• Nuclear Engineering and Technology
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• 제28권2호
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• pp.150-161
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• 1996

연구로용 우라늄-실리사이드 분산 핵연료에서의 응력 및 변형율 분포를 계산할 수 있는 변형모델을 개발하였다. 이 변형모델은 탄소성이론 및 지수법칙 크리프이론을 기초로 한 것이며, 또한 등방 핵연료팽윤 및 열팽창을 가정하였다. 개발된 모델을 HANARO 및 카나다의 NRU 핵연료에 적용하여 본 결과 핵연료의 변형을 성공적으로 계산하는 것으로 판단되었다. 계산결과에 따르면, 연구로용 우라늄-실리사이드 분산핵연료가 연소할 때 핵연료심에서 가장 중요한 변형기구는 팽윤이며, 피복관에서 가장 중요한 변형기구는 크리프이다. 또한, 피복관에서 원주방향 최대응력은 항상 5 MPa 이하로서 항복응력보다 훨씬 낮게 유지되었다. 여기서 고려한 두 핵연료설계에 대해서 전 연소도 범위에서 핵연료봉의 부피변화는 10% 이하로 예측되었다.

• 한국정밀공학회지
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• 제18권2호
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• pp.171-176
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• 2001

This study focuses on the energy partition and heat flux distribution in creep-feed grinding. From the measurements of transient grinding temperature in the workpiece which the thermocouple was embedded, the overall energy partition to the workpiece was estimated with moving heat source theory using the developed scalene triangle heat model. The energy partition was calculated as 3.75% in down grinding smaller than 5.3% in up grinding. Also, the scalene triangle heat model was confirmed as the most optional heat model in correspond to the experimental data. Then, the heat flux distribution was calculated from temperature responses. The heat flux is negative behind the grinding zone where fluid was applied. In this experimental result, the total heat flow to the workpiece per unit width obtained by integrating the positive heat flux was 0,7W/mm for down grinding.

• Steel and Composite Structures
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• 제39권1호
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• pp.21-33
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• 2021

This paper aims to improve the current state-of-the-art in long-term deflection prediction in steel-concrete composite beams. The efficiency of a time-dependent finite element model based on linear creep theory is verified with available experimental data. A parametric numerical study is then carried out, which focuses on the effects of concrete creep and/or shrinkage, ultimate shrinkage strain and reinforcing bars in the slab. The study shows that the long-term deformations in composite beams are dominated by concrete shrinkage and that a higher area of reinforcing bars leads to lower long-term deformations and steel stresses. The AISC model appears to overestimate the shrinkage-induced deflection. A modified ACI equation is proposed to quantify time-dependent deflections in composite beams. In particular, a modified reduction factor reflecting the influence of reinforcing bars and a coefficient reflecting the influence of ultimate shrinkage are introduced in the proposed equation. The long-term deflections predicted by this equation and the results of extensive numerical analyses are found to be in good agreement.

• 한국초전도ㆍ저온공학회논문지
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• 제25권3호
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• pp.28-33
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• 2023

Magnetic relaxation properties of pure MgB₂ powder samples and diluted water-treated MgB₂ powder samples were investigated. The magnetic field H-dependence, m(H), and the time t-dependence, m(t), of the magnetic moment m were measured and analyzed using the PPMS-VSM magnetometer equipment, respectively. The m(t) reduction rates of pure MgB₂ powder samples and diluted water-treated MgB₂ powder samples decreased to about 0.7 ~ 1.8% and 0.6 ~ 1.0% for about 7200 s, respectively, at temperature T = 15 K. The magnetic relaxation properties of the two types of MgB₂ powders were analyzed by calculating the magnetic relaxation rate S = -dln(M_irr)/dln(t) values according to Anderson-Kim theory. The magnetic relaxation ratio S values of the two types of MgB₂ powder samples were almost similar. As a result of the quantum creep effect, the constant magnetic relaxation rate S characteristic was confirmed at a temperature range of T = 10 K or less.

• 대한기계학회논문집A
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• 제26권10호
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• pp.1953-1960
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• 2002

The viscoplasticity theory based on overstress, one of the unified state variable theories, is generalized to model zero (no influence of loading rate) and negative (flow stress decreases with loading rate) as well as positive (flow stress increases with loading rate) rate sensitivity in a consistent way. On the basis of the long-time asymptotic solution the different types of rate sensitivity are classified with respect to an augmentation function that is introduced in the evolution law fur a state variable equilibrium stress. The theory predicts normal relaxation and creep behaviors even if unusual rate sensitivity is modeled. The constitutive model fir the behavior of a modified 9Cr-1 Mo steel at various temperatures is then compared with experimental data found in the literature.

• 콘크리트학회지
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• 제9권2호
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• pp.153-161
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• 1997

콘크리트가 외기에 노출되면 수분확산 현상으로 수분의 이동이 일어나고, 콘크리트 내부는 부등수분분포상태가 된다. 이러한 부등수분분포에 의하여 콘크리트 구조물에는 부등건조수축이 발생하며, 부등건조수축은 콘크리트 표면에 인장응력을 발생시키고, 표면균열의 원인이 되기도 한다. 또한 부등건조수축으로 인한 잔류응력은 현저한 크리이프의 영향을 받고, 이 때 각 위치에서 수분의 차이로 인하여 부등크리이프가 발생한다. 따라서 이 연구에서는 수분확산이론을 근거로 하여 콘크리트 내부의 수분분포를 합리적으로 예측할 수 있는 해석프로그램을 개발하였다. 그리고 콘크리트 내부의 각 위치에서 부등크리이프의 영향을 고려한 부등건조수축 해석방법을 제시하였다. 또한 콘크리트 내부의 각 위치에서의 부등건조수축 측정실험을 실시하여, 부등건조수축 해석방법의 타당성을 검증하였다.

• 소성∙가공
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• 제14권5호
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• pp.466-472
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• 2005

Inelastic deformation behavior of metals and alloys is considered rate dependent. Uniaxial ratcheting experiments performed by Ruggles and Krempl, and Hassan and Kyriakides exhibited that higher mean stress for a fixed stress amplitude resulted in higher ratchet strain within a rate independent framework and higher stress rate resulted in lower ratchet strain, respectively. These phenomena are qualitatively investigated by numerical experiments through unified viscoplasticity theory. The theory does not separate rate-independent plasticity and rate-dependent creep, and thus uses only one inelastic strain to describe inelastic deformation processes with the concept of the yield surface. The growth law for the kinematic stress, which is a tensor valued state variable of the constitutive equations, is modified to predict the linear evolution of long-term ratchet strain.

• Computers and Concrete
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• 제32권4호
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• pp.383-392
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• 2023

The mechanical properties of Recycled Aggregate Concrete (RAC) with 100 percent Recycled Coarse Aggregate (RCA) under loading rates were investigated in depth. The theoretical model was validated utilizing the RAC elastic modulus obtained from cylindrical specimens subjected to various strain rates. Viscoelastic theories have traditionally been used to describe creep and relaxation of viscoelastic materials at low strain rates. In this study, viscoelastic theories were extended to the time domain of high strain rates. The theory proposed was known as reversed viscoelastic theory. Normalized Dirichlet-Prony theory was used as an illustration, and its parameters were determined. Comparing the predicted results to the experimental data revealed a high level of concordance. This methodology demonstrated its ability to characterize the strain rate effect for viscoelastic materials, as well as its applicability for determining not only the elastic modulus for viscoelastic materials, but also their shear and bulk moduli.

• 대한기계학회논문집A
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• 제24권8호
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• pp.1991-1999
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• 2000

This research has been performed to reveal the hysteresis phenomena of the hunting motion in a railway passenger car having a bolster. Since linear analysis can not explain them, bifurcation analysis is used to predict its outbreak velocities in this paper. However bifurcation analysis is attended with huge computing time, thus this research proposes more effective numerical algorithm to reduce it than previous researches. Stability of periodic solution is obtained by adapting of Floquet theory while stability of equilibrium solutions is obtained by eigen-value analysis. As a result, linear and nonlinear critical speed are acquired. Full scale roller rig test is carried out for the validation of the numerical result. Finally, it is certified that there are many similarities between numerical and test results.

• Steel and Composite Structures
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• 제7권1호
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• pp.71-85
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• 2007

This study suggests modular composite profile beams, where the prefab concept is applied to existing composite profile beams. The prefab concept produces a beam of desired size having two types of profile: side module and bottom module. Module section will improve construction efforts because it offers several benefits : reduction of deflections due to creep and shrinkage, which might be found in existing composite profile beams; increase in span/depth ratio; and free prefabrication of any required beams. Based on the established analysis theory of composite profile beams, an analysis theory of modular composite profile beams was suggested, and analysis values were compared with experimental ones. The behavior of individual modules with increase of load was measured with a strain gauge, and the shear connection ratio between modules was analyzed by using the measured values. As a result of experiment, it was found that theoretical flexural strength on condition of full connection was 57%-80% by connection of modules for each specimen, and it is expected that flexural strength will approximate the theoretical levels through further module improvement.