• Journal of Welding and Joining
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• 제28권4호
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• pp.26-32
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• 2010

The purpose of this study is to evaluate the residual stresses at the multi-pass FCA weldment using the finite element analysis (FEA). In order to do it, an H-type specimen was selected as a test specimen. The variable used was in-plane restraint intensity. The temperature distribution at the multi-pass FCA butt weldment was evaluated in accordance with the relevant guidance recommended by the KWJS. The effective conductivity for the weld metal corresponding to each welding pass was introduced to control the maximum temperature below the vaporization temperature of weld metal. The heat flux caused by welding arc was assumed to be applied to the weld metal corresponding to welding pass. With heat transfer analysis results, the distribution of transverse residual stresses was evaluated using the thermo-mechanical analysis and compared with the measured results by XRD and uniaxial strain gage. In thermo-mechanical analysis, the plastic strain resetting at the temperature above melting temperature of $1450^{\circ}C$ was considered and the weld metal and base metal was assumed to be bilinear kinematics hardening continuum. According to the comparison between FEA and experiment, transverse residual stresses at the multi-pass FCA butt weldment obtained by FEA had a good agreement with the measured results, regardless of in-plane rigidity. Based on the results, it was concluded that thermo-mechanical FE analysis based on temperature distribution calculated in accordance with the KWJS’s guidance could be used as a tool to predict the distribution of residual stress of the multi-pass FCA butt weldment.

• Smart Structures and Systems
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• 제26권4호
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• pp.435-449
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• 2020

To effectively extract the vast wind resource, offshore wind turbines are designed with large rotor and slender tower, which makes them vulnerable to external vibration sources such as wind and wave loads. Substantial research efforts have been devoted to mitigate the unwanted vibrations of offshore wind turbines to ensure their serviceability and safety in the normal working condition. However, most previous studies investigated the vibration control of wind turbines in one direction only, i.e., either the out-of-plane or in-plane direction. In reality, wind turbines inevitably vibrate in both directions when they are subjected to the external excitations. The studies on both the in-plane and out-of-plane vibration control of wind turbines are, however, scarce. In the present study, the NREL 5 MW wind turbine is taken as an example, a detailed three-dimensional (3D) Finite Element (FE) model of the wind turbine is developed in ABAQUS. To simultaneously control the in-plane and out-of-plane vibrations induced by the combined wind and wave loads, another carefully designed (i.e., tuned) spring and dashpot are added to the perpendicular direction of each Tuned Mass Damper (TMD) system that is used to control the vibrations of the tower and blades in one particular direction. With this simple modification, a bi-directional TMD system is formed and the vibrations in both the out-of-plane and in-plane directions are simultaneously suppressed. To examine the control effectiveness, the responses of the wind turbine without control, with separate TMD system and the proposed bi-directional TMD system are calculated and compared. Numerical results show that the bi-directional TMD system can simultaneously control the out-of-plane and in-plane vibrations of the wind turbine without changing too much of the conventional design of the control system. The bi-directional control system therefore could be a cost-effective solution to mitigate the bi-directional vibrations of offshore wind turbines.

• 한국해양환경ㆍ에너지학회지
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• 제17권3호
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• pp.174-181
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• 2014

선형발전기가 연결된 부이의 수직운동에 대한 시간영역 해석을 수행하여 얻은 시계열 자료를 가지고 선형발전기를 설계하고 전기에너지 출력특성 및 효율에 대해 살펴보았다. 1차 변환장치로 원통형 부이를 선택하였고, 2차 변환장치로 양측식 할박(Halbach) 배열 영구자석 가동자와 철심형 슬롯리스(Slotless) 고정자로 구성된 선형발전기를 사용하였다. 시간영역에서 부이의 수직운동 속도와 파랑하중을 입력자료로 직선형 영구자석 발전기가 설계되었고, 설계된 발전기는 규칙파 조건에서 유한요소 해석법을 적용하여 발전특성해석을 수행함으로써, 그 타당성이 입증되었다. 또한 불규칙파 조건에서 합리적이고 빠른 해석을 위해, 등가회로법을 적용하여 발전특성 해석을 수행하였는데, 그 결과 역시 매우 타당함을 확인하였다.

• 한국자동차공학회논문집
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• 제17권1호
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• pp.114-119
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• 2009

The authors have investigated the NVH problems of drive system in full vehicle test. However it is difficult to define the NVH problems of driveline system. Since it is hard to measure the rotating part and it is vague that only the drive system induces the NVH problem. Vibration in a driveline is presented in this paper. In the experiment, the rear sub-frame and propeller shafts and axle were composed and mounted with rubber each other. For applying the vibration input instead of the torsional vibration effect of an engine, the shaker was taken. In particular, torsional vibration due to fluctuating forced vibration excitation across the joint between driveline and rear sub-frame was carefully examined. Accordingly, the joint response was checked from experiments and the FE-simulation using FRF (frequency response function) analysis was performed. All test results were signal processed and validated against numerical simulations. In present study, the new test bench for measuring the vibration signal and simulating the vehicle chassis system was proposed. The modal value and the mode shape of components were analyzed using the CAE model to identify the important components affecting driveline noise and vibration. It could be reached that the simplified test bench could be well established and be used for design guide and development of the vehicle chassis components.

• Structural Engineering and Mechanics
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• 제43권2호
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• pp.179-197
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• 2012

This study presents the findings of the structural health monitoring and the real time system identification of one of the first large scale building instrumentations in Turkey for earthquake safety. Within this context, a thorough review of steps in the instrumentation, monitoring is presented and seismic performance evaluation of structures using both nonlinear pushover and nonlinear dynamic time history analysis is carried out. The sensor locations are determined using the optimal sensor placement techniques used in NASA for on orbit modal identification of large space structures. System identification is carried out via the stochastic subspace technique. The results of the study show that under ambient vibrations, stocky buildings can be substantially stiffer than what is predicted by the finite element models due to the presence of a large number of partitioning walls. However, in a severe earthquake, it will not be safe to rely on this resistance due to the fact that once the partitioning walls crack, the bare frame contributes to the lateral stiffness of the building alone. Consequently, the periods obtained from system identification will be closer to those obtained from the FE analysis. A technique to control the validity of the proportional damping assumption is employed that checks the presence of phase difference in displacements of different stories obtained from band pass filtered records and it is confirmed that the "proportional damping assumption" is valid for this structure. Two different techniques are implemented for identifying the influence of the soil structure interaction. The first technique uses the transfer function between the roof and the basement in both directions. The second technique uses a pre-whitening filter on the data obtained from both the basement and the roof. Subsequently the impulse response function is computed from the scaled cross correlation between the input and the output. The overall results showed that the structure will satisfy the life safety performance level in a future earthquake but some soil structure interaction effects should be expected in the North South direction.

• Nuclear Engineering and Technology
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• 제38권4호
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• pp.375-382
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• 2006

Characterization tests (load vs. displacement curve) are conducted for the springs of Zirconium alloy spacer grids for an advanced LWR fuel assembly. Twofold testing is employed: strap-based and assembly-based tests. The assembly-based test satisfies the in situ boundary conditions of the spring within the grid assembly. The aim of the characterization test via the aforementioned two methods is to establish an appropriate assembly-based test method that fulfills the actual boundary conditions. A characterization test under the spacer grid assembly boundary condition is also conducted to investigate the actual behavior of the spring in the core. The stiffness of the characteristic curve is smaller than that of the strap-wised boundary condition. This phenomenon may cause the strap slit condition. A spacer grid consists of horizontal and vertical straps. The strap slit positions are differentiated from each other. They affords examination of the variation of the external load distribution in the grid spring. Localized legions of high stress and their values are analyzed, as they may be affected by the spring shape. Through a comparison of the results of the test and FE analysis, it is concluded that the present assembly-based analysis model and procedure are reasonably well conducted and can be used for spring characterization in the core. Guidelines for improving the mechanical integrity of the spring are also discussed.

• 한국철도학회논문집
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• 제19권1호
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• pp.1-10
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• 2016

본 논문은 고속열차 객실 내 유리창의 차음 성능을 향상시키기 위한 기반연구로, 접합유리에 사용되는 고분자 필름의 물성과 음향학적 특성을 분석하여 그 상관관계를 살펴보고자 하였다. 기존에 사용되고 있는 PVB(polyvinyl butyal) 필름 및 다른 단량체 비율을 가지는 PVB 유도체, 그리고 유리 사이 차음재로 사용가능 한 PC(polycarbonate)의 동적기계적 특성을 분석(DMA)하였다. DMA 분석을 통해 상온에 가까운 온도범위에서 유리전이온도($T_g$)를 가지며 tan ${\delta}$(loss tangent) 값이 큰 것으로 나타난 PVB-HEMU 시편의 댐핑(damping) 성능이 가장 우수할 것으로 예측하고, 이를 고속철도 차량의 접합유리창과 동일한 구조에서의 투과손실 시험과 해석을 통해 검증하고자 하였다. 접합필름의 종류에 따른 유리창 투과손실 값을 투과손실 측정(인텐시티법)과 FE 해석을 이용해 도출하고, 고분자 필름의 물성과 음향성능 간 상관관계 분석을 위한 기초자료를 구축하였다.

• 한국철도학회논문집
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• 제11권3호
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• pp.257-262
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• 2008

반복하중을 받는 도로 노반체의 경우 회복탄성계수를 이용한 해석 및 설계 방법이 이루어지고 있는데 유사한 형태의 하중조건인 철도 노반체의 경우에도 그 거동은 크게 다르지 않을 것으로 판단된다. 이러한 현상적인 특성을 감안하여 본 논문에서는 회복탄성계수를 기반으로 하는 구성방정식을 상용 유한요소 해석 프로그램에 적용하였다. 일반적으로 K-${\theta}$ 혹은 Uzan 모델을 기반으로 수치해석 프로그램과의 접목이 이루어져 왔다. 이러한 모델의 기본 가정은 반복하중으로 인하여 재료의 상태는 비선형탄성 상태까지 도달한 조건으로 보고 있으므로 추가적인 반복하중으로 인한 재료의 거동은 탄성구간 내에서만 발생하는 것으로 가정하고 있다. 그러나 점성토에 대한 회복탄성실험 결과를 보면 하중의 재하 횟수 증가에 따라 영구변형이 발생하는 상황이므로 기존의 모델을 사용한다면 발생하는 영구변형을 충분히 모델링 할 수 없게 된다. 따라서 본 논문에서는 실험결과로부터 도출된 소성특성과 간단한 파괴기준을 적용하여 영구변형이 발생하는 조건에서 구성모델을 개발하였으며 개발된 모델의 적용결과는 실험 결과값과 잘 일치하는 것으로 확인되었다.

• 한국전산구조공학회논문집
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• 제20권4호
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• pp.463-468
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• 2007

최근 일본에서 발생한 지진으로 이동통신용 전자장비들이 많이 파손되었으나. 국내에서 제작되어 일본에 설치된 모제품은 강진에서도 대부분 구조적 손상이 없었다. 본 논문에서는 이동통신 네트워크 장비의 정/동적 특성을 평가하기 위하여 제품의 유한요소 모델을 생성하여 정적 및 동적 구조해석을 수행하였다. 또한 Zone 3 GR-63-CORE 동적 실험을 수행하여 제품의 안정성을 검증하였고, 유한요소 구조해석 결과와 비교하여 실험과 해석 결과가 유사함이 입증되었다. 구조해석 결과인 동적 응답특성은 실험보다 다소 크게 나왔으며, 부재의 특성파악을 위해 구조물의 치수를 설계변수로 하여 진동특성에 대한 민감도 해석으로 고유진동수에 민감한 부재를 판별하였고, 치수변경으로 경량화 설계치를 산출하였다. 경량화된 디자인의 동적 응답변위가 원래 디자인보다 작게 나와 최적화 결과가 유용할 것으로 보인다.

• Elastomers and Composites
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• 제47권3호
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• pp.194-200
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• 2012

고무재료는 배합조건에 따라 기계적 거동이 다르기 때문에 피로수명평가를 위해 부품소재에 대한 물성데이터 확보가 선행되어야 한다. 그러나 모든 종류의 배합조건을 고려하여 표준화된 재료물성을 확보하는 것은 현실적으로 불가능하다. 따라서 본 연구에서는 최소한의 제한된 실험으로부터 피로수명평가를 위한 입력물성을 선택하기 위한 방법을 제시하였으며, 피로수명 평가를 위한 최적 피로손상 파라미터를 결정하였다. 이러한 결과를 활용하여 부품의 피로수명 평가결과와 내구시험결과를 비교한 후, 본 연구에서 제안된 피로수명 평가절차의 타당성을 검토하였다.