• 한국전산구조공학회논문집
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• 제30권4호
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• pp.275-282
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• 2017

본 논문에서는 변형에 의해 유발된 패턴변화(pattern transformation)에 기반하여 압축(compression)과 인장(tension) 하중 모두에서 음의 포아송 비(negative poisson's ratio)를 나타내는 다공성(porous) 구조를 제안한다. 기존에 개발된 원형 구멍을 이용한 구조는 연결선(ligament)의 회전 모멘트 부족으로 인해 인장 시 양의 포아송 비를 나타내는 한계점이 있었으며, 타원형 구멍을 이용한 구조는 응력집중 현상으로 인하여 내구성(durability)이 약한 문제점이 있었다. 이에 본 연구에서는 휘어진 연결선의 배열을 통하여 인장하중 하에서의 회전 모멘트를 증가시키는 동시에 응력집중 현상을 완화하고 변형에너지(strain energy)를 구조물 전반에서 고르게 흡수하도록 설계하였다. 이를 통해 10%의 공칭 변형률(nominal strain) 범위 내의 압축과 인장 모두에서 음의 포아송 비를 가지며, 기존 모델에 비하여 강성(stiffness)과 내구성이 개선된 구조를 개발하였다. 비선형 유한요소해석을 통하여 기존 타원형 구멍 모델과의 비교를 수행하였으며 제안된 모델이 구조의 강성과 내구성 측면에서 현저히 개선됨을 확인하였다.

• 한국소음진동공학회논문집
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• 제26권2호
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• pp.179-186
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• 2016

This paper presents a numerical method that can evaluate the effect of crack for the in-plane bending vibration of Timoshenko beam. The method is a transfer matrix method that the element transfer matrix is deduced from the element dynamic stiffness matrix. An edge crack is expressed as a rotational spring, and then is formulated as an independent transfer matrix. To demonstrate the accuracy of this theory, the results computed from the present are compared with those obtained from the commercial finite element analysis program. Based on these comparison results, a parametric study is performed to analyze the effects for the size and locations of crack.

• Structural Engineering and Mechanics
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• 제67권5호
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• pp.439-455
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• 2018

Within a probabilistic framework, this paper addresses the determination of the static structural response of beams and frames with partially restrained (semi-rigid) connections. The flexibility of the nodal connections is incorporated via an idealized linear-elastic behavior of the beam constraints through the use of rotational springs, which are here considered uncertain for taking into account the largely scattered results observed in experimental findings. The analysis is conducted via the Probabilistic Transformation Method, by modelling the spring stiffness terms (or equivalently, the fixity factors of the beam) as uniformly distributed random variables. The limit values of the Eurocode 3 fixity factors for steel semi-rigid connections are assumed. The exact probability density function of a few indicators of the structural response is derived and discussed in order to identify to what extent the uncertainty of the beam constraints affects the resulting beam response. Some design considerations arise which point out the paramount importance of probability-based approaches whenever a comprehensive experimental background regarding the stiffness of the beam connection is lacking, for example in steel frames with semi-rigid connections or in precast reinforced concrete framed structures. Indeed, it is demonstrated that resorting to deterministic approaches may lead to misleading (and in some cases non-conservative) outcomes from a design viewpoint.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2002년도 춘계학술대회 논문집
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• pp.82-86
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• 2002

The thermal characteristics of high-speed air spindle system with built-in motor are studied. Experiment and finite difference method analysis obtain temperature rise and temperature distribution of housing. For the analysis, air fluid film model is built and temperature rise and distribution in thermal steady state are computed for each rotational speed. Generally, it is said that the heat generation of air bearing is negligible. But the heat generation in air film by heat dissipation can not be negligible especially into high-speed region of the journal. In case that the heat generation of air spindle system is high, natural frequency of the spindle system becomes lower when the thermal state is in steady-state and it means the changes of air bearing stiffness due to the change of bearing clearance. It is shown that the temperature rise of air spindle system causes thermal expansion and induces the variation of bearing clearance. In consequence the stiffness of air bearing becomes smaller.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2005년도 춘계학술대회논문집
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• pp.221-226
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• 2005

In this paper, a dynamic model for the rotor shaft-coupling-blade system is developed. The blades are attached to a disk and driven by an electric motor shaft which is flexible in torsion. We assumed that the shaft torsional flexibility is lumped in the flexible coupling which is usually adopted in rotor systems. The Lagrangian approach with the small deformation theory for both blade-bending and shaft-torsional deformations is employed for developing the equation of the motion. The assumed modes method is used for estimating the blade transverse deflection. The numerical results highlight the effects of both structural damping of the system and the torsional stiffness of the flexible coupling to the dynamic response of the blade. The results showed strong coupling between the blade bending and shaft torsional vibrations in the form of inertial nonlinearif, stiffness hardening and softening.

• 한국소음진동공학회논문집
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• 제13권4호
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• pp.247-257
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• 2003

This paper presents an analytical method to Investigate the stability of a hydrodynamic journal bearing with rotating herringbone grooves. The dynamic coefficients of the hydrodynamic Journal bearing are calculated using the FEM and the perturbation method. The linear equations of motion can be represented as a parametrically excited system because the dynamic coefficients have time-varying components due to the rotating grooves, even in the steady state. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving Hill's infinite determinant of these algebraic equations. The validity of this research is proved by the comparison of the stability chart with the time response of the whirl radius obtained from the equations of motion. This research shows that the instability of the hydrodynamic journal bearing with rotating herringbone grooves increases with increasing eccentricity and with decreasing groove number, which play the major roles in increasing the average and variation of stiffness coefficients, respectively. It also shows that a high rotational speed is another source of instability by increasing the stiffness coefficients without changing the damping coefficients.

• 한국정밀공학회지
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• 제21권2호
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• pp.203-209
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• 2004

Compliant bi-stable mechanism allows two stable states within its operation range staying at one of the local minimum states of the potential energy. Energy storage characteristics of the bi-stable mechanism offer two distinct and repeatable stable states, which require no power input to maintain it at each stable state. This paper suggests an equivalent model of the chevron-type bi-stable microactuator using the equivalent spring stiffness in the rectilinear and the rotational directions. From this model the range of spring stiffness where the bi-stable mechanism can be operated is analyzed and compared with the results of the FEA (Finite Element Analysis) using ANSYS for the buckling analysis, both of which show a good agreement. Based on the analysis, a newly designed chevron-type bi-stable MEMS actuator using hinges is suggested for the latch-up operation. It is found that the experimental response characteristics of around 36V for the bi-stable actuation for the 60$mu extrm{m}$ stroke correspond very well to the results of the equivalent model analysis after the change in cross-sectional area by the fabrication process is taken into account. Together with the resonance frequency experiment where 1760Hz is measured, it is shown that the chevron-type bi-stable MEMS actuator using hinges is applicable to the optical switch as an actuator.

• 한국강구조학회 논문집
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• 제11권4호통권41호
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• pp.385-395
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• 1999

철골구조물에 각형강관기둥과 H형강보가 많이 사용되는 추세에 있다. 각형강관기둥과 H형강보의 접합부는 그 회전강성이 약한 것으로 알려져 있다. 그러한 약점을 보완하기 위해서 콘크리트 충전된 각형강관과 H형강보 접합부에 대한 많은 고안이 이루어지고 있다. 그런데 이렇게 고안된 모든 접합부 모델에 대해서 실험을 행할 수는 없으므로 수치해석 모델링과 수치해석에 의해 그 강도를 규명해야 한다. 본 논문에서는 유한요소 모델링기법을 연구하고 접합부의 강성을 좌우하는 여러요소 즉 콘크리트 강도, 각형강관두께, 축력의 크기 및 편심위치 등을 변화시켜 접합부 강성변화에 어떤 영향을 미치는지 살펴보았다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2009년도 춘계학술대회 논문집
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• pp.266-271
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• 2009

A modeling method for the modal analysis of a multi-packet blade system having a crack undergoing rotational motion is presented in this paper. Each blade is assumed as a slender cantilever beam. The stiffness coupling effects between blades due to the flexibilities of the disc and the shroud are modeled with discrete springs. Hybrid deformation variables are employed to derive the equations of motion. The flexibility due to crack, which is assumed to be open during the vibration, is calculated basing on a fracture mechanics theory. To obtain more general information, the equations of motion are transformed into dimensionless forms in which dimensionless parameters are identified. The effects of the dimensionless parameters related to the angular speed, the depth and location of a crack on the modal characteristics of the system are investigated with some numerical examples.

• 한국정밀공학회지
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• 제24권2호
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• pp.127-133
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• 2007

The spindle system with a built-in motor can be used to simplify the structure of machine tools, to improve the machining flexibility of machine tools, and to perform the high speed machining. To improve the competition power of price to quality, spindle design is very important. Because it possesses over 10 percent of machine tool's price. The latest machine tools have rotational frequency and excellent about might and precision cutting. So it requires static and dynamic strength in the load aspect. In conclusion, the deformation of the spindle end have to extremely small displacement in static and dynamic load. In this study, On the assumption that the bearings that are supporting 24,000rpm high-speed spindle are selected in the most optimum condition, the natural frequency and deformation of the spindle end is obtained by FEM mode analysis. The Taguchi Method was used to draw optimized condition of bearing position and it's stiffness.