• 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.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2001년도 추계학술대회논문집 II
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• pp.995-999
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• 2001

This paper deals with the free vibrations of arches with general boundary condition. Based on the dynamic equilibrium equations of a arch element acting the stress resultants and the inertia forces, the governing differential equation is derived for the in-plane free vibration of such arches. Differential equations are solved numerically to calculate natural frequencies. In numerical examples, the parabolic arch is considered. The effects of the arch rise to span length ratio, the slenderness ratio, the vertical spring coefficient and the rotational spring coefficient on the natural frequencies are analyzed.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2009년도 제40회 하계학술대회
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• pp.724_725
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• 2009

This paper deals with control parameters deduction and dynamic analysis of hybrid thrust magnetic bearing(HTMB). The flux density at air-gap is obtained from system modeling which considers permanent magnet and electro magnet. The vertical force is derived from flux density using maxwell's stress tensor. An accurate linear model is obtained by using linear approximations of the attraction force around the nominal equilibrium point. The dynamic simulation of the HTMB using the PD controller is conducted and control parameters are deducted.

• 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.

• 한국산학기술학회논문지
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• 제18권6호
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• pp.263-268
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• 2017

상용 마그네슘 합금의 경우, 상온에서 낮은 성형성을 갖기 때문에, 온간 성형 조건 하에서 성형 공정을 수행하는 것이 일반적이다. 마그네슘 합금은 온간 성형 과정 중에 동적 재결정(dynamic recrystallization, DRX)이 발생하여, 초기 결정립 사이즈가 급격하게 작아지며, 내부 전위 밀도가 낮아지게 된다. 이에 따라, 유동 응력 곡선은 세 단계의 복잡한 변형 경화 및 연화 현상을 보이게 된다. 첫 번째 구간에서는 변형률이 증가함에 따라, 가공 경화에 의해 응력이 증가하는 경향을 보이며, 두 번째 구간에서는 동적 재결정 현상에 의한 가공 연화로 응력이 갑작스럽게 감소한다. 세 번째 구간에서는 가공 경화와 가공 연화 사이의 평형에 의해, 응력이 일정하게 나타난다. 본 연구에서는, 성형 온도 $300^{\circ}C$, 변형률 속도는 0.001, 0.1, 1, 10/sec에서 AZ80 합금의 구성 방정식의 18개 변수들을 체계적으로 최적화하며, 유동 곡선의 정확도를 높일 수 있는 방식에 대해 제안하려고 한다. 또한 AZ80외에 AZ61도 추가적으로 최적화여 본 논문에서 제안한 최적화 방식의 성능을 증명하였다.

• 국제초고층학회논문집
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• 제11권4호
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• pp.331-346
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• 2022

Reproducing the horizontally homogeneous atmospheric boundary layer in computational wind engineering is essential for predicting the wind loads on structures. One of the important issues is to use fully developed inflow conditions, which will lead to the consistence problem between inflow condition and internal roughness. Thus, by analyzing the previous results of computational fluid dynamic modeling turbulent horizontally homogeneous atmospheric boundary layer, we modify the past hypotheses, detailly derive a new type of inflow condition for standard k-ε turbulence model. A group of remedial approaches including formulation for wall shear stress and fixing the values of turbulent kinetic energy and turbulent dissipation rate in first wall adjacent layer cells, are also derived to realize the consistence of inflow condition and internal roughness. By combing the approaches with four different sets of inflow conditions, the well-maintained atmospheric boundary layer flow verifies the feasibility and capability of the proposed inflow conditions and remedial approaches.

• Structural Engineering and Mechanics
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• 제56권1호
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• pp.137-156
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• 2015

In this paper, an analytical solution of displacement, strain and stress field for rotating thick-walled cylinder made of functionally graded material subjected to the uniform external magnetic field and thermal field in plane strain state has been studied. Stress, strain and displacement field as a function of radial coordinates considering magneto-thermo-elasticity are derived analytically. According to the Maxwell electro-dynamic equations, Lorentz force in term of displacement is obtained in cylindrical coordinates. Also, symmetric temperature distribution along the thickness of hollow cylinder is obtained by solving Fourier heat transfer equation in cylindrical coordinates. Using equation of equilibrium and thermo-mechanical constitutive equations associated with Lorentz force, a second-order inhomogeneous differential equation in term of displacement is obtained and will be solved analytically. Except Poisson's ratio, other mechanical properties such as elasticity modulus, density, magnetic permeability coefficient, heat conduction coefficient and thermal expansion coefficient are assumed to vary through the thickness according to a power law. In results analysis, non-homogeneity parameter has been chosen arbitrary and inner and outer surface of cylinder are assumed to be rich metal and rich ceramic, respectively. The effect of rotation, thermal, magnetic field and non-homogeneity parameter of functionally graded material which indicates percentages of cylinder's constituents are studied on displacement, Von Mises equivalent stress and Von Mises equivalent strain fields.

• Environmental Engineering Research
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• 제22권3호
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• pp.281-287
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• 2017

The joint effect of different concentrations (0.05-0.375 mmol/L) of $Cd^{2+}$ and various concentrations (50-400 mg/L) of [$C_3mim$][OAc] on the growth and physiology of wheat seedlings were investigated. The toxicity of $Cd^{2+}$ could be significantly reduced by lower concentrations (50-200 mg/L) of [$C_3mim$][OAc]. With higher concentration (${\geq}300mg/L$) of [$C_3mim$][OAc], the reduction became less, and compared to $Cd^{2+}$ stress only, the toxicity of 400 mg/L [$C_3mim$][OAc] increased. As for different $Cd^{2+}$ concentrations, the optimal reduction depends on the balance between the exact concentration of $Cd^{2+}$ and [$C_3mim$][OAc]: With low $Cd^{2+}$ concentration (0.05-0.250 mmol/L) requiring 50 mg/L [$C_3mim$][OAc], it could reach a dynamic equilibrium, while high $Cd^{2+}$ concentration (0.375 mmol/L) requiring 100 mg/L [$C_3mim$][OAc]. The growth and physiological indexes of wheat seedlings show a decrease in toxicity compared to the Cd alone treatment, when the dynamic equilibrium is reached. The concentration of $Cd^{2+}$ in leaf tissues showed that low concentration (50-200 mg/L) of [$C_3mim$][OAc] can reduce the toxicity of $Cd^{2+}$ (0.05-0.375 mmol/L) by decreasing the level of $Cd^{2+}$ concentration in the tissues, whereas the higher concentration (${\geq}300mg/L$) of [$C_3mim$][OAc] can increase the concentration of $Cd^{2+}$.

• 한국전산구조공학회논문집
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• 제15권1호
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• pp.189-195
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• 2002

이 논문은 크로소이드 완화곡선을 갖는 수평 곡선보의 자유진동에 관한 연구이다. 곡선보가 진동할 때 부재의 미소요소에 발생하는 합응력과 관성력에 대한 동적 평형방정식을 이용하여 변화곡률을 갖는 수평 곡선보의 자유진동을 지배하는 무차원 상미분방정식과 단부조건에 대한 무차원 경계조건을 유도하였다. 유도된 미분방정식을 크로소이드 완화 곡선을 갖는 수평 곡선보에 적용하여 회전-회전 및 고정-고정 보의 고유진동수 및 변위와 합응력의 진동형을 산출하였다. 구조해석용 범용프로그램인 ADINA와 본 연구의 결과를 비교하여 본 연구의 타당성을 검증하였으며, 수치해석의 결과로 무차원 고유진동수와 곡선보의 변수들 사이의 관계를 표 및 그림에 나타내었다.

• Smart Structures and Systems
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• 제18권1호
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• pp.139-154
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• 2016

This paper is concerned with the dynamics of hyperelastic solids and structures. We seek for a smart control actuation that produces a desired (prescribed) displacement field in the presence of transient imposed forces. In the literature, this problem is denoted as displacement tracking, or also as shape morphing problem. One talks about shape control, when the displacements to be tracked do vanish. In the present paper, it is assumed that the control actuation is provided by imposed eigenstrains, e.g., by the electric field in piezoelectric actuators, or by thermal actuators, or via analogous physical effects, such as magneto-striction or pre-stress. Structures with a controlled eigenstrain-type actuation belong to the class of smart structures. The action of the eigenstrains can be conveniently characterized by actuation stresses. Our theoretical derivations are performed in the framework of the theory of small incremental dynamic deformations superimposed upon a statically pre-deformed configuration of a hyperelastic solid or structure. We particularly ask for a distribution of incremental actuation stresses, such that the incremental displacements follow exactly a prescribed trajectory field, despite the imposed incremental forces are present. An exact solution of this problem is presented under the assumption that the actuation stresses can be tailored freely and applied everywhere within the body. Extending a Neumann-type solution strategy, it is shown that the actuation stresses due to the distributed control eigenstrains must satisfy certain quasi-static equilibrium conditions, where auxiliary body-forces and auxiliary surface tractions are to be taken into account. The latter auxiliary loading can be directly computed from the imposed forces and from the desired displacement field to be tracked. Hence, despite the problem is a dynamic one, a straightforward computation of proper actuator distributions can be obtained in the framework of quasi-static equilibrium conditions. Necessary conditions for the functioning of this concept are presented. Particularly, it must be required that the intermediate configuration is infinitesimally superstable. Previous results of our group for the case of shape control and displacement tracking in linear elastic structures are included as special cases. The high potential of the solution is demonstrated via Finite Element computations for an irregularly shaped four-corner plate in a state of plain strain.