Eiji Yanagisawa

Contact Tensor in Constitutive Model for Granular Materials

Summary The roles of the contact tensor in the constitutive model for granular materials are considered. After deriving the micromechanically based contact tensor and the modified stress tensor, we give possible physical meanings of them. The constitutive model with these tensors and their rates can account for more features of deformation of granular materials. The anisotropic hardening feature and the non-coaxiality between the rate of deformation and the stress tensors are given special attention in this paper.

Mode switching and recursive bifurcation in granular materials

Abstract The existence of the “mode switching” and “recursive bifurcation” behavior in granular materials (sand triaxial-compression-test specimens) under shearing is revealed. Group-theoretic bifurcation theory is employed to categorize bifurcation modes through the observation of deformation patterns of these materials, and hence to detect the occurrence of recursive bifurcation. A bifurcation point search technique is developed and applied to a strain versus stress curve of a specimen to identify the presence of a series of bifurcation points associated with the recursive bifurcation. In addition, an imperfection sensitivity law is employed to testify the occurrence of mode switching between two dominant bifurcation modes. Curves of strain versus stress undergoing mode switching and recursive bifurcation are simulated well by the bifurcation equation. The predominant role of mode switching and recursive bifurcation in shearing behavior of the granular materials is clearly demonstrated and fully described with the use of the bifurcation theory.

Imperfection sensitive strength variation of soil specimens

The mechanism of the statistical variation of the shearing behavior of soil specimens is investigated from the viewpoint of bifurcation, under the major assumption that these specimens undergo elastic bifurcation, and that such variation is ascribed to the variation of initial imperfections among specimens. Through a synthesis and an extension of the pre-existing bifurcation theory, we advance a method to describe that variation. Two series of triaxial compression tests are conducted to construct a data bank of the shearing behavior of sand specimens. An imperfection sensitivity law is employed to describe the variation in their axial strain versus axial stress curves and volumetric strain versus axial stress curves, and hence to assess the validity of those assumptions to demonstrate the predominant role of elastic bifurcation in soil shearing behavior. The theory of the stochasticity of initial imperfections is employed to explain the probabilistic variation of the maximum strength.

Description of Cyclic Deformation Behavior of Sand by an Anisotropic Hardening Model

Abstract A simple anisotropic hardening model for granular material is proposed to describe cyclic behavior of sands during earthquake excitations. Special emphasis was placed on the formulation of unloading and reloading behavior of the model. Some numerical results of response of a sand element under repeated loading condition are presented in order to demonstrate some features of the present model. Considerations are made on the calculation results by comparing to the laboratory test results and possible alternative formulation of the constitutive model is suggested.