• Steel and Composite Structures
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• v.45 no.4
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• pp.547-554
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• 2022

This paper studies the static stability of an axially graded column with the power-law gradient varying along the axial direction. For a nonhomogeneous column with one end linked to a rotational spring and loaded by a compressive force, respectively, an Euler problem is analyzed by solving a boundary value problem of an ordinary differential equation with varying coefficients. Buckling loads through the characteristic equation with the aid of the Bessel functions are exactly given. An alternative way to approximately determine buckling loads through the integral equation method is also presented. By comparing approximate buckling loads with the exact ones, the approximate solution is simple in form and enough accurate for varying power-law gradients. The influences of the gradient index and the rotational spring stiffness on the critical forces are elucidated. The critical force and mode shapes at buckling are presented in graph. The critical force given here may be used as a benchmark to check the accuracy and effectiveness of numerical solutions. The approximate solution provides a feasible approach to calculating the buckling loads and to assessing the loss of stability of columns in engineering.

• Advances in aircraft and spacecraft science
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• v.11 no.1
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• pp.55-81
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• 2024

This paper presents the exact solutions and closed forms for of nonlinear stability and vibration behaviors of straight and curved beams with nonlinear viscoelastic boundary conditions, for the first time. The mathematical formulations of the beam are expressed based on Euler-Bernoulli beam theory with the von Karman nonlinearity to include the mid-plane stretching. The classical boundary conditions are replaced by nonlinear viscoelastic boundary conditions on both sides, that are presented by three elements (i.e., linear spring, nonlinear spring, and nonlinear damper). The nonlinear integro-differential equation of buckling problem subjected to nonlinear nonhomogeneous boundary conditions is derived and exactly solved to compute nonlinear static response and critical buckling load. The vibration problem is converted to nonlinear eigenvalue problem and solved analytically to calculate the natural frequencies and to predict the corresponding mode shapes. Parametric studies are carried out to depict the effects of nonlinear boundary conditions and amplitude of initial curvature on nonlinear static response and vibration behaviors of curved beam. Numerical results show that the nonlinear boundary conditions have significant effects on the critical buckling load, nonlinear buckling response and natural frequencies of the curved beam. The proposed model can be exploited in analysis of macrosystem (airfoil, flappers and wings) and microsystem (MEMS, nanosensor and nanoactuators).

• Steel and Composite Structures
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• v.52 no.5
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• pp.557-569
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• 2024

Nonlinear internal modals interactions analysis of axially functionally graded nanorods is evaluated on the basis of nonlocal elasticity theory and Rayleigh beam model for the first time. Functionally graded materials can be determined as nonhomogeneous composites which are obtained by combining of two various materials in order to get a new ideal material. In this research, material properties of nanorods are supposed to be calmly varied along the axial direction. Hamilton's principle is used to derive the equations with consideration of Von-Kármán's geometrically nonlinearity. Harmonic Differential Quadrature (HDQ) and Multiple Scale (MS) solution techniques are used to derive an approximate-analytic solution to the linear and nonlinear free axial vibration problem of non-classical nanorods for clamped-clamped and clamped-free boundary conditions. A parametric study is carried out to indicate the effects of index of AFG, aspect ratio, mode number, internal resonances and nonlinear amplitude on nonlinear nonlocal frequencies of axially functionally graded nanorods. Also, the effects of nonlocal and nonlinear coefficients and AFG index on relationships of internal resonances have been investigated. The presented theatrical-semi analytical model has the ability to predict very suitable results for extracting the internal modal interactions in the AFG nanorod.

• Proceedings of the Korean Geotechical Society Conference
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• 1999.03a
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• pp.217-224
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• 1999

This Paper shows the results of a series of model tests on the behavior of single rigid Pile, which subjected to lateral load, in non-homogeneous Nak-Dong River sands, consisted of two layers, upper and lower layers. The purpose of the present paper is to investigate the effect of ratio of lower layer thickness to embedded pile length ratio of soil modules of upper to lower layer (E$\sub$h1//E$\sub$h2/) and pile head constraint condition on the characteristics of lateral behavior of single pile. These effects can be quantified only by the results of model tests. As a model test result, in non-homogeneous sand, it shows that the lateral behavior depends upon the ratio of soil modules of upper to lower layer more than other factors. And, in respect of deflection, it was found that the reduction ratio of deflection by pile head fixity is the value of 0.5 and 0.6 for E$\sub$h1//E$\sub$h2/=0.18 and E$\sub$h1//E$\sub$h2/=5.56, respectively. The critical thickness of lower layer on the change of deflection is about 25 - 50% of pile embedded length. Also, in respect of maximum bending moment it was found that the reduction ratio of maximum bending moment by pile head fixity is the value of 0.55 and 0.7 for E$\sub$h1//E$\sub$h2/=0.18 and E$\sub$h1//E$\sub$h2/=5.56, respectively.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.1
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• pp.7-14
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• 2006

The finite failure NHPP models proposed in the literature exhibit either constant, monotonic increasing or monotonic decreasing failure occurrence rates per fault. In this paper, we propose the Erlang reliability model, which can capture the increasing nature of the failure occurrence rate per fault. Equations to estimate the parameters of the Erlang finite failure NHPP model based on failure data collected in the form of inter-failure times are developed. For the sake of proposing shape parameter of the Erlang distribution, we used to the goodness-of-fit test of distribution. Data set, where the underlying failure process could not be adequately described by the existing models, which motivated the development of the Erlang model. Analysis of the failure data set which led us to the Erlang model, using arithmetic and Laplace trend tests, goodness-of-fit test, bias tests is presented.

• Spatial Information Research
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• v.11 no.3
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• pp.227-240
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• 2003

The primary focus of this study is to provide a general methodology which can be utilized to understand and analyze environmental issues such as long term ecosystem dynamics and land use/cover change by development of 2D dynamic landscape models and model-based simulation. Change processes in land cover and ecosystem function can be understood in terms of the spatial and temporal distribution of land cover resources. In development of a system to understand major processes of change and obtain predictive information, first of all, spatial heterogeneity is to be taken into account because landscape spatial pattern affects on land cover change and interaction between different land cover types. Therefore, the relationship between pattern and processes is to be included in the research. Landscape modeling requires different approach depending on the definition, assumption, and rules employed for mechanism behind the processes such as spatial event process, land degradation, deforestration, desertification, and change in an urban environment. The rule-based models are described in the paper for land cover change by natural fires. Finally, a case study is presented as an example using spatial modeling and simulation to study and synthesize patterns and processes at different scales ranging from fine-scale to global scale.

• Journal of the Korea Society of Computer and Information
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• v.10 no.6 s.38
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• pp.27-36
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• 2005

Finite failure NHPP models presented in the literature exhibit either constant, monotonic increasing or monotonic decreasing failure occurrence rates Per fault. This Paper Proposes reliability model using the generalized gamma distribution, which can capture the monotonic increasing(or monotonic decreasing) nature of the failure occurrence rate per fault. Equations to estimate the parameters of the generalized gamma finite failure NHPP model based on failure data collected in the form of interfailure times are developed. For the sake of proposing shape parameter of the generalized gamma distribution, used to the special pattern. Data set, where the underlying failure process could not be adequately described by the knowing models, which motivated the development of the gamma or Weibull model. Analysis of failure data set for the generalized gamma modell, using arithmetic and Laplace trend tests . goodness-of-fit test, bias tests is presented.

• Journal of the Korean GEO-environmental Society
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• v.2 no.3
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• pp.81-88
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• 2001

The behavior of porous medium is modeled by linear thermoporoelastic behavior, linear poroviscoelastic behavior, poroplastic behavior, and poroviscoplastic behavior, etc. The behavior has, in general, a complicated aspect which makes a mechanical description of the problem with time. Constitutive modeling for deformation behavior of porous medium with coupling effects is needed since there is interaction between the constituents in pores with a relative velocity to each other. In this work, it is explained 3-dimensional behavior depending on degree of saturation for porous medium composed of homogeneous, isotropic materials. It is obtained the governing equations based on continuum porous mechanics. In addition, it is developed constitutive model which can be understood of behavior for porous medium which can be understood, analysed behavior of porous medium. It can be accomplished exact analysis and prediction of behavior in porous medium. The behavior for porous medium is analysed exactly, and the prediction of deformation behavior is accomplished. Consequently, it will be basis to analyze 3-dimensional behavior in municipal solid waste landfill, and the practical using of porous medium ground which are composed of nonhomogeneous, anisotropic materials can be done widely.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1367-1370
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• 2005

Microcellular foam processing of polymers requires a nucleated cell density greater than $10^9\;cells/cm^3$ so that the fully grown cells are smaller than 10 mm. A microcellular foam can be developed by first saturating a polymer sample with a volatile blowing agent, followed by rapidly decreasing its solubility in the polymer. In general, the cellular structure of crystalline polymer foams is difficult to control, compared to that of amorphous polymer foams. Since the gas does not dissolved in the crystallites, the polymer/gas solution formed during the microcellular processing is nonuniform. Moreover, the bubble nucleation is nonhomogeneous because of the heterogeneous nature of the crystalline polymer. In this paper, the effects of the crystallinity and morphology of crystalline polymers on the microcellular foam processing and on reflectivity of products are investigated. First, polymer specimens with various morphology and amount of solved blowing agent were prepared by varying the saturation pressure, saturation time and foaming condition. Then, cell morphologies according to several conditions were studied. The specimens with differing gas amount of solved and morphologies were foamed and their cellular structures were compared. The experimental results of reflectivity are compared to raw specimen and another specimen of different experimental conditions. After the experiments, recognize whether how reflectivity changes according to solved gas amount. And the effect of cell density and cell size on reflectivity is studied

• Earthquakes and Structures
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• v.10 no.5
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• pp.1143-1179
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• 2016

Offshore wind turbines are considered as a fundamental part to develop substantial, alternative energy sources. In this highly flexible structures, monopiles are usually used as support foundations. Since the monopiles are large diameter (3.5 to 7 m) deep foundations, they result in extremely stiff short monopiles where the slenderness (length to diameter) may range between 5 and 10. Consequently, their elastic deformation patterns under lateral loading differ from those of small diameter monopiles usually employed for supporting structures in offshore oil and gas industry. For this reason, design recommendations (API and DNV) are not appropriate for designing foundations for offshore wind turbine structures as they have been established on the basis of full-scale load tests on long, slender and flexible piles. Furthermore, as these facilities are very sensitive to rotations and dynamic changes in the soil-pile system, the accurate prediction of monopile head displacement and rotation constitutes a design criterion of paramount importance. In this paper, the Fourier Series Aided Finite Element Method (FSAFEM) is employed for the determination of static impedance functions of monopiles for OWT subjected to horizontal force and/or to an overturning moment, where a non-homogeneous soil profile has been considered. On the basis of an extensive parametric study, and in order to address the problem of head stiffness of short monopiles, approximate analytical formulae are obtained for lateral stiffness $K_L$, rotational stiffness $K_R$ and cross coupling stiffness $K_{LR}$ for both rough and smooth interfaces. Theses expressions which depend only on the values of the monopile slenderness $L/D_p$ rather than the relative soil/monopile rigidity $E_p/E_s$ usually found in the offshore platforms designing codes (DNV code for example) have been incorporated in the expressions of the OWT natural frequency of four wind farm sites. Excellent agreement has been found between the computed and the measured natural frequencies.