• Steel and Composite Structures
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• 제14권4호
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• pp.367-377
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• 2013

Damage in structures often leads to failure. Thus it is very important to monitor structures for the occurrence of damage. When damage happens in a structure the consequence is a change in its modal parameters such as natural frequencies and mode shapes. Artificial Neural Networks (ANNs) are inspired by human biological neurons and have been applied for damage identification with varied success. Natural frequencies of a structure have a strong effect on damage and are applied as effective input parameters used to train the ANN in this study. The applicability of ANNs as a powerful tool for predicting the severity of damage in a model steel girder bridge is examined in this study. The data required for the ANNs which are in the form of natural frequencies were obtained from numerical modal analysis. By incorporating the training data, ANNs are capable of producing outputs in terms of damage severity using the first five natural frequencies. It has been demonstrated that an ANN trained only with natural frequency data can determine the severity of damage with a 6.8% error. The results shows that ANNs trained with numerically obtained samples have a strong potential for structural damage identification.

• Journal of Power Electronics
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• 제19권1호
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• pp.307-315
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• 2019

Condition monitoring has been recognized as an effective and low-cost method to enhance the reliability and improve the maintainability of power electronic converters. In power electronic converters, high-frequency oscillation occurs during the switching transients of power transistors, which is known as ringing. The ringing frequency mainly depends on the values of the parasitic capacitance and stray inductance in the oscillation loop. Although circuit stray inductance is an important factor that leads to the ringing, it does not change with transistor aging. A shift in either the inside inductance or junction capacitance is an important failure precursor for power transistors. Therefore, ringing frequency can be used to monitor the health of power transistors. However, the switching actions of power transistors usually result in a dynamic behavior that can generate oscillation signals mixed with background noise, which makes it hard to directly extract the ringing frequency. A frequency extraction method based on empirical mode decomposition (EMD) and Fast Fourier transformation (FFT) is proposed in this paper. The proposed method is simple and has a high precision. Simulation results are given to verify the ringing analysis and experimental results are given to verify the effectiveness of the proposed method.

• 전력전자학회논문지
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• 제25권6호
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• pp.459-468
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• 2020

The pole interaction of the line-commutated converter high-voltage direct current (HVDC) is analyzed, and a practical solution that uses a surge arrester is proposed. Jeju HVDC No. 2 is a double-monopole HVDC link that has a rated power capacity of 2 × 200 MW and was commissioned in 2012. During normal operation, Jeju HVDC No. 2 is operated in the bipolar mode to minimize the loss caused by the dedicated metallic return. However, when one pole of the inverter valve is bypassed, a commutation failure can occur in the other pole. This phenomenon is called pole interaction in this work. This pole interaction interrupts the HVDC power transfer for almost 2 s and may affect the stability of the power system. This research proposes the installation of a surge arrester at the inverter neutral, which can be an effective and practical solution for pole interaction. The HVDC system is analyzed, and the residual voltage of the surge arrester is determined. Detailed simulation using PSCAD/EMTDC demonstrates that the proposed method eliminates the pole interaction of the bipolar-operated HVDC.

• 한국산학기술학회논문지
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• 제17권10호
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• pp.68-78
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• 2016

최근 철도 열차제어시스템은 최첨단 기술로 인한 무선통신기반 개발의 수요와 이에 따른 환경의 다변화로 인해 안전에 대한 쟁점도 증가하고 있다. 이에 따라 기존의 열차제어시스템에서 다루는 지상 설비 설계에 대한 단계별 안전 활동 강화의 필요성 역시 강조되고 있다. 국내 철도 산업은 대다수 열약한 산업환경으로 구성되어 있다. 이러다 보니, 설계 산출물에 대한 생성에 초점이 맞춰져 있지, 산출물에 대한 검증 방안 및 적용에 대한 역량이 상당히 부족한 실정이다. 따라서, 본 논문에서는 열차제어시스템 설계 단계의 아키텍처 산출물 검증 방안을 확보하기 위해 안전분석 산출물을 바탕으로 확보 가능한 방법에 대해 기술하고 있다. 이렇듯 설계적 산출물에 대한 검증방안을 구축하고 직접 수행에 따른 설계적 무결성을 높이고자 한다. 특히, FMEA 안전 분석 기법을 활용해, 안전 분석 기법을 통해 개별 단계별 산출물의 활용을 통한 아키텍처 산출물과의 연계성 확보를 통해 무결성 확보를 위한 기술적 접근법을 기술하였다. 본 연구의 결과를 토대로 안전성 향상 접근에 대한 시험 활동 재정립을 통해 개선함으로써 향후 모델 기반 열차제어시스템 개발 시 개념설계에서 발생할 수 있는 안전성 이슈를 제거 함에 따라, 설계적 비용 및 시간을 절감 및 안전성 향상을 기대할 수 있을 것으로 기대된다.

• Structural Engineering and Mechanics
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• 제75권3호
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• pp.401-414
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• 2020

Unlike the existing truss models for shear and torsion analysis, in this study, the torsional capacities of reinforced concrete (RC) members were estimated by introducing multi-potential capacity criteria that considered the aggregate interlock, concrete crushing, and spalling of concrete cover. The smeared truss model based on the fixed-angle theory was utilized to obtain the torsional behavior of reinforced concrete member, and the multi-potential capacity criteria were then applied to draw the capacity of the member. In addition, to avoid any iterative calculation in the existing torsional behavior model, a simple strength model was suggested that considers key variables, such as the effective thickness of torsional member, principal stress angle, and strain effect that reduces the resistance of concrete due to large longitudinal tensile strain. The proposed multi-potential capacity concept and the simple strength model were verified by comparing with test results collected from the literature. The study found that the multi-potential capacity could estimate in a rational manner not only the torsional strength but also the failure mode of RC members subjected to torsional moment, by reflecting the reinforcing index in both transverse and longitudinal directions, as well as the sectional and material properties of RC members.

• 한국구조물진단유지관리공학회 논문집
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• 제15권5호
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• pp.178-189
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• 2011

본 연구에서는 CFRP 표면부착 공법의 대안으로 최근에 관심을 끌고 있는 NSM(Near Surface Mounted)기법으로 전단 보강된 RC 부재의 전단강도를 평가하기 위한 실험과 해석을 수행하였다. 전단철근이 없는 7개의 실험체에 대해 4점 휨실험을 실시하였다. 실험변수로는 CFRP 스트립의 경사($45^{\circ}$, $90^{\circ}$)와 스트립의 간격(250mm, 200mm, 150mm, 100mm)이 고려되었다. 실험적 연구를 통해 NSM공법으로 전단 보강된 RC 부재의 전단강도와 파괴모드에 대한 각 실험변수의 영향을 평가하였다. 실험결과는 $45^{\circ}$ 경사로 스트립을 보강한 실험체들은 스트립의 파단으로 파괴된 반면, 수직으로 스트립을 보강한 실험체들은 스트립의 슬립으로 파괴됨을 보였다. 또한, $45^{\circ}$ 경사 스트립이 수직 스트립보다 전단저항력 증가시킬뿐만 아니라 파괴시의 처짐을 크게 증가시키는 것으로 나타났다. 추가적으로 RBSN 해석은 NSM기법으로 전단 보강된 RC 부재의 균열형상 및 하중-처짐관계를 적절하게 예측하였다.

• 치위생과학회지
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• 제9권4호
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• pp.405-412
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• 2009

본 연구는 도재소부용 비귀금속에 Au 코팅 층을 형성하고, Au 코팅 층이 금속-도재의 결합력에 미치는 영향을 알아보고자 2출 굴곡 시험 후에 SEM/EDS 방법으로 조사하여 비교하였다. 1. Ni-Cr ally 군과 Co-Cr alloy 군, 그리고 Ti 군의 절단면 시편의 전자현미경 사진에서 Au 코팅 층은 산화층의 확산을 제어하는 것으로 관찰되었다. 2. 2축 굴곡 시험 후에 EDS 분석 결과 Ni-Cr alloy 군과 Co-Cr alloy 군에서 Si 함량은 Au 코팅 층을 형성한 시편에서 약간 감소하였고, 결합실패의 형태는 cohesive failure와 adhesive failure가 혼재된 양상으로 관찰되었다. 3. Ni-Cr alloy 군과 Co-Cr alloy 군에서 Au 코팅 층은 도재-금속 결합력에 미치는 영향은 미미하였다. 4. Ti 군의 결합실패 형태는 Ni-Cr alloy 군과 Co-Cr alloy 군에 비해서 adhesive failure 양상이 두드러지게 관찰되었고, Au 코팅 층은 도재-금속 결합력에 영향을 미치지 않았다. 이상의 결과 비귀금속에 적용한 Au 코팅은 산화층의 확산을 제어 하는 것으로 관찰 되었으나, 금속-도재 간의 결합력 증징에 미치는 효과는 미미하였다. Au 코팅을 이용하여 금속-도재 간의 결합력 증진을 위해서는 많은 연구가 지속되어야 할 것으로 사료된다.

• The Journal of Advanced Prosthodontics
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• 제10권5호
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• pp.374-380
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• 2018

PURPOSE. The effect of silica-based glass-ceramic liners on the tensile bond strength between zirconia and resin-based luting agent was evaluated and compared with the effect of 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing primers. MATERIALS AND METHODS. Titanium abutments and zirconia crowns (n = 60) were fabricated, and the adhesive surfaces of the specimens were treated by airborne-particle abrasion. The specimens were divided into 5 groups based on surface treatment: a control group, 2 primer groups (MP: Monobond Plus; ZP: Z Prime Plus), and 2 liner groups (PL: P-containing Liner; PFL: P-free Liner). All specimens were cemented with self-adhesive resin-based luting agent. After 24-hour water storage and thermocycling (5,000 cycles, $5^{\circ}C/55^{\circ}C$), the tensile bond strength was measured using a universal testing machine. Failure mode analysis and elemental analysis on the bonding interface were performed. The data were analyzed using Kruskal-Wallis test, Dunn's post hoc test, and Fisher's exact test. RESULTS. The liner groups and primer groups showed significantly higher tensile bond strengths than that of the control group (P<.05). PFL showed a significantly higher tensile bond strength than the primer groups (P<.05). The percentage of mixed failure was higher in the primer groups than in the control group (P<.001), and all the specimens showed mixed failure in the liner groups (P<.001). A chemical reaction area was observed at the bonding interface between zirconia and liner. CONCLUSION. The application of liner significantly increased the tensile bond strength between zirconia and resin-based luting agent. PFL was more effective than MDP-containing primers in improving the tensile bond strength with the resin-based luting agent.

• Steel and Composite Structures
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• 제49권2호
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• pp.143-159
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• 2023

Recently, the design of scaffolding systems has garnered considerable attention due to the increasing number of scaffold collapses. These incidents arise from the underestimation of imposed loads and the site-specific conditions that restrict the application of lateral restraints in scaffold assemblies. The present study is committed to augmenting the buckling resistance of vertical support members, obviating the need for supplementary lateral restraints. To achieve this objective, experimental and computational analyses were performed to assess the axial load buckling capacity of steel props, composed of two hollow steel pipes that slide into each other for a certain length. Three full-scale steel props with various geometric properties were tested to construct and validate the analytical models. The total unsupported length of the steel props is 6 m, while three pins were installed to tighten the outer and inner pipes in the distance they overlapped. Finite Element (FE) modeling is carried out for the three steel props, and the developed models were verified using the experimental results. Also, theoretical analysis is utilized to verify the FE analysis. Using the FE-verified models, a parametric study is conducted to evaluate the effect of different inserted pipe lengths on the steel props' axial load capacity and lateral displacement. Based on the results, the typical failure mode for the studied steel props is global elastic buckling. Also, the prop's elastic buckling strength is sensitive to the inserted length of the smaller pipe. A threshold of minimum inserted length is one-third of the total length, after which the buckling strength increases. The present study offers a prop with enhanced buckling resistance and introduces an equation for calculating an equivalent effective length factor (k), which can be seamlessly incorporated into Euler's buckling equation, thereby facilitating the determination of the buckling capacity of the enhanced props and providing a pragmatic engineering solution.

• 한국구조물진단유지관리공학회 논문집
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• 제25권5호
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• pp.173-181
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• 2021

철근콘크리트 구조물은 다양한 환경에 노출되어 수분 침투로 인한 철근 부식이 발생하며, 부식으로 인한 구조물의 내구성능 저하 문제가 발생할 수 있다. 따라서 이러한 문제점을 해결하기 위해 철근에 비해 인장강도, 비부식성, 경량화 등 뛰어난 장점을 가진 FRP 보강근에 대한 연구가 활발히 진행되고 있다. FRP 보강근은 철근과 달리 항복 구간이 없으므로 파괴 시까지 선형탄성거동이 나타나고 탄성계수가 낮아 과도한 처짐이 발생할 수 있으므로 한계상태 조건에 대한 적용성 검토가 필요하다. 한계상태에서 FRP 보강 콘크리트 의 휨 설계 시 ACI 440.1R은 FRP 보강근의 재료적 불확실성을 고려하여 환경감소계수와 강도감소계수를 모두 적용하여 휨강도가 크게 낮아진다. 따라서 본 연구에서는 국내·외 다양한 문헌을 조사하여 유효단면이차모멘트 제안식의 처짐 해석 결과와 실험결과를 비교하였으며, ACI 440.1R 및 Fib bulletin 40의 설계휨강도를 분석하였다. 실험 결과에 따른 휨강도는 ACI 440.1R에 비해 Fib bulletin 40의 설계휨강도와 유사한 경향이 확인되었으며, 인장지배단면에서 ACI 440.1R은 설계휨강도를 보수적으로 평가하는 것으로 나타났다.