Papers by Author: Ryouji Kondou

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Authors: Chobin Makabe, K. Kuniyoshi, Masaki Fujikawa, Ryouji Kondou, D. Shinohara
Abstract: If the crystal grain size of a metal is made smaller, its strength is higher. So, many methods of grain refinement have been proposed. In this study, from the viewpoint of basic plastic working, the variations of static ultimate tensile strength and fatigue strength after the application of plastic torsional deformation on face centered cubic crystal metals, that are, aluminum and copper, were investigated. Tensile test, Vickers harness test and Rotating bending fatigue test were performed. The hardness of the materials varied from surface layer to center section in cross sectional area. In the case of aluminum, the tensile strength and fatigue limit were improved after application of torsional deformation. However, in the case of copper, the fatigue limit was not improved. This is strongly related to hardness distribution around the surface layer of the specimen. Also, it was found that the crack growth mode was changed by applying the pre-strain. From these results, one of an idea for improvement of material strength will be considered.
Authors: Takuya Yamane, Ryouji Kondou, Chobin Makabe
Abstract: A new grain refinement and strengthening technique by modified ECAP (equal-channel angular pressing) technique was proposed in this study. ECAP technique is an effective technique for grain refinement and strengthening of metal material. This technique gives high shear plastic strain to the material without geometric transformation of the specimen. However, traditional ECAP technique is restricted by material type and size, especially length. Such kinds of restrictions cause various problems in practical use. We modified traditional ECAP dies and processes to allow high plastic strain for long-length pure-aluminum specimens. Then, grain distribution was observed using a microscope, grain size was determined by the Jeffries and the Heyn methods, and strengthening was investigated by micro-Vickers hardness test. Then the effectiveness of proposed grain refinement or strengthening techniques was discussed.
Authors: Ryouji Kondou, Tetsuya Ohashi
Abstract: Slip deformation phenomena in compatible type multi crystal models subjected to tensile load are analyzed by a finite element crystal plasticity analysis code, and accumulation of geometrically-necessary and statistically-stored dislocations (GNDs and SSDs) are evaluated in detail. Crystal orientations for the grains are chosen so that mutual constraint of deformation through grain boundary planes does not take place. We call these models as compatible type multi crystals, because “compatibility requirements” at grain boundaries are automatically maintained by slip deformation only on the primary systems and uniform deformation is expected to occur in each grain. Results of the analysis, however, show non-uniform deformation with high density of GNDs accumulated in a form of band. Growth of such kind of structure of GNDs caused localized accumulation of SSDs at grain boundary triple junctions. Mechanism for the band-shaped accumulation of GNDs in the compatible type multi crystals are discussed from the viewpoint of multi body interactions which arise from shape change of crystal grains after slip deformation.
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