• Computers and Concrete
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• 제17권2호
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• pp.173-188
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• 2016

Generally, thermal stress induced by hydration heat causes cracking in mass concrete structures, requiring a thorough control during the construction. The prediction of the thermal stress is currently undertaken by means of numerical analysis despite its lack of reliability due to the properties of concrete varying over time. In this paper, a method for the prediction of thermal stress in concrete structures by adjusting thermal stress measured by a thermal stress device according to the degree of restraint is proposed to improve the prediction accuracy. The ratio of stress in concrete structures to stress under complete restraint is used as the degree of restraint. To consider the history of the degree of restraint, incremental stress is predicted by comparing the degree of restraint and the incremental stress obtained by the thermal stress device. Furthermore, the thermal stresses of wall and foundation predicted by the proposed method are compared to those obtained by numerical analysis. The thermal stresses obtained by the proposed method are similar to those obtained by the analysis for structures with internally as well as externally strong restraint. It is therefore concluded that the prediction of thermal stress for concrete structures with various boundary conditions using the proposed method is suggested to be accurate.

• 대한기계학회논문집A
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• 제39권2호
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• pp.143-156
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• 2015

함수구배재료의 모드 III 균열이 물성치 구배방향과 다른 방향으로 비정상적으로 전파할 때 전파균열선단부근의 응력 및 변위장에 대하여 연구하였다. 함수구배재료는 밀도가 일정한 상태에서 전단탄성계수가 선형적으로 변화하는 경우와 밀도와 전단탄성계수가 지수형적으로 변화하는 경우로 가정했다. 조화함수의 해를 얻기 위하여 일반적인 편미분방정식의 동적평형방정식을 라플라스 방정식으로 변환하였다. 라플라스 방정식으로부터 균열속도 변화률, 응력확대계수의 변화률 등에 의존되는 응력장과 변위장을 근접해법으로 얻었다. 본 연구에서 얻어진 응력장과 변위장을 사용하여 재료의 비 균질성, 균열속도의 변화률, 응력확대계수의 변화률 등을 고려한 상태에서 균열이 임의의 방향으로 전파할 때 균열선단부근의 응력 및 변위 그리고 응력확대계수에 대하여 연구하였다.

• Journal of Welding and Joining
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• 제6권4호
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• pp.35-43
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• 1988

Welding residual stresses were calculated by two dimensional thermal elasto-plastic analysis using element method. Complicated plastic behavior during heat transfer was simulated with time. Fist, temperature distributions. To consider time varying behavior of material properties and loading and unloading processes, iterative calculation based on initial stiffness method was carried out. The method proposed by Yamata was used in time increment control which determined the accuracy of claculation. comparison with other caculated and experimental results shows fairly good agreement.

• 한국정밀공학회지
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• 제10권3호
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• pp.87-96
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• 1993

In this study, the peak stress of a fuillet in elastic structure was optimized to have minimum value by using quadratic isoparametric element. The method of auomatic gridding was also developed along with shape algorithm and design element technique was adopted in selecting design variables. The computer program developed was combined with the Hooke-Jeeves direct algorithm of optimization techniques in order to minimize the peak stress of the fillet. The imployment of design element technique significantly cut down computer time by the reduction in design variables, and the opitmum fillet shape with uniform minimum stress was obtained by varying design variables along x and y directions in improving the shape compared to other results. By using automatic gridding, in which Bezier surfaces and Coons surfaces of cubic interpolation were employed, the irregular boundary was removed resulting in smoother anbd more accurate fillet shape possessing uniform minimum stress.

• Journal of Biosystems Engineering
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• 제17권1호
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• pp.79-90
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• 1992

Viscoelastic characteristics of agricultural products may be determined through three basic tests ; stress relaxation, creep, and dynamic test. Considering the changeability of living materials, dynamic test in which information is derived in a relatively short time appears to be highly desirable, in which either cyclic stress or cyclic strain is imposed and the remaining quantity (strain or stress) is measured. The periodically varying stress will also result in periodically varying strain which in a viscoelastic material should theoretically be out of phase with the stress, because part of the energy subjected to sample is stored in the material as potential energy and part is dissipated as heat. This behavior results in a complex frequency-dependent compliance denoted by J($i{\omega}$). The complex compliance and therefore the storage compliance, the loss compliance, the phase angle, and percent energy loss for the sample should be obtainable with a given static viscoelastic property of the material under static load. The complex compliance of the rough rice kernel were computed from the Burger's model describing creep behavior of the material which were obtained in the previous study. Also, the effects of cyclic load and moisture content of grain on the dynamic viscoelastic behavior of the samples were analyized. The results obtained from this study were summarized as follows ; 1. The storage compliance of the rough rice kernel slightly decreased with the frequency applied but at above the frequency of 0.1 Hz it was nearly constant with the frequency, and the loss compliance of the sample very rapidly decreased with increase in the frequency on those frequency ranges. 2. It was shown that the storage compliance and the loss compliance of the sample increased with increase in grain moisture content. Effect of grain moisture content on the storage compliance of the sample was highly significant than effect of the frequency applied, but effect of the frequency on the loss compliance of the sample was more significant than effect of grain moisture content. 3. In low moisture content, the percent energy loss of Japonica-type rough rice was much higher than that of Indica-type rough rice, but, in high moisture content, vice versa.

• 한국전산구조공학회논문집
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• 제24권3호
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• pp.337-342
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• 2011

매우 큰 편심하중을 지지하면서 상하로 수직위치를 제어하는 의료용 리프트 칼럼은 수직위치의 정확도를 확보하기 위해 매우 낮은 구조적 변형과 진동은 물론 높은 피로강도가 요구된다. 리프트 칼럼은 일반적으로 3단 슬라이딩 붐구조로 유도전동기로 작동되며, 상하운동에 따라 변동응력을 나타낸다. 본 연구에서는 리프트 칼럼의 상하운동으로 야기되는 변동응력에 따른 피로강도를 수치해석적으로 예측하였다. 한 주기 상하운동에 따른 응력변동은 유한요소해석으로 구하였으며, 피로수명은 Palmgren-miner기법과 재료의 S-N선도를 적용하여 예측하였다. 수치해석의 신뢰성을 확보하기 위하여 리프트 칼럼의 상세형상과 체결부위을 반영한 3차원 FEM모델을 생성하였으며, 칼럼과 패드사이 계면은 접촉조건으로 처리하였다.

• 한국식품영양과학회지
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• 제28권6호
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• pp.1293-1297
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• 1999

Effect of rice protein and lipid on rheological properties of gelatinized rice flour solutions(4%) with three rice varieties(Dongjin, Jinmi, Tamjin) known for varying taste of cooked rice was investigated with Haake viscometer. The rheological behaviors of all rice flour solutions were illustrated by Herschel Bulkley equation and exhibited pseudoplastic behavior with yield stress. When rice flour solutions treated with protease and dithiothreitol, there was decreased in flow behavior index value. Flow behavior index was decreased by dealbumin and deglutelin rice flour solutions among deprotein groups. The Jinmi rice flour solutions exhibited slightly lower consistency index than Dongjin and Tamjin. Defatted rice flour solutions exhibited lower consistency index than rice flour solutions, while dealbumin, deglutelin rice flour solutions exhibited high consistency index. Protease treated rice flour solutions exhibited increase in Dongjin and Tamjin. The yield stress was increased in sequence eating quality. Yield stress of defatted rice flour solutions was decreased, while deglutelin and rice starch flour solutions was increased. The time dependent charac teristics of all rice flour solutions appeared forming hysteresis loop and thixotropic behavior showed. The time dependent characteristics was appeared in sequence eating quality. Rice starch and deglutelin flour solutions appeared greatly time dependent characteristics, but defatted rice flour solutions appeared very little.

• 한국지반환경공학회 논문집
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• 제11권4호
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• pp.19-24
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• 2010

사면의 거동에 관한 종래의 해석 기법은 사면거동을 시간과 변위의 2차원 크리프로 해석하는 것이었으나, 이러한 해석 기법은 강우에 의한 응력변화를 고려 않고 시간에 따른 변위만을 고려하여, 강우 시 사면의 거동 및 붕괴를 설명하지 못한다. 또한 강우에 의한 이력현상으로 인해 파괴면내의 전단에너지가 감소한다. 크리프 해석은 응력항을, 이력현상 해석은 시간항을 무시한 해석이므로, 두 가지 해석을 결합하여, 사면의 거동을 해석한다면, 응력변화, 변위 및 시간을 모두 고려한 실제 사면의 거동해석이 됨을 알 수 있다. 그러나 응력변화에 관한 항은, 강우로 인한 실제 사면의 침투 및 배수 시, 전단응력의 변화 및 전단강도의 변화를 동시에 유발하므로, 이 두 항을 동시에 고려할 수 있는 안전계수항으로 바꾸어야 한다. 본 논문은 강우의 지반침투 및 배수로 인하여 사면의 단위중량이 변하는 과정에 대한 크리프와 이력현상을 고려할 수 있는 결합해석으로 고찰하고 이에 따른 사면붕괴에 관한 전단응력과 전단강도의 변화 및 안전계수변화를 3차원으로 나타내고자 수행한 결과이다.

• 대한기계학회논문집A
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• 제26권12호
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• pp.2647-2655
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• 2002

This research presents an analytical model to investigate the stability due to the ball bearing waviness i n a rotating system supported by two ball bearings. The stiffness of a ball bearing changes periodically due to the waviness in the rolling elements as the rotor rotates, and it can be calculated by differentiating the nonlinear contact forces. The linearized equations of motion can be represented as a parametrically excited system in the form of Mathieu's equation, because the stiffness coefficients have time -varying components due to the waviness. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as the simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving the Hill's infinite determinant of these algebraic equations. The validity of this research is proved by comparing the stability chart with the time responses of the vibration model suggested by prior researches. This research shows that the waviness in the rolling elements of a ball bearing generates the time-varying component of the stiffness coefficient, whose frequency is called the frequency of the parametric excitation. It also shows that the instability takes place from the positions in which the ratio of the natural frequency to the frequency of the parametric excitation corresponds to i/2 (i=1,2,3..).

• 한국정밀공학회지
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• 제12권8호
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• pp.147-155
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• 1995

Recent experimental studies have been shown that strain gages can be employed to determine either static or dynamic stress intensity factors $K_{I}$ wiht relatively simple experiments. However, it does not usually provide a reliable value of stress intensity factor because of local yielding and limited regions for strain gage placement at the vicinity of the crack tip. This paper attempted to define a valid region and to indicate procedures for locating and orienting the strain gage to determine static toughness $K_{Is}$ accurately form one strain gage readings with respect to varying loadings. The strain gage methods was used for compact tension specimens made of Polycarbonate and PMMA(polymethyl methacrylate). Series expansions of the static and dynamic strain fields are applied. Strain gage orientation and location are then studied to optimize the strain response. Especially, in the dynamic experiment, the specimen employed is an oversized Charpy V-notch specimen which has been modified to provide significant constraint with a large elevation of the flow stress. The impact behavior of the specimen is monitored by placing strain gage near the crack tip. The dynamic toughness $K_{Id}$ is determined from the strain time traces of this gage.e.