• 대한기계학회논문집A
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• 제34권4호
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• pp.487-493
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• 2010

대부분의 구조해석 문제는 외부에서 주어진 하중에 대한 변형과 응력에 관심을 두고 있지만 많은 경우에서 표면 또는 내부에 주어진 응력이나 트랙션을 구하는 역문제 해석이 필요하게 된다. 본 연구에서는 구하고자 하는 트랙션에서 멀리 떨어진 영역의 변위를 측정하여 미지의 트랙션을 평가하는데 유한요소법을 사용한 역문제 수식화를 적용하였다. 일반적으로 역시스템의 불안정으로 인하여 측정 변위의 작은 오차는 해석 결과에 큰 영향을 주게 된다. 이와 같은 역시스템의 불안정성을 개선하기 위하여 본 연구에서는 구하고자 하는 트랙션에 가까운 단면의 변위를 Gram-Schmidt 수직화 기법을 통한 수직기저함수 사용하여 예측하고 보다 안정된 역문제를 해석하는 방법을 개발하였고 장점들을 수치 예제를 통하여 보여주었다.

• Steel and Composite Structures
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• 제26권6호
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• pp.673-691
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• 2018

The main goal of this research is to examine the in-plane and out-of-plane forced vibration of a curved nanocomposite microbeam. The in-plane and out-of-plane displacements of the structure are considered based on the first order shear deformation theory (FSDT). The curved microbeam is reinforced by functionally graded carbon nanotubes (FG-CNTs) and thus the extended rule of mixture is employed to estimate the effective material properties of the structure. Also, the small scale effect is captured using the strain gradient theory. The structure is rested on a nonlinear orthotropic viscoelastic foundation and is subjected to concentrated transverse harmonic external force, thermal and magnetic loads. The derivation of the governing equations is performed using energy method and Hamilton's principle. Differential quadrature (DQ) method along with integral quadrature (IQ) and Newmark methods are employed to solve the problem. The effect of various parameters such as volume fraction and distribution type of CNTs, boundary conditions, elastic foundation, temperature changes, material length scale parameters, magnetic field, central angle and width to thickness ratio are studied on the frequency and force responses of the structure. The results indicate that the highest frequency and lowest vibration amplitude belongs to FGX distribution type while the inverse condition is observed for FGO distribution type. In addition, the hardening-type response of the structure with FGX distribution type is more intense with respect to the other distribution types.

• 한국산림과학회지
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• 제95권4호
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• pp.381-387
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• 2006

본 연구는 우리나라 산림휴양량의 장기 예측을 가능하게 할 수 있는 모형개발을 시도하였으며, 특히, 계량경제모형에 부합하고 동시에 산림휴양에 대한 정책적 조절변수들을 발굴하여 산림휴양정책에 활용가능한 실용적 모형개발을 목표로 수행되었다. 이전의 선행연구들이 사용한 횡단면 분석의 문제점을 극복하기 위하여 횡단면자료와 시계열자료를 통합한 패널자료를 사용하여 분석하였다. 분석결과에 의하면 산림휴양량은 두 지역간의 거리와는 음(-)의 관계를 보이는 반면 출발지 인구와 목적지 산림면적, 그리고 정책변수로 상정된 목적지의 사회개발세출액과는 양(+)의 관계가 존재하는 것으로 나타났다. 또한 거리변수는 탄력적인 반면, 나머지 독립변수들은 모두 산림휴양량과는 비탄력적인 것으로 나타났다. 이러한 결과는 두 지역간의 산림휴양량은 일반적인 중력모형(gravity model) 이론과 일치하는 패턴은 따르고 있다 할 수 있다.

• 대한기계학회논문집A
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• 제20권1호
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• pp.88-99
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• 1996

The explicit scheme for finite element analysis of sheet metal forming problems has been widely used for providing practical solutions since it improves the convergency problem, memory size and computational time especially for the case of complicated geometry and large element number. The explicit schemes in general use are based on the elastic-plastic modeling of material requiring large computataion time. In the present work, a basic formulation for rigid-plastic explicit finite element analysis of plain strain sheet metal forming problems has been proposed. The effect of some basic parameters involved in the dynamic analysis has been studied in detail. Thus, the effective ranges of parameters have been proposed for numerical simultion by the rigid-plastic explicit finite element method. A direct trial-and-error method is introduced to treat contact and friction. In computation, sheet material is assumed to possess normal anisotropy and rigid-plastic workhardening characteristics. In order to show the validity and effectiveness of the proposed explicit scheme, computations are carried out for cylindrical punch stretching and the computational results are compared with those by the implicit scheme as well as with a commercial code. The proposed rigid-plastic exlicit finite element method can be used as a robust and efficient computational method for analysis of sheet metal forming.

• Design & Manufacturing
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• 제12권2호
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• pp.34-39
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• 2018

The biggest topics in the automobile industry are light weightening and fuel efficiency improvement. There's a lot of research going on. It is focused on light weight materials. Light weight material is seen as the best way to reduce fuel consumption and to solve the problem of environmental pollution and resource depletion. For the light weight materials, new materials such as aluminum, magnesium, and carbon-hardening materials can be found. Research on the joining techniques of dual materials, improvement of material properties by improving the method of manufacture of existing materials, and studies on ultra-high strength steel sheets are expected to take up the most weight in lightweight materials. As the strength of the ultra-high strength steel sheets increases during forming, it is difficult to obtain dimensional precision due to the increase in elastic restoring force compared to mild or high strength steel sheets. Spring back is known to be affected by a number of factors due to poor plastic molding, and can be divided into the effects of the material spraying and the process. The study on the plasticitic variables were studied as plasticitic factors that can be controlled by a part company. Tensile testing of ultra-high strength materials was conducted to derive properties for plasticitic analysis and to analyze spring back with two factors controlling the height of the bead and blank holding force by adding tensile force and controlling the flow rate.

• 비파괴검사학회지
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• 제19권2호
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• pp.100-109
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• 1999

본 연구에서는 산란 초음파의 주파수 특성을 이용하여 단일 섬유 복합재료내 인터페이즈의 물리적 성질을 측정하는 방법을 제안하고, 그 가능성을 이론적으로 고찰하였다. 그 결과 주파수 영역에서 나타나는 산과 골의 위치 및 크기가 인터페이즈의 성질에 의해 상당한 영향을 받음을 보임으로써 이 방법의 타당성을 확인하였다. 산란 초음파의 주파수 거동은 산란체의 공명과 관련되어 있지만 공명 피크가 음향파의 경우만큼 두드러지지 않아 기존의 음향 공명산란이론을 바로 적용할 수 없었다. 산란초음파의 주파수특성으로부터 인터페이즈의 성질을 구하는 역문제의 해법으로서 신경회로망을 구성하였으며, 이를 사용하면 대부분의 경우 인터페이즈의 성질을 상당히 정확하게 추정할 수 있었다.

• 한국항공우주학회지
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• 제33권2호
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• pp.32-38
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• 2005

본 연구에서는 근사모델을 이용하여 설계공간의 타당성을 높일 수 있는 방법을 제시하였다. 이때 설계공간을 이동시키기 위한 기준으로 Chebyshev Inequality를 사용하였다. 이를 공탄성을 고려한 항공기 익형 설계문제에 적용함으로써 타당성이 크게 향상됨을 확인하였으며 이렇게 구한 설계공간 내에서 최적화를 수행함으로써 보다 우수한 최적값도 얻을 수 있었다. 즉 설계공간 내에서 주어진 제약조건을 만족할 확률이 증가하였으며, 설계공간을 이동시킴으로써 보다 우수한 최적점이 설계공간 내에 포함되었다고 할 수 있다. 또한 이 과정에서 반응면 모델과 크리깅 모델, 두 가지 근사모델을 사용하여 정확성과 효율성, 실험점에 대한 강건성 등을 비교하였으며, 본 연구에서 설계한 문제의 경우 비교적 선형적인 특징으로 인해 반응면이 보다 우수한 결과를 보여줌을 확인하였다.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2005년도 International Meeting on Information Displayvol.I
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• pp.700-702
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• 2005

A 5.0-inch transmissive type plastic TFT arrays were successfully fabricated on a plastic substrate at the resolution of $400{\times}3{\times}300$ lines (100ppi). All of the TFT processes were carried out below $150^{\circ}C$ on PES plastic films. After thin film deposition using PECVD, thin film failures such as film delamination and cracking often occurred. For successful growth of thin films (about 1um) without their failures, it is necessary to solve the critical problem related to the internal compressive stress (some GPa) leading to delamination at a threshold thickness value of the films. The Griffith's theory explains the failure process by looking at the excess of elastic energy inside the film, which overcomes the cohesive energy between film and substrate. To increase the above mentioned threshold thickness value there are two possibilities: (i) the improvement of the interface adhesion (for example, through surface micro-roughening and/or surface activation), and (ii) the reduction of the internal stress. In this work, reducing a-Si layer film thickness and optimizing a barrier SiNx layer have produced stable CVD films at 150oC, over PES substrates

• Structural Engineering and Mechanics
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• 제38권6호
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• pp.697-714
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• 2011

Assessing the ductility of reinforced concrete sections and members has been a complex and intractable problem for many years. Given the complexity in estimating ductility, members are often designed specifically for strength whilst ductility is provided implicitly through the use of ductile steel reinforcing bars and by ensuring that concrete crushing provides the ultimate limit state. As such, the empirical hinge length and neutral axis depth approaches have been sufficient to estimate ductility and moment redistribution within the bounds of the test regimes from which they were derived. However, being empirical, these methods do not have a sound structural mechanics background and consequently have severe limitations when brittle materials are used and when concrete crushing may not occur. Structural mechanics based approaches to estimating rotational capacities and rotation requirements for given amounts of moment redistribution have shown that FRP plated reinforced concrete (RC) sections can have significant moment redistribution capacities. In this paper, the concept of moment redistribution in beams is explained and it is shown specifically how an existing RC member can be retrofitted with FRP plates for both strength and ductility requirements. Furthermore, it is also shown how ductility through moment redistribution can be used to maximise the increase in strength of a member. The concept of primary and secondary hinges is also introduced and it is shown how the response of the non-hinge region influences the redistribution capacity of the primary hinges, and that for maximum moment redistribution to occur the non-hinge region needs to remain elastic.

• Earthquakes and Structures
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• 제10권6호
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• pp.1253-1272
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• 2016

For a nonlinear control system, there are many uncertainties, such as the structural model, controlled parameters and external loads. Although the significant progress has been achieved on the robust control of nonlinear systems through some researches on this issue, there are still some limitations, for instance, the complicated solving process, weak conservatism of system, involuted structures and high order of controllers. In this study, the computational structural mechanics and optimal control theory are adopted to address above problems. The induced norm is the eigenvalue problem in structural mechanics, i.e., the elastic stable Euler critical force or eigenfrequency of structural system. The segment mixed energy is introduced with a precise integration and an extended Wittrick-Williams (W-W) induced norm calculation method. This is then incorporated in the market-based control (MBC) theory and combined with the force analogy method (FAM) to solve the MBC robust strategy (R-MBC) of nonlinear systems. Finally, a single-degree-of-freedom (SDOF) system and a 9-stories steel frame structure are analyzed. The results are compared with those calculated by the $H{\infty}$-robust (R-$H{\infty}$) algorithm, and show the induced norm leads to the infinite control output as soon as it reaches the critical value. The R-MBC strategy has a better control effect than the R-$H{\infty}$ algorithm and has the advantage of strong strain capacity and short online computation time. Thus, it can be applied to large complex structures.