• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1998년도 금형가공 심포지엄
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• pp.54-59
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• 1998

In this study, two kinds of life prediction method for hot forming die are developed . One is empirical method requiring some experiment that evaluate thermal softening of die material accoring to operating conditions. The other is analyticl method that calcuate wear quantity of die occuring during the forming process. Wear is a predominant factor as well as plastic deformation and heat checking . And, these methods are applied to prodict tool life real die producting part for automobile. Thus , the applicability and the accuracy of the presented methods are investigated. Using the verified life prediction method above , optimal blocker die design minimizing the finisher die is done.

• 소성∙가공
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• 제20권6호
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• pp.427-431
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• 2011

In general, hot stamped component is trimmed by costly and time consuming laser cutting when the material strength is over 1,500MPa. The aim of this work was to demonstrate that the trimming die life is improved and the trimming load is decreased by lowering the strength of the region to be trimmed. The model employed in this study was a hat shape, similar to the cross section of many hot stamped products. FE-analysis of hot stamping process was performed to evaluate the effect of tool shape on cooling rate at the area to be trimmed. The best tool shape was thus identified, which created slower cooling and lower hardness at the region to be trimmed. The wear at the cutting tool edge was also reduced.

• Coupled systems mechanics
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• 제7권6호
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• pp.669-690
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• 2018

This paper deals with numerical modeling of dynamic failure phenomena in rate-sensitive brittle and/or ductile materials. To this end, a two-dimensional continuum viscodamage-embedded discontinuity model, which is based on our previous work (see Do et al. 2017), is developed. More specifically, the pre-peak nonlinear and rate-sensitive hardening response of the material behavior, representing the fracture-process zone creation, is described by a rate-dependent continuum damage model. Meanwhile, an embedded displacement discontinuity model is used to formulate the post-peak response, involving the macro-crack creation accompanied by exponential softening. The numerical implementation in the context of the finite element method exploiting the second-order mid-point scheme is discussed in detail. In order to show the performance of the model several numerical examples are included.

• 지질공학
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• 제32권1호
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• pp.1-12
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• 2022

Based on the extended spatial mobilization plane (SMP) criterion, we present an elastic-brittle-plastic solution for an axisymmetric cylindrical tunnel. The influences of the intermediate principal compressive stress and material strain-softening behavior are considered. Closed-form formulas for the critical support force, radius of plastic zone, and distributions of stress and displacement in surrounding rock are proposed. The elastic-plastic solution based on SMP is compared with the Kastner solution to verify the credibility of the obtained elastic-plastic solution. The elastic-brittle-plastic solution following the SMP criterion and the current solution based on the Mohr-Coulomb criterion are also compared. The rock strain-softening rate and the intermediate principal stress affect the stability of the surrounding rock. The results provide guidance for optimizing the design of support systems for tunnels.

• 대한토목학회논문집
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• 제7권2호
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• pp.69-77
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• 1987

철근 콘크리트 구조물의 재료적 비선형 해석을 위해 유한요소법을 적용하였다. 2 축응력 상태에서의 콘크리트 거동은 인장균열과 균열사이의 인장증강효과(tension stiffening effect) 그리고 최대압축 강도를 넘어서의 변형연화(strain-softening) 효과를 고려하는 비선형 구성 방정식으로 나타냈다. 콘크리트를 직교성 (orthotropic) 재료로 가정함으로써 비선형 탄성체로 간주하고, 등가일축변형도 개념을 사용한 등가 일축 응력-변형도(equivalent uniaxial stress-strain) 관계식으로 모형화하고, 철근 보강재는 Bauschinger 효과를 갖는 탄소성 변형 경화재료(elasto-plastic strain-hardening material)로 모형화 했다. 평면 응력 상태에서 철근콘크리트 보의 모형화는 각 절점에 2 개의 자유도를 갖는 사각형요소로 모형화하여 적용 시쳤으며, 이로부터 구한 유한요소해석의 결과치를 실험결과치의 중앙처짐, 응력, 변형율 그리고 균열성장과정에 대하여 비교 검토 하였다.

• Tribology and Lubricants
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• 제33권3호
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• pp.98-105
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• 2017

A pin-on-disk test is performed to measure the wear volume of a ductile cast iron (DCI) roll when it wears down using a high carbon steel and two alloy steels at different sliding velocities between the roll and the material (steel). Normal pressure is set as constant and test temperatures are 400, 500 and $600^{\circ}C$. In addition, thermal softening behavior of the DCI roll is examined using a high-temperature micro-hardness tester and the surface hardness variation of the DCI roll is expressed in terms of temperature and heating time. Based on experimental data, a wear coefficient used in Archard's wear model for each material is obtained. The wear volume is clearly observed when the test temperature is $400^{\circ}C$ and sliding velocity varies. However, it is not measured at temperatures of $500^{\circ}C$ and $600^{\circ}C$ even with variations in sliding velocity. From the optical photographs of the pin and disk, the abrasive wear is observed at $400^{\circ}C$ clearly, but no at $500^{\circ}C$ and $600^{\circ}C$. At higher temperatures, the pin surface is not smooth and has many tiny caves distributed on it. It is found that wear volume is dependent on the carbon contents rather than alloy contents. Results also reveal that the variations of wear coefficients are almost linearly proportional to the carbon contents of the material.

• Journal of Welding and Joining
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• 제27권2호
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• pp.69-75
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• 2009

The effects of welding speed were investigated on penetration characteristics, defects and mechanical properties including formability test in Nd:YAG laser welded 1000MPa grade DP steels. A shielding gas was not used and bead-on-plate welding was performed with various welding speeds at 3.5kW laser power. Defects of surface and inner beads were not observed in all welding speeds. As the welding speed increased, the weld cross-section varied from the trapezoid having wider bottom bead, through X type, finally to V type in partial penetration range of welding speeds. The characteristic of hardness distribution was also investigated. The center of HAZ had maximum hardness, followed by a slight decrease of hardness as approaching to FZ. Significant softening occurred at the HAZ near BM. Regardless of the welding speed, the weld showed approximately the same hardness distribution. In the perpendicular tensile test with respect to the weld direction, all specimens were fractured at the softening zone. In the parallel tensile test to the weld direction, the first crack occurred at weld center and then propagated into the weld. Good formability over 80% was taken for all welding conditions.

• 한국지반공학회논문집
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• 제26권1호
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• pp.63-73
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• 2010

포항 일원을 포함하여 신생대에 형성된 지역의 경우, 매립을 위한 양질의 재료원이 부족하며, 미고결된 신생대 퇴적 이암을 도로성토재나 항만공사의 매립재로 활용하려는 시도가 진행되고 있다. 그러나 매립재로 사용되는 미고결 이암의 공학적 특성 및 환경적 영향 등이 아직까지도 명확하게 밝혀지지 않고 있다. 이암을 매립재로 활용하기 위해서는 흡수, 연화, 슬레이킹과 지질학적 메커니즘을 파악해야 하며 침하나 수침에 따른 강도 저하 등에 대한 특성 파악이 필요하다. 본 연구에서는 실내시험과 현장 시험시공을 통해 도로성토재나 매립재로 사용되는 이암의 적합성을 여부를 파악코자 하였다. 특히, 이암을 매립재로 활용하는 경우, 이암의 환경적인 영향을 반드시 고려해야 함을 알 수 있다. 본 연구결과, 이암을 도로성토재나 매립재로 활용하는 경우 발생할 수 있는 문제들을 미리 예방하는데 활용 할 수 있을 것으로 본다.

• 한국식품저장유통학회지
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• 제8권1호
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• pp.66-73
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• 2001

본 연구에서는 복숭아 '미백도', '대구보' 및 '유명' 과실의 발육단계별로 과실의 경도, 세포벽성분 및 g1ycisidase 활성의 변화를 조사함으로써 수확후의 저장중 과실에서와 같이 발육중인 과실의 연화에서 도 $\beta$-galactosidase가 중요한 역할을 하는지 알고자 하였다. 조사시기는 5월 13일, 6월 16일, 7월 16일, 8월 5일이었으며, 수확기가 늦은 '유명'은 8월 28일에 한번 더 조사하였다. Total sugar와 비섬유성 중성당의 함량은 각 품종의 세포벽물질을 증류수, 0.05M CDTA 0.05M $Na_2$CO$_3$, 4% KOH, 2.4% KOH로 차례로 분획하여 조사하였다. 과실의 발육에 따라 경도는 세 품종 모두에서 감소하였으며, '유명' 과실의 경도는 모든 발육단계에서 '미백도'와 '대구보'보다 높았다. 발육단계에 따른 각 분획별 total sugar의 함량 변화는 품종간에 뚜렷한 차이가 없었다. 세 품종의 세포벽 물질과 각 분획중의 주요 중성당은 arabinose와 galactose였다. '미백도'와 '대구보'의 수확일인 8월 5일의 증류수 가용성 분획의 rhamnose의 mol % 변화는 품종별 경도 변화와 상관관계가 있었다. 가용성 $\beta$-galactosidase의 활성은 세 품종 모두에서 파실 발육초기에는 높았으나 초기 이후에는 매우 낮은 수준이었다. 세포벽결합형 $\beta$-galactosidase는 세 품종 모두에서 발육초기에 높았던 활성이 수확기가지 계속적으로 감소하였다. 다른 glycosidase의 활성들도 품종간에 뚜렷한 차이를 보이지는 않았다.

• Computers and Concrete
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• 제7권4호
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• pp.317-329
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• 2010

The paper describes localization of deformation in a bar under tensile loading. The material of the bar is considered as non-linear viscous elastic and the bar consists of two symmetric halves. It is assumed that the model represents behavior of the quasi-brittle viscous material under uniaxial tension with different loading rates. Besides that, the bar could represent uniaxial stress-strain law on a single plane of a microplane material model. Non-linear material property is taken from the microplane material model and it is coupled with the viscous damper producing non-linear Maxwell material model. Mathematically, the problem is described with a system of two partial differential equations with a non-linear algebraic constraint. In order to obtain solution, the system of differential algebraic equations is transformed into a system of three partial differential equations. System is subjected to loadings of different rate and it is shown that localization occurs only for high loading rates. Mathematically, in such a case two solutions are possible: one without the localization (unstable) and one with the localization (stable one). Furthermore, mass is added to the bar and in that case the problem is described with a system of four differential equations. It is demonstrated that for high enough loading rates, it is the added mass that dominates the response, in contrast to the viscous and elastic material parameters that dominated in the case without mass. This is demonstrated by several numerical examples.