• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.4
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• pp.321-328
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• 2021

This paper derives numerically new buckling Knockdown factors for the lightweight design of the composite propellant tanks for space launch vehicles. A nonlinear finite element analysis code, ABAQUS, is used for the present postbuckling analysis of composite cylinders under compressive loads. Various thickness ratios (R/t) and slenderness ratios (L/R) are considered and Single Perturbation Load Approach is applied to represent the geometric initial imperfection of the composite cylinder. For the composite cylinder with thickness ratio of 500 and slenderness ratio of 2.04, the buckling Knockdown factor derived in this work is higher by 84.38% than NASA's previous buckling design criteria. Therefore, it is investigated that a lightweight design is possible when the present Knockdown factors are used for the design of composite propellant tanks. In addition, it is shown that global buckling loads and buckling Knockdown factors decrease as the thickness ratio or slenderness ratio of composite cylinders increases.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.3
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• pp.196-203
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• 2007

The postbuckling behavior and failure of composite cylinders subjected to external hydrostatic pressure are investigated by a finite element method and test. A nonlinear finite element program, ACOS, is used for the postbuckling progressive failure analysis of composite cylinders. A total of 5 carbon/epoxy composite cylinders were fabricated and tested to verify the finite element results. For comparison, analyses by MSC/NASTRAN and MSC/MARC are additionally conducted. Among the softwares, the finite element program, ACOS, predicts the buckling loads the best with about 11 to 26% deviation from experimental results except for one specimen. While the finite element analysis shows global buckling modes with 4 waves in hoop direction, in the experiments the local buckling appears first and results in the final failure without global buckling.

• Journal of the Korea Concrete Institute
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• v.16 no.1 s.79
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• pp.27-35
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• 2004

Most of concrete fatigue tests currently used are flexural tension or compression methods to investigate the tensile or compressive properties, respectively. However, the concrete pavement or concrete slab is actually subjected to a combined stress condition such as biaxial or triaxial. The split tension test may result in similar stress condition to biaxial stress condition. The purposes of this study were to evaluate the split tension fatigue test method for application in concrete. These were done by a finite element analysis and experimental series. The results were as follows: The optimum configuration of split tension fatigue test was a cylinder of 15cm in diameter and 7.5cm in thickness, which had a little different thickness compared to the KS standard cylinder of ${\phi}15{\times}30cm$. The concrete stress ratio of compressive against horizontal from FEA was 3.1, while that from theory was 3.0. The stress distributions of mortar and steel were almost similar at different thicknesses. The measured static split tensile strengths of concrete and mortar were quite similar at 30cm and 7.5cm thickness cylinders. The measured stress-strain relationship showed their consistency at all specimens regardless of thickness, and confirmed the results from FEA. As a results, the concrete split tension specimen, cylinder of 15cm in diameter and 7.5cm in thickness, could be used at fatigue test because of its accuracy, simplicity and convenience.

• Journal of the Korean Wood Science and Technology
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• v.38 no.3
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• pp.170-177
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• 2010

Finite element numerical analysis was conducted with using the knot data which has a strong influence on the prediction of capacity for the structural wood member. Wood is a orthotropic property unlike other structural materials, so orthotropic property was applied. Knot was modelled as a cylinder shape, cone shape, and cubic shape. Compressive test was carried out to investigate the failure types and to calculate ultimate strengths for the wood members. Numerical model which can reflect the member size, number of knot, location of knot, size of knot was created and analyzed. By the numerical analysis using the ultimate compressive strength, numerical stress distribution types of each specimen was compared to real failure types for the test specimen. Cylinder shape modelling might be most reasonable, according to the necessary time for the analysis, the difficulty of element meshing, and the similarity of stress transfer around knot. Moreover, according to the stress and deformation distribution for the numerical analysis, failures or cracks of real specimen were developed in the vicinity of stress concentrated section and most transformed section. Based on the those results, numerical analysis could be utilized as a useful method to analyze the performance of bending member and tensile member, if only orthotropic property and knot modelling were properly applied.

• Bulletin of the Society of Naval Architects of Korea
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• v.22 no.3
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• pp.9-18
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• 1985

The numerical calculation for solving boundary-value problem related to potential flows with a free surface is carried out by application of the localized finite element method. Only forced motion of 2-D body in infinitely deep fluid is considered, although this schemes is equally applicable to any first order time-harmonic problems of similar nature. The infinite domain of the fluid is separated into the inner flow field and the outer flow field with common inter-surface boundary. The finite element method is applied to obtain the solution in the inner flow field and the Green functions are utilized to represent the solution in the outer flow field. At the inter-surface boundary, the continuity of the value of potential and the normal derivative of the potential(i.e. matching condition) is conserved. The present method has better computational efficiency than the previous LFEM and the integral equation method of Frank. This enhanced computational efficiency is presumably due to the fact that the present method gives a symmetric coefficient matrix and requires less computational time in calculating the influence coefficient matrix of Green function than the integral equation method. And the irregular frequency desen't exist because the uniqueness of the solution is assured by the such that the exact free surface condition is satisfied on the boundary of the localized finite element region(i.e. inner region). As an example of the above method, the hydrodynamic forces for the circular cylinder and the rectangular cylinders are calculated. In the computed results, the small number of singularity distribution segments($3{\sim}6$) give good result relative to Ursell's and Vugts'.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.4
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• pp.449-455
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• 2011

Recently, international collaborative research which was organized at Bhabha Atomic Research Centre in India, was conducted to develop for pressure capacity and nonlinear behavior of PHWR 1/4 scale nuclear containment building between experimental test and numerical code. In this paper, a nonlinear finite element analysis was carried out in order to predict ultimate pressure capacity and nonlinear behavior of the 1/4 scale containment building. The 1/4 scale containment building is consisted of basemat, cylinder wall, dome and 4-buttress. For the finite element analysis, commercial program ABAQUS was used. Finite element models including concrete, rebar and tendon have been developed for assessment of ultimate pressure capacity and failure mode for nuclear containment building. From the analysis results, first crack of the concrete, the yielding of the rebar and ultimate capacity pressure occurred at $1.6P_d$(design pressure), $3.36P_d$ and $4.0P_d$, respectively.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.14 no.7
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• pp.3121-3126
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• 2013

The integrity evaluation of pipes in nuclear power plant are essential for the safety of reactor vessel, and integrity must be assured when flaws are found. Accurate stress intensity analyses and crack growth rate data of surface-cracked components are needed for reliable prediction of their fatigue life and fracture strengths. Fatigue design and life assessment are the essential technologies to design the structures such as pipe, industrial plant equipment and so on. The effect of crack spacing on stress intensity factor K values was studied using three-dimensional finite element method (FEM). For the case of cylinder under internal pressure, a significant increase in K values observed at the deepest point of the surface crack. Also, this paper describes the fatigue analysis for cracked structures submitted to bending loads.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.29 no.3
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• pp.225-232
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• 2018

This paper proposed the optimization method of the extremely low numerical-dispersion finite-difference time-domain (ELND-FDTD) method based on the H(2,4) scheme for wideband and extremely accurate electromagnetic properties of lossy material, which has a constant conductivity and relative permittivity. The optimized values of three variables are calculated for the minimum numerical dispersion errors of the proposed FDTD method. The excellent accuracy of the proposed method is verified by comparing the calculated results of three different FDTD methods and the analytical results of the two-dimensional dielectric cylinder scattering problem.

• Journal of the Society of Naval Architects of Korea
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• v.55 no.4
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• pp.321-329
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• 2018

Resonance phenomena occurs at large vertical pump which is operating to cool down the hot steam using sea water in the power plant. To avoid the resonance, the natural frequency needs to be isolated about 20% from motor operating speed. Yet, excessive vibration occurs especially at low tide. At first, natural frequency of the whole pump system and each part is calculated using ANSYS. As it is revealed in the previous journal papers that only circular pipe part is related to resonance, the FSI technique is applied for free vibration analysis. The natural frequency is reduced to 60% (compared to that) of the frequency measured in air as it is similar to other published results. And the frequency obtained by finite element analysis is almost same to that obtained from modal test. Based on the accurate finite element model and analysis, design change is tried to avoid the resonance by changing the thickness of pipe and base supporting plate. In stead of doing optimization process, design sensitivity is computed and used to find such designs to avoid resonance.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.6
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• pp.779-787
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• 2002

The cooling heat transfer by impinging water spray jets on a rotating roll with a relatively large diameter has been investigated under various experimental conditions with 3 different sizes of flat type nozzle. The local heat transfer coefficients were calculated by finite difference method using measured surface temperatures of the circular cylinder as boundary conditions. Results show that a peak value of the heat transfer coefficient is located at the center of sprayed area and there may be a secondary peak at the downstream. The average heat transfer coefficients on the sprayed area were found to be 10 to 22 ㎾/$m^2$$^{\circ}C$, and were not related to spraying pressure, but approximately linearly to flow rate of sprayed water. Also it is found that increasing the distance from roll to nozzle could improve the cooling efficiency by increasing the sprayed area.