• Structural Engineering and Mechanics
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• v.58 no.5
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• pp.931-947
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• 2016

The effects of nanotechnology and smartness on the buckling reduction of pipes are the main contributions of present work. For this ends, the pipe is simulated with classical piezoelectric polymeric cylindrical shell reinforced by armchair double walled boron nitride nanotubes (DWBNNTs), The structure is subjected to combined electro-thermo-mechanical loads. The surrounding elastic foundation is modeled with a novel model namely as orthotropic nonhomogeneous Pasternak medium. Using representative volume element (RVE) based on micromechanical modeling, mechanical, electrical and thermal characteristics of the equivalent composite are determined. Employing nonlinear strains-displacements and stress-strain relations as well as the charge equation for coupling of electrical and mechanical fields, the governing equations are derived based on Hamilton's principal. Based on differential quadrature method (DQM), the buckling load of pipe is calculated. The influences of electrical and thermal loads, geometrical parameters of shell, elastic foundation, orientation angle and volume percent of DWBNNTs in polymer are investigated on the buckling of pipe. Results showed that the generated ${\Phi}$ improved sensor and actuator applications in several process industries, because it increases the stability of structure. Furthermore, using nanotechnology in reinforcing the pipe, the buckling load of structure increases.

• Structural Engineering and Mechanics
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• v.81 no.4
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• pp.523-527
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• 2022

The objective of this study is to simulate the flow in pipes with various boundary conditions. Free-pressure fluid model, is used in the pipe based on Navier-Stokes equation. The models are solved by using the numerical method. A problem called "stability of pipes" is used in order to compare frequency and critical fluid velocity. When the initial conditions of problem satisfied the instability conditions, the free-pressure model could accurately predict discontinuities in the solution field. Employing nonlinear strains-displacements, stress-strain energy method the governing equations were derived using Hamilton's principal. Differential quadrature method (DQM) is used for obtaining the frequency and critical fluid velocity. The results of this paper are analyzed by hyperbolic numerical method. Results show that the level of numerical diffusion in the solution field and the range of well-posedness are two important criteria for selecting the two-fluid models. The solutions for predicting the flow variables is approximately equal to the two-pressure model 2. Therefore, the predicted pressure changes profile in the two-pressure model is more consistent with actual physics. Therefore, in numerical modeling of gas-liquid two-phase flows in the vertical pipe, the present model can be applied.

• Steel and Composite Structures
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• v.18 no.2
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• pp.443-465
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• 2015

In this paper, an efficient and simple trigonometric shear deformation theory is presented for thermal buckling analysis of functionally graded plates. It is assumed that the plate is in contact with elastic foundation during deformation. The theory accounts for sinusoidal distribution of transverse shear stress, and satisfies the free transverse shear stress conditions on the top and bottom surfaces of the plate without using shear correction factor. Unlike the conventional trigonometric shear deformation theory, the proposed sinusoidal shear deformation theory contains only four unknowns. It is assumed that the mechanical and thermal non-homogeneous properties of functionally graded plate vary smoothly by distribution of power law across the plate thickness. Using the non-linear strain-displacement relations, the equilibrium and stability equations of plates made of functionally graded materials are derived. The boundary conditions for the plate are assumed to be simply supported on all edges. The elastic foundation is modelled by two-parameters Pasternak model, which is obtained by adding a shear layer to the Winkler model. The effects of thermal loading types and variations of power of functionally graded material, aspect ratio, and thickness ratio on the critical buckling temperature of functionally graded plates are investigated and discussed.

• Journal of Ocean Engineering and Technology
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• v.21 no.1 s.74
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• pp.31-39
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• 2007

Recently, the nonlinear dynamic responses among waves, submarine pipeline and seabed have become a target of analyses for marine geotechnical and coastal engineers. Specifically, the velocity field around the submarine pipeline and the wave-induced responses of soil, such as stress and strain inside seabed, have been recognized as dominant factors in discussing the stability of submarine pipeline. The aim of this paper is to investigate nonlinear dynamic responses of soil in seabed, around submarine pipeline, under wave loading. In order to examine wave-induced soil responses, first, the calculation is conducted in the whole domain, including wave field and the seabed, using the VOF-FDM method. Then, velocities and pressures, which are obtained on the boundary between the wave field and the seabed, are used as the boundary condition to compute the wave-induced stress and strain inside seabed, using the poro-elastic FEM model, which is based on the approximation of the Biot's equations. Based on the numerical results, the characteristics of wave-induced soil responses around submarine pipeline are investigated, in detail, inrelation to relative separate distance of the submarine pipeline from seabed. Also, the velocity field around the submarine pipeline is discussed.

• Geophysics and Geophysical Exploration
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• v.19 no.3
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• pp.153-163
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• 2016

In order to prevent ground collapses by groundwater level drawdown, we need to understand the groundwater flow and also make an analytical approach to the cause of the collapses. In this study, we used the result of the soil lab tests to compare and review the suitability of the various interaction equations about the relation between volumetric water content and the dielectric constant. In addition, using GPR (Ground-Penetrating Radar), we reviewed the possibility of calculating an estimate of dielectric constant. Lastly, we applied seepage analysis and stress-strain analysis to the sandy ground given by ground excavation. In comparison with the previous result of the soil lab tests, we similarly predicted the suction of unsaturated soil from results of stress-strain analysis considered the seepage force for the unsaturated soil.

• Journal of the Korean Geotechnical Society
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• v.35 no.4
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• pp.15-26
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• 2019

The stability of the gravity quay wall against earthquakes is evaluated on the basis of the allowable displacement of the wall. To estimate the displacement caused by external forces, empirical equations based on the Newmark sliding block method or numerical analysis are widely used. In numerical analysis, it is possible to analyze precisely a complicated site and structure, but difficult to set the appropriate parameters and environments; there are limitations in obtaining reliable results, depending on one's level of expertise. The Newmark method, with only seismic motions, is widely used because it is simpler than numerical simulations when estimating permanent displacement. However, the empirical equations do not have any parameters for the response characteristics and sliding block of the structure, and sliding blocks being assumed as rigid bodies does not consider the nonlinear behavior of the soil and interaction with the structure. Therefore, in order to evaluate the seismic stability of the gravity quay wall, a newly-developed empirical equation is needed to overcome the above-mentioned limitations. In this study, numerical simulations are performed to analyze the response characteristics of the backfill of the structure, and to propose an optimal method of calculating the active area. For this purpose, finite element analyses were performed to analyze the response characteristics, and stress-strain relationships for various seismic motions. As a result, the response characteristics, sliding block, and failure surface of the backfill vary depending on the input seismic motions.

• Structural Engineering and Mechanics
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• v.65 no.4
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• pp.491-504
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• 2018

Because of sandwich structures with low weight and high stiffness have much usage in various industries such as civil and aerospace engineering, in this article, buckling and free vibration analyses of coupled micro composite sandwich plates are investigated based on sinusoidal shear deformation (SSDT) and most general strain gradient theories (MGSGT). It is assumed that the sandwich structure rested on an orthotropic elastic foundation and make of four composite face sheets with temperature-dependent material properties that they reinforced by carbon and boron nitride nanotubes and two flexible transversely orthotropic cores. Mathematical formulation is presented using Hamilton's principle and governing equations of motions are derived based on energy approach and applying variation method for simply supported edges under electro-magneto-thermo-mechanical, axial buckling and pre-stresses loadings. In order to predict the effects of various parameters such as material length scale parameter, length to width ratio, length to thickness ratio, thickness of face sheets to core thickness ratio, nanotubes volume fraction, pre-stress load and orthotropic elastic medium on the natural frequencies and critical buckling load of double-bonded micro composite sandwich plates. It is found that orthotropic elastic medium has a special role on the system stability and increasing Winkler and Pasternak constants lead to enhance the natural frequency and critical buckling load of micro plates, while decrease natural frequency and critical buckling load with increasing temperature changes. Also, it is showed that pre-stresses due to help the axial buckling load causes that delay the buckling phenomenon. Moreover, it is concluded that the sandwich structures with orthotropic cores have high stiffness, but because they are not economical, thus it is necessary the sandwich plates reinforce by carbon or boron nitride nanotubes specially, because these nanotubes have important thermal and mechanical properties in comparison of the other reinforcement.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.1
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• pp.82-96
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• 2019

A semi-submersible structure has been widely used for offshore drilling and production of oil and gas. The small water plane area makes the structure very sensitive to weight increase in terms of payload and stability. Therefore, it is necessary to lighten the substructure from the early design stage. This study aims at an optimization of hull structure based on a sophisticated yield and buckling strength in accordance with classification rules. An in-house strength assessment system is developed to automate the procedure such as a generation of buckling panels, a collection of required panel information, automatic buckling and yield check and so on. The developed system enables an automatic yield and buckling strength check of all panels composing the hull structure at each iteration of the optimization. Design variables are plate thickness and stiffener section profiles. In order to overcome the difficulty of large number of design variables and the computational burden of FE analysis, various methods are proposed. The steepest descent method is selected as the optimization algorithm for an efficient search. For a reduction of the number of design variables and a direct application to practical design, the stiffener section variable is determined by selecting one from a pre-defined standard library. Plate thickness is also discretized at 0.5t interval. The number of FE analysis is reduced by using equations to analytically estimating the stress changes in gradient calculation and line search steps. As an endeavor to robust optimization, the number of design variables to be simultaneously optimized is divided by grouping the scantling variables by the plane. A sequential optimization is performed group by group. As a verification example, a central column of a semi-submersible structure is optimized and compared with a conventional optimization of all design variables at once.

• Steel and Composite Structures
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• v.25 no.2
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• pp.157-175
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• 2017

The novelty of this work is the use of a new displacement field that includes undetermined integral terms for analyzing thermal buckling response of functionally graded (FG) sandwich plates. The proposed kinematic uses only four variables, which is even less than the first shear deformation theory (FSDT) and the conventional higher shear deformation theories (HSDTs). The theory considers a trigonometric variation of transverse shear stress and verifies the traction free boundary conditions without employing the shear correction factors. Material properties of the sandwich plate faces are considered to be graded in the thickness direction according to a simple power-law variation in terms of the volume fractions of the constituents. The core layer is still homogeneous and made of an isotropic material. The thermal loads are assumed as uniform, linear and non-linear temperature rises within the thickness direction. An energy based variational principle is employed to derive the governing equations as an eigenvalue problem. The validation of the present work is checked by comparing the obtained results the available ones in the literature. The influences of aspect and thickness ratios, material index, loading type, and sandwich plate type on the critical buckling are all discussed.

• Journal of the Korean Geotechnical Society
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• v.15 no.6
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• pp.45-55
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• 1999

Wave-induced response, liquefaction and stability of unsaturated seabed are studied. The unsaturated seabed is modeled as a fluid-filled polo-elastic medium. The coupled process of fluid flow and the deformation of soil skeleton is formulated in the framework of Biot's theory. The resulting governing equations are solved using a semi-analytical method to evaluate the stresses and pore water pressure of unsaturated and multi-layered seabed. The semi-analytical method can be applied to calculate a pore pressure and the stresses of in anisotropic inhomogeneous seabed. The results indicate that the degree of saturation influences mostly on the magnitudes of a pore pressure and the stresses of unsaturated and multi-layed seabed. Based on the pore pressure and stresses in seabed, the analysis on the possibilities of liquefaction and shear failure was performed. The results show that the maximum depth of shear failure occurrence is deeper than the maximum liquefaction depth.