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최덕기,김창수,Choi, Deok-Kee,Kim, Chang-Soo 대한기계학회 1998 大韓機械學會論文集A Vol.22 No.1
This paper addresses a method which can be used for analyzing thermal stresses of a functionally graded material(FGM) using semi-analytical approach. FGM is a nonhomogeneous material whose composition is changed continuously from a metal surface to a ceramic surface. An infinite one dimensional FGM plate is considered. The temperature distribution in the FGM is obtained by approximate Green's function solution. To expedite the convergence of the solutions, alternative Green's function solution is derived and shows good agreement with results from finite difference method. Thermal stresses are calculated using temperature distribution of the plate.
분자 동역학 방식을 사용한 전역 최적화 기법에 관한 연구
최덕기,김재윤,Choi, Deok-Kee,Kim, Jae-Yoon 대한기계학회 1999 大韓機械學會論文集A Vol.23 No.7
This paper addresses a novel optimization technique based on molecular dynamics simulation which has been utilized for physical model simulation at various disciplines. In this study, objective functions are considered to be potential functions, which depict molecular interactions. Comparisons of typical optimization method such as the steepest descent and the present method for several test functions are made. The present method shows applicability and stability in finding a global optimum.
최덕기,김창수,Choi, Deok-Kee,Kim, Chang-Soo 대한기계학회 1997 大韓機械學會論文集A Vol.21 No.9
This paper addresses method which can be used for analyzing thermal stresses of a functionally graded material(FGM) using semi-analytical approach. FGM is a nonhomogeneous material whose composition changes continuously from a metal surface to a ceramic surface. An infinite one dimensional FGM plate is considered. The temperature distribution in the FGM is obtained by approximate Green's function solution. To expedite the convergence of the solutions, alternative Green's function solution is derived and shows good agreement with results from finite difference method. Thermal stresses are calculated using temperature distribution of the plate.
PC Cluster를 사용한 나노 재료 모사에 관한 연구
최덕기(Deok-Kee Choi),류한규(Han-Kyu Ryu) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.5
Not a few scientists have paid attention to application of molecular dynamics to chemistry, biology and physics. With recent popularity of nano technology, nano-scale analysis has become a major subject in various engineering fields. A underlying nano scale analysis is based on classical molecular theories representing molecular dynamics. Based on Newton's law of motions of particles, the movement of each particles is to be determined by numerical integrations. As the size of computation is closely related with the number of molecules, materials simulation takes up huge amount of computer resources so that it is not until recent days that the application of molecular dynamics to materials simulations draw some attention from many researchers. Thanks to high-performance computers, materials simulation via molecular dynamics looks promising. In this study, a PC cluster consisting of multiple commodity PCs is established and nano scale materials simulations are carried out. Micro-sized crack propagation inside a nano material is displayed by the simulation.
고변형률 변형하에서 재료 내부의 온도상승 계산을 위한 재료 모델링
최덕기(Deok-Kee Choi),유한규(Han-Kyu Ryu) 한국항공우주학회 2004 韓國航空宇宙學會誌 Vol.32 No.7
고속으로 비행하는 물체가 다른 물체와 충돌하는 경우에는 극히 짧은 시간에 커다란 변형이 일어나게 된다. 고변형률 변형(high-strain-rate deformation)에서는 소성변형이 일어나면서 상당한 열을 발생시키고 재료의 온도를 상승시킨다. 온도의 상승은 재료의 동적인 물성에 많은 영향을 미치게 되므로, 변형 시의 온도상승을 예측하는 것은 매우 중요하다. 변형시의 온도상승은 주로 전위( dislocation)의 움직임과 공공(vacancy)으로 인한 재료내의 저장에너지와 밀접한 관계를 갖게되므로, 저장 에너지의 양을 파악하는 것은 매우 중요하다. 고변형률 변형시 전위가 빠르게 움직이면서 평형상태에서의 경우보다 많은 과공공(excess vacancies)을 발생시키게 된다. 본 논문에서는 과공공을 포함하는 미시적 재료 모델을 구성하고 분자동역학(molecular dynamics, MD) 기법을 사용하여 면심입방격자(fcc) 구조를 가지는 재료(구리)에 대한 저장 에너지를 계산하였다. High velocity impacts are accompanied with large deformations, which generate a large amount of heat due to plastic works, resulting in a significant temperature rise of the material. Because the elevated temperature affects the dynamic properties of materials, it is important to predict the temperature rise during high-strain-rate deformations. Both existing vacancies and excess vacancies are credited to the stored energy, yet it is difficult to distinguish one from another in contribution to the stored energy using macroscopic level materials models. In this study, an atomistic material model for fcc materials such as copper is set up to calculate the stored energy using molecular dynamics (MD) simulations. It is concluded that excess vacancies play an important role for the stored energy during a high-strain-rate deformation.
다양한 변형률 에너지 함수를 사용한 전방 및 후방 십자인대의 모델링에 관한 연구
임준택(Lim Jun-taek),최덕기(Choi Deok-kee),김영은(Kim young-en) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.11
The mechanical study on the anterior and posterior cruciate ligament(ACL, PCL) is of importance because the recent increase of outdoor and indoor activities is directly related to causing sport injuries on the knee joints. Constitutive models for many biological tissues have been known as hyperelasticitic models. The elastic behavior of ACL and PCL may be described by the free energy function which accounts for the matrix and the collagen fibers. This paper addresses a comparison of different types of the free energy function to the existing results.
뇌 조직의 기계적 물성에 관한 젤라틴을 이용한 수치해석 및 실험적 연구
반용(Yong Bahn),최덕기(Deok-Kee Choi) 대한기계학회 2015 大韓機械學會論文集B Vol.39 No.2
생체 조직에 대한 물리적 특성은 생체공학의 주된 관심사다. 특히 뇌 조직과 같은 매우 무른 생체 조직의 특성은 아직까지 정확히 밝혀지지 않고 있는 실정이다. 이는 윤리적, 사회적인 문제로 실험이 매우 제한적이고 어렵기 때문이다. 하지만 의료 응용분야에서의 로봇 수술이 발달함에 따라 이런 매우 무른 조직에 대한 정확한 특성이 요구되어지고 있는 실정이다. 이에 본 논문에서는 뇌 조직과 유사한 거동을 보이는 젤라틴으로 시편을 제작하여 기존연구와 비교하고 유사한 거동을 보이는 시편 제작 조건을 찾아내고 이 조건으로 만들어진 시편을 이용하여 반복적인 실험을 실시하였다. 이렇게 얻어진 실험 데이터를 이용하여 초탄성 모델에 적용시켜 재료 상수들을 찾고 이를 FE 해석에 적용시켜 실험데이터와 비교하여 일치함을 보였다. The mechanical properties of living tissues have been major subjects of interest in biomechanics. In particular, the characteristics of very soft materials such as the brain have not been fully understood because experiments are often severely limited by ethical guidelines. There are increasing demands for studies on remote medical operations using robots. We conducted compression tests on brain-like specimens made of gelatin to find substitutes with the mechanical properties of brain tissues. Using a finite element analysis, we compared our experimental data with existing data on the brain in order to establish material models for brain tissues. We found that our substitute models for brain tissues effectively simulated their mechanical behaviors.