• The Korean Journal of Rheology
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• v.8 no.3_4
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• pp.215-225
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• 1996

평판형의 열가소성 수지를 유리전이온도 이상으로 가열한 다음 압력을 가함으로써 원하는 형상의 제품을 성형하는 열성형공정은 대상 수지가 큰 변형을 일으킬 뿐만 아니라 비선형적 거동을 보이게 된다. 따라서 수지의 변형거동 예측과 최적성형조건의 설정에 많은 어려움과 시행착오를 거치게 되는 바, 열성형 공정의 최적화를 위한 연구의 일환으로 원형 평판위 수지를 대상으로 수지의 부풀림 거동과 이에 따른 두께 분포를 예측할수 있는유한요 소법의 수치모사 알고리듬을 개발하고자 하였다. Piola-Kirchhoff 응력 텐서와 Green 변형 텐서 및 lagrangian 변형 텐서를 사용하여 평판상의 응력-변형에 대한 비선형의 에너지 수 지식을 수립하고 Newton-Raphson 반복수렴법을 이용하여 근사적으로 해석하였으며 수지의 유변학적 구성방정식으로는 neo-Hookean 모델, Mooney-Rivlin 모델 및 Ogden 모델등의 초탄성 모델을 사용하여 그결과를 비교하였다. 수치모사에는 두께가 매우 얇기 때문에 두께 방향의 응력변화를 무시할수 있는 membrane 가정을 도입한 2차원적 해석과 두께 방향의 응력 변화를 고려하는 3차원적 해석을 모두 수행하고 그 차이를 비교하였으며 3차원적 해석 의 경우에는 penalty법을 이용하여 비 압축성을 만족하였다. 일차적으로 내압을 받는 두꺼 운 원통계에 대한 수치모사 해석을 수행하고 완전해와 비교함으로써 개발된 수치모사 알고 리듬의 열성형 공정에의 적용 타당성을 검증하였으며 이를 이용하여 원형 평판의 자유부풀 림거동을 예측한 결과 Treloar 등의 실험결과와 잘 부합함을 확인하였다. 또 간단한 형상의 금형이 있는 경우와 반지름 방향으로의 온도변화에 따른 수지의 변형거동을 해석함으로써 실제 열성형 공정에서 요구되고 있는 성형품의 두께 분포를 균일하게 하기 위한 방안을 제 시하였다.

• Journal of the Korean Geotechnical Society
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• v.22 no.1
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• pp.5-14
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• 2006

A simple and useful plane strain testing device was newly developed and its mechanical features were presented in this paper. The new testing device was designed to be capable of testing various stress paths expected under plane strain condition with only the conventional triaxial loading system. The applicability of the new testing device was systematically checked both by theoretical evaluation and by experiments. As a result, it was found that the new testing device has much wider range of application than the conventional plane strain testing devices.

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
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• 2000.09a
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• pp.95-102
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• 2000

선형분산을 가정한 Berkhoff(1972)의 완경사방정식은 단일주기파(monochromaticwave)에 대해 심해로부터 천해까지 수심에 제한 없이 파랑의 변형을 해석할 수 있으나 식의 유도과정 중 바닥이 완경사(｜∇h｜/kh≪1) 라는 가정을 도입함으로써, 바닥곡률항(∇$^2$h)과 바닥경사의 제곱항(｜∇h｜$^2$)으로 대표되는 고차수심효과를 무시하였다. (중략)

• Journal of the Korean Society of Hazard Mitigation
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• v.7 no.3
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• pp.19-31
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• 2007

Humidity and strain were estimated for understanding the relation between humidity change by self-desiccation and shrinkage in high-performance concrete with low water binder ratio and containing fly ash and blast furnace slag. Internal humidity change and shrinkage strain were about 10%, 10%, 7%, 11%, 11% and $320{\times}10^{-6}$, $270{\times}10^{-6}$, $231{\times}10^{-6}$, $371{\times}10^{-6}$, $350{\times}10^{-6}$ respectively on OPC30, O30F10, O30F20, O30G40, O30G50 and from the results, fly ash made humidity change and strain decrease but slag increase comparing with ordinary portland cement. Considering only relation internal humidity and shrinkage by self-desiccation, humidity change and shrinkage represented the strong linear relation regardless of mineral admixture. For specifying the relation on internal humidity change and autogenous shrinkage strain, shrinkage model was established which is driven by capillary pressure in pore water and surface energy in hydrates on the assumption of a single network and extended meniscus in pore system of concrete. This model and experimental results had a similar tendency so it would be concluded that the internal humidity change by self-desiccation in HPC originated in small pores less than 20nm, therefore controlling plan on autogenous shrinkage might be focused on surface tension of water and degree of saturation in small pore.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.12
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• pp.5199-5206
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• 2010

In this paper, we investigate the buckling analysis of laminated composite plates, using a improved assumed natural strain shell element. In order to overcome membrane and shear locking phenomena, the assumed natural strain method is used. The eigenvalues of the laminated composite plates are calculated by varying the width-thickness ratio and angle of fiber. To improve an shell element for buckling analysis, the new combination of sampling points for assumed natural strain method was applied and the refined first-order shear deformation theory which allows the shear deformation without shear correction factor. In order to validate the present solutions, the reference solutions are used and discussed. The results of laminated composite plates under the in-plane shear loading may be the benchmark test for the buckling analysis.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.6
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• pp.2284-2291
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• 2010

In this paper, we investigate the vibration analysis of plates, using an 8-node shell element that accounts for the transverse shear strains and rotary inertia. The forced vibration analysis of plates subjected to arbitrary loading is investigated. In order to overcome membrane and shear locking phenomena, the assumed natural strain method is used. To improve an 8-node shell element for forced vibration analysis, the new combination of sampling points for assumed natural strain method was applied. The refined first-order shear deformation theory based on Reissner-Mindlin theory which allows the shear deformation without shear correction factor and rotary inertia effect to be considered is adopted for development of 8-node assumed strain shell element. In order to validate the finite element numerical solutions, the reference solutions of plates are presented. Results of the present theory show good agreement with the reference solution. In addition the effect of damping is investigated on the forced vibration analysis of plates.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.15 no.1
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• pp.69-84
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• 2002

In this papers, an analytical model which can simulate the post-cracking behavior and tension stiffening effect in a reinforced concrete(RC) tension member is proposed. Unlike the classical approaches using the bond stress-slip relationship or the assumed bond stress distribution, the tension stiffening effect at post-cracking stage is quantified on the basis of polynomial strain distribution functions of steel and concrete, and its contribution is implemented into the reinforcing steel. The introduced model can be effectively used in constructing the stress-strain curve of concrete at post-cracking stage, and the loads carried by concrete and reinforcing steel along the member axis can be directly evaluated on the basis of the introduced model. In advance, the prediction of cracking loads and elongations of reinforced steel using the introduced model shows good agreement with results from the previous analytical studies and experimental data.

• Journal of the Korean Society for Railway
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• v.11 no.3
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• pp.257-262
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• 2008

Design and analysis of road subgrade system, which is exposed to repetitive loading condition, uses resilient modulus. The behavior of railway subgrade system will not be quite different from that of road system. Following this phenomenological feature of the subgrade system, this paper introduces the implementation of the resilient modulus based constitutive model on a commercial finite element software. The implementation of the resilient modulus models such as K-${\theta}$ and Uzan on a FE program has been conducted previously. These model assumes that the material state reaches to the nonlinear elastic condition and with further application of repetitive loads, the response of material is completed in elastic condition. According to the recent test results performed on cohesive subgrade soils, however, permanent deformation occurs with repetitive loads. With aids of previously suggested models the permanent deformation cannot be modeled. To overcome such limitation a plastic potential derived from the test results and simple failure criterion based constitutive model is developed. The comparison between the analysis and test results shows a good correlation.

• Journal of the Korea institute for structural maintenance and inspection
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• v.23 no.1
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• pp.54-62
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• 2019

In this paper, a stress-strain model for high-strength confined concrete is proposed using compressive fracture energy. In the compression test performed by author in Reference [6], an acrylic bar with strain gauges was embedded in the center of the specimen to measure the local strain distribution. It was found from the test that the local strain measurement by this acrylic rod is very effective. The local fracture zone length was defined based on the local strain distribution measured by the acrylic rod. Specifically, it was defined as the length where the local strain increases more than twice of the strain corresponding to maximum stress. In addition, the stress-strain relationship of confined concrete with compressive fracture energy is proposed on the assumption that the amount of energy absorbed by the compressive members subjected to the given lateral confining pressure is constant regardless of the aspect ratio and size. The proposed model predicts even results from other researchers accurately.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.13 no.2
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• pp.197-208
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• 2000

Choi et al./sup 1)/ presented the total Lagrangian formulation based upon the degenerated shell element. Geometrically correct formulation is developed by updating the direction of normal vectors and taking into account the second order rotation terms in the incremental displacement field. Assumed strain concept is adopted in order to overcome the shear locking phenomena and to eliminate the spurious zero energy mode. In this paper, for the ultimate strength analysis of stiffened shell structures considering effects of residual stresses, the return mapping algorithm based on the consistent elasto-plastic tangent modulus is applied to anisotropic shell structures. In addition, the load/displacement incremental scheme is adopted for non-linear F.E. analysis. Based on such methodology, the computer program is developed and numerical examples to demonstrate the accuracy and the effectiveness of the proposed shell element are presented and compared with the results in literatures.