Kazuyoshi Iwashita

Micro-deformation mechanism of shear banding process based on modified distinct element method

Numerical simulation tests were carried out using the distinct element method (DEM) by paying much attention to the micro-deformation mechanism leading to the development of shear bands. To do this, the conventional DEM was modified slightly such that the effect of rolling resistance at contact points could be taken into account (called MDEM). It is found that MDEM can be a powerful tool for simulating not only the generation of large voids inside a shear band but also the high gradient of particle rotation along the shear band boundaries, in a quite similar manner to those observed in natural granular soils. It is concluded, based on the numerical simulation tests, that the basic micro-deformation mechanism ending up with the formation of shear bands is in the generation of a column-like structure during the hardening process and its collapse in the softening process.

Study on couple stress and shear band development in granular media based on numerical simulation analyses

From the reasonable accordance between the simulation and laboratory tests, it is concluded that the simulation test using the distinct element method can provide a powerful tool to simulate the micro- as well as macro-behavior of granular media. This is true, in particular, when the rotational resistance is introduced into the conventional one. Based on both the simulation and laboratory tests, we reached the following conclusions: One of the most important changes in microstructure, which takes place during a strain hardening process, is the formation of column-like structure growing parallel to the major principal stress direction. After failure, the column-like structure is reconstructed during a strain softening process by means of rolling, not sliding, at contact points so that a high gradient of particle rotation is generated, changing from negative to positive in a relatively narrow shear zone. Large voids appear in the shear band, and the resulting local void ratio can exceed the corresponding maximum one determined by a standard method. This fact strongly suggests that unique stress condition, which leads to such special microstructure, may develop in the shear band. In fact, couple stresses exist in a shear band in a manner consistent with the change of the particle rotation gradient from negative to positive. In spite of the presence of the couple stress, the stress tensor is nearly symmetric, indicating that the couple stress is very small in magnitude. The presence of the small couple stress still plays an important role in the development of microstructure in shear bands.

Influence of Bedding on Shear Band Formation of Quasistatic Granular Media

We study the influence of inherent anisotropy (i.e., bedding angle) on stress‐strain behavior shear band formation in quasistatic granular media. Plane strain biaxial tests are carried out using two‐dimensional Distinct Element Method (DEM). Oval/elliptical shaped particles of aspect ratio 1.5 generated by overlapping discrete circular elements are used in the simulations. Formation of shear band with different bedding angles is investigated. It is found that assembly of elliptical particles exhibits shear band formation with standard DEM without considering rolling resistance.

IN-SITU STUDY ON STRENGTH AND HYDRAULIC LOSSES OF FIBER REINFORCED PLASTIC PENSTOCK IN SEAWATER PUMPED STORAGE POWER PLANT

海水揚水発電は, 海域を下部調整池とすることによって, コストダウンや立地に関わる制約条件の緩和などが期待できる. 一方で, 水圧管路の海水腐食や海生生物の固着による水理機能低下が課題である. 海水揚水発電技術実証プラントでは, 岩盤埋設型水圧管路の材料として初めてFRP管を適用し, その強度特性や水理特性に着目した検証を1999年3月から2004年2月まで行った. その結果, FRP管の挙動はクリープによる影響が認められるが, 内圧の岩盤負担効果が期待できること, また海生生物の固着に伴う水頭損失は設計値を大きく下回り, 発電所の水路として将来にわたって良好な水理特性を維持できるものと判断された. これらは海水揚水商用プラントの実現に向け, 極めて有意義な情報を与えるものと考えられる.