• 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.

• 한국자기학회지
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• 제12권2호
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• pp.73-79
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• 2002

자성유체는 고액혼상유체로 뉴턴유체와 달리 자기력에 반응하는 유체이다. 본 연구에서는 금속과 유체의 특성을 겸비한 자성유체의 열전달 특성을 밀도 있게 연구함을 목적으로, 그 대상을 이중원관으로 하여 열대류현상을 고찰하기 위해 내부원관을 가열하고, 외부원관을 냉각하면서, 또한 외부에서 인가자장의 세기와 방향에 따라 자연대류 현상의 변화 및 열전달 특성을 수치해석적으로 연구하여, 실험결과와 비교 검토하였다. 자성유체의 자연대류현상은 인가자장에 따라 제어할 수 있었고, 평균 뉴셀트수를 구한 결과로서 자장을 가하지 않았을 경우와 비교해 보면 열전달은 자장을 중력방향으로 가하거나 중력과 반대방향으로 그세기를 -14 mT이상 가했을 때 증가하였고, 중력과 반대방향으로 감했을 때 감소하였다. 특히 자장의 세기가 -14 mT가 되면 열전달은 최소가 되었다.

• Communications for Statistical Applications and Methods
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• 제11권1호
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• pp.161-168
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• 2004

A single-index model is useful in fields which employ multidimensional regression models. Many methods have been developed in parametric and nonparametric approaches. In this paper, posterior inference is considered and a wavelet series is thought of as a function approximated to a true function in the single-index model. The posterior inference needs a prior distribution for each parameter estimated. A prior distribution of each coefficient of the wavelet series is proposed as a hierarchical distribution. A direction $\beta$ is assumed with a unit vector and affects estimate of the true function. Because of the constraint of the direction, a transformation, a spherical polar coordinate $\theta$, of the direction is required. Since the posterior distribution of the direction is unknown, we apply a Metropolis-Hastings algorithm to generate random samples of the direction. Through a Monte Carlo simulation we investigate estimates of the true function and the direction.

• 한국산업융합학회 논문집
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• 제9권1호
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• pp.37-44
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• 2006

Stress and displacement fields of a Mode III crack propagating along the normal to gradient in an orthotropic functionally gradient materials (OFGM), which has (1) an exponential variation of shear modulus and density, and (2) linear variation of shear modulus with a constant density, are derived. The equations of motion in OFGM are developed and solution to the displacement and stress fields for a propagating crack at constant speed though an asymptotic analysis. The first three terms in expansion of stress and displacement are derived to explicitly bring out the influence of nonhomogeneity. When the FGM constant ${\zeta}$ is zero or $r{\rightarrow}0$, the fields for OFGM are almost same as the those for homogeneous orthotropic material. Using the stress components, the effects of nonhomogeneity on stress components are discussed.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.113-118
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• 2003

Stress and displacement fields for a propagating crack in a functionally gradient material (FGM) which has exponentially varying elastic and physical properties along the direction of the crack propagation, are derived. The equations of motion in nonhomogeneous material are developed using displacement potentials. The solutions to the displacement fields and the stress fields for a crack propagating at constant speed along the gradient are obtained through an asymptotic analysis. The influences of nonhomogeneity on the higher order terms of the stress fields are explicitly brought out. Using these stress components, isochromatic fringes around the stationary crack are generated at crack for different nonhomogeneity and the effects of nohonhomgeneity on these fringes are discussed.

• Structural Engineering and Mechanics
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• 제13권5호
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• pp.491-504
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• 2002

Recently we formulated a 2D hybrid stress element from the 3D Hellinger-Reissner principle for the analysis of thick bodies that are symmetric to the thickness direction. Polynomials have typically been used for all the displacement and stress fields. Although the element predicted the dominant stress and all displacement fields accurately, its prediction of the out-of-plane shear stresses was affected by the very high order terms used in the polynomials. This paper describes an improved formulation of the 2D element using Fourier series expansion for the out-of-plane displacement and stress fields. Numerical results illustrate that its predictions have markedly improved.

• 대한기계학회논문집
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• 제12권5호
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• pp.1150-1157
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• 1988

본 연구에서는 수치계산을 통해 진행하는 방향에 수직으로 일정한 압력구배를 받는 2차원 난류 자유제트의 발달을 고려해 보고자 한다.속도장을 구한 후에는 제 트의 온도가 주위온도 보다 높을 경우와 또 제트를 사이에 둔 양쪽 유체에 온도차가 있을때의 열확산 특성도 함께 알아보려는 것이다. 수직 방향으로 압력구배가 있는 유동장은 최근 활발히 연구가 진행되는 분야인 제트축에 수직방향으로 주위유동이 있 는 경우와 흡사하며 제트를 통한 열확산현상은 air curtain등 실제 응용도가 큼에도 불구하고 문헌에 보고된 것이 별로 없는 것으로 보여진다.

• 천문학회지
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• 제37권5호
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• pp.601-603
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• 2004

We present the results of the linear analysis for the Parker-Jeans instability in the magnetized gas disks including the effect of cosmic-ray diffusion along the magnetic field lines. We adopted an uni-formly rotating two temperature layered disk with a horizontal magnetic fields and solved the perturbed equations numerically. Fragmentation of gases takes place and filamentary structures are formed by the growth of the instability. Nagai et al. (1998) showed that the direction of filaments being formed by the Parker-Jeans instability depends on the strength of pressure outside the unperturbed gas disk. We found that at some range of external pressures the direction of filaments is also governed by the value of the diffusion coefficient of CR along the magnetic field lines k.

• Journal of Mechanical Science and Technology
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• 제19권11호
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• pp.1967-1974
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• 2005

The subsurface stress field caused by both normal loads and tangential loads has been evaluated using the rectangular patch solution. The effect of tangential loading on the subsurface stress field has been investigated in detail for both the cylinder-on-cylinder contact and a spur gear teeth contact. For the cylinder-on-cylinder contact, the subsurface stress fields are moved more to the direction of tangential loads and the positions where the maximum stress occur are getting closer to the surface with the increasing tangential loads. The subsurface stress fields of the gear teeth contact are expanded more widely to the direction of tangential loads with the increasing tangential loads. The friction coefficient of a gear teeth contact is low because they are operated in a lubricated condition, and therefore surface tractions in the EHL condition hardly affect on the subsurface stress field.

• 정보저장시스템학회논문집
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• 제2권4호
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• pp.230-235
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• 2006

The machine locking system is an important device for the safety of persons using the machine. In this study, a locking system using electromagnetic fields is proposed to decrease the defects and the cost for repairing and maintenance of the existing locking system using structural mechanism. We analyze the electromagnetic locking system and calculate the generated force considering direction by the finite element method. Also, we set up two design domains for the topology optimization; first domain is optimized to reduce the volume and the other is optimized to maximize the generated force keeping the volume, especially. The optimal design is obtained by integration of the two optimized results. An improved design is obtained by the optimal topology and it is confirmed by comparison with the initial locking system.