• 대한기계학회논문집A
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• 제28권7호
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• pp.930-939
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• 2004

Constitutive models for final stage densification of metal powder compacts with power-law creep deformation were investigated. The constitutive models were implemented into a finite element program (ABAQUS) by using user subroutine CREEP and, from FEM results, useful densification curves were obtained when hydrostatic and uniaxial stress were applied to the powder compacts at various pressures and temperatures. Because the densification behavior varied as the constitutive models, the equivalent stress surface on each constitutive equation was investigated to analyze the difference of densification behavior.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2001년도 봄 학술발표회 논문집
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• pp.121-127
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• 2001

To investigate the stability and integrity of offshore structures, it is necessary to estimates the significant ocean environmental loadings. They include hydrostatic pressure, wind, wave, current, tide, ice, earthquake, temperature, marine growth and scouring. Waves are of major importance among them because of the large forces acting on submerged parts of the structure caused by accompanying water motions. For the comparison of stress and deflection analysis tools, a vertical pile is applied under environmental loads. The analysis is concerned with SACS(Structural Analysis Computer System), ANSYS and C program. SACS and C program have nearly the same results but not ANSYS. This study shows the comparison of results obtained from 3 different approaches.

• 농업생명과학연구
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• 제44권6호
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• pp.133-140
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• 2010

초고압 처리가 홍삼의 진세노사이드 함량 및 항산화 활성에 미치는 영향에 대하여 조사하였다. 전통적인 방법으로 제조한 일반 홍삼과 초고압 처리 홍삼의 조사포닌 함량은 각각 21.93 및 27.29 mg/g이었고, 총 페놀성 화합물과 조사포닌 및 진세노사이드 함량은 초고압 처리에 의해 증가하였다. 일반 홍삼과 초고압 처리 홍삼에 가장 많이 함유되어 있는 진세노사이드는 Rb1으로 각각 14.10 mg/g과 17.71 mg/g 이었으며, 초고압 처리에 의하여 25.60% Rb1이 증가하였다. 또한 홍삼의 특징적인 유효 성분인 Rg3는 일반 홍삼에 비하여 초고압 처리한 홍삼에서 10.46% 증가하였다. DPPH 및 ABTS 라디칼 소거 활성을 측정한 결과, 열수 추출물의 농도가 증가함에 따라 DPPH 및 ABTS 라디칼 소거 활성이 증가하는 경향을 보였으며 초고압 처리 홍삼이 일반 홍삼 열수 추출물에 비해 높은 라디칼 소거 활성을 보였다. 환원력과 FRAP 활성도 분획물의 농도가 증가함에 따라 환원력과 FRAP 활성이 증가하는 경향을 보였으며, 일반 홍삼과 초고압 처리 홍삼 열수 추출물의 FRAP 활성은 농도 10 mg/mL일 때 각각 0.30 및 0.36의 흡광도를 보였다. 따라서 초고압 처리 홍삼 열수 추출물은 자유 라디칼에 의해 유발된 산화적 스트레스를 효과적으로 방어할 수 있을 것으로 판단된다.

• 한국전산구조공학회논문집
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• 제12권3호
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• pp.353-362
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• 1999

본 연구에서는 콘크리트 중력식 댐의 임계균열길이가 계산되었으며 또한 복합균열의 균열선단에서 유효응력 확대계수의 변화량이 조사되었다. 작용하중으로는 댐 상부면에 작용하는 정수압과 댐의 균열면에 작용하는 수압 및 자중으로 구성된 정하중, 그리고 댐 주위에서 발파작업이 수행되는 경우에 고려될 수 있는 발파진동 및 동수압으로 구성된 동하중이 사용되었다. 균열이 발생한 위치와 방향 및 발파진동의 크기에 따라 임계균열길이가 계산되었으며, 또한 복합균열의 형태 및 균열선단 간의 이격거리에 따른 유효응력확대계수의 변화량이 검토되었다.

• 한국정밀공학회지
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• 제15권1호
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• pp.130-136
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• 1998

A procedure of stress and strength analysis has been proposed for the centrifuge rotor of composite materials of quasi-isotropic laminates. The goal in this study is to maximize the allowable rotating speed, that is, to minimize maximum strength ratio with the given path length by changing the geometric parameter-outer radius and ply angles in quasi-isotropic laminates. Optimum values of the geometric parameter-outer radius and ply angles are obtained by multilevel optimization. All the geometric dimensions and stresses are normalized such that the result can be extended to a general case. Two dimensional analysis at each cross section with an elliptic tube hole subjected to internal hydrostatic pressures by samples as well as the centrifugal body forces has been performed along the height to calculate the stress distribution with the plane stress assumption, and Tsai-Wu failure criterion is used to calculate the strength ratio. The maximum allowable rotating speed can be increased by changing the radii of the outer surface along the height with the maximum strength ratio under the unit value : The optimal number of ply angles maximizing the allowable rotating speed in quasi-isotropic laminates is found to be the half number of tube hole, and the optimal laminate rotation angle is the half of $[{\pi}/m]$. A $[{\pi}/3]$ laminate, for instance, is stronger than a $[{\pi}/4]$ laminate for the centrifuge rotor of 6 tube hole number even though they have the same stiffness.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2007년도 추계학술대회 논문집
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• pp.50-52
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• 2007

Large size forged parts usually were made by hot open die forging because of the die cost, high applied load and small manufacturing quantities. Cast ingots were used in open die forging and the ingots almost included the cavities in its inside. Therefore, one of the aims for forging processes is to close and remove the cavities. However, its criteria were well not defined since the studies have many difficulties to investigate the cavity behaviors because of its large size. In this study, the cavity closure behavior was investigated by experimental and FE analysis. The FEM analysis is performed to investigate the overlap defect of cast ingots during free forging stage. The measured flow stress data were used to simulate the forging process of cast ingot using the practical material properties. Also the analysis of cavity closure is performed by using the $DEFORM^{TM}$-3D. The calculated results of cavity closure behavior are compared with the measured results before and after forging, which are scanned by the X-ray scanner. From this result, the criteria for deformation amounts effect on the cavity closure can be investigated by the comparison between practical experiment and numerical analysis.

• 소성∙가공
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• 제32권5호
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• pp.287-293
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• 2023

This study aims to develop a computational framework based on the finite element method for modeling the hydrogen embrittlement in martensitic steel. The hydrogen embrittlement is a well-known phenomenon, in which the hydrogen penetrates into the surface, flows through the microstructure and finally leads to pre-mature fracture under external or internal stresses. The current numerical model takes into account the effect of hydrogen on the plasticity and failure behavior of martensitic steel under various stress states. This allows for the construction of a failure criterion that accounts for conventional stress states and hydrogen concentration. The developed model is capable of simulating hydrogen diffusion through the lattice based on the distribution of hydrostatic stress. Additionally, it can calculate the hydrogen concentration in trapped sites, such as dislocations, using a local equilibrium assumption, often referred to as Oriani's equilibrium. The developed model parameters are identified through the tensile tests with and without hydrogen environment, and the performance of model can be validated by analyzing fractured automotive part in the hydrogen environment.

• 한국해양공학회지
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• 제25권3호
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• pp.53-65
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• 2011

This is the third of several companion papers dealing with the derivation of material constants for ductile failure criteria under hydrostatic stress. It was observed that the ultimate engineering stresses and elongations at fracture from tensile tests for round specimens with various notch radii tended to increase and decrease, respectively, because of the stress triaxiality. The engineering stress curves from tests are compared with numerical simulation results, and it is proved that the curves from the two approaches very closely coincide. Failure strains are obtained from the equivalent plastic strain histories from numerical simulations at the time when the experimental engineering stress drops suddenly. After introducing the new concept of average stress triaxiality and accumulated average strain energy, the material constants of the Johnson-Cook failure criterion for critical energies of 100%, 50%, and 15% are presented. The experimental results obtained for EH-36 steel were in relatively good agreement with the 100% critical energy, whereas the literature states that aluminum fits with a 15% critical energy. Therefore, it is expected that a unified failure criterion for critical energy, which is available for most kinds of ductile materials, can be provided according to the used materials.

• 대한기계학회논문집A
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• 제36권10호
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• pp.1235-1239
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• 2012

원자력발전소의 원자로압력용기 상부헤드에는 출력제어 및 정지용 제어봉이 통과하는 노즐이 있으며 이 노즐은 상부헤드 노즐과 J 형태의 홈으로서 용접 되어 있다. 최근 외국의 원자력발전소에서 이 용접영역 주변의 노즐 및 용접부에서 일차수응력부식 균열이 발생한 사례가 보고되고 있다. 본 논문에서는 이 용접부의 용접잔류응력과 운전 중에서의 응력상태를 유한요소해석을 이용하여 평가함으로써 고응력 위치를 확인하고 응력관점에서 균열발생 가능성이 높은 지역을 예측하고자 하였다. 해석결과 용접에 의해서 형성된 잔류응력이 수압시험과 운전조건에 의해 다소 변동되기는 하나 응력분포형태는 큰 변화가 없었다. 전반적으로 노즐내면에서는 용접이 시작되는 지점 주변에서 최대 인장응력이 형성되고 노즐외면에서는 용접이 끝나는 지점 주변에서 최대인장응력이 형성되는 것을 확인하였다.

• 소성∙가공
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• 제31권6호
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• pp.376-383
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• 2022

The potential damage region of a WC-Co cemented carbide die is investigated for cold forging process of a wheel-nut by numerical simulation with its chemical composition considered. Numerical simulation is utilized to calculate internal stress, especially for the WC-Co die, during the forging process. Finite element model is established, in which the elasto-plastic properties are applied to the work-piece of bulk steel, and elastic properties are considered for the lower die insert of the WC-Co alloy. This stress analysis enables to distinguish the potential damage regions of the WC-Co die. The regions from calculation are comparatively analyzed along with the crack area observed in the die after repetitive manufacturing. Effect of chemical composition of the WC-Co is also evaluated on characteristics of potential damage region of the die with variance of mechanical properties considered. Derived from Mohr-Coulomb fracture model, furthermore, a new stress index is presented and used for die stress analysis. This index inherently considers hydrostatic pressure and is then capable of deducing wide range of its distribution for representing stress state by modification of its parameter implying pressure sensitivity.