• 한국전산구조공학회논문집
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• 제34권3호
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• pp.151-157
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• 2021

본 연구에서는 비국부 적분 연산기로 표현되는 페리다이나믹 방정식의 수렴성을 검토한다. 정적/준정적 손상 해석 문제를 효율적으로 해석하기 위해 페리다이나믹 방정식의 implicit 정식화가 필요하다. 이 과정에서 페리다이나믹 비국부 적분 방정식으로부터 대수방정식 형태가 나타나게 되어 시스템 행렬 계산을 위해 많은 시간이 소요되기 때문에, 효율적인 계산을 위해 수렴성이 중요한 요소가 된다. 특히 radial influence 함수를 적분 kernel로 사용하는 경우 fractional Laplacian 적분 방정식이 유도된다. 비국부 적분 연산기의 교윳값 성질에 의해 대수방정식의 condition number가 radial influence 함수의 차수 및 비국부 영역의 크기에 영향을 받는 것이 수학적으로 확인되었다. 본 연구에서는 이를 토대로 균열이 있는 페리다이나믹 정적 해석 문제를 Newton-Raphson 방법으로 해석할 때 적분 커널의 차수, 비국부 영역의 크기 등이 대수방정식의 condition number와 preconditioned conjugate gradient (PCG) 방법으로 계산 시 수렴성 및 계산 시간에 미치는 영향을 수치적으로 분석한다.

• Structural Engineering and Mechanics
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• 제78권5호
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• pp.599-610
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• 2021

Early detection of small concrete crack or reinforcement corrosion is necessary for Structural Health Monitoring (SHM). Global vibration based methods are advantageous over local methods because of simple equipment installation and cost efficiency. Among vibration based techniques, FRF based methods are preferred over modal based methods. In this study, a new coupled method using frequency response function (FRF) and proper orthogonal modes (POM) is proposed by using the dynamic characteristic of a damaged beam. For the numerical simulation, wave finite element (WFE), coupled with traditional finite element (FE) method is used for effectively incorporating the damage related information and faster computation. As reported in literature, hybrid combination of wave function based wave finite element method and shape function based finite element method can addresses the mid frequency modelling difficulty as it utilises the advantages of both the methods. It also reduces the dynamic matrix dimension. The algorithms are implemented on a three-dimensional reinforced concrete beam. Damage is modelled and studied for two scenarios, i.e., crack in concrete and rebar corrosion. Single and multiple damage locations with different damage length are also considered. The proposed methodology is found to be very sensitive to both single- and multiple- damage while being computationally efficient at the same time. It is observed that the detection of damage due to corrosion is more challenging than that of concrete crack. The similarity index obtained from the damage parameters shows that it can be a very effective indicator for appropriately indicating initiation of damage in concrete structure in the form of spread corrosion or invisible crack.

• 한국지열·수열에너지학회논문집
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• 제18권3호
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• pp.7-18
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• 2022

Secondary batteries used in electric vehicles have a potential risk of ignition and explosion. Various safety measures are being taken to prevent these risks. A numerical study was performed using a computational fluid dynamics code on the cases where pressure relief vents that can reduce the blast overpressures of batteries were installed in the through-compression test room, short-circuit drop test room, combustion test room, and immersion test room in facilities rleated to battery used in electric vehicles. This study was conducted using the weight of TNT equivalent to the energy release from the battery, where the the thermal runaway energy was set to 324,000 kJ for the capacity of the lithium-ion battery was 90 kWh and the state of charge (SOC) of the battery of 100%. The explosion energy of TNT (△H_TNT) generally has a range of 4,437 to 4,765 kJ/kg, and a value of 4,500 kJ/kg was thus used in this study. The dimensionless explosion efficiency coefficient was defined as 15% assuming the most unfavorable condition, and the TNT equivalent mass was calculated to be 11 kg. The internal explosion generated in a test room shows the very complex propagation behavior of blast waves. The shock wave generated after the explosion creates reflected shock waves on all inner surfaces. If the internally reflected shock waves are not effectively released to the outside, the overpressures inside are increased or maintained due to the continuous reflection and superposition from the inside for a long time. Blast simulations for internal explosion targeting four test rooms with pressure relief vents installed were herein conducted. It was found that that the maximum blast overpressure of 34.69 bar occurred on the rear wall of the immersion test room, and the smallest blast overpressure was calculated to be 3.58 bar on the side wall of the short-circuit drop test room.

• Advances in concrete construction
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• 제13권1호
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• pp.83-99
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• 2022

The two, NBD and SCB tests using gypsum circular discs each containing a single notch have been experimentally accomplished in a rock mechanics laboratory. These specimens have also been numerically modelled by a two-dimensional particle flow which is based on Discrete Element Method (DEM). Each testing specimen had a thickness of 5 cm with 10 cm in diameter. The specimens' lengths varied as 2, 3, and 4 cm; and the specimens' notch angles varied as 0°, 45° and 90°. Similar semi-circular gypsum specimens were also prepared each contained one edge notch with angles 0° or 45°. The uniaxial testing machine was used to perform the experimental tests for both NBD and SCB gypsum specimens. At the same time, the numerical simulation of these tests were performed by PFC2D. The experimental results showed that the failure mechanism of rocks is mainly affected by the orientations of joints with respect to the loading directions. The failure mechanism and fracturing patterns of the gypsum specimens are directly related to the final failure loading. It has been shown that the number of induced tensile cracks showing the specimens' tensile behavior, and increases by decreasing the length and angle of joints. It should be noted that the fracture toughness of rocks' specimens obtained by NBD tests was higher than that of the SCB tests. The fracture toughness of rocks usually increases with the increasing of joints' angles but increasing the joints' lengths do not change the fracture toughness. The numerical solutions and the experimental results for both NDB and SCB tests give nearly similar fracture patterns during the loading process.

• 한국광학회지
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• 제32권4호
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• pp.180-186
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• 2021

불화폴리이미드는 우수한 광투과율과 광학적 굴절률이 높은 특징을 가지며 이를 이용하여 제작된 폴리머 광도파로 위상변조기는 광손실이 작고 저전력으로 안정적인 위상제어가 가능하게 된다. 본 논문에서는 고굴절률 불화폴리이미드를 이용하여 광도파로 위상변조기를 설계하고 제작하여 특성을 분석하였다. 광도파로를 제작하여 손실을 측정하기에 효율적인 새로운 방법을 제시하였고, 제작된 폴리이미드 광도파로의 전파손실은 0.9 dB/cm로 확인되었다. 위상변조기는 π 위상변화를 위해 9.1 mW의 전력을 필요로 하였으며, 기존 폴리머 위상변조기 대비 응답속도가 10배 정도 향상된 290 μs 정도의 응답시간을 가지는 것을 확인하였다.

• KEPCO Journal on Electric Power and Energy
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• 제8권1호
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• pp.5-7
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• 2022

KEPCO's plan is undergoing a trial operation to replace the open-loop section with ring main units configuration where underground distribution lines are installed, by linking the multi-way circuit breakers auto (MCA) on the power side of each pad-mounted transformer. However, ring main units application mentioned above may cause the ripple effects, when implementing the configuration without a study of protection coordination. Because ring main units with classical pre-set protection devices contribution in fault condition didn't consider yet. For the reliable ring main units operation, it is necessary to resolve several protection issues such as the protection coordination with substation side, prevention of the transformer inrush current. These issues can radically deteriorate the distribution system reliability Hence, it is essential to design proper protection coordination to reduce these types of problems. This paper presents a scheme of ring main units' configuration and MCA's settings of time-current curves to preserve the performance of protection coordination among the switchgears considering constraints, e.g. prevention of the ripple effects (on the branch section when a transformer failure occurs and the mainline when a branch line failure occurs). It was confirmed that the propagation of the failure for each interrupter segment could be minimized by applying the proposed TCC and the interrupter settings for the MCAs (branch, transformer). Further, it was verified that the undetected area of the distribution automation system (DAS) could be supplemented by having the MCA configurated ring main units operate first, instead of the internal protection equipment in the transformer such as the fuse, STP when a transformer failure occurs.

• 한국압력기기공학회 논문집
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• 제17권2호
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• pp.145-156
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• 2021

Ultrasonic Nanocrystal Surface Modification (UNSM) is a peening technology that generates elastic-plastic deformation on the material surface to which a static load of a air compressor and a dynamic load of ultrasonic vibration energy are applied by striking the material surface with a strike pin. In the UNSM-treated material, the structure of the surface layer is modified into a nano-crystal structure and compressive residual stress occurs. When UNSM is applied to welds in a reactor coolant system where PWSCC can occur, it has the effect of relieving tensile residual stress in the weld and thus suppressing crack initiation and propagation. In order to quantitatively evaluate the compressive residual stress generated by UNSM, many finite element studies have been conducted. In existing studies, single-path UNSM or UNSM in a limited area has been simulated due to excessive computing time and analysis convergence problems. However, it is difficult to accurately calculate the compressive residual stress generated by the actual UNSM under these limited conditions. Therefore, in this study, a minimum finite element peening analysis area that can reliably calculate the compressive residual stress is proposed. To confirm the validity of the proposed analysis area, the compressive residual stress obtained from the experiment are compared with finite element analysis results.

• 한국전자통신학회논문지
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• 제18권2호
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• pp.291-298
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• 2023

SRAM 기반 인 메모리 컴퓨팅은 폰 노이만 구조의 병목 현상을 해결하는 기술 중 하나이다. SRAM 기반의 인 메모리 컴퓨팅을 구현하기 위해서는 효율적인 SRAM 비트 셀 설계가 필수적이다. 본 논문에서는 전력 소모를 감소시키고 회로 성능을 개선시키는 저 전력 차동 감지 8+T SRAM 비트 셀을 제안한다. 제안하는 8+T SRAM 비트 셀은 SRAM 읽기와 비트 연산을 동시에 수행하고 각 논리 연산을 병렬로 수행하는 리플 캐리 가산기에 적용한다. 제안하는 8+T SRAM 기반 리플 캐리 가산기는 기존 구조와 비교 하여 전력 소모는 11.53% 감소하였지만, 전파 지연 시간은 6.36% 증가하였다. 또한 이 가산기는 PDP(: Power Delay Product)가 5.90% 감소, EDP(: Energy Delay Product)가 0.08% 증가하였다. 제안한 회로는 TSMC 65nm CMOS 공정을 이용하여 설계하였으며, SPECTRE 시뮬레이션을 통해 타당성을 검증하였다.

• 한국지반공학회논문집
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• 제23권2호
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• pp.5-17
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• 2007

기존의 표면파 기법들은 근거리장 효과를 피하기 위하여 적절한 실험 방법 및 필터 기준을 제시하고 있다. HWAW 기법은 다른 표면파와는 다르게 이러한 근거리장 영역에서의 파의 거동을 적극적으로 활용하여 지반의 전단파 속도 주상도를 도출한다. 에너지가 작은 저주파 신호를 시간-주파수 해석을 통해 획득하며 체적파의 영향을 고려한 단일 배열 역산을 통해 가능하다. 이러한 사항을 검증하기 위해서 근거리장에서의 탄성파의 전파 형상을 수치해석을 통하여 모사하였으며 다양한 지반 형태를 대표하기 위해 5개의 지반모델을 구성하였다. 수치모델링을 통해 획득한 시간영역 신호를 이용하여 HWAW 기법으로 실험분산곡선을 도출하였다. 도출된 실험 분산곡선과 3차원 정모델링(Kausel의 방법)을 통해 계산된 이론 분산곡선의 형상이 각 위치에서 저의 일치하였다. 그러므로 HWAW 기법을 수행하는데 있어 근거리장 영역이 포함된 실험 분산곡선을 획득하여도 Kausel의 방법을 이용하여 역산을 수행할 경우 신뢰성 있는 지반의 전단파 속도를 도출하는 것이 가능하다는 것을 확인할 수 있었다.

• 한국지반공학회논문집
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• 제23권1호
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• pp.51-65
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• 2007

본 논문의 목적은 초음파 탐사 방법을 이용하여 지반내의 불연속 물질을 탐측할 수 있는 고해상도의 시스템을 개발하고 실내에서 적용하는 것이다. 초음파 탐측 방법은 불연속면의 경계면에서 반사되어온 반사파를 탐측하고, 이를 배열하여 불연속면의 존재를 찾는 방법이다. 본 논문은 암반내 초음파의 전달 양상, 최적화된 트랜스듀서의 선택, 데이터 획득, 신호처리 방법, 영상화 기법, 그리고 실내 적용실험을 포함하고 있다. 실내실험은 수평이동장치와 회전이동장치를 이용하여 수행된다. 수중에서 수평이동 및 회전실험 결과, 불연속면의 위치와 크기가 정확하게 평가 되었다. 또한 석고시료에서 회전실험결과 석고 내에 존재하는 균열과 공동이 비교적 정확하게 영상화됨을 알 수 있다. 본 논문은 새롭게 제시된 방법이 암반 불연속면 탐측에 매우 경제적이고 효과적인 방법이 될 수 있음을 보여준다.