well as the normal and tangential components of interparticle contact forces, which evolve independently during the loading process. In other terms, the virtual simple shear stress is an intermediate quantity in the upscaling process from the microscopic level (characterized by contact distribution and interparticle contact forces) to the macroscopic stress. The stress space fabric produces macroscopic stress through the tensorial average. Thus, the stress space fabric characterizes the fundamental and higher modes of anisotropy induced in granular materials. Herein, the induced fabric is associated with monotonic and cyclic loadings, loading with the rotation of the principal stress, and general loading. Upon loading with the rotation of the principal stress axis, some of the virtual simple shear mechanisms undergo loading whereas others undergo unloading. This process of fabric evolution is the primary cause of noncoaxiality between the axes of principal stresses and strains. Although cyclic behavior and behavior under the rotation of the principal stress axis seem to originate from two distinct mechanisms, the strain space multiple mechanism model demonstrates that these behaviors are closely related through the hysteretic damping factor. Copyright © 2011 John Wiley & Sons, Ltd. Source


Tobita T.,Kyoto University | Manzari M.T.,George Washington University | Ozutsumi O.,Meisosha Co. | Ueda K.,Railway Technical Research Institute | And 2 more authors.
Geotechnics for Catastrophic Flooding Events - Proceedings of the 4th International Conference on Geotechnical Engineering for Disaster Mitigation and Rehabilation, GEDMAR 2014 | Year: 2015

Over the past twenty years after the VELACS project, with major advancement of computer simulation technologies, research works on numerical constitutive modelling of liquefiable sand have been conducted intensively. Contribution of the VELACS project on the development of numerical modeling in this field has never been underrated. However, with the development of new and innovative models, we believe that now is the time to revisit the project and help define direction of research for possibly another twenty years'. As a pilot project for the future international collaborations to examine the capabilities of existing numerical techniques for liquefaction analysis through laboratory experiments and centrifuge tests, a Class A prediction is conducted by numerical modelers of four institutes for a series of centrifuge experiments of flat and inclined saturated sand deposits.A brief description of the experiments, results of the numerical predictions, and general remarks for the next step are presented in this paper. © 2015 Taylor & Francis Group, London. Source


Iai S.,Kyoto University | Tobita T.,Kyoto University | Hussien M.N.,Kyoto University | Rollins K.M.,Brigham Young University | Ozutsumi O.,Meisosha Co.
Soil-Foundation-Structure Interaction - Selected Papers from the International Workshop on Soil-Foundation-Structure Interaction, SFSI 09 | Year: 2010

The paper reviews (1) soil-pile interaction in horizontal plane, (2) a simplified approach for idealizing the soil-pile interaction in terms of a non-linear spring, and (3) effect of soil-pile separation. The review is based on the experimental (laboratory and full scale in-situ tests) and numerical studies performed by the authors over recent years. © 2010 Taylor & Francis Group. Source

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