• Smart Structures and Systems
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• 제22권3호
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• pp.291-302
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• 2018

This paper presents a flexibility based method for damage identification from static measurements in beam-type structures. The response of the beam at the Damaged State is decomposed into the response at the Reference State plus the response at an Incremental State, which represents the effect of damage. The damage is localized by detecting slope discontinuities in the deflection of the structure at the Incremental State. A denoising filtering technique is applied to reduce the effect of experimental noise. The extent of the damage is estimated through comparing the experimental flexural stiffness of the damaged cross-sections with the corresponding values provided by analytical models of cracked beams. The paper illustrates the method by showing a numerical example with two cracks and an experimental case study of a simply supported steel beam with one artificially introduced notch type crack at three damage levels. A Digital Image Correlation system was used to accurately measure the deflections of the beam at a dense measurement grid under a set of point loads. The results indicate that the method can successfully detect and quantify a small damage from the experimental data.

• 천문학회보
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• 제36권1호
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• pp.52.1-52.1
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• 2011

Black hole masses of Active Galactic Nuclei (AGN) are one of the most important parameters in AGN physics. Based on the virial assumption, black hole masses can be determined from the product of the width of the broad emission lines and the continuum/line luminosities. Using the Low Resolution Imaging Spectrometer(LRIS) at the Keck telescope, we obtained high quality spectra (S/N~100), covering 2300-5500A in the rest-frame, for a sample of 37 intermediate-luminosity AGN at z~0.4, in order to calibrate various black hole mass estimators based on the Mg II (2798A) and the Hbeta (4861A) emission lines. After subtracting continuum and complex FeII emission under Mg II and Hbeta, we fit the broad emission lines using high order Guass-Hermite models to best constrain the profile and the width of the emission lines. Combining the SDSS spectra covering Halpha emission line with the Keck spectra, we determine a set of 6 black hole masses for each object, based on the line width (MgII, Hbeta, and Halpha) and the luminosity (LMgII, LHbeta, LHalpha, L3000, L5100), and calibrate each black hole mass estimator. We will present uncertainties and limitations of each mass estimator.

• Structural Engineering and Mechanics
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• 제40권2호
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• pp.257-277
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• 2011

In this paper the geometrically nonlinear continuum plate finite element model, hitherto not reported in the literature, is developed using the total Lagrange formulation. With the layerwise displacement field of Reddy, nonlinear Green-Lagrange small strain large displacements relations (in the von Karman sense) and linear elastic orthotropic material properties for each lamina, the 3D elasticity equations are reduced to 2D problem and the nonlinear equilibrium integral form is obtained. By performing the linearization on nonlinear integral form and then the discretization on linearized integral form, tangent stiffness matrix is obtained with less manipulation and in more consistent form, compared to the one obtained using laminated element approach. Symmetric tangent stiffness matrixes, together with internal force vector are then utilized in Newton Raphson's method for the numerical solution of nonlinear incremental finite element equilibrium equations. Despite of its complex layer dependent numerical nature, the present model has no shear locking problems, compared to ESL (Equivalent Single Layer) models, or aspect ratio problems, as the 3D finite element may have when analyzing thin plate behavior. The originally coded MATLAB computer program for the finite element solution is used to verify the accuracy of the numerical model, by calculating nonlinear response of plates with different mechanical properties, which are isotropic, orthotropic and anisotropic (cross ply and angle ply), different plate thickness, different boundary conditions and different load direction (unloading/loading). The obtained results are compared with available results from the literature and the linear solutions from the author's previous papers.

• Steel and Composite Structures
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• 제47권5호
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• pp.569-585
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• 2023

This paper proposes an efficient approach for the structural topology optimization of bi-directional functionally graded structures by incorporating popular radial basis functions (RBFs) into an implicit level set (ILS) method. Compared to traditional element density-based methods, a level set (LS) description of material boundaries produces a smoother boundary description of the design. The paper develops RBF implicit modeling with multiquadric (MQ) splines, thin-plate spline (TPS), exponential spline (ES), and Gaussians (GS) to define the ILS function with high accuracy and smoothness. The optimization problem is formulated by considering RBF-based nodal densities as design variables and minimizing the compliance objective function. A LS-RBF optimization method is proposed to transform a Hamilton-Jacobi partial differential equation (PDE) into a system of coupled non-linear ordinary differential equations (ODEs) over the entire design domain using a collocation formulation of the method of lines design variables. The paper presents detailed mathematical expressions for BiDFG beams topology optimization with two different material models: continuum functionally graded (CFG) and mechanical functionally graded (MFG). Several numerical examples are presented to verify the method's efficiency, reliability, and success in accuracy, convergence speed, and insensitivity to initial designs in the topology optimization of two-dimensional (2D) structures. Overall, the paper presents a novel and efficient approach to topology optimization that can handle bi-directional functionally graded structures with complex geometries.

• Computers and Concrete
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• 제7권6호
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• pp.483-496
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• 2010

This work deals with the determination of crack openings in 2D reinforced concrete structures using the Finite Element Method with a smeared rotating crack model or an embedded crack model. In the smeared crack model, the strong discontinuity associated with the crack is spread throughout the finite element. As is well known, the continuity of the displacement field assumed for these models is incompatible with the actual discontinuity. However, this type of model has been used extensively due to the relative computational simplicity it provides by treating cracks in a continuum framework, as well as the reportedly good predictions of reinforced concrete members' structural behavior. On the other hand, by enriching the displacement field within each finite element crossed by the crack path, the embedded crack model is able to describe the effects of actual discontinuities (cracks). This paper presents a comparative study of the abilities of these 2D models in predicting the mechanical behavior of reinforced concrete structures. Structural responses are compared with experimental results from the literature, including crack patterns, crack openings and rebar stresses predicted by both models.

• 멤브레인
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• 제10권3호
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• pp.139-147
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• 2000

불소고분자(ETFE, ECTFE, PVDF)내로 방향족 유기용매(벤젠, 톨루엔, 클로로벤젠)의 비정상 흡수실험에서 non-Fickian (혹은 비이상적인) 확산이 관측되었다. 본 연구에서는 Fick's 법칙에 바탕을 둔 확산모델식(Crank, Long ＆ Richman, Berens ＆ Hopfenberg, Neogi, Li)을 이용하여 방향족 유기용매의 흡수실험에서 관측된 비이상적 흡수데이터론 분석하였다. 모델식의 매개변수 값은 실험데이터와 모델 예측 값의 차이를 최소화하는 least square 법을 이용하여 결정하였다. Fickian 확산으로부터 약간 벗어나는 ETFE 흡수데이터는 앞에서 언급한 모델식들을 이용하여 모두 만족할 만한 결과를 얻었다. 특히 Neogi 모델식은 ETFE-용매계의 고유확산계수(0.4~0.8$\times$$10^-5{cm}^2$/day) 및 평형 확산계수(0.13~0.31$\times$$10^-4{cm}^2$/day), 고분자구조의 이완 속도상수 값을 예측해주었다. PVDF의 전형적인 sigmoidal 흡수데이터에 대해서는 Crank 모델이 비교적 잘 적용되었으며, 초기 확산계수와 평형 확산계수간의 비($D_{\infty}/D_i$)는 80~200의 값을 나타내주었다. 가속적인 흡수데이터를 나타내주는 ECTFE의 경우에는 모든 모델식들의 예측 결과가 상당히 벗어났다. Fickian 확산으로부터 많이 벗어나는 비이상적인 흡수데이터로부터 확산 이동성질과 고분자구조의 이완현상에 대한 정보를 얻기 위해서는 열역학이나 continuum mechanics에 바탕을 둔 새로운 모델식을 적용해야 할 것으로 사료된다.

• 한국컴퓨터그래픽스학회논문지
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• 제12권1호
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• pp.21-25
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• 2006

This paper proposes a real-time simulation technique for thin shells undergoing large deformation. Thin shells are almost two-dimensional structures visually well approximated as surfaces, such as leaves, paper sheets, hats, aluminum cans. Unfortunately accurate simulation of these structures requires one of the most complex formulations in continuum mechanics, shell theory [4]. Moreover, there has not yet been any work reported to produce visually convincing animation of them while achieving real-time performance. Motivated by discrete shells [5] and modal warping [3], we formulate dynamics of thin shells using mass-spring models instead of finite element models, and then apply the modal warping technique to cope with large rotational deformation of thin shells. Experiments show that the proposed technique runs in real-time, and that it can simulate large bending and/or twisting deformations with acceptable realism.

• Computers and Concrete
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• 제5권4호
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• pp.401-416
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• 2008

The paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuum Strong Discontinuous Approach (CSDA). Mixture theory concepts are used to model reinforced concrete as a 3D composite material constituted of concrete with long fibers (rebars) bundles oriented in different directions embedded in it. The effects of the rebars are modeled by phenomenological constitutive models devised to reproduce the axial non-linear behavior, as well as the bond-slip and dowel action. The paper presents the constitutive models assumed for the components and the compatibility conditions chosen to constitute the composite. Numerical analyses of existing experimental reinforced concrete members are presented, illustrating the applicability of the proposed methodology.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.561-564
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• 2002

For the reduction of fuel consumption of high speed, the aerodynamic drag must be reduced. In early vehicle design process, it is very important to have information about aerodynamic characteristics of design models. In this phase CFD methods are usually used to predict the aerodynamic forces. But commercial programs using turbulence models cannot give a good agreement with experimental result and have also problems with convergence. PowerFLOW employs a new technology called DIGITAL PHYSICS, which provides a different approach to simulating fluids. DIGITAL PHYSICS uses a lattice-based approach (extended from lattice-gas and lattice-Boltzmann methods) where time, space and velocity are discrete. This discrete system represents the Wavier-Stokes continuum behavior without the numerical instability Issues of traditional CFD solvers, such as convergence. In this paper, aerodynamic performance of vehicles are simulated using PowerFLOW by Exa and results are compared with experimental wind tunnel data.

• Steel and Composite Structures
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• 제24권5호
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• pp.579-589
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• 2017

The objective of this work is to analyze post-buckling of functionally graded (FG) beams with porosity effect under compression load. Material properties of the beam change in the thickness direction according to power-law distributions with different porosity models. It is known that post-buckling problems are geometrically nonlinear problems. In the nonlinear kinematic model of the beam, total Lagrangian finite element model of two dimensional (2-D) continuum is used in conjunction with the Newton-Raphson method. In the study, the effects of material distribution, porosity parameters, compression loads on the post-buckling behavior of FG beams are investigated and discussed with porosity effects. Also, the effects of the different porosity models on the FG beams are investigated in post-buckling case.