• 한국가스학회지
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• 제15권1호
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• pp.30-34
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• 2011

본 논문은 104L의 저장용량과 70MPa의 사용압력을 갖는 수소연료탱크용 복합소재 압력용기에 관한 강도안전성을 해석한 평가연구이다. 탄소섬유 복합용기의 내측은 6061-T6의 알루미늄 라이너를 사용하고, 외경측은 원주방향을 따라서 탄소섬유 후프층을 형성하고, 또한 $12^{\circ}C$ 적층과 $70^{\circ}C$ 적층을 헤리컬방향으로 경사지게 감아서 제작하였다. 복합소재 연료탱크의 강도안전성에 대한 FEM 해석결과는 US DOT-CFFC와 KS B ISO 11119-2 규정에 따라 평가되었다. FEM 해석결과에 의하면, 104L의 복합용기로 제시된 설계모델을 US DOT-CFFC와 KS B ISO 11119-2 기준으로 평가할 때 모두 안전하다할 수 있다. 그렇지만, 탄소섬유 연료탱크에 대한 계산결과는 한국의 평가기준을 사용하기보다는 미국의 안전기준을 적용하는 것이 더 안전하다는 해석결과를 알 수 있다. 따라서 70MPa의 수소연료 압력용기의 강도안전성을 충분히 확보하기 위해서는 미국의 DOT-CFFC 안전기준에 의거 평가하고 설계하는 것이 바람직하다.

• Smart Structures and Systems
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• 제9권2호
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• pp.105-125
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• 2012

A modified Pagano method is developed for the three-dimensional (3D) free vibration analysis of simply-supported, multilayered functionally graded material (FGM) circular hollow cylinders with a constant rotational speed with respect to the meridional direction of the cylinders. The material properties of each FGM layer constituting the cylinders are regarded as heterogeneous through the thickness coordinate, and then specified to obey a power-law distribution of the volume fractions of the constituents, and the effects of centrifugal and Coriolis accelerations, as well as the initial hoop stress due to rotation, are considered. The Pagano method, which was developed for the static and dynamic analyses of multilayered composite plates, is modified in that a displacement-based formulation is replaced by a mixed formulation, the complex-valued solutions of the system equations are transferred to the real-valued solutions, a successive approximation method is adopted to extend its application to FGM cylinders, and a propagator matrix method is developed to reduce the time needed for its implementation. These modifications make the Pagano method feasible for multilayered FGM cylinders, and the computation in the implementation is independent of the total number of the layers, thus becoming less time-consuming than usual.

• 한국항공우주학회지
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• 제35권7호
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• pp.592-600
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• 2007

최근에 발사체의 경량화를 위해 추진제 탱크의 재료를 복합재료로 대체하기 위한 연구가 많이 진행되고 있다. 본 연구에서는 극저온용으로 개발된 복합재와 알루미늄 라이너로 구성된 타입 3 추진제 탱크를 제작하고 실제 극저온 상태의 운용환경을 고려한 실험을 수행하였다. 이를 위해 액체 질소를 제작된 타입 3 탱크에 주입하고 기체 질소를 이용하여 가압하였다. 실험수행과정에서 헬리컬 층과 후프 층 사이에서 층간 분리 현상이 관찰되었으며, 이에 대한 원인을 분석하기 위해 해석적 방법과 실험적 방법이 사용되었다. 해석적 방법에서는 점진적 파손 해석을 고려한 열탄성 해석으로부터 파손 지수를 평가하였으며 실험적 방법에서는 타입 3 탱크를 쉽게 모사할 수 있는 복합재/알루미늄 링 시편의 액체질소 담금 시험을 통해 헬리컬 층과 후프 층 사이의 계면을 관찰하였다.

• 한국초전도ㆍ저온공학회논문지
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• 제17권3호
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• pp.13-17
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• 2015

In superconducting coil applications particularly in wet wound coils, coated conductor (CC) tapes are subjected to different type of stresses. These include hoop stress acting along the length of the CC tape and the Lorentz force acting perpendicular to the CC tape's surface. Since the latter is commonly associated with delamination problem of multi-layered CC tapes, more understanding and attention on the delamination phenomena induced in the case of coil applications are needed. Difference on the coefficient of thermal expansion (CTE) of each constituent layer of the CC tape, the bobbin, and the impregnating materials is the main causes of delamination in CC tapes when subjected to thermal cycling. The CC tape might also experience cyclic loading due to the energizing scheme (on - off) during operation. In the design of degradation-free superconducting coils, therefore, characterization of the delamination behaviors including mechanism and strength in REBCO CC tapes becomes critical. In this study, transverse tensile tests were conducted under different loading conditions using different size of upper anvils on the GdBCO CC tapes. The mechanical and electromechanical delamination strength behaviors of the CC tapes under transverse tensile loading were examined and a two-parameter Weibull distribution analysis was conducted in statistical aspects. As a result, the CC tape showed similar range of mechanical delamination strength regardless of cross-head speed adopted. On the other hand, cyclic loading might have affected the CC tape in both upper anvil sizes adopted.

• 한국초전도ㆍ저온공학회논문지
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• 제19권2호
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• pp.25-28
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• 2017

In superconducting coil applications particularly in wet wound coils, coated conductor (CC) tapes are subjected to different type of stresses that could affect its electromechanical transport property. These include hoop stress acting along the length of the CC tape and the Lorentz force acting perpendicular to the CC tape's surface. Since the latter is commonly associated with the delamination problem of multi-layered REBCO CC tapes, more understanding and attention on the delamination phenomena induced in the case of coil applications are needed. Difference on the coefficient of thermal expansion (CTE) of each constituent layer of the CC tape, the bobbin, and the impregnating materials is the main causes of delamination in CC tapes when subjected to thermal and mechanical cycling. In the design of degradation-free superconducting coils, therefore, characterization of the delamination behaviors including mechanism and strength in the multi-layered REBCO CC tapes becomes a critical issue. Various trials to increase the delamination strength by improving interface characteristics at interlayers have been performed. In this study, in order to investigate the influences of laser cleaning and Ag annealing treated at the substrate side surface, transverse tensile tests were conducted under different sample configurations using $4.5mm{\times}8mm$ upper anvil. The mechanical delamination strength of differently processed CC samples was examined at room temperature (RT). As a result, the Sample 1 with the additional laser cleaning and Ag annealing processes and the Sample 2 with additional Ag annealing process only showed higher mechanical delamination strength as compared to the Sample 3 without such additional treatments. Sample 3 showed quite different behavior when the loading direction is to the substrate side where the delamination strength much lower as compared to other cases.

• 콘크리트학회논문집
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• 제17권4호
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• pp.521-526
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• 2005

원전 격납건물의 축대칭 모델은 해석상의 간편성으로 인하여 널리 사용된다. 하지만, 일반적인 돔 텐던의 배치는 축대칭 형상이 아니며 곡률을 가진 돔에 임의로 배치된 관계로 축대칭 근사화시 좀 더 엄밀한 수학적 유도가 요구된다. 본 연구에서는 국내의 CANDU형 및 한국형 격납건물 돔에 비축대칭으로 배치된 텐던을 축대칭 모델에 적용하기 위한 합리적인 변환 절차를 제안하였다. 텐던 강성의 모델링에서는 실제 3차원으로 배치된 돔 텐던의 자오선방향 및 원환방향으로의 강성 기여를 고려할 수 있도록 텐던을 등가의 층으로 근사화하였다. 프리스트레싱의 효과는 등가하중법 및 초기응력법 관점에서 고찰하였으며, 축대칭 모델의 방법론에 적합하도록 등가하중 및 초기응력을 유도하였다. 후속 논문에서는 제안된 모델을 적용한 수치 예제들을 범용구조해석 프로그램으로 해석하고 타당성을 검증하였다.

• 한국농공학회지
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• 제39권4호
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• pp.106-113
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• 1997

Dynamic loading of structures often causes excursions of stresses well into the inelastic range, and the influence of the geometric changes on the dynamic response is also significant in many cases. Therefore, both material and geometric nonlinearity effects should be considered in case that a dynamic load acts on the structure. A structure in a nuclear power plant is a structure of importance which puts emphasis on safety. A nuclear container is a pressure vessel subject to internal pressure and this structure is constructed by a reinforced concrete or a pre-stressed concrete. In this study, the material nonlinearity effect on the dynamic response is formulated by the elasto-viscoplastic model highly corresponding to the real behavior of the material. Also, the geometrically nonlinear behavior is taken into account using a total Lagrangian coordinate system, and the equilibrium equation of motion is numerically solved by a central difference scheme. The constitutive relation of concrete is modeled according to a Drucker-Prager yield criterion in compression. The reinforcing bars are modeled by a smeared layer at the location of reinforcements, and the steel layer model under Von Mises yield criteria is adopted to represent an elastic-plastic behavior. To investigate the dynamic response of a nuclear reinforced concrete containment structure, the steel-ratios of 0, 3, 5 and 10 percent, are considered. The results obtained from the analysis of an example were summarized as follows 1. As the steel-ratio increases, the amplitude and the period of the vertical displacements in apex of dome decreased. The Dynamic Magnification Factor(DMF) was some larger than that of the structure without steel. However, the regular trend was not found in the values of DMF. 2. The dynamic response of the vertical displacement and the radial displacement in the dome-wall junction were shown that the period of displacement in initial step decreased with the steel-ratio increases. Especially, the effect of the steel on the dynamic response of radial displacement disapeared almost. The values of DMF were 1.94, 2.5, 2.62 and 2.66, and the values increased with the steel-ratio. 3. The characteristics of the dynamic response of radial displacement in the mid-wall were similar to that of dome-wall junction. The values of DMF were 1.91, 2.11, 2.13 and 2.18, and the values increased with the steel-ratio. 4. The amplitude and the period of the hoop-stresses in the dome, the dome-wall junction, and the mid-wall were shown the decreased trend with the steel-ratio. The values of DMF were some larger than those of the structure without steel. However, the regular trend was not found in the values of DMF.

• 한국재료학회지
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• 제18권10호
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• pp.547-551
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• 2008

Crack-free joining of $Si_3N_4\;and\;Al_2O_3$ using 15 layers has been achieved by a unique approach introducing Sialon polytypoids as a functionally graded materials (FGMs) bonding layer. In the past, hot press sintering of multilayered FGMs with 20 layers of thickness $500{\mu}m$ each has been fabricated successfully. In this study, the number of layers for FGM was reduced to 15 layers from 20 layers for optimization. For fabrication, model was hot pressed at 38 MPa while heating up to $1700^{\circ}$, and it was cooled at $2^{\circ}$/min to minimize residual stress during sintering. Initially, FGM with 15 layers had cracks near 90 wt.% 12H / 10 wt.% $Al_2O_3$ and 90 wt.% 12H/10 wt.% $Si_3N_4$ layers. To solve this problem, FEM (finite element method) program based on the maximum tensile stress theory was applied to design optimized FGM layers of crack free joint. The sample is 3-dimensional cylindrical shape where this has been transformed to 2-dimensional axisymmetric mode. Based on the simulation, crack-free FGM sample was obtained by designing axial, hoop and radial stresses less than tensile strength values across all the layers of FGM. Therefore, we were able to predict and prevent the damage by calculating its thermal stress using its elastic modulus and coefficient of thermal expansion. Such analyses are especially useful for FGM samples where the residual stresses are very difficult to measure experimentally.