• Structural Engineering and Mechanics
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• 제75권6호
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• pp.771-784
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• 2020

Reliability analysis techniques combining with various surrogate models have attracted increasing attention because of their accuracy and great efficiency. However, they primarily focus on the structures with continuous response, while very rare researches on the reliability analysis for structures with discontinuous response are carried out. Furthermore, existing adaptive reliability analysis methods based on importance sampling (IS) still have some intractable defects when dealing with small failure probability, and there is no related research on reliability analysis for structures involving discontinuous response and small failure probability. Therefore, this paper proposes a novel reliability analysis method called AGPC-IS for such structures, which combines adaptive Gaussian process classification (GPC) and adaptive-kernel-density-estimation-based IS. In AGPC-IS, an efficient adaptive strategy for design of experiments (DoE), taking into consideration the classification uncertainty, the sampling uniformity and the regional classification accuracy improvement, is developed with the purpose of improving the accuracy of Gaussian process classifier. The adaptive kernel density estimation is introduced for constructing the quasi-optimal density function of IS. In addition, a novel and more precise stopping criterion is also developed from the perspective of the stability of failure probability estimation. The efficiency, superiority and practicability of AGPC-IS are verified by three examples.

• JSTS:Journal of Semiconductor Technology and Science
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• 제9권1호
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• pp.8-13
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• 2009

In this paper, discontinuous interconnect lines are modeled as a cascaded line composed of many uniform interconnect lines. The system functions of respective uniform interconnect lines are determined, followed by its time domain response. Since the time domain response expression is a transcendental form, the waveform expression is reconfigured as an approximated linear expression. The proposed model has less than 2% error in the delay estimation.

• 대한기계학회논문집
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• 제12권6호
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• pp.1282-1289
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• 1988

본 연구에서는 형태와 특성이 다른 솔레노이드 밸브를 사용한 위치제어 시스 템에 위에서 언급한 두가지 단속적 제어 방법을 적용하여 이들 밸브의 형태와 특성이 시스템의 응답특성에 미치는 영향을 실험적으로 규명하고 실린더 내의 압력 변화에 미 치는 영향도 조사하였다.

• 한국터널공학회:학술대회논문집
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• 한국터널공학회 2005년도 학술발표회 논문집
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• pp.348-358
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• 2005

Although the evaluation of the mechanical properties and behavior of discontinuous rock masses is very important for the design of underground openings, it has always been considered the most difficult problem. One of the difficulties in describing the rock mass behavior is assigning the appropriate constitutive model. This limitation may be overcome with the progress in discrete element software such as PFC, which does not need the user to prescribe a constitutive model for rock mass. Instead, the micro-scale properties of the intact rock and joints are defined and the macro-scale response results from those properties and the geometry of the problem. In this paper, a $30m{\times}30m{\times}30m$ jointed rock mass of road tunnel site was analyzed. A discrete fracture network was developed from the joint geometry obtained from core logging and surface survey. Using the discontinuities geometry from the DFN model, PFC simulations were carried out, starting with the intact rock and systematically adding the joints and the stress-strain response was recorded for each case. With the stress-strain response curves, the mechanical properties of discontinuous rock masses were determined and compared to the results of empirical methods such as RMR, Q and GSI. The values of Young's modulus, Poisson's ratio and peak strength are almost similar from PFC model and Empirical methods. As expected, the presence of joints had a pronounced effect on mechanical properties of the rock mass. More importantly, the mechanical response of the PFC model was not determined by a user specified constitutive model.

• 열처리공학회지
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• 제33권4호
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• pp.173-179
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• 2020

The purpose of this study was to investigate comparatively the microstructural characteristics and hardness of discontinuous precipitates (DPs) in Mg-9%Al alloy, which were formed by continuous cooling (CC) from 678 K to RT and isothermal aging (IA) at 413 K, respectively. In as-cast state, the Mg-9%Al alloy consisted of partially divorced eutectic β(Mg₁₇Al₁₂) particles with a small amount of DPs showing (α+β) lamellar morphology adjacent to the β particles. The DPs formed by CC had interlamellar spacings in a broad range of 0.85~2.12 ㎛ (1.51 ㎛ in average) owing to the various formation temperatures in response to continuous cooling process. Meanwhile, the DPs formed by IA had relatively narrower interlamellar spacings of 0.14~0.29 ㎛ (0.21 ㎛ in average), which is associated with the low and constant formation temperature. Thinner and higher volume fraction of β phase layers were noticeable in the DPs formed by IA. Higher hardness values were obtained in the DPs formed by IA than the DPs formed by CC, which may well be ascribed to the finer lamellar structure and higher β phase content of the DPs formed by IA.

• 열처리공학회지
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• 제33권5호
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• pp.219-225
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• 2020

The objective of this study was to investigate the tensile properties and thermal conductivities of Mg9.3%Al alloy in as-cast state and heat-treated state consisting of fully discontinuous precipitates (DPs), respectively. The fully DPs microstructure was obtained by solution treatment at 405℃ for 24 h, followed by furnace cooling to RT. The as-cast alloy showed a partially divorced eutectic β(Mg₁₇Al₁₂) phase particles formed along the α-(Mg) cell boundaries. The DPs had various apparent (α+β) interlamellar spacings, which is related to different transformation temperatures during the furnace cooling. The DPs microstructure exhibited better tensile strength than the as-cast one, resulting from the higher value of elongation in response to its more homogeneous microstructure. It is noticeable that the DPs microstructure had 12.4% higher thermal conductivity in average than the as-cast one between RT and 200℃. The XRD analyses revealed that the lower Al concentration in the α-(Mg) matrix may well be responsible for the better thermal conductivity of the DPs microstructure.

• 한국통신학회논문지
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• 제17권5호
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• pp.460-471
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• 1992

본 논문에서는 최근에 주로 사용되는 디지털 PLL중 Tri-State 방식과 sample-Hold 방식을 사용한 PLL루프의 시간 불연속 동작을 묘사하기 위한 새로운 모델을 설정하여 비선형 PLL의 안정도 해석을 Z영역에서 하였으며 과도응답을 구하기 위한 상태방정식을 유도하였다. 종래에는 디지털 PLL의 시간 불연속 동작을 시간 연속 동작으로 근사화 시켜 선형적 해석을 하므로써 실제로는 시간 불연속 동작을 하는 디지털 PLL의 불안정한 영역을 정확히 찾아내지 못하였으나 새로운 모델에 의한 Z영역에서의 해석에서는 시간연속 해석에서 발견할 수 없었던 불안정 영역을 밝혀냄으로써 디지털 PLL의 최적 설계가 가능하도록 루프계수의 한계를 구하였다.

• IUGG한국위원회:학술대회논문집
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• IUGG한국위원회 2003년도 정기총회 및 학술발표회
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• pp.18-18
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• 2003

Three-dimensional finite-difference simulation of earthquake ground motion is performed using a locally variable time-step (LVTS) scheme matching with discontinuous grids. Discontinuous grids in three directions and extension of the discontinuous grids' boundary to the free-surface in the LVTS scheme minimize the cost of both the computational memory and the CPU time for models like the localized sedimentary basin. A simplified model of sedimentary basin is dealt to show the feasibility and efficiency of the LVTS scheme. The basin parameters are examined to understand the main characteristics on ground-motion response in the basin. The results show that the seismic energy is concentrated on a marginal area of the basin far from the source. This focusing effect is mainly due to the constructive interference of the direct S-wave with the basin-edge induced surface waves. The ground-motion amplification over the deepest part of the basin is relatively lower than that above the shallow basin edge. Therefore the ground-motion amplification may be more related to the source azimuth or the direction of the incident waves into the basin rather than the depth of it.

• International Journal of Naval Architecture and Ocean Engineering
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• 제11권1호
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• pp.572-583
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• 2019

This paper aims to assess the applicability of the Runge Kutta Discontinuous Galerkin-Direct Ghost Fluid Method to the internal explosion inside a water-filled tube, which previously was studied by many researchers in separate works. Once the explosive charge located at the inner center of the water-filled tube explodes, the tube wall is subjected to an extremely high intensity fluid loading and deformed. The deformation causes a modification of the field of fluid flow in the region near the water-structure interface so that has substantial influence on the response of the structure. To connect the structure and the fluid, valid data exchanges along the interface are essential. Classical fluid structure interaction simulations usually employ a matched meshing scheme which discretizes the fluid and structure domains using a single mesh density. The computational cost of fluid structure interaction simulations is usually governed by the structure because the size of time step may be determined by the density of structure mesh. The finer mesh density, the better solution, but more expensive computational cost. To reduce such computational cost, a non-matched meshing scheme which allows for different mesh densities is employed. The coupled numerical approach of this paper has fewer difficulties in the implementation and computation, compared to gas dynamics based approach which requires complicated analytical manipulations. It can also be applied to wider compressible, inviscid fluid flow analyses often found in underwater explosion events.

• 열처리공학회지
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• 제35권4호
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• pp.177-184
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• 2022

The present study aims to investigate the influence of Zn addition on hardness and microstructural characteristics of discontinuous precipitates (DPs) formed by isothermal aging in Mg-9%Al and Mg-9%Al-1%Zn alloys. To obtain large DPs volume fractions in the microstructure, the alloy specimens were solution-treated at 688 K for 24 h followed by water quenching, and then aged at 413 K for 48 h. The aged Mg-9%Al-1%Zn alloy had higher DPs content than the Mg-9%Al alloy, indicating that the Zn addition plays a beneficial role in enhancing age-hardening response. The DPs in the Zn-containing alloy possessed the higher hardness than those of the Zn-free alloy. Microstructural examination revealed that the increased hardness of the DPs resulting from the Zn addition is closely associated with the lower α-(Mg)/β(Mg₁₇Al₁₂) interlamellar spacing and the higher volume fraction of β phase layer of the DPs.