• Journal of Astronomy and Space Sciences
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• 제22권4호
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• pp.441-450
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• 2005

유기재료(organic material)는 위성이나 우주비행체의 열을 조절하고 우주환경에 직접 노출되는 것을 차단하기 위하여 가장 흔하게 사용되는 재료 중 하나이다. 본 논문에서는 지상설비를 이용하여 모사한 우주환경에서 유기재료의 물성변화를 관찰하였다. 대표적인 위성용 유기 열조절 재료 중 하나인 2mil ITO(Indium Tin Oxide) coated aluminized KAPTON을 실험 대상으로 선택하였다. 양성자에 의한 단일 우주환경효과를 실험하기 위하여, 한국 원자력의학원의 MC-50 싸이클로트론(cyclotron)을 이용하여 양성자를 조사(irradiation)하였으며, 조사조건은 지구궤도 주변 최고의 양성자 발생기록인 1972년 8월의 최고치 상황을 적용하였다. 조사에너지는 평균 관측에너지인 30MeV으로 고정하였으며, 등가 조사량은 우주노출 시간 1년, 3년, 5년 및 10년을 기준으로 설정하였다. 분석과정은 인장강도를 측정하여 정량적 물성저하를 확인하였고, 전계방출 전자주사현미경 등으로 결정성변화와 노출표면의 손상을 분석하였다.

• Restorative Dentistry and Endodontics
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• 제22권2호
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• pp.505-518
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• 1997

The objective of this investigation was to compare the effects of water storage on the aspect of hardness and diametral tensile strengths of four hybrid glass ionomer cements(two compomers and two resin-reinforced glass ionomers) with a resin composite material. One composite resin(Degufill Ultra), two compomers(Dyract, Compoglass Cavifil), and two resin-reinforced glass ionomers(Fuji Duet, Vitremer) were used in this study. Cylindrical specimens were prepared and stored at $36{\pm}1^{\circ}C$ in distilled water for 10 minutes after set, and then tested on an Instron testing machine(No.4467) at 1.0 mm/min displacement rate. Vicker's hardness and diametral tensile strengths as time elapsed were measured after aging in water for 10 minutes, 1 hour, 3 hours, 1 day, 3 days, 5 days and 7 days at $36{\pm}1^{\circ}C$. During the test of diametral tensile strength, stress-strain curves were obtained, from which the compressive modulus were calculated and compared. The structure of four set glass ionomer cement mass was observed on SEM(Hitachi, S-2300) after being etched with 9.6% hydrofluoric acid for 1 minute. The results were as follows; 1. The hardness of the experimental group(compomer and the resin reinforced glass ionomer cement) did not exceed the value of control group(Degufill Ultra). 2. Vicker's hardness of the Fuji Duet tended to increase succeedingly, Dyract was decreased after 3 hours in water, and Vitremer was the lowest. 3. The control group(Degufill Ultra) presented progressively on increased diametral tensile strength with time, Fuji Duet were decreased after 3 days, Compoglass Cavifil and Vitremer were decreased after 5 days in water storage. 4. Compressive modulus of the control group(Degufill Ultra) and Dyract were increased sharply timely, Fuji Duet and Vitremer were increased smoothly by lapse of time in water. Fuji Duet were stronger than Vitremer. On the other hand, Vitremer exhibited the lowest toughness. 5. The microstructure of compomer was similar with that of the composite resin(Degufill Ultra), and the fillers in resin-reinforced glass ionomer cements were noticed. It can be concluded that mechanical properties of hybrid glass ionomer cements is weaker than composite resin, and that the compomers or the resin-reinforced glass ionomers can not substitute the composite resins. A plenty of considerations should be done on the application of them to the area under the loading and high wear has a little adverse effect on the mechanical properties on the water storage for 7 days. The further research should be needed to confirm the advantage of the compomer.

• Structural Engineering and Mechanics
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• 제83권4호
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• pp.495-513
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• 2022

This paper aims to study the fracture mechanism of rocks under the 'u'shape cutters considering the effects of crack (pre-existing crack) distances, crack spacing and crack inclination angles. The effects of loading rates on the rock fragmentation underneath these cutters have been also studied. For this purpose, nine experimental samples with dimensions of 5 cm×10 cm×10 cm consisting of the non-persistent cracks were prepared. The first three specimens' sets had one non-persistent crack (pre-existing crack) with a length of 2 cm and angularity of 0°, 45°, and 90°. The spacing between the crack and the "u" shape cutter was 2 cm. The second three specimens" set had one non-persistent crack with a length of 2 cm and angularity of 0°, 45°, and 90° but the spacing between pre-existing crack and the "u" shape cutter was 4 cm. The third three specimens'set has two non-persistent cracks with lengths of 2 cm and angularity of 0°, 45° and 90°. The spacing between the upper crack and the "u" shape cutter was 2 cm and the spacing between the lower crack and the upper crack was 2 cm. The samples were tested under a loading rate of 0.005 mm/s. concurrent with the experimental investigation. The numerical simulations were performed on the modeled samples with non-persistent cracks using PFC2D. These models were tested under three different loading rates of 0.005 mm/s, 0.01 mm/sec and 0.02 mm/sec. These results show that the crack number, crack spacing, crack angularity, and loading rate has important effects on the crack growth mechanism in the rocks underneath the "u" shape cutters. In addition, the failure modes and the fracture patterns in the experimental tests and numerical simulations are similar to one another showing the validity and accuracy of the current study.

• Structural Engineering and Mechanics
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• 제30권6호
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• pp.713-732
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• 2008

Serviceability and durability of the concrete members can be seriously affected by the corrosion of steel rebar. Carbonation front and or chloride ingress can destroy the passive film on rebar and may set the corrosion (oxidation process). Depending on the level of oxidation (expansive corrosion products/rust) damage to the cover concrete takes place in the form of expansion, cracking and spalling or delamination. This makes the concrete unable to develop forces through bond and also become unprotected against further degradation from corrosion; and thus marks the end of service life for corrosion-affected structures. This paper presents an analytical model that predicts the weight loss of steel rebar and the corresponding time from onset of corrosion for the known corrosion rate and thus can be used for the determination of time to cover cracking in corrosion affected RC member. This model uses fully the thick-walled cylinder approach. The gradual crack propagation in radial directions (from inside) is considered when the circumferential tensile stresses at the inner surface of intact concrete have reached the tensile strength of concrete. The analysis is done separately with and without considering the stiffness of reinforcing steel and rust combine along with the assumption of zero residual strength of cracked concrete. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel rebar. The capability of the model to produce the experimental trends is demonstrated by comparing the model's predictions with the results of experimental data published in the literature. The effect of considering the corroded reinforcing steel bar stiffness is demonstrated. A sensitivity analysis has also been carried out to show the influence of the various parameters. It has been found that material properties and their inter-relations significantly influence weight loss of rebar. Time to cover cracking from onset of corrosion for the same weight loss is influenced by corrosion rate and state of oxidation of corrosion product formed. Time to cover cracking from onset of corrosion is useful in making certain decisions pertaining to inspection, repair, rehabilitation, replacement and demolition of RC member/structure in corrosive environment.

• Structural Engineering and Mechanics
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• 제73권3호
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• pp.271-286
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• 2020

Passive control technologies are commonly used in several areas to suppress structural vibrations by the addition of supplementary damping, and some modal damping may be heavy beyond critical damping even for regular structures with energy dissipation devices. The design of passive control structures is typically based on (complex) mode superposition approaches. However, the conventional mode superposition approach is predominantly applied to cases of under-critical damping. Moreover, when any modal damping ratio is equal or close to 1.0, the system becomes defective, i.e., a complete set of eigenvectors cannot be obtained such that some well-known algorithms for the quadratic eigenvalue problem are invalid. In this paper, a generalized complex mode superposition method that is suitable for under-critical, critical and over-critical damping is proposed and expressed in a unified form for structural displacement, velocity and acceleration responses. In the new method, the conventional algorithm for the eigenvalue problem is still valid, even though the system becomes defective due to critical modal damping. Based on the modal truncation error analysis, modal corrected methods for displacement and acceleration responses are developed to approximately consider the contribution of the truncated higher modes. Finally, the implementation of the proposed methods is presented through two numerical examples, and the effectiveness is investigated. The results also show that over-critically damped modes have a significant impact on structural responses. This study is a development of the original complex mode superposition method and can be applied well to dynamic analyses of non-classically damped systems.

• Structural Engineering and Mechanics
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• 제63권3호
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• pp.329-345
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• 2017

This paper devoted to study dynamic interaction between crack and inclusion or void by developing the eXtended Finite Element Methods (XFEM). A novel XFEM approximation is presented for these structures containing multi discontinuities (void, inclusion, and crack). The level set methods are used so that elements that include a crack segment, the boundary of a void, or the boundary of an inclusion are not required to conform to discontinuous edges. The investigation covers the effects of a single circular or elliptical void / stiff inclusion, and multi stiff inclusions on the crack propagation path under dynamic loads. Both the void and the inclusion have a significant effect on the dynamic crack propagation path. The crack initially curves towards into the void, then, the crack moves round the void and propagates away the void. If a large void lies in front of crack tip, the crack may propagate into the void. If an enough small void lies in front of crack tip, the void may have a slight or no influence on the crack propagation path. For a stiff inclusion, the crack initially propagates away the inclusion, then, after the crack moves round the inclusion, it starts to propagate along its original path. As ${\delta}$ (the ratio of the elastic modulus of the inclusion to that of the matrix) increases, a larger curvature of the crack path deflection can be observed. However, as ${\delta}$ increases from 2 to 10, the curvature has an evident increase. By comparison, the curvature has a slight increase, as ${\delta}$ increases from 10 to 1000.

• 한국철도학회논문집
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• 제19권3호
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• pp.351-362
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• 2016

본 연구의 목적은 고속철도 역사시설 및 열차서비스 전반에 관한 서비스 만족도를 측정하여 고객만족에 영향을 미치는 주요 서비스요인을 도출하고 그 관계를 밝히는 데 있다. 이를 위해 고속철도 이용객 202명을 대상으로 설문을 수행하여 분석한 결과, 역사 및 열차 관련 서비스에 관한 설문문항 63개 중, 주차관련서비스, 역사편의시설관련서비스, 발권서비스, 열차 내 서비스, 열차 내 환경 등의 다섯 가지 주요 서비스요인이 도출되었다. 구조방정식모형을 통한 가설검증 결과, 주차관련서비스는 열차 내 서비스에 직접적인 영향을 미치지 않으며, 그 외 역사편의시설 및 발권서비스는 열차 내 서비스 및 환경에 각각 긍정적인 영향을 미치는 선행요소로 나타났다. 또한 열차 내 서비스와 환경요인은 고객의 만족을 강화시키는 주요한 변수로 검증되었다. 따라서 역사시설 및 열차서비스의 경험은 철도서비스만족에 긍정적인 영향을 미치는 것을 알 수 있었다.

• 한국융합학회논문지
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• 제8권12호
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• pp.343-349
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• 2017

본 연구는 산업단지 입지요인을 선행연구를 중심으로 종합적으로 검토하여 본 연구 목적에 맞게 연구모델을 개발과 관련변수를 투입하여 실증 분석을 실시하였다. 본 연구에서는 입지결정요인을 기반시설의 안정화와 산업자원의 접근도 그리고 매개변수인 주거 근린환경요인군의 세 가지 요인 군으로 대별하여 산업입지결정요인과의 관계정도를 분석하였다. 본 연구 분석을 위해 연구모형에 근거하여 설정된 가설을 통해 구조방정식 모형의 경로분석을 통해 가설을 분석하였다. 분석결과 독립변수인 기반시설요인군의 교통과 산업기반시설 요인과 자원접근 요인인 노농과 생산자원 접근성 모두 산업입지에 영향관계에 있는 것으로 분석되었다. 또한 매개변수인 근린환경요인과 산업입지결정요인간의 분석에서도 통계적으로 유의한 상관관계가 있는 것으로 분석되었다. 본 연구 결과는 지속적으로 추진되고 있는 국가 및 지방 자치단체의 산업단지 개발에 있어서 산업기반시설의 구축과 산업단지 개발 사업에 유용한 지침을 제공할 수 있을 것으로 기대된다.

• 한국콘텐츠학회논문지
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• 제9권12호
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• pp.261-269
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• 2009

본 연구의 목적은 민간경호경비 회사의 시장지향성과 서비스품질 및 경영성과의 관계를 규명하는데 있다. 이 연구는 2009년 서울 소재 민간경호경비 회사를 모집단으로 선정한 다음, 유의표집법을 활용하여 250명의 연구대상을 표집하였다. 그러나 응답이 불성실하고 무의미한 자료를 제외하고 최종분석에 사용된 사례수는 총 235명이었다. 이 연구에서는 설문지의 타당도 및 신뢰도를 검증하기 위하여 신뢰도분석의 알파값과 공변량구조분석의 최대우도법을 이용하였다. 설문지의 타당도는 외생변수의 경우 .675, 내생변수의 경우 .732이상으로 나타났으며, 설문지의 신뢰도는 Cronbach's $\alpha$값이 .732이상으로 나타났다. 이 연구에서는 신뢰도분석을 위해 SPSSWIN 16.0 프로그램을 활용하였으며, 확인적 요인분석과 공변량구조분석을 위하여 AMOS 5.0 프로그램을 적용하였다. 이상과 같은 연구방법 및 절차를 통해 이 연구에서 도출한 결론은 다음과 같다. 이 연구는 민간경호경비 회사의 시장지향성이 첫째, 간경호경비 회사가 시장지향적일수록 서비스품질은 향상된다. 둘째, 민간경호경비 회사가 시장지향적일수록 경영성과는 향상된다. 셋째, 서비스품질이 좋을수록 경영성과는 높아진다. 이상의 결론을 종합하면, 민간경호경비 회사의 시장지향성은 경영성과에 직접적인 영향을 미칠 뿐만 아니라, 서비스품질을 통하여 경영성과에 간접적인 영향을 미치고 있음을 알 수 있다.

• 인터넷정보학회논문지
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• 제12권4호
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• pp.145-156
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• 2011

본 연구는 e-저작권침해 일탈행위를 유발하는 원인을 규명하기 위해 진행되었다. 이를 위해 대학생 531명을 대상으로 설문을 실시하였고, 여기서 얻어진 데이터를 분석하기 위해서 구조방정식모델(SEM)을 사용하였다. 구조방정식모델의 설계는 익명상황, 규범의식, 처벌인지를 e-저작권침해에 대한 직접적인 변인으로 구성하였고, 또한 익명상황을 규범의식과 처벌인지에 대한 변인으로 설정하였다. 변인들 간의 직 간접적 인과관계를 분석한 결과, 익명상황은 e-저작권침해에 대한 직접적인 원인으로 나타났으나, 규범의식과 처벌인지 변인은 통계적으로 유의하지 않았다. 또한, 익명상황은 규범의식과 처벌인지에 대한 직접적으로 원인이며, 규범의식을 매개로 처벌인지에 간접적으로 영향을 미치는 것으로 밝혀졌다. 이러한 연구결과로 볼 때, e-저작권침해를 예방하기 위해서는 익명상황에서 규범을 지키는 것을 훈련할 수 있는 적절한 교육적 기회를 제공할 필요가 있다. 이를 위한 다양하고 효과적인 정보윤리 교육방법에 대한 모색이 이루어져야 할 것이다.