• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1994년도 추계학술대회 논문집
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• pp.55-63
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• 1994

The analysis of superplastic sheet forming process is studied by the use of the finite element method using a convected coordinate system and a skew boundary condition. In the formulation, the large inelastic behavior of the superplastic material is described as incompressible, nonlinear, viscous flow. The formulation is then approximated to the finite dimensional space with the use of membrane elements, which results in algebraic linear equations. In addition to the finite element formulation, a pressure cycle control algorithm is combined in the analysis for optimization of the forming time, which deals with the maximization of the strain rate sensitivity, the protection of the thickness reduction, the consistency of the desired strain rate and improvement of formability.

• 한국전산구조공학회논문집
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• 제27권3호
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• pp.147-154
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• 2014

충격 및 폭발하중으로 인한 위험으로부터 구조물의 안정성을 확보하기 위한 필요성의 증대에 따라 고율변형을 받는 콘크리트의 거동은 중요한 연구주제가 되었다. 콘크리트의 고율변형 거동은 정적인 상태와는 다른 독특한 거동을 보이기 때문에 다양한 고율변형모델들이 제안되어 고율변형 상태를 수치해석하는데 사용되고 있다. 이러한 수치해석 과정에서 발생하는 문제가 요소의 크기에 따라 수치해석결과가 크게 변하는 요소의존성 문제이다. 본 논문에서는 파괴에너지이론에 기초하여 요소의존성을 최소화할 수 있는 기준식을 제안하고 HJC(Holmquist Johnson Cook)모델을 이용한 관통수치해석을 통해 기준식을 검증하였다. 그 결과 기준식을 통해 산정된 파괴변형률을 수치해석상에 적용해줌으로써 해석결과의 요소의존성이 감소하였고 해의 정확성 또한 향상되는 것을 파악할 수 있었다.

• 대한기계학회논문집A
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• 제25권12호
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• pp.2056-2061
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• 2001

Non-proportional loading tests of Nylon 66 at room temperature exhibit path dependent behavior and plasticity-relaxation interactions. The uniaxial formulation of the viscoplasticity theory based on overstress (VBO), which has been used to reproduce the nonlinear strain rate sensitivity, relaxation, significant recovery and cyclic softening behaviors of Nylon 66, is extended to three-dimensions to predict the response in strain-controlled, comer-path tests. VBO consists of a flow law that is easily written for either the stress or the strain as the independent variable. The flow law depends on the overstress, the difference between the stress and the equilibrium stress that is a state variable in VBO. The evolution law of the equilibrium stress in turn contains two additional state variables, the kinematic stress and the isotropic stress. The simulations show that the constitutive model is competent at modeling the deformation behavior of Nylon 66 and other solid polymers.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2003년도 봄 학술발표회 논문집
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• pp.137-144
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• 2003

Since quasi-brittle material like concrete shows strain localization behavior accompanied by strain softening, a numerical drawback such as mesh sensitivity is appeared in the finite element analysis. In this study, a homogenized crack model which overcomes the drawback and considers rate discontinuity in the constitutive equation is proposed for modeling of cracking in concrete and its propagation in strain softening regime. Then, a series of finite element analysis of the concrete under various loading conditions has been performed. From comparison of analysis results with experimental data, it is shown that failure behavior due to localized cracking of concrete under both compressive loading condition and tensile loading condition is well predicted by the homogenized crack model.

• 소성∙가공
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• 제23권3호
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• pp.178-183
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• 2014

Continuous casting is a process where molten metal is solidified into a semi-finished billet on a large scale with either a rectangular or round cross section for subsequent processing. The use of continuous casting provides an opportunity for producing material on a practical industrial scale with lower cost than conventional casting. In the current study, the material was fabricated by continuous casting and subsequent extrusion. Tensile tests were conducted on continuous cast ZK60A after extrusion over a range of strain rates at temperatures from 473K to 623K. The alloy exhibits a quasi-superplastic behavior with a maximum recorded elongation of ~250% at 523K when tested with an initial strain rate of $10^{-5}/s$. The experiments give a strain rate sensitivity exponent of 0.3~0.4 and an activation energy of 108 kJ/mol. From the current investigation, it was found that the high-temperature plastic flow of the ZK60A is controlled by a dislocation viscous glide mechanism.

• 대한화학회지
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• 제23권4호
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• pp.217-236
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• 1979

소성변형에 대한 著者들의 이론을 초소성합금(Zn-Al eutectoid, A1-Cu, Pb-Sn, Sn-Bi, Mg-Al eutectics)에 적용하였다. 그 결과 초소성합금의 소성변형은 두 개의 grain boundary流動單位의 平行連結로 나타낼 수 있었다. 이 두 개의 流動單位는 流動式에 나타나는 parameter $X_{gj}/{\alpha}_{gj}$와 ${\beta}_{gj}$(j=1 혹온 2)로 表現할 수 있으며 이들을 實驗的으로 求할 수 있었다. 著者들의 流動式은 實驗과 잘 一致하였다. Strain rate sensitivity 對 -In(strain rate) 곡선을 이론으로 구한 결과 유동단위수만큼의 peak가 ${\beta}_{gj}$(j=1 or 2) 값에 따라 분리되어 나타났고 초소성의 조건도 ${beta}_{gj}$값에 의하여 결정됨을 알았다. ${\beta}_{gj}값의 粒子크기 依存性을 구하였고 온도변화에 따른 ${\beta}_{gj}$값 변화로부터 각 流動單位의 활성화엔탈피, ${\Delta}H_{gj}^{\neq}$도 구하였다. 그 결과 ${\Delta}H_{gj}^{\neq}$는 재료성분원소들의 grain boundary 자체확산에 의한 활성화엔탈피와 같이 나타났고 또 이들은 粒子 크기 증가에 마라 증가함을 보였다

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2005년도 춘계학술발표대회 개요집
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• pp.19-21
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• 2005

The ball shea. tests for ball grid array (BGA) and flip chip packages were carried out with different displacement rates to find out the optimum condition of the displacement rate for this test. The BGA packages consisted of two different kinds of solder balls (eutectic Sn-37wt.%Pb and Sn-3.5wt.%Ag) and electroplated Au/Ni/Cu substrate, whereas the flip chip package consisted of electroplated Sn-37Pb solder and Cu UBM. The packages were reflowed up to 10 times, or aged at 443 K up to 21 days. The variation of the displacement rate resulted in the variations of the shear properties such as shear force, displacement rate at break, fracture mode and strain rate sensitivity. The increase in the displacement rate led to the increase of the shear force and brittleness of solder joints.

• Structural Engineering and Mechanics
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• 제55권4호
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• pp.857-869
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• 2015

The aluminum 7075-T6 is known as an alloy widely used in aircraft structural applications, which does not exhibit strain rate sensitivity during dynamic compressive tests. Despite mechanical importance of the material, there is not enough attention to determine appropriate sample dimensions such as a sample diameter relative to the device bar diameter and sample length to diameter (L/D) ratio for dynamic tests and how these two parameters can change mechanical behaviors of the sample under dynamic loading condition. In this study, various samples which have different diameters of 31.8, 25.4, 15.9, and 9.5 mm and sample L/D ratios of 2.0, 1.5, 1.0, 0.5, and 0.25 were tested using Split Hopkinson Pressure Bar (SHPB), as this testing device is proper to characterize mechanical behaviors of solid materials at high strain rates. The mechanical behavior of this alloy was examined under ${\sim}200-5,500s^{-1}$ dynamic strain rate. Aluminum samples of 2.0, 1.5 and 1.0 of L/D ratios were well fitted into the stress-strain curve, Madison and Green's diagram, regardless of the sample diameters. Also, the 0.5 and 0.25 L/D ratio samples having the diameter of 31.8 and 25.4 mm followed the stress-strain curve. As results, larger samples (31.8 and 25.4 mm) in diameters followed the stress-strain curve regardless of the L/D ratios, whereas the 0.5 and 0.25 L/D ratios of small diameter sample (15.9 and 9.5 mm) did not follow the stress-strain diagram but significantly deviate from the diagram. Our results indicate that the L/D ratio is important determinant in stress-strain responses under the SHPB test when the sample diameter is small relative to the test bar diameter (31.8 mm), but when sample diameter is close to the bar diameter, L/D ratio does not significantly affect the stress-strain responses. This suggests that the areal mismatch (non-contact area of the testing bar) between the sample and the bar can misrepresent mechanical behaviors of the aluminum 7075-T6 at the dynamic loading condition.

• 한국전산구조공학회논문집
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• 제25권3호
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• pp.211-217
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• 2012

본 연구에서는 유리단섬유로 보강된 분사식 섬유보강 복합재료의 인장거동 평가를 위한 실험 및 해석연구를 수행하였다. 이를 위해 다양한 변형율속도(strain rate)에 따른 에폭시수지 및 분사식 섬유보강 복합재료의 인장강도 실험을 수행하였다. 본 연구에 사용된 분사식 섬유보강 복합재료는 15mm 길이로 절단된 유리단섬유가 25% 부피비율로 혼입된 보수 보강용 재료이다. 에폭시수지의 점탄성 특성을 고려하기 위해 역산모델링(inverse simulation)을 수행하여 변형율속도에 따른 점성변화를 함수식으로 제안하였다. 역산모델링을 통해 제안된 함수식을 미세역학 기반의 점탄성 손상모델(micromechanics-based viscoelastic damage model; Yang et al., 2012)에 적용하여 분사식 섬유보강 복합재료의 인장거동을 수치적으로 해석하였다. 분사식 섬유보강 복합재료의 인장거동 해석결과와 실험결과를 비교하여 미세역학 기반의 점탄성 손상모델의 정확성을 검증하였다.

• 한국재료학회지
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• 제4권7호
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• pp.828-836
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• 1994

YBCO산화물초전도체의 고온변형 특성을 조사하기 위하여 $890^{\circ}C$ ~ $930^{\circ}C$의 온도범위에서 $1.0 x 10^{-5}s^{-1}\sim 1.0^{-4}s^{-1}$의초기변형속도로 압축시험을 수행하였다. 변형온도가 증가함에 따라 또한 초기변형속도가 감소함에 따라 flow stress는 감소하였다. 변형률속도 민감지수는 0.41-0.46이었다. 이는 초소성 변형이 일어났음을 보여준다. 초소성변형에 대한 활성화 에너지는 약 500 ~ 580KJ/mol이었으며 Ag첨가량이 증가할수록 활성화에너지는 감소하였다. 초소성변형된 시편들의 미세조직 관찰결과 변형중에 결정립 성장이 일어났으며 Ag양이 증가함에 따라 이러한 현상을 뚜렷하였다. 변형후 결정립 형태는 등축상을 유지하였다. 이러한 결과로 볼때, YBCO 초전도체의고온변형기구는 확산을 동반한 결정립계 미끄러짐으로써 그 비율은 전 변형량중 약 65%정도였다.