• 소성∙가공
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• 제10권3호
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• pp.206-213
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• 2001

Dynamic deformation of metallic materials mostly accompanies substantial amounts of deformation heat. Since the flow stress of deformation is sensitive to temperature, implication of heat due to plastic work is essential to the evaluation of constitutive relations. In this study, a series of compression tests were conducted for SAF 2507 super duplex stainless steel at various temperatures and strain rates. The accumulation of plastic work was calculated through numerical integration and converted into the elevation of temperature. Subsequent logarithmic interpolation deduced isothermal flow surfaces, which were primary input data of finite element analysis. Simple closed die forging process was analyzed and optimized with commercial FEM code applying both raw and calibrated material database. The effect of accounting deformation heat was more noticeable in high-speed forming process.

• 소성∙가공
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• 제24권5호
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• pp.323-331
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• 2015

Impact extrusion is an economical and productive process that can replace the multistage deep drawing process for producing deep rectangular cases. In the current work, a three-dimensional finite element analysis of the impact extrusion process of a commercial purity aluminum alloy (AA1070) was performed to predict loads, material flow, and deformed shapes using the Hansel-Spittel rheology law, which describes the flow stress at various temperatures and strain rates. The role of various process parameters such as friction, clearance between punch and die, aspect ratio and thickness of billet on the process and the shapes was analyzed.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1994년도 추계학술대회 논문집
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• pp.80-94
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• 1994

As for axisymmetirc sheet metal forming, a kind of pick-up apparatus for body-wrinkling is deviced. Experiments with both hemispherical and flat headed punches, with various clearances between punch and die, with respect to three kinds of materials each of which has two thicknesses, are performed. Firstly the process of evolution of body-wrinkling is observed. Then the critical blank-holding force (or meridional tensile force) for suppression of body-wrinkling at a specified punch-stroke is measured for all cases mentioned above. An empirical formula for it is proposed. Deformation patterns and stress distributions are analysed by the use of FEM. A simplified critical condition for body-wrinkling is formulated and introduced into the FEM program. And its effectiveness is checked by comparison with the experimental results. Using this FEM system, the governing factors of body-wrinkling are cleared up.

• 대한기계학회논문집A
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• 제20권12호
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• pp.3715-3727
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• 1996

Compression behavior of semi-solid aluminum alloys with controlled solid fractions was investigated in the present study. The stress and strain relationships were obtained from the compression test. Variations of the solid fraction distribution and the material behaviour were investigated for various friction coeffieiants and die speedsd. For a finite element analysis, the semi-solid material was described by a compressible regid viscoplastic model for the solid region and darcy's law for the liquid region. The computed results were compared with experimental data for the validity of the yield criteria.

• 구강회복응용과학지
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• 제19권3호
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• pp.219-237
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• 2003

he purpose of this study was to compare the fracture resistance of the IPS Empress ceramic crown with 1.0mm width rounded shoulder, which is usually recommended in all ceramic crown, and 0.5mm width chamfer finish lines on the maxillary first premolar. 30 sound maxillary first premolars were selected and then storaged in 5% NaOCl and saline. 15 teeth were performed preparation for each group(1.0mm rounded shoulder, 0.5mm chamfer). After 30 stone dies were made for each group, the IPS Empress ceramic crowns were fabricated and cemented with resin cement(Bistite resin cement, Tokuyama Soda Co. LTD., Japan) on the natural teeth. The cemented crowns were mounted on the positioning jig and the universal testing machine(Zwick Z020, Zwick Co., Germany)was used to measure the fracture strength, with stress loading on the occlusal surface between buccal and lingual cusp. And also, three-dimensional finite element model was used to measure the stress distribution with two types of the finish lines(1.0mm rounded shoulder, 0.5mm chamfer) and two loading conditions(both buccal and lingual cusp inclination, lingual cusp inclination only). The result of the this study were as follows. In the fracture resistance experiment according to the finish line, the mean fracture strength of rounded shoulder(842N) showed higher value than that of the chamfer(590N) (p<0.05). In the three dimensional finite element analysis of all ceramic crown, metal die and natural teeth model did not show any differences in stress distribution between finish lines. Generally, when force was loaded on the occlusal inclination of buccal and lingual cusp, the stress was concentrated on the loading point and the central groove of occlusal surface. When force was loaded only on the occlusal inclination of lingual cusp, the stress was concentrated on the lingual finish line and loading point.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2003년도 추계학술발표대회 논문집
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• pp.46-49
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• 2003

Cu have been widely used as signal transmission materials for electrical electronic components owing to its high electrical conductivity. However, it's size have been limited to small ones due to its poor mechanical properties, Until now, strengthening of the copper at toy was obtained either by the solid solution and precipitation hardening by adding alloy elements or the work hardening by deformation process. Adding the at toy elements lead to reduction of electrical conductivity. In this aspect, if carbon nanofiber is used as reinforcement which have outstanding mechanical strength and electric conductivity, it is possible to develope Cu matrix nanocomposite having almost no loss of electric conductivity. It is expected to be innovative in electric conduct ing material market. The unidirectional alignment of carbon nanofiber is the most challenging task developing the copper matrix composites of high strength and electric conductivity In this study, the unidirectional alignment of carbon nanofibers which is used reinforced material are controlled by drawing process in order to manufacture the intermediary materials for the carbon nanofiber reinforced Cu matrix nanocomposite and align mechanism as well as optimized drawing process parameters are verified via experiments and numerical analysis. The materials used in this study were pure copper and the nanofibers of 150nm in diameter and of $10~20\mu\textrm{m}$ In length. The materials have been tested and the tensile strength was 75MPa with the elongation of 44% for the copper it is assumed that carbon nanofiber behave like porous elasto-plastic materials. Compaction test was conducted to obtain constitutive properties of carbon nanofiber. Optimal parameter for drawing process was obtained by experiments and numerical analysis considering the various drawing angles, reduction areas, friction coefficient, etc Lower reduction areas provides the less rupture of cu tube is not iced during the drawing process. Optimal die angle was between 5 degree and 12 degree. Relative density of carbon nanofiber embedded in the copper tube is higher as drawing diameter decrease and compressive residual stress is occurred in the copper tube. Carbon nanofibers are moved to the reverse drawing direct ion via shear force caused by deformation of the copper tube and alined to the drawing direction.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 춘계학술대회 논문집
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• pp.138-143
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• 2005

Finite Element analysis is widely applied to elevated temperature forging processes and shows a lot of information of plastic deformation such as strain, stress, defects, damages and temperature distributions. In highly elevated temperature deformation processes, temperature of material and tool have significant influence on tool life, deformation conditions and productivities. To predict temperature related properties accurately, adequate coefficients of not only contact heat transfer between material and dies but also convection heat transfer due to coolants are required. In most F.E analysis, too higher value of contact heat transfer coefficient is usually applied to get acceptable temperature distribution of tool. For contact heat transfer coefficients between die and workpiece, accurate values were evaluated with different pressure and lubricants conditions. But convection heat transfer coefficients have not been investigated for forging lubricants. In this research, convection heat transfer coefficients for cooling by emulsion lubricants are suggested by experiment and Inverse method. To verify acquired convection and contact heat transfer coefficients, tool temperature was measured for the comparison between measured tool temperature and analysis results. To increase analysis accuracy, repeated analysis scheme was applied till temperature of the tool got to be in the steady-state conditions. Verification of heat transfer coefficients both contact and convection heat transfer coefficients was proven with good accordance between measurement and analysis.

• 열처리공학회지
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• 제13권6호
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• pp.383-390
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• 2000

The static creep behaviour of AlSl 420F stainless steel was investigated over the temperature range of $540{\sim}585^{\circ}C$ and the stress range of $13{\sim}19kg/mm^2$ (127.4~186.2MPa). Constant stress creep tests were carried out in the experiment. Measured stress exponent, n, for the creep deformation of the alloy under the given conditions was found to vary at the range of 9.59, 9.15, 8.78, and 8.53 for the temperature of 540, 555, 570, and $585^{\circ}C$ respectively. The activation energy, Qc, for the creep deformation was 106.42, 102.58,97.81, and 94.58 kcal/mole for the stress of 13, 15, 17, and $19kg/mm^2$, respectively. Lason-Miller parameter, P, for the crept specimens for AlSl 420F stainless steel was measured as $P=T(log\;t_T+21)$. The empirical static creep rate obtained by the regression analysis was as follows. $${\varepsilon}={\exp}[(3.79{\times}10^{-2}{\sigma}+2.722)T-3.0747{\sigma}+28.109]{\times}{\sigma}^{(-2.367{\times}10^{-2}T+22.33)}{\exp}\left[-\frac{(-2.015{\sigma}+132.580){\times}10^3}{RT}\right]$$ The failure plane were observed, intergranular fracture was dominated by r (round) type crack over the experimental range.

• Journal of the Korean Academy of Child and Adolescent Psychiatry
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• 제4권1호
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• pp.54-67
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• 1993

본 연구는 자폐아 어머니 160명을 대상으로 자폐아 어머니의 스트레스와 스트레스 정도에 영향을 끼치는 자폐아 어머니와 아동의 특성을 파악하여, 자폐아 어머니를 위한 간호전략 수립을 위한 기초 자료를 제공하는데 그 목적을 두고 본 연구를 시도하였다. 이를 위하여 본 연구자가 개발한 자폐아 어머니의 스트레스에 관한 도구는, 어머니의 일상 생활에 관련된 스트레스(11문항),어머니의 가족, 사회적 관계 에서의 스트레스(26문항), 아이의 장애 원인, 치료, 교육 및 예후에 관련된 스트레스 (28문항) 및 어머니의 부정적 심리 상태에 관련된 스트레스(9문항) 등 74개 문항으로 구성되어 있으며 도구 신뢰도는 Cronbach's ${\alpha}$ =0.94였다. 연구 결과는 다음과 같다1) 자폐아 어머니의 스트레스 정도는 영역별로 볼 때 장애원인, 치료, 교육 및 예후에 관련된 스트레스 평점이 3.19로 가장 높았으며 어머니의 부정적 심리상태에 관련된 스트레스 평점은 2.85, 어머니의 일상생활애 관련된 스트레스 평점은 2.45,그리고 가족, 사회적 관계에서의 스트레스 평점은 2.05의 순으로 나타났다. 2) 총 74개의 스트레스 항목 인지정도의 평점은 2.62였다 스트레스 정도가 가장 높게 인지된 항목은 '자폐아에 대한 사회의 이해가 부족하다', '부모가 사망한다면 누가 이 아이를 돌볼 수 있을지 걱정이다', '아이가 성장함에 따라 단계적으로 교육받을 수 있는 시설이 없다' 등이었으며, 낮은 스트레스로 인지된 항목은 '남편이 아이를 미워한다', '남편과 이혼하고 싶다' 등이었다. 3) 자폐아 어머니 특성 중에서 자녀의 수, 아이 문제에 대한 인지 정도는 스트레스 정도에 유의한 차이를 나타내었는데 자녀의 수가 많을수록, 문제 인지가 심할수록 어머니가 느끼는 스트레스가 높았다.

• 소성∙가공
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• 제18권1호
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• pp.26-38
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• 2009

It was reported that the semi-solid forming process has many advantages over the conventional forming process, such as a long die life, good mechanical properties and energy savings. It is very important, however, to control liquid segregation to gain mechanical property improvement of materials. During forming process, rheology material has complex characteristics, thixotropic behavior. Also, difference of velocity between solid and liquid in the semi-solid state material makes a liquid segregation and specific stress variation. Therefore, it is difficult for a numerical simulation of the rheology process to be performed. General plastic or fluid dynamic analysis is not suitable for the behavior of rheology material. The behavior and stress of solid particle in the rheology material during forging process is affected by viscosity, temperature and solid fraction. In this study, compression experiments of aluminum alloy were performed under each other tool shape which is rectangle shape(square array), rectangle shape(hexagonal array), and free shape tool. In addition, the dynamics behavior compare with Okano equation to power law model which is viscosity equation.