• 한국해양공학회지
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• 제28권2호
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• pp.133-139
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• 2014

Offshore wind turbines have been constructed extensively throughout the world. These turbines are subjected to approximately $10^8$ horizontal load cycles produced from wind, waves, and current during their lifetimes. Therefore, the accumulated displacement of the foundation under horizontal cyclic loading has significant effects on the foundation design of a wind turbine. Akili(2006) and Achmus et al.(2009) performed cyclic triaxial tests on dry sands and proposed an empirical model for predicting the accumulated plastic strain of sands under long-term cyclic loading. In this study, cyclic triaxial tests were performed to analyze the cyclic loading behaviors of dry sands. A total of 27 test cases were performed by varying three parameters: the relative density of the sands, cyclic load level, and confining stress. The test results showed that the accumulated plastic strain increased with an increase in the cyclic load level and a decrease in the relative density of the sand. The confining stress had less effect on the plastic strain. In addition, the plastic strain at the 1st loading cycle was about 57% of the accumulated strain at 1,000 cycles. Finally, the input parameters of the empirical models of Akili(2006) and Achmus et al.(2009) were evaluated by using the relative density of the sand and the cyclic load level.

• 한국재난정보학회 논문집
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• 제7권3호
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• pp.206-219
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• 2011

포장 설계기준에서 중요한 요인은 응력과 변형률 분포도이다. 합리적인 응력과 변형률 분포도를 달성하기 위한 차량타이어의 접촉면적과 공기압은 매우중요하다. 본 연구에서는 이동하중 하에서 연성포장의 점탄성 특성에 관한 내용을 다루고 있으며 현장 측정시험을 통하여 실제 도로의 종횡변형률을 수치해석 결과와 비교분석 하였다. 포장거동에 대한 차량이동하중의 영향을 적절히 모사하기 위하여 단계하중을 이용한 3차원 유한요소 해석이 수행되었다. 점탄성 해석을 위하여 아스팔트 혼합물의 이완계수, E(t), 가 실험실에서 제작된 시료의 실험으로부터 측정되었다. 현장조사 결과에 의하면, 종횡변형률은 서로 상이한 값을 보였으며 전반적으로 변형률의 크기는 차량의 속도가 증가함에 따라 감소함을 보였다. 전반적으로 횡방향 변형률은 종방향 변형률에 비하여 작은값을 보였으며 그 차이는 횡방향에서 더욱 두드러진 경향을 보였다.

• 콘크리트학회논문집
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• 제21권2호
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• pp.159-168
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• 2009

본 연구에서는 기존의 연구방법을 개선한 프리스트레스가 도입된 아라미드 스트랩으로 횡 구속된 RC 원형기둥의 일축압축 거동에 대하여 강도와 변위에 의한 강성 및 구속효과에 대하여 연구하였으며, 그 프리스트레싱 보강방법 및 과정에 대하여 비교적 상세히 소개하였다. 이 방법으로 보강된 기둥의 파괴형태는 초기 능동 구속압의 작용으로 인하여 변위 증가율이 매우 낮으며 이후 최종 파괴시까지도 그 형상이 유지되는 매우 안정적인 파괴형태를 보이게 된다. 파괴하중 비교 결과 일반 RC 기둥과 비교하여 73% 이상 강도증진 효과를 나타내었다. 또한 변형 증가율이 매우 작으며 이로 인한 변위구속효과가 매우 우수함을 확인할 수 있었다. 기존의 보강방법 (CFS, GFS)과의 비교를 통하여, 상대적으로 작은 면적으로 우수한 강도증진을 보이며 강성증가로 인한 구속효과 역시 우수하며 체적 증가율도 팽창하지 않는 결과를 보였다. 특히, 콘크리트 팽창 이후부터 발휘되는 기존 방법의 구속효과와는 달리, 초기부터 도입되는 프리스트레스트 긴장력에 의한 초기변형 구속효과가 매우 탁월하며, 최종 파괴시까지 체적 변형률이 팽창하지 않는 안정적인 결과를 나타낸다.

• 생명과학회지
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• 제19권3호
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• pp.322-326
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• 2009

간 질환 개선용 영양보충제로 시판되고 있는 글루타티온 고함유 Saccharomyces cerevisiae FF-8 효모 균체(FF-8) 급여가 사염화탄소 및 알코올-유발 흰쥐 조직 스트레스에 대하여 억제효과가 있다는 것을보고한 바 있다. 본 실험에서는 글루타티온 고함유 효모 FF-8 급여에 의한 오르트산-유발 지방간에서 각 조직 중 과산화지질 농도와 간 조직 내 미네랄 성분과의 관계를 검토하였다. 체중 증가량은 정상군은 두 실험군에 비해 증가하였으나, 오르트산 급여 실험군들에서는 체중 증가량이 5% 수준에서 유의적으로 감소하였다. 각 조직 중량은 오르트산 급여군에서 간 조직에서 5% 수준에서 유의적으로 증가하였고 FF-8 투여에 의해 다소 감소하는 것으로 나타났다. 한편, 신장 및 비장에서도 오르트산 급여에 의해 5% 수준에서 유의적으로 감소하였다. 간 조직 중의 과산화지질 농도는 오르트산 급여군에서 5% 수준에서 유의적으로 증가하였고, FF-8 균체 급여군에서 감소하였다. 이때 간 조직 중의 글루타티온 농도도 유사한 경향을 보였다. 천연 항산화 미네랄로 알려진 간 조직 중의 아연 농도는 정상군에 비해 오르트산 유발 지방간에서 감소하였고, FF-8 균체 급여에 의해 다소 증가되는 것으로 나타났다. 이상의 실험 결과에서 오르트산-유발 지방간에서 지질과산화 농도의 증가는 간 조직내 천연 항산화 물질인 글로타티온과 아연 농도의 현저한 감소에 기인하였고, 항산화 물질인 글루타티온을 고함유한 S. cerevisiae FF-8 효모 균체 동시 급여에 의해서는 글로타티온과 아연 농도의 증가에 의해 과산화지질 농도가 경감됨으로서 오르트산 유발 산화스트레스를 경감시키는 효과가 있는 것으로 확인되었다.

• 소성∙가공
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• 제18권8호
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• pp.640-645
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• 2009

Recently, boss and rib test based on backward extrusion process was proposed to quantitatively evaluate the interfacial friction condition in bulk forming process. In this test, the tube-shaped punch with hole pressurizes the workpiece so that the boss and rib are formed along the hole and outer surface of the punch. It was experimentally and numerically revealed that the height of boss is higher than that of the rib under the severe friction condition. This work is focused on the effect of the punch design and flow stress on deformation pattern in boss and rib test. From the boss and rib test simulations, it was found that there is slight variation in both the heights of boss and rib according to the length of punch land, nose radius, and face angle. However the hole diameter of the punch and the clearance between the punch and die have a significant influence on the calibration curves showing the heights of the boss and rib. In addition, the effect of flow stress on the calibration curves was investigated through FE simulations. It was found that there is no effect of strength coefficient of the workpiece on the calibration curves for estimation of friction condition. On the other hand, the strain-hardening exponent of the workpiece has a significant influence on the calibration curve.

• Structural Engineering and Mechanics
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• 제39권3호
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• pp.303-316
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• 2011

Experimental tests have shown that glass exhibits very different strengths when tested under biaxial and uniaxial conditions. This paper presents a study on the effects of biaxial stresses on the notional ultimate strength of glass. The study involved applying the theory of elasticity and finite element analysis of the Griffith flaw in the micro scale. The strain intensity at the tip of the critical flaw is used as the main criterion for defining the limit state of fracture in glass. A simple and robust relationship between the maximum principal stress and the uniaxial stress to cause failure of the same glass specimen has been developed. The relationship has been used for evaluating the strength values of both new and old annealed glass panels. The characteristic strength values determined in accordance with the test results based on 5% of exceedance are compared with provisions in the ASTM standard.

• Korea-Australia Rheology Journal
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• 제11권4호
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• pp.293-304
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• 1999

Refinements of classical theories for entangled or crosslinked polymeric systems have led to incommensurable models for rubber networks and polymer melts, contrary to experimental evidence, which suggests a great deal of similarity. Uniaxial elongation and compression data of linear and branched polymer melts as well as of crosslinked rubbers were analyzed with respect to their nonlinear strain measure. This was found to be the result of two contributions: (1) affine orientation of network strands, and (2) isotropic strand extension. Network strand extension is caused by an increasing restriction of lateral movement of polymer chains due to deformation, and is modelled by a molecular stress function which in the tube concept of Doi and Edwards is the inverse of the relative tube diameter. Up to moderate strains, $f^2$ is found to be linear in the average stretch for melts as well as for rubbers, which corresponds to a constant tube volume. At large strains, rubbers show maximum extensibility, while melts show maximum molecular tension. This maximum value of the molecular stress function governs the ultimate magnitude of the strain-hardening effect of linear and long-chain branched polymer melts in extensional flows.

• Journal of Advanced Marine Engineering and Technology
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• 제32권6호
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• pp.938-946
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• 2008

Fe-base amorphous alloy and two crystalline phases composite were fabricated. The effect of temperature and strain rate on mechanical properties was evaluated utilizing compression test. Mixture of non-crystalline and crystalline phases were found using X-ray diffraction (XRD) and differential thermal analysis (DTA) tests. Based on glass transition temperature and crystallization temperature. compression tests were performed in the temperature ranging from $560^{\circ}C$ to $700^{\circ}C$ with $20^{\circ}C$ interval. Relationship between microstructure, including fracture surface morphology, and mechanical behavior was studied. The peak stress of Fe-base amorphous alloy was over 2GPa and expected to have a good wear resistance, but it is expected hard to deform because of low ductility. The peak stress and elongation of two crystalline phases composite was over 1GPa and about 20%, therefore it is possible to deform high strength wear resistant materials such as engine valve.

• Steel and Composite Structures
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• 제29권2호
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• pp.161-173
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• 2018

Point bending is often used for cambering and curving structural steel girders. An analytical solution, applicable in the elasto-plastic range only, that relates applied loads to the desired curve was recently developed for inducing horizontal curves using four-point bending. This solution does not account for initial residual stresses and geometric imperfections built-in hot-rolled sections. This paper presents results from a full-scale test on a hot-rolled steel section curved using four-point bending. In parallel, a numerical analysis, accounting for both initial geometric imperfections and initial residual stresses, was carried out. The models were validated against the experimental results and a good agreement for lateral offset and for strain in the elasto-plastic and post-plastic ranges was achieved. The results show that the effect of initial residual stresses on deformation and strain is minimal. Finally, residual stresses due to cold bending calculated from the numerical analysis were assessed and a revised stress value for the service load design of the curved girder is proposed.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2004년도 춘계 학술발표대회 개요집
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• pp.246-248
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• 2004

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameters criterion based on equivalent plastic strain and stress triaxiality. This study provides the fundamental clarification of the effect of strength mismatching, which can elevate plastic constraint due to heterogeneous plastic straining, loading mode and loading rate on critical condition to initiate ductile crack from notch root using equivalent plastic strain and stress triaxiality based on the two-parameter criterion obtained on homogeneous specimens under static tension. The critical condition to initiate ductile crack from notch root for strength mismatched bend specimens under both static and dynamic loading would be almost the same as that for homogeneous tensile specimens with circumferential sharp notch under static loading.