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Doh, Jaehyeok,Lee, Jongsoo Elsevier 2016 Computers & structures Vol.169 No.-
<P><B>Abstract</B></P> <P>Carbon nanotubes (CNTs) have distinct features in their remarkable mechanical, electrical, thermal, and chemical properties. However, material properties of CNTs can often not be validated due to experimental limitations. In this study, we developed finite element models of single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) based on molecular mechanics theory to evaluate mechanical properties such as Young’s modulus, ultimate strength, and strain in accordance with chirality. We performed tensile analyses with armchair/zigzag SWCNTs and armchair–zigzag/zigzag–armchair DWCNTs composed of nonlinear beam elements. We validated the proposed FE model of SWCNTs by comparing ultimate stress and strain with conventional approaches.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Description of nonlinear behaviors of CNTs based on Morse potential energy model. </LI> <LI> Evaluation of mechanical properties of SWCNTs/DWCNTs using molecular mechanics-based finite element method. </LI> <LI> Evaluation of mechanical properties of DWCNTs with the combination of Armchair–zigzag and zigzag–armchair. </LI> <LI> Validation of the proposed finite element model that is formulated with nonlinear beam elements. </LI> <LI> Comparison of Young’s modulus, ultimate stress and strain with other research results. </LI> </UL> </P>
Bayesian estimation of the lethargy coefficient for probabilistic fatigue life model
Doh, Jaehyeok,Lee, Jongsoo Society for Computational Design and Engineering 2018 Journal of computational design and engineering Vol.5 No.2
In this study, a model for probabilistic fatigue life that is based on the Zhurkov model is suggested using stochastically and statistically estimated lethargy coefficients. The fatigue life model was derived using the Zhurkov life model, and it was deterministically validated using real fatigue life data as a reference. For this process, firstly, a lethargy coefficient that is related to the failure of materials must be obtained with rupture time and stress from a quasi-static tensile test. These experiments are performed using HS40R steel. However, the lethargy coefficient has discrepancies due to the inherent uncertainty and the variation of material properties in the experiments. The Bayesian approach was employed for estimating the lethargy coefficient of the fatigue life model using the Markov Chain Monte Carlo (MCMC) sampling method and considering its uncertainties. Once the samples are obtained, one can proceed to the posterior predictive inference of the fatigue life. This life model was shown to be reasonable when compared with experimental fatigue life data. As a result, predicted fatigue life was observed to significantly decrease in accordance with increasing relative stress conditions.
Bayesian estimation of the lethargy coefficient for probabilistic fatigue life model
Jaehyeok Doh,Jongsoo Lee 한국CDE학회 2018 Journal of computational design and engineering Vol.5 No.2
In this study, a model for probabilistic fatigue life that is based on the Zhurkov model is suggested using stochastically and statistically estimated lethargy coefficients. The fatigue life model was derived using the Zhurkov life model, and it was deterministically validated using real fatigue life data as a reference. For this process, firstly, a lethargy coefficient that is related to the failure of materials must be obtained with rupture time and stress from a quasi-static tensile test. These experiments are performed using HS40R steel. However, the lethargy coefficient has discrepancies due to the inherent uncertainty and the variation of material properties in the experiments. The Bayesian approach was employed for esti-mating the lethargy coefficient of the fatigue life model using the Markov Chain Monte Carlo (MCMC) sampling method and considering its uncertainties. Once the samples are obtained, one can proceed to the posterior predictive inference of the fatigue life. This life model was shown to be reasonable when compared with experimental fatigue life data. As a result, predicted fatigue life was observed to signifi-cantly decrease in accordance with increasing relative stress conditions.
강도조건을 고려한 벽걸이 모니터 브라켓 암의 다중목적 근사최적설계
도재혁(Jaehyeok Doh),이종수(Jongsoo Lee) 대한기계학회 2015 大韓機械學會論文集A Vol.39 No.5
본 연구에서는 벽걸이 모니터 브라켓 암의 다중목적 근사최적설계를 수행하였다. 이를 위해 브라켓 암의 자유도를 고려하여 평면내의 회전 각도를 선정해 응력과 처짐량이 크게 발생하는 경우에 대한 최적화 문제를 정식화 하였다. 직교배열표와 반응표면법을 사용하여 평균 및 파라미터 분석을 통해 성능지수에 대한 설계변수 민감도를 확인하였으며, 중심합성계획법과 D-최적 계획법을 사용하여 목적함수와 제한조건함수에 대하여 반응표면 근사모델을 생성하고 R<SUP>2</SUP> 값을 통해 정확도를 평가하였다. 이를 비지배 분류 유전알고리즘에 적용하여 최적화를 수행하고 유한요소해석을 통해 검증하였다. 또한, 중심합성 계획법과 D-최적 계획법을 이용한 최적해를 비교 분석하였다. In this study, an approximate multi-objective optimization of a wall-mounted monitor bracket arm was performed. The rotation angle of the bracket arm was determined considering the inplane degree of freedom. We then formulated an optimization problem on maximum stress and deflection. Analyses of mean and design parameters were conducted for sensitivity regarding performance with orthogonal array and response surface method (RSM). RSM models of objective and constraint functions were generated using central composite (CCD) and D-optimal design. The accuracy of approximate models was evaluated through R<SUP>2</SUP> value. The obtained optimal solutions by non-dominant sorting genetic algorithm (NSGA-II) were validated through the finite element analysis and we compared the obtained optimal solution by CCD and D-optimal design.