• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2004년도 춘계 학술발표회 제16권1호
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• pp.466-471
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• 2004

Efficient numerical finite element analysis of creeping concrete structures requires the use Kelvin or Maxwell chain model, which is most conveniently identified from a continuous retardation or relaxation spectrum, the spectrum in turn being determined from the given compliance or relaxation function. The method of doing that within the context of solidification theory for creep with aging was previously worked out by Bazant and Xi, but only for the case of a continuous retardation spectrum based on Kelvin chain. The present paper is motivated by the need to incorporate concrete creep into the recently published microplane model M4 for nonlinear triaxial behavior of concrete, including tensile fracturing and behavior under compression. In that context. the Maxwell chain is more effective than Kelvin chain. because of the kinematic constraint of the microplanes used in M4. Determination of the continuous relaxation spectrum for Maxwell chain. based on the solidification theory, is outlined and numerical examples are presented.

• 한국주조공학회지
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• 제9권1호
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• pp.73-79
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• 1989

Microstructural characteristics of Al-3.63wt.%Pb hyper-monotectic alloy rapidly-solidified by melt spinning were examined. Possibility of forming a planar liquid -solid interface during rapid solidification of this alloy was also considered with a morphological stability theory, and a mechanism of forming banded structure observed at the bottom parts of melt-spinned specimens was considered as well. Application of the absolute stability criterion predicts the liquid-solid interface of the primary aluminium phase to be able to maintain a planar interface during the early stage of rapid solidification. Formation of banded structure was supposed to be resulted from the release of latent heat during solidification, which affect the stability of a planar liquid-solid interface.

• 대한기계학회논문집B
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• 제22권5호
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• pp.698-707
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• 1998

A simplified model for predicting microsegregation during columnar-dendritic solidification of binary alloys is developed, in which back diffusion, dendrite arm coarsening and dendrite tip undercooling are simultaneously incorporated. The inclusion of tip undercooling is accomplished by modifying the initial conditions of the existing solute diffusion model, in such forms that tip undercooling depresses the beginning of solidification below the liquidus temperature, and that the secondary arm spacing evolves in accordance with the minimum undercooling theory. Sample calculations for the well-known benchmark system show that the present predictions not only consist with the extablished limiting cases, but also agree favorably with the available experimental data within a reasonable tolerance. In particular, a typical decreasing trend in the eutectic fraction at high cooling rates is successfully resolved. Comparison of the individual and combined effects of characteristic parameters in reference with the limiting cases reveals the interactions among parameters. Every parameter plays the role of reducing the eutectic fraction, and the degree of influence depends primarily on the cooling rate. Coarsening enhances the effect of tip undercooling, while suppressing that of back diffusion. A vigorous back diffusion seems to restrain the apperance of the undercooling effect. Overall, each contribution of the three parameters to microsegregation is estimated to be of the same order, which suffices to justify the present study.

• Structural Engineering and Mechanics
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• 제81권5호
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• pp.617-631
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• 2022

In this paper the viscoelastic responses of four experimental steel-concrete composite beams subjected to highly variable environmental conditions are investigated by means of a finite element (FE) model. Concrete specimens submitted to stepped stress changes are also evaluated to validate the current formulations. Here, two well-known approaches commonly used to solve the viscoelastic constitutive relationship for concrete are employed. The first approach directly solves the integral-type form of the constitutive equation at the macroscopic level, in which aging is included by updating material properties. The second approach is postulated from a rate-type law based on an age-independent Generalized Kelvin rheological model together with Solidification Theory, using a micromechanical based approach. Thus, conceptually both approaches include concrete hardening in two different manners. The aim of this work is to compare and analyze the numerical prediction in terms of long-term deflections of the studied specimens according to both approaches. To accomplish this goal, the performance of several well-known model codes for concrete creep and shrinkage such as ACI 209, CEB-MC90, CEB-MC99, B3, GL 2000 and FIB-2010 are evaluated by means of statistical bias indicators. It is shown that both approaches with minor differences acceptably match the long-term experimental deflection and are able to capture complex oscillatory responses due to variable temperature and relative humidity. Nevertheless, the use of an age-independent scheme as proposed by Solidification Theory may be computationally more advantageous.

• 대한토목학회논문집
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• 제29권1A호
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• pp.85-93
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• 2009

본 논문은 콘크리트의 크리프 현상에 대한 보다 단순하고 합리적인 유변학적 모델을 구성하고, 크리프 예측 모델 개발에 대한 하나의 방법론을 제시하는 데에 일차적인 목표가 있다. 장기적인 응력에 의한 콘크리트의 변형은 발생 메커니즘과 시간 의존성 여부에 따라 즉각적인 탄성 변형과 시간 의존적 단기 크리프, 시간 독립적 단기 크리프, 장기 크리프의 합으로 볼 수 있으며, 이들 변형을 모사하기 위해 6개의 매개변수를 갖는 유변학 모델을 구성하였다. 각 매개변수의 구성에는 미세 프리스트레스 고체화 이론과 기존 설계기준을 활용하였고, 이론적 접근이 어려운 경우에 한해 수치적 접근을 시도하였다. 수립된 모델의 검증은 실제 실험 데이터를 사용하였고, 기존 모델 및 설계식과 비교 평가하여 그 합리성을 확인하였다.

• 한국주조공학회지
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• 제11권1호
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• pp.79-84
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• 1991

The process of solidification of metal is accompanied by liquid-solid change and known as Stefan's heat conduction problem on the moving boundary. In this study the temperature distribution in ingot and metallic mould during casting was analyzed by the two dimensional heat conduction theory. The transient temperature distribution was numerically calculated using a finite element method on the nodal point of mesh screen representing ingot and mould cross section. The theory was applied on the casting of aluminum(purity ; 99%) in flat ingot mould of GC25. The analysis will make it possible to calculate an optimum mould shape of which temperature gradient becomes minimum.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 춘계학술대회 논문집
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• pp.176-179
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• 2005

Spray casting of hypereutectic Al-Si based alloy has been reported to provide distinct advantages over ingot metallurgy (IM) or rapid solidification/powder metallurgy (RS/PM) process in terms of microstructure refinement. Hypereutectic Al-Si based alloys have been regarded attractive for automotive and aerospace application, due to high specific strength, good wear resistance, low coefficient of thermal expansion, high thermal stability, and good creep resistance. In this study, hypereutectic Al-25Si-2.0Cu-1.0Mg alloy was prepared by OSPREY spray casting process. High temperature deformation behavior of the hypereutectic Al-Si based alloy has been investigated by applying the internal variable theory proposed by Chang et al. The change of strain rate sensitivity and Creep transition were analyzed by using the load relaxation test and constant creep test.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2006년도 학술발표회 논문집
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• pp.89-96
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• 2006

Recently, many researches on the dissipation of excess pore pressure in liquefied sand grounds have been performed to evaluate post-liquefaction behaviors of structures. In this paper. centrifuge tests were performed to simulate liquefaction behaviors of prototype soil. The evaluation model of solidified layer thickness was developed to simulate non-linear variation of solidified layer thickness with time. Also, the dissipation of excess pore pressure in liquefied sand was evaluated by applying the solidification theory and the consolidation theory. The developed model gives a good estimation of the solidified layer thickness and the time history of excess pore pressure.

• 소성∙가공
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• 제14권6호
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• pp.502-508
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• 2005

Hypereutectic Al-Si alloys have been regarded attractive for automotive and aerospace application, due to high specific strength, good wear resistance, high thermal stability, low thermal expansion coefficient and good creep resistance. Spray casting of hypereutectic Al-Si alloy has been reported to provide distinct advantages over ingot metallurgy (IM) or rapid solidification/powder metallurgy (RS/PM) process in terms of microstructure refinement. In this study, hypereutectic Al-25Si-2.0Cu-1.0Mg alloy was prepared by OSPREY spray casting process. The change of strain rate sensitivity and Creep transition were analyzed by using the load relaxation test and constant creep test. High temperature deformation behavior of the hypereutectic Al-Si alloy has been investigated by applying the internal variable theory proposed by Chang et al. Especially, the creep resistance of spray casted hypereutectic Al-Si alloy can be enhanced considerably by the accumulation of prestrain.

• 재활간호학회지
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• 제5권2호
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• pp.217-225
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• 2002

Purpose : This research is aimed at developing a substantive theory related to the experience of adjustmented hemodialysis patients by identifying and analyzing the nature, process, and types of adjustment experienced by them. For this purpose, this study takes a grounded theory approach. Method : Data were collected from April, 2002 through September, 2002 through in-depth interviews and close observations of eleven hemodialysis patients who have experienced adjustment. With their consent, the interviews were recorded by audio tapes and later transcribed. Observation memos were also prepared on the subjects' behavior during the interviews. Data collection continued until saturated. The data were analyzed into concepts, subcategories, and categories with the open coding process. The axial coding was done to identify the relationships of the concepts and categories. And the selective coding was done to develop a core category, which is the central phenomenon of the hemodialysis patients who experienced adjustment. Result : This process resulted in 88 concepts, which may be grouped into 24 subcategories and 6 core categories. The 6 categories, in fact, depict the process of changes the patients experience from the sense of crisis, self-control, new life meaning, support system, coping ability, and quality of life improvement. And its core is self-control, which encompasses the dynamic stages from recognition, change attempts, belief solidification, and empowerment. Conclusion : The results of the research offer the following implications. First, research on adjustment is a formative stage in nursing, it is imperative for nursing researchers to develop them, which may be more relevant to nursing. Second, it is important to develop nursing intervention techniques that may be most effective in adjustment of hemodialysis patients and at the same time for each stage of changes taking place in adjustmented hemodialysis patients.