• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.5
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• pp.273-278
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• 2019

In this study, a new simple method for the estimation of hyperelastic material properties by indentation tests is proposed. Among hyperelastic material models, the Yeoh model with three material properties ($C_{10}$, $C_{20}$, $C_{30}$) is adopted to describe the strain energy density in terms of strain invariants. Finite element simulations of the spherical indentation of hyperelastic materials of the Yeoh model with different material properties are performed to establish a database of indentation force-displacement curves. The indentation force-displacement curves are fitted by cubic polynomials, which are approximated as a product of third-order polynomials of ($C_{10}$, $C_{20}$, $C_{30}$). A regression analysis is conducted to determine the coefficients of the equations for the indentation force-displacement curve approximations. A regression equation is used to estimate the hyperelastic material properties. The present method is verified by comparing the estimated material properties with true values.

• Journal of the Korean Society for Railway
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• v.12 no.3
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• pp.396-404
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• 2009

With developing the next generation high speed railway, there need to be plans to make sure of running safety though researchs on the crack and break of rail by rolling contact fatigue. Therefore, this study performed the parametric analysis effecting on the fatigue life of rail using simplified equations. It analyzed the internal stress of rail according to the track quality, train velocity, wheel radius, track stiffness, sleeper space, wheel load. For the more, via the finite element method, it analyzed shear force on the rail head which could be changed by the early length of crack, angle of crack and temperature. As a result, this study continued the main parameter effecting on the fatigue life of rail.

• 한국방재학회:학술대회논문집
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• 2011.02a
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• pp.32-32
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• 2011

최근 국내외에서 친환경건축물에 관한 관심이 매우 높아짐으로 인해 콘크리트의 물량을 절감하여 이산화탄소량을 줄이는 중공슬래브는 다양한 형태로 세계적으로 개발이 되고 있는 추세이다. 특히 이방향 중공슬래브는 환경적인 측면에서 이방향 중공슬래브는 중공부 생성에 재생플라스틱을 활용하여 폐자원을 재사용하고, 콘크리트와 철근의 사용량 절감에 따른 화석에너지 및 이산화탄소 발생량을 감소한다는 장점이 있다. 또한 시스템 측면에서 이방향 중공슬래브는 기존의 철근콘크리트 플랫플레이트 바닥구조 시스템의 자중을 절감하여 구조체를 경량화 시키고, 이에 따라 장스팬 구현이 가능하며, 단열효과가 뛰어나다. 이와 같이 이방향 중공슬래브는 장점이 많지만 플랫플레이트 슬래브의 취약점인 뚫림전단 파괴에 주의해야 한다. 이에 본 연구에서는 선행으로 실시된 이방향 중공슬래브-기둥 접합부 뚫림전단 성능평가 실험을 바탕으로 하여 경량체가 이방향 중공슬래브-기둥 접합부 뚫림전단 성능에 미치는 영향을 살펴보기 위해 범용 유한요소해석 프로그램인 ABAQUS를 사용하여 경량체량 및 위치를 주요변수로 한 해석적인 변화를 검토하였다. 본 연구를 통해 경량체가 삽입된 이방향 중공슬래브의 뚫림전단 성능에 대해, 해석결과 경량체 량과 위치에 따라 최대 뚫림전단강도는 기준 실험체에 비해 74.3%, 73%의 강도저하를 나타내는 것으로 알 수 있었다. 이는 실험상의 강도저하 값인 84.1%, 56.4%와 다소 차이가 있으며, 해석에서 중공부 주위의 응력집중 현상이 제대로 반영되지 않은 것으로 판단된다. 또한 이방향 슬래브에 경량체를 삽입 할 경우 경량체가 시작하는 부분에서 응력이 급격히 감소하는 현상이 나타났으며, 이러한 급격한 응력감소는 기둥 주위 위험단면의 변화를 가져오는 것으로 추정된다. 즉, 위험단면의 변화는 기둥으로부터 경량체 사이의 거리에 따라 달라지며, 위험단면 내의 콘크리트 단면 손실은 뚫림전단 강도를 감소시킨다. 본 연구에서는 이방향 중공슬래브의 뚫림전단강도를 산정할 수 있는 근사식을 제안하였으며, 보다 정확한 이방향 중공슬래브의 뚫림전단강도의 산정식을 위해서는 위험단면의 변화와 콘크리트 단면손실로 인한 전단강도 저하의 관계에 대한 추가적인 연구가 필요하다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.4
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• pp.336-345
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• 2012

In crack growth life, uncertainties are caused by variance of geometry, applied loads and material properties. Therefore, the reliability estimation for these uncertainties is required to keep the robustness of calculated life. The stress intensity factors are the most important variable in crack growth life calculation, but its equation is hard to know for complex geometry, therefore they are processed by the finite element analysis which takes long time. In this paper, the response surface is considered to increase efficiency of the reliability analysis for crack growth life of a turbine wheel. The approximation model of the stress intensity factors is obtained by the regression analysis for FEA data and the response surface of crack growth life is generated for selected factors. The reliability analysis is operated by the Monte Carlo Simulation for the response surface. The results indicate that the response surface could reduce computations that need for reliability analysis for the turbine wheel, which is hard to derive stress intensity factor equation, successfully.

• The Korean Journal of Rheology
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• v.11 no.1
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• pp.18-27
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• 1999

Predicting the deformation behaviors of sheets in thermoforming processes has been a daunting challenge due to the strong nonlinearities arising from very large deformations, mold-polymer contact condition and hyperelasticity constitutive equations. Nonlinear numerical analysis is always required to face this challenge especially for realistic processing conditions. In this study a 3-D algorithm and the membrane approximation are developed for thermoforming processes. The constitutive equation is expressed in terms of the 2nd Piola-Kirchhoff stress tensor and the Cauchy-Green deformation tensor. The 2-term Mooney-Rivlin model is used for the material model equation. The algorithm is established by the finite element formulation employing the total Lagrangian coordinate. The deformation behavior and the stress distribution results of 3-D algorithm with various point boundary conditions are compared to those of the membrane approximation algorithm. Also, the slip boundary condition and the no-slip boundary condition are applied for the systems that have molds. Finally, the effect of sheet temperatures on the final thickness distribution is investigated for the ABS material.

• Journal of the Society of Naval Architects of Korea
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• v.39 no.2
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• pp.52-60
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• 2002

Line heating is a forming process which makes the curved surface with the residual strain created by applying heat source of high temperature to steel plate. in order to control the residual strain, it is necessary to understand not only conductive heat transfer between heat source and steel plate, but also temperature distribution of steel plate. In this paper we attempted to analyze is temperature distribution of steel plate by simplifying a line heating process to collision-effusive flux of high temperature and high velocity, and conductive heat transfer phenomenon. To analyze this, combustion in the torch is simplified to collision effusive phenomenon before analyzing turbulent heat flux. The distribution of temperature field between the torch and steel plate is computed through turbulent heat flux analysis, and the convective heat transfer coefficient between effusive flux and steel plate is calculated using approximate empirical Nusselt formula. The velocity of heat flux into steel plate is computed using the temperature distribution and convective heat transfer coefficient, and temperature field in the steel plate is obtained through conductive heat transfer analysis in which the traction is induced by velocity of heat flux. In this study, Finite Element Method is used to accomplish turbulent heat flux analysis and conductive heat transfer analysis. FEA results are compared with empirical data to verify results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.5
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• pp.499-505
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• 2015

To properly design and assess a piezoelectric vibration energy harvester, it is necessary to consider the application of an efficiency measure of energy conversion. The energy conversion efficiency is defined in this work as the ratio of the electrical output power to the mechanical input power for a piezoelectric vibration energy harvester with an impedance-matched load resistor. While previous research works employed the electrical output power for approximate impedance-matched load resistance, this work derives an efficiency measure considering optimally matched resistance. The modified efficiency measure is validated by comparing it with finite element analysis results for piezoelectric vibration energy harvesters with three different values of the electro-mechanical coupling coefficient. New findings on the characteristics of energy conversion and conversion efficiency are also provided for the two different impedance matching methods.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.12
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• pp.8654-8664
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• 2015

Beam-type structures with non-uniform cross sections are widely used in mechanical, architectural, and civil engineering fields. This paper deals with dynamic characteristics and vibration problems. Governing equations are first derived by using local coordinates. Their solutions are then assumed by using Galerkin's mode summation method. Bisection method is also applied in solving the determinant of the matrix which can provide natural frequencies. Whereas finite element methods adopt admissible functions satisfying only geometric boundary condition, in this study we apply Galerkin's mode summation method which uses eigen-functions satisfying both governing equations and boundary conditions. Modal analysis and experimental tests are finally performed using simply-supported beams with four different non-uniform cross-sections. Our analytical results then show good agreement with experimental ones.

• Journal of Korea Water Resources Association
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• v.31 no.5
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• pp.553-564
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• 1998

For the proper design and management of underground storage cavern, groundwater flow around cavern should be analyzed. Since this flow is influenced by spatial nonuniformity of hudraulic conductivity, the two-dimensional finite element flow model incorporating stochastic concepts was developed to analyze influences due to this nonuniformity. Monte Carlo technique was applied to obtain an approximate solution for two-dimensional, steady flow in a stochastically defined nonuniform medisu. For this purpose, the values of hydraulic conductivity were generated for each element with known mean and standard deviations. The uncertainty in model prediction depends on both the nonuniformity in hydraulic conductivity and the natures of the flow system such as water curtain and boundary condition. Therefore the uncertainties in predicted hydraulic head and gradient are the greatest where the mean hydraulic gradients are relatively large and far from the boundaries. Especially, we relate these uncertainties with well known gas tightness condition.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.6
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• pp.147-154
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• 2014

The cable damages of the bridge structures induce very important impact on the structural safety, which implies the close monitoring of the cable damage is required to secure sustained safety of the bridges. Most usual available maintenance techniques are based on the monitoring the change of the natural frequency of the structures by damages. However, existing method are based on vibration method to calculate lateral vibration and system identification can calculate the axial stiffness using sensitivity equation by trial error method. But the frequency study by the longitudinal movement need because of the sag effect in system identification. This study proposes a new method to investigate the damage magnitudes and status. The method improves the accuracies in the magnitudes and status of damages by adopting the natural frequency of longitudinal movement. The study results have been validated by comparing them with the approximate solution of FEM. Thus, the relationship of cable damage and frequency appear with relation that the severe damage has the little frequency. If we know the real frequency we can estimate the cable damage severity using this relationship. This method can be possible the efficient management of the cable damage.