Papers by Author: Yutaka Imaida

Abstract: The purpose of this study is to verify the validity of sheet buckling design based on the effective width theory investigation of impact crushing properties in high strength steel sheets. We clarify the need to make full sections effectively without elastic buckling occurring and consider the application of the effective width theory under high speed deformation. We report our findings of our investigation into the sheet buckling phenomenon with numerical simulation by varying deformation strain rate, mechanical property of material and member configuration. The results demonstrate that, with increasing crush speed, the cross section of a steel sheet become effective, while there is a high possibility of the buckling phenomenon that does not function as efficient impact absorption and is not evaluated with the existing theory.

Abstract: Recently, semi-solid processing is paid to attention in the field of the light alloys. By this method, it is improved ductility and fatigue strength. Although, because of those mechanical properties of the elongation and toughness is not excellent, the range that can be the application to parts is limited. On the other hand, it is reported that grain refinements cause improvement of ductility and appearance of super plasticity. Then, Equal-Channel Angular Pressing (ECAP) method is reported to be effective to the sample making of a bulk and ultra fine grain in various alloys in recent years. In this study, it tried to improve ductility and durability due to making ultra fine grain in AC4CH alloys by the ECAP method, and the influence of ECAP processing on the mechanical property of AC4CH was investigated. As the result, the ductility of AC4CH has improved by ECAP processing. However, the tensile strength of AC4CH declined along with the increase in the number of passes. So, for the purpose of additional improvement of tensile strength, ECAP-Back Pressure (ECAP-BP) method that was reported to be more effective for grain refinements than ECAP method was applied to semi-solid AC4CH and compared with ECAP method. As the result, the tensile strength of AC4CH was maintained by use of ECAP-BP. Moreover, both ductility and toughness of that have been also improved.

Abstract: This paper proposes new impact testing methods applicable to sheet metals both under tension and compression based on widely used split Hopkinson pressure bar (SHPB) technique. Explicit dynamic finite element simulations by using LS-DYNA 3D are systematically conducted for several specimen clamping conditions to seek the appropriate methodologies to realize the two tests. For the tensile test, a method which can reduce stress oscillations that usually appear in the measured stress-strain curves is proposed and is devised to be used with SHPB technique. For the compression test, a candidate which can restrict buckling of the specimen is proposed. The method uses a special die-set sandwiching the sheet metal specimen which is simultaneously compressed without disturbing the planar stress wave to be propagated.

Abstract: This paper proposes a seamless microforming technique from 100 micrometer order bulging all the way down to micrometer-order embossments by using hydro spark forming method. Sub-millimeter order bulging with 300 and 100 m diameter aluminum thin foils are demonstrated to be easily available without accurate positioning as in the conventional methods. The present technique is successfully applicable also to surface embossments of micrometer order, for a coin surface, an IC chip and hologram surfaces.