• Computers and Concrete
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• 제19권2호
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• pp.143-152
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• 2017

This paper has numerically investigated the changes of loading-induced stress in concrete with the corrosion time in the sulfate-containing environment. Firstly, based on Fick's law and reaction kinetics, a diffusion-reaction equation of sulfate ion in concrete is proposed, and it is numerically solved to obtain the spatial and temporal distribution of sulfate ion concentration in concrete by the finite difference method. Secondly, by fitting the existed experimental data of concrete in sodium sulfate solutions, the chemical damage of concrete associated with sulfate ion concentration and corrosion time is quantitatively presented. Thirdly, depending on the plastic-damage mechanics, while considering the influence of sulfate attack on concrete properties, a simplified chemo-mechanical damage model, with stress-based plasticity and strain-driven damage, for concrete under axial loading and sulfate attack is determined by introducing the chemical damage degree. Finally, an axially compressed concrete prism immersed into the sodium sulfate solution is regarded as an object to investigate the time-varying stress in concrete subjected to the couplings of axial loading and sulfate attack.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.1331-1336
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• 2008

This paper suggests a method using Bayesian inference to estimate the parameters of Weibull distribution and acceleration parameters under the condition that the stresses are time-dependent functions. A Bayesian model based on the discrete time approximation is formulated to infer the parameters of interest from the failure data of the virtual tests and a statistical analysis is considered to decide the most probable mean values of the parameters for reasoning of the failure data.

• 한국해양학회지
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• 제17권2호
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• pp.41-50
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• 1982

수평적으로 무한히 넓은 천해에 시간에 따라 크기와 방향이 바뀌는 바람이 불 때의 해수의 유량transports)에 대한 이론적인 계산을 할 수 있는 모델을 만들 었다. 이 모델에서 사용하는 선형화된 이론식은 가속력, 편향력(Coriolis force), 바람의 응력(wind stress) 및 저면 마찰력을 포함하고 있으며, 이 모델로부터 해수 유량에 대한 다음과 같은 결과를 얻었다. 일정한 바람이 계속 불 때의 해수유량은 해저면 마찰로 인하여 바람 오른쪽 방향 90 보다 작다. 바람이 불기 시작한 후의 천이상태(transient state)의 유량은 정상류적(steady)인 에크만류( Ekman current) 에다가 시간이 지남에 따라서 크기가 감소하는 관성진동(inertial oscillation)과의 합성으로 나타난다. 바람의 크기가 정현적(sinrsoidal)으로 바뀌는 경우는 유량 벡터 는 바람 변화의 주기와 같은 시간동안 일주회전을 하는 유량타원(transport ellipse) 을 그린다. 바람의 방향이 바뀔 경우 해수유량은 자유관성진동(free inertial oscillation)과 강제진동의 합으로 나타나는데, 이 경우 강제진동의 주기는 바람 방향의 회전주기(rotation period)와 같다. 이 이론적인 해수유량모델은 해에서 시간에 따라 바람이 바뀔 경우 해수와 영양염의 이동에 대한 해석을 하는데 유용 하다.

• 대한토목학회논문집
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• 제34권3호
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• pp.755-761
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• 2014

콘크리트의 크리프와 건조수축 변형에 대한 내적 혹은 외적 구속은 크리프 발생 응력조건을 변화시키며 이에 따라 크리프 변형의 발생은 응력변화에 종속적으로 변화한다. 시간이력 하중을 받는 크리프 거동문제로서 이해되는 이러한 크리프 거동현상의 수학적 모델링은 일반적으로 재하시의 콘크리트 재령과 물성값 및 하중이력을 기본 구성인자로 고려하여 시간적분 혹은 점증적 형태로 유도되었다. 본 논문에서는 시간이력하중을 받는 크리프 모델 가운데 단일 크리프 곡선을 사용하는 초기 크리프 모델인 평행 크리프 법의 단순성을 고려하여 이 방법이 갖는 단점과 한계성을 극복하고 성능을 개선한 평행 크리프 법을 유도하였다. 유도된 크리프 모델의 성질을 분석하고 예측 성능을 검증하기 위한 목적으로 원통형 콘크리트 공시체를 제작하고 시간이력 하중 하의 크리프 실험을 수행하였다. 끝으로, 콘크리트 공시체의 크리프 변형으로 인한 초기하중의 변화가 공시체의 재령에 따른 거동에 미치는 영향정도를 실험을 통해 분석하였으며, 크리프 시험기의 스프링계수를 측정하여 이로 인한 실험오차를 보정하였다.

• 한국해안해양공학회지
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• 제14권4호
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• pp.286-294
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• 2002

본 연구에서는 응력이력과 시간이력이 과압밀 상태에서의 간극수압계수, A(Skempton, 1954)에 미치는 영향에 관하여 실험적으로 규명하였다. 응력경로 회전각, 과압밀비 및 접근길이로 정의되는 응력이력과 재하속도 이력 그리고 정지기간으로 정의되는 시간이력에 따른 간극수압계수(A)의 값과 변화 양상을 보았다. 응력경로 회전각에 따른 간극수압계수(A)는 그 값과 변화 경향에서 상이하게 나타났고, A의 값에 있어서는 과압밀비 의존성을 보였지만, 변화 양상에서는 의존성을 찾아 볼 수 없었다. 접근길이에 따른 간극수압계수(A)는 초기 미소 변형률 구간을 제외하고는 큰 영향을 받지 않았다. 또한, A의 값은 재하속도 이력에 영향을 받았지만, 변화 경향은 모두 동일하게 나타났다. 마지막으로 정지기간에 따른 과압밀점토의 간극수압계수, A는 정지기간의 유무에 따라 변화 경향과 값이 큰 차이를 보이다가 일정 축차응력 또는 변형률을 넘어서면 거의 유사한 값을 나타내었다.

• Smart Structures and Systems
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• 제20권5호
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• pp.525-537
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• 2017

This paper explores different vibration control strategies for the cancellation of human-induced vibration on a structure with time-varying modal parameters. The main motivation of this study is a lively urban stress-ribbon footbridge (Pedro $G\acute{o}mez$ Bosque, Valladolid, Spain) that, after a whole-year monitoring, several natural frequencies within the band of interest (normal paring frequency range) have been tracked. The most perceptible vibration mode of the structure at approximately 1.8 Hz changes up to 20%. In order to find a solution for this real case, this paper takes the annual modal parameter estimates (approx. 14000 estimations) of this mode and designs three control strategies: a) a tuned mass damper (TMD) tuned to the most-repeated modal properties of the aforementioned mode, b) two semi-active TMD strategies, one with an on-off control law for the TMD damping, and other with frequency and damping tuned by updating the damper force. All strategies have been carefully compared considering two structure models: a) only the aforementioned mode and b) all the other tracked modes. The results have been compared considering human-induced vibrations and have helped the authors on making a decision of the most advisable strategy to be practically implemented.

• Korea-Australia Rheology Journal
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• 제20권4호
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• pp.253-260
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• 2008

A method based on transient shear flow dynamics of red cell aggregates was developed to investigate reversible re-aggregation processes with decreasing shear flow. In the microchannel-flow aggregometry, the aggregated red blood cells that are subjected to continuously decreasing shear stress in microchannel flow were measured with the use of a laser-scattering technique. Both the laser-backscattered intensity and pressure were simultaneously measured with respect to time, resulting in shear stress ranging from $0{\sim}35\;Pa$ for a time period of less than 30 seconds. The time dependent recording of the backscattered light intensity (syllectogram) yielded an upward convex curve with a peak point, which reflected the transition threshold of aggregation in the RBC suspensions. Critical-time and critical-shear stress corresponding to the peak point were examined by varying the initial pressure-differential and the micro channel depth, and these results showed good potential for being used as new aggregation indices. In the present study, these newly proposed indices were also validated by differentiating the effect of fibrinogen on RBC aggregation and then these indices were compared to the conventional indices that were measured by a rotational aggregometer.

• Nuclear Engineering and Technology
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• 제56권5호
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• pp.1863-1873
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• 2024

In the Korean domestic nuclear industry, to analyze the reliability of instrumentation and control (I&C) systems, the failure rates of the electronic components constituting the I&C systems are predicted based on the MIL-HDBK-217F standard titled 'Reliability Prediction of Electronic Equipment'. Based on these predicted failure rates, the mean time to failure of the I&C systems is calculated to determine the replacement period of the I&C systems. However, this conventional approach to the prediction of electronic component failure rates assumes that factors affecting the failure rates such as ambient temperature and operating voltage are static constants. In this regard, the objective of this study is to propose a prediction method for the remaining useful life (RUL) of electronic components considering mean time to failure calculations reflecting dynamic environments, such as changes in ambient temperature and operating voltage. Results of this study show that the RUL of electronic components can be estimated depending on time-varying temperature and electrical stress, implying that the RUL of electronic components can be predicted under dynamic stress conditions.

• International Journal of Precision Engineering and Manufacturing
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• 제2권2호
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• pp.17-25
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• 2001

In this paper, a theoretical study has been made to reduce the residual stress and birefringence in the injection-molded parts. An optimization program has been used to minimize the residual stresses and birefringence calculated from a simulation program. The thermally induced stress has been calculated using a linear viscoelasticity model. The flow stress and birefringence has been calculated using the Leonov's viscoelasticity model. This has been applied to the injection molding of a circular disc and a plate. the optimization has been done either by changing process variables while maintaining the mold temperature constant or by varying the mold-wall temperature with time. This study shows the significant reduction in residual stress and birefringence is possible through the optimization of processing conditions.

• 소성∙가공
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• 제3권4호
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• pp.415-426
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• 1994

Multistage deformation behavior of Nb-microalloyed steel and carbon steel was studied by torsion test with declining temperature T, constant pass strain $\varepsilon_i$, interrupt time $(t_i)$, and varying strain rate $(\.{\varepsilon})$. In the range of $1000^{\circ}C~790^{\circ}C$ and 4.00/sec~0.38/sec, the flow stress at each pass was correlated to the deformation variables. As the finished deformation temperatures are decreased to the range of $790^{\circ}C~900^{\circ}C$, Nb precipitates play an important role on the grain refinement of Nb-microalloyed steel. the flow stress of Nb-microalloyed steel was higher than the carbon steel's while the grain size of Nb-microalloyed steel was smaller than carbon steel below the temperature of $900^{\circ}C$.