• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.78-89
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• 1993

A new theory based on the modified momentum and energy conservation has been proposed in order to overcome the drawbacks included in previous theories which are used for the determination of dynamic yield stresses and the investigation of dynamic behavior of metals. Then the improvements suggested by the new theory have been manifested through the analysis of the error included in the measurement of deformed length and through the comparison between the new theory, existing theories, and experimental results performed by previous workers. Meanwhile rod impact test has been performed which uses a compressed- air system for the acceleration of flat-ended cylindrical free-cutting brass rods. From the geomtrical measurements of deformed length, the dynamic yield stress of free-cutting brass has determined.

• Nuclear Engineering and Technology
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• v.21 no.2
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• pp.89-98
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• 1989

The expanding copper mandrel test performed at three strain rates (3.2$\times$10E -5/s, 2.0$\times$10E-6/s and 1.2$\times$10E-7/s) over 553-873 K temperature range by varying the heating rates (8-1$0^{\circ}C$/s, 1-2$^{\circ}C$/s and 0.5$^{\circ}C$/s) in air and in vacuum (5$\times$10E-5 torr). The yield stress peak, the strain rate sensitivity minimum and the activation volume peaks could be explained in terms of the dynamic strain aging. The activation energy for dynamic strain aging obtained from the yield stress peak temperature and strain rate was 196 KJ/mol and this value was in good agreement with the activation energy for oxygen diffusion in $\alpha$-zirconium and Zircaloy-2 (207-220 KJ/mol). Therefore, oxygen atoms are responsible for the dynamic strain aging which appeared between 573 K and 673 K. The yield stress increase due to the oxidation was obtained by comparing the yield stress in air with that in vacuum and represented by the percentage increase of yield stress ( $\sigma$$^{a}$ $_{y}$ - $\sigma$$^{v}$ $_{y}$ / $\sigma$$^{v}$ $_{y}$ ). The slower the strain rate, the greater the percentage increase occurs. In order to estimate the yield stress of PWR fuel cladding material under the service environment, the yield stress in water was obtained by comparing the oxidation rate in air that in water assuming the relationship between the oxygen pick-up amount and the yield stress increase.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.8
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• pp.5654-5663
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• 2015

In the unsaturated soil, suction, the negative pore water pressure leads to increases of the yield stress and the plastic shear stiffness of the soil skeleton due to the growth in interparticle stress. Therefore, in this study, the stress-strain relationship based on cyclic elasto-plastic constitutive model extended for unsaturated soil considering the 1st and the 2nd yield functions was induced in order to account for these effects of suction under the dynamic loading condition such as the earthquake. Through the program code considering this relationship and the routine of the cyclic loading with the reversion of loading direction, the numerical simulation of the cyclic triaxial test under the unsaturated condition would be possible. It is expected that the results of this study possibly contribute to the accuracy improvement on the prediction of unsaturated soil behavior under the dynamic loading condition.

• The Korean Journal of Rheology
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• v.6 no.1
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• pp.49-59
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• 1994

높은 전기장 하에서 다분산, 비구형 실리카/실리콘 오일 현탁액의 전기유변 (electrorheological, ER)현상을 살펴 보았다. 전기유변유체는 전기장 방향으로 사슬구조를 형 성하는 특성을 보이며 이것이 유변물성의 향상에 기여하는 것으로 알려졌다. 동적(dynamic) 상태 실험에서 전기장 하의 실리카 현탁액은 매우낮은 임계변형율(${\gamma}$c =0.1%)이상에서 비선 형 점탄성을 보였다. 저장탄성계수(G＇)는 변형율 변화에 손실탄성계수(G＂)는 매질의 점도 에 더 민감한 의존성을 보였다. 또한 겉보기 항복응력은 입자의 부피분율과 전기장에$\Phi$1.9E1.4 의 의존성을 보였는데 부피분율에 대한 의존성이 큰 이유는 0.1 이상의 부피분율에서 복합 사슬 구조 내의 입자들 간의 상호 정전효과가 지배적으로 나타나기 때문이라고 생각된다. 정상상태 실험에서는 부피분율이 크거나 높은 전기장 하에서 전단속도가 0.1sec-1 정도 이하 로 감소함에 따라 전단응력이 급겨히 증가하는 현상을 보였다. 그러므로 본질적인 동적 항 복응력을 얻기 위해서는 매우 낮은 전단속도 영역의 특이한 응력거동을 고려해야한다. 큰 전단속도 하에서는 hydrodynamic interaction의 영향으로 전단속도의 증가에 따라 전단응력 이 증가하였다. 이같은 전단응력의 거동을 계단전단실험으로 확인하였다.

• Journal of the Korean GEO-environmental Society
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• v.16 no.10
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• pp.5-14
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• 2015

Suction affects the unsaturated soil as the negative pore pressure, and leads to increases of the yield stress and the plastic shear stiffness of the soil skeleton due to the growth in interparticle stress. Hence, in this study, in order to account for these effects of suction under the dynamic loading condition such as the earthquake, the element simulation of the cyclic triaxial test using induced stress-strain relation based on cyclic elasto-plastic constitutive model extended for unsaturated soil considering the $1^{st}$ and the $2^{nd}$ yield functions was conducted. Through the stress path, stress-strain relation and relation between volumetric strain and axial strain, it was seen in all the cases that the simulation results demonstrated a good agreement with the experimental results. It is expected that the results of this study possibly contribute to the accuracy improvement on the prediction of unsaturated soil behavior under the dynamic loading condition.

• Korean Journal of Food Science and Technology
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• v.32 no.4
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• pp.974-978
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• 2000

The vane method was used to measure yield stresses of five commercial kochujang samples under a controlled shear-rate operating condition. Magnitudes of vane yield stress were higher than those of yield stress using Casson model with a concentric cylinder viscometer. Magnitudes of vane yield stresses showed great differences between the static $({\sigma}_s)$ and dynamic yield stresses $({\sigma}_d)$ of kochujang samples with undisturbed structure (UDS) and with broken down structure (BDS). A dimensionless yield number $(N{\sigma}_o)$ was determined from the ratio of ${\sigma}s$ to ${\sigma}d$ in order to describe the existence of temporary structure of kochujang.

• The Korean Journal of Rheology
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• v.5 no.2
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• pp.99-108
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• 1993

시멘트 페이스트의 비선형적 점탄성 거동을 연구하기 위해 동적인 전단 유동 시험 이 수행되었다. 전단응력과 전단변형 또는 전단변형율간의 관계를 보여주는 히스테리시스 곡선을 얻기 위하여 전단응력이 연속적으로 측정되었다. 이는 기존의 주파수 혹은 변형의 증가에 의한 실험(frequency or strain sweep experiment)과는 달리 저자에 의해 수정된 점 성계(HAAKE Model RV20/RC20/CV20N)의 조정프로그램을 이용하여 수행되었다. 동적 전 단유동시험에서 얻어진 히스테리시스곡선은 시멘트 페이스트가 전단변형을 받는 동안 선형 탄성, 입자간 연결고리의 파괴 및 점성유체 거동을 보여준다. 측정된 항복전단응력은 전단변 형율의 증가에 따라 파우어함수(Power low equation)에 의해 증가함을 보여준다.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.10a
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• pp.849-853
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• 2002

A specific experimental method, the split Hopkinson pressure bar (SHPB) technique has been widely used to determine the dynamic material properties under the impact compressive loading conditions with strain-rate of the order of 103/s~104/s. In this paper, dynamic deformation behaviors of rubber materials widely used for the isolation of vibration from varying structures under dynamic loading are determined using a Split Hopkinson Pressure Bar technique.

• The Korean Journal of Rheology
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• v.7 no.3
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• pp.203-210
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• 1995

전기장 하에서 폴리아닐린/미네랄 오일 현탁액의 유변학적 특성에 관한 실험적인 연 구를 큐엣 셀 형태의 레오미터를 사용하여수행하였다. 폴리아닐린 현탁액은 전기장을 가해 줄 때 점도가 크게 상승하는 현상을 보였고 부피분율과 전기장의 3/2승에 비례하는 동적 항 복응력을 나타내었다. 작은 변형 진폭의 동적 상태 실험을 통하여 저장계수와 손실계수를 변형진폭, 변형의 구동 주파수 및 전기장의 함수로 나타내었다. 저장계수는 전기장을 증가시 킬 때 증가하나 손실계수(5wt%)는 약한 전기장 의존성을 보였다. 낮은 응력을 가해줄때의 크립과 회복곡선은 초기의 순간적인 변형 증가와 지연되는 변형 그리고 회복 불가능한 영구 적 변형으로 구성되어진다. 탄성 한계 항복응력은 전기장의 세기가 증가함에 따라 증가하였 다. 매우 작은 변형에서는 응력과 변형사이의 선형적 관계를 보여 고체와 유사한 거동을 나 타내었다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.4
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• pp.1041-1050
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• 1995

The deformed shape of rod specimen of copper alloys was measured after the high-velocity impact against a rigid anvil and analyzed with one-dimensional theory to determine dynamic yield stress and strain-rate sensitivity which is defined as the ratio of dynamic yield stress to static flow stress. The evvect of two-dimensional deformation on the determination of dynamic yield stress by the one-dimensional theory, was investigated through comparison with the analysis by hydrocode. It showed that the one-dimensional theory is relatively consistent with two-dimensional hydrocode in spite of its simplicity in analysis.