• 소성∙가공
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• 제8권2호
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• pp.188-199
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• 1999

Semi-Solid Forming Process(Thixoforming, Rheocasting) is a novel forming process which has some advantages compared with conventional die casting, squeeze casting and hot/cold forging. In this study. Thixoforming process was selected as analysis processing in terms of billet handling and easiness of automation process. The Thixoforming process consists of reheating process of billet, billet handling, filling inot the die cavity and solidification of SSM part. In filling process, two rheology models which were Newtonian and Non-Nettonian model (Ostwald-deWaele)were verified with experimental results. The Ostwald-deWaele model shows the good agreement to the real flow and filling phenomena in die cavity. To give a boost the economical efficiency of Thixoforming process and to ensure the good forming result, reheating device coupled die set was proposed and the initial billet temperature for system that was found from experimental resluts. This study presents an overview of application of numerical analysis for simulation of semi-solid metal forming process to reduce the lead time for development of manufacturing part in industrial field.

• 한국소음진동공학회논문집
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• 제12권3호
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• pp.243-250
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• 2002

Semi-active control devices have received significant attention in recent Years because they offer the adaptability of active-control devices without requiring the associated large power sources. Magnetorheological(MR) dampers are semiactive control devices that use MR fluids to produce controllable dampers. This paper applies sliding mode control method using target variation rate of Lyapunov function for the control of structures by use of MR dampers. The three-story building model under earthquake excitation is analyzed by installing a MR damper in the first-story. The performance of semi-active controllers designed by clipped-optimal algorithm and modified sliding mode control algorithm is compared to the performance of passive-type MR dampers. The results indicate that semi-active controllers achieve a greater reduction of responses than passive-type system and especially the controller by modified sliding mode control method shows a good applicability in the view of response control and control force.

• Ecology and Resilient Infrastructure
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• 제10권4호
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• pp.161-170
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• 2023

본 연구에서는 제주도에서 자생하는 준맹그로브인 황근 (Hibiscus hamabo)과 갯대추나무 (Paliurus ramosissimus)의 탄소 저장량을 원격탐사로 추정하고 기후요인에 의하여 공간변이를 예측하는 인공지능 모델을 구축하고자 하였다. 준맹그로브의 지상부 탄소 축적량은 Global Ecosystem Dynamics Investigation (GEDI) 라이다에 의하여 제공되는 지상부 생물량 밀도(aboveground biomass density, AGBD)를 Sentinel-2 영상으로부터 추출한 normalized difference vegetation index (NDVI)으로 해상도를 상향하여 추정하였다. 제주도에서 단위면적당 탄소 축적량은 황근이 16.6 t C/ha, 갯대추나무가 21.1 t C/ha이었다. 제주도 전 해안에서 준맹그로브의 탄소 축적량은 11.5 t C로 추정되었다. 환경요인에 따른 준맹그로브의 탄소 축적량을 예측하기 위하여 랜덤 포레스트 기술을 적용하였다. 제주도 준맹그로브림의 분포면적 대비 지상부 생물량의 잔차를 계산하였다. 이 잔차에 영향을 미치는 주요 환경요인으로는 가장 습한 달의 강수량, 가장 더운 달의 최고온도, 등온성 및 가장 습한 달의 평균 온도가 선정되었다. 제주도에서 랜덤 포레스트 분석으로 예측된 준맹그로브의 탄소 축적량은 12.0 t C/ha - 27.6 t C/ha 범위의 공간적 변이를 나타내었다. 본 연구에서 개발된 탄소 축적량의 원격탐사 추정법과 환경요인에 따른 인공지능 예측법은 한반도에서 탄소흡수원으로서 맹그로브의 보전과 조성에 필요한 기초자료로 활용할 수 있을 것이다.

• 대한기계학회논문집A
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• 제21권9호
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• pp.1400-1413
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• 1997

Semi-solid forging(SSF) process of A356 aluminium alloy has been studied to assess the effect of process variables on the component integrity. Semi-solid material(SSM) was fabricated by mechanical and electro-magnetic stirring process. The fabricated SSM by using mechanical stirring process has been carried out on cooling rate of 0.022.deg. C/sec 0.0094.deg. C/sec and stirring speed n=600, 1000 rpm, respectively. The fabricated SSM by using electro-magnetic stirring process is supplied by Pechiney. The holding time and temperature in the semi-solid state before forging also affects the globular microstructure of alloy. Therefore, the influence of these two parameters is discussed in terms of the microstructure of alloy. The SSF process has been conducted with three different die temperatures($T_{die}$=250.deg. C, 300.deg. C, 350.deg. C) and two kinds of gate types(straight gate and orifice gate). This paper is to investigate the influence of gate shapes of die on filling phenomena in SSF process more deeply. The mechanical properties of forged components were also investigated for variation of process conditions such as die temperature, gate shape and SSM.

• 소성∙가공
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• 제8권5호
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• pp.437-448
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• 1999

The reheating of the billet in the semi-solid state as quickly and homogeneously as possible is one of the most imposrtant parts. To obtain a fine globular microstructure in cross section of billet, the optimal design of the induction coil for variation of alloys and specimen sizes is necessary. For the thixo-forging process the construction of the reheating data base is very important, because the reheating conditions are different for variation of SSM and billet sizes. So in this study, the optimal coil design of A356 (ALTHIX) and Aι2024 with d×ι=60×90 (mm) to obtain the globular microstructure is theoretically proposed. The suitability of an optimal coil design will be demonstrated by reheating experiments. Finally, the thixoformability of an arbitrarily shaped product is evaluated by its forming variables. The defects and mechanical properties are also investigated.

• 충청수학회지
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• 제15권1호
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• pp.1-6
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• 2002

David Gavid [3] proved that in many familiar cases the upper semi-finite topology on the set of closed subsets of a space is the largest topology making the coincidence function continuous, when the collection of functions is given the graph topology. Considering G-spaces and taking the coincidence set to consist of points where orbits coincidence, we obtain G-version of many of his results.

• Composites Research
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• 제13권2호
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• pp.51-60
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• 2000

본 논문에서는 두께 방향의 와인딩 각도 변화를 고려한 필라멘트 와인딩 된 압력탱크의 해석을 수행하였다. 두께방향의 와인딩 각도를 고려하기 위하여 임의의 맨드릴 표면에 대한 필라멘트 와인딩 궤적을 찾아낼 수 있는 semi-geodesic path식을 도입하였다. 필라멘트 와인딩 된 압력탱크의 유한요소 모델링을 위하여 3차원 적층고체요소를 사용하였으며, 상용프로그램ABAQUS를 이용하여 유한요소해석을 수행하였다. 해석에서는 모재 파손에 의한 강성저하를 모사하기 위하여 재료 비선형성을 고려하였으며, 이를 위하여 각 하중 증분 후에 섬유 수직방향 응력을 모재의 인장강도와 비교하는 최대응력파손이론을 도입하였고, 파손이 일어난 층에 대해서 모재 방향의 재료 상수를 1/10로 저하시켰다. 그리고, 점진적 파손해석을 수행한 결과를 문헌에 주어진 실험데이터와 비교하였다.

• Journal of Astronomy and Space Sciences
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• 제38권4호
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• pp.229-236
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• 2021

We present for the first time the characteristics of upper atmospheric horizontal winds over the Korean Peninsula. Winds and their variability are derived using four-year measurements by the Korea Astronomy and Space Science Institute (KASI) meteor radar. A general characteristic of zonal and meridional winds is that they exhibit distinct diurnal and seasonal variations. Their changes indicate sometimes similar or sometimes different periodicities. Both winds are characterized by either semi-diurnal tides (12 hour period) and/or diurnal tides (24 hour period) from 80-100 km. In terms of annual change, the annual variation is the strongest component in both winds, but semi-annual and ter-annual variations are only detected in zonal winds.

• The Plant Pathology Journal
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• 제21권3호
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• pp.207-213
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• 2005

Cultural, morphological and pathogenic variation in Indian isolates of Ascochyta rabiei, the causal agent of blight of chickpea, was investigated. Fungal isolates representative of seven agroclimatic regions in north western plain zones (NWPZ) of India showed variation in colony colour as mouse gray with green hue, light mouse gray with slate gray centre and gray with dark brown centre, when grown on chickpea dextrose agar (CDA). Conidiomatal color of the isolates varied from brown to slate gray and black. The number of conidiomata and conidia formed on CDA ranged from 49.7 to 90.7 and $5.5\times10^4\;to\;3\times10^5cm^{-2}$, respectively. The size of conidiomata and conidia of A. rabiei isolates varied from $274\times232{\mu}m\;to\;156\times116{\mu}m$, and from $14.0\times6.2{\mu}m\;to\;10.7\times4.6{\mu}m$, respectively. Fourteen A. rabiei isolates from the seven agroclimatic regions of NWPZ were evaluated for their virulence on 180 chickpea genotypes in controlled environment. Cluster analysis based on the disease rating on a 1-9 scale indicated higher similarity coefficient (> 0.65) between isolates from different agroecological regions, while few isolates from the same region had less similarity. The 14 isolates were grouped into eight pathotypes at > 0.5 similarity coefficient. Sixteen genotypes were identified as probable differentials to distinguish A. rabiei isolates.

• 소성∙가공
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• 제8권4호
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• pp.374-383
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• 1999

To determine the flow stress of semi-solid materials, a new combined method has been studied by experimental and analytic technique in the current approach. Using backward extrusion experiment and its numerical analysis, the characterization scheme of semi-solid materials according to the change of initial solid volume fraction has been proposed. Because that solid volume fraction is sensitive to temperature change, it is required to precisely control the temperature setting. Model materials can guarantee the establishment of material characterization technique from the noise due to temperature change. Thus, clay mixed with bonded abrasives was used for experiment and the change of initial solid fraction was copied out through the variation of mixing ratio. Upper bound method was adapted to increase in efficiency of the calculation in numerical analysis and new kinematically admissible velocity field was employed to improve the accuracy of numerical solution. It is thought that the material characterization scheme proposed in this study can be applied to not only semi-solid materials, but also other materials that is difficult to obtain the simple stress state.