탄성체의 점탄성 및 이력 마찰 특성에 대한 멀티스케일 해석

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 기계항공공학부(멀티스케일 기계설계전공), 2019. 2. 조맹효.In this dissertation, a systematic multiscale simulation framework is proposed to capture the viscoelastic and frictional behavior of natural rubber materials. To overcome the limitation of time and length scale in a convention atomistic molecular dynamics simulation, a coarse-grained mapping scheme is employed. With an extended applicable scale in the coarse-grained domain, the relaxation characteristics of natural rubber system can be accessed and quantified. For the unvulcanized natural rubbers, the effect of molecular weight on the time-dependent relaxation behavior is examined. Moreover, micro-structural changes in the bulk natural rubber are consideredvulcanization and silica nanofillers, those are relevant design parameters when producing rubber compounds as engineering applications. The coarse-grained molecular dynamics simulation results demonstrate that the relaxation characteristics of rubber are substantially influenced by the chain length. With increasing molecular weight of system, the relaxation time is also increased, which is manifested by a slowed relaxation spectrum. This feature becomes more obvious with the formation of vulcanized networks between the rubber chains. However, the effect of silica particles is relatively marginal due to a weak interfacial interaction with rubber matrix. From the relaxation spectra, the frequency-dependent dynamic modulus is calculated using the Fourier transformation. Thus, the storage and loss modulus in frequency domain are computed and compared with experimental results. The frictional behavior of rubber exhibits a complex nature since various factors substantially alter the frictional feature, such as contact surface condition, roughness, velocity, material property, and temperature. Herein, the hysteresis friction characteristics attributed to the internal viscoelastic energy dissipation is examined when a rubber block slides on a hard rough substrate. Based on the viscoelastic properties obtained from the coarse-grained simulations, the hysteresis friction coefficients are predicted using an analytical approach depending on velocity, material property, normal pressure, and surface roughness. Moreover, the numerical simulation with a finite element model is conducted for the O-ring seal system to investigate the frictional response. With the present multiscale framework, the viscoelastic behavior of natural rubber system can be assessed considering various design parameters. Plus, the hysteresis frictional feature of rubber is predicted with the consideration of a range of velocity and roughness length scale. Furthermore, the frictional response of rubber in contact with moving part can be examined via the numerical simulation approach.본 논문에서는 고무의 점탄성 거동 및 이력 마찰 특성을 규명하기 위한 멀티스케일 해석 방법론을 제시한다. 기존의 분자동역학 전산모사는 고분자 재료의 물성을 예측하는데 큰 기여를 해왔지만, 시간과 공간 스케일의 한계로 이하여 메조스케일에 대한 특성을 다루는데 한계를 보여왔다. 본 연구에서는 여러 원자로 구성된 단위 구조를 가상의 입자로 치환하여 전체 시스템의 자유도를 줄임으로써 기존 분자동역학 전산모사의 시간/공간 스케일의 한계를 극복하는 축소 분자동역학 기법을 도입하였다. 이를 통해 확장된 시간/공간 스케일에서 고무의 시간에 따른 안정화 특성을 정량적으로 다룰 수 있으며 고무 재료의 점탄성 거동의 규명을 가능케 하였다. 이를 바탕으로 고무의 분자량과 가황 특성 및 실리카 강화입자 삽입에 따른 점탄성 거동의 변화를 도출하였다. 특히, 분자량 증가와 가황 과정은 고무의 시간에 따른 안정화 특성을 크게 변화시킴을 확인하였다. 반면, 실리카 입자에 대한 점탄성 거동의 변화는 크지 않다. 이는 실리카 입자와 고무의 약한 상호작용력에 기인한다. 축소 분자동역학 전산모사를 통해 도출된 시간에 따른 안정화 특성은 퓨리에 변환을 통해 주파수 영역에 대한 종탄성계수 및 손실계수를 제공한다. 탄성체의 마찰 특성은 마찰 표면 특성, 재료의 동적 물성, 속도, 온도 등에 영향을 받는다. 탄성체의 경우 이력 마찰 특성이 전체 마찰 특성을 좌우하게 되는데, 이는 마찰 과정에서 표면에서 발생하는 변형에 의한 에너지 소산과 관련이 있다. 본 연구에서는 축소 전산모사를 통해 도출된 동적 물성에 기반하여, 주어진 마찰 조건에서 이력 마찰 계수를 도출하였다. 이를 통해 탄성체의 마찰 특성을 종합적으로 규명할 수 있는 멀티스케일 해석 방법론을 정립하였다. 본 연구는 탄성체의 점탄성 특성 및 마찰 특성을 종합적으로 규명할 수 있는 해석 기반 멀티스케일 방법론을 제안함으로써 다양한 산업 분야에 적용될 수 있는 가능성을 제시한다. 표면과 맞닿아 작동하는 다양한 고분자 소재의 마찰 특성을 해석적 접근을 통해 순차적으로 규명할 수 있으므로 폭넓은 활용이 기대된다.1. Introduction 1 1.1. Rubber friction 1 1.2. Viscoelastic nature of polymer 3 1.3. Viscoelastic property with molecular dynamics simulation 4 1.4. Outline of the thesis 5 2. Coarse-grained modeling methodology 7 2.1. Overview of coarse-grained molecular dynamics 7 2.2. Iterative Boltzmann inversion 8 2.3. Coarse-grained modeling of rubber system 16 2.3.1. Coarse-grained potential for bulk natural rubber 16 2.3.2. Coarse-grained modeling of rubber vulcanization 27 2.4. Introduction of nanoparticle into rubber 37 3. Viscoelastic behavior of natural rubber 47 3.1. Coarse-grained molecular dynamics simulation setup 47 3.2. Characterization of viscoelastic property 50 3.3. Dynamic scaling between coarse-grained system and all-atomistic system 51 3.4. Viscoelastic property of natural rubber system 60 4. Hysteresis friction behavior of natural rubber 71 4.1. Characterization of surface roughness 71 4.2. Analytical model for hysteresis friction 77 4.3. Frictional behavior of natural rubber system 79 4.4. Numerical application: dynamic seal system 90 5. Conclusions and recommendations 94 Appendix A. Multiscale modeling of interphase in crosslinked epoxy nanocomposites 96 A.1. Overview 96 A.2. Interfacial nature of crosslinked epoxy with nanofiller 98 A.3. Characterization of interphase property with multiscale modeling 103 A.4. Summary of appendix A 107 Appendix B. Multiscale analysis of load transfer in crosslinked epoxy nanocomposites 109 B.1. Overview 109 B.2. Internal stress transfer in interfacial region 111 B.3. Multiscale modeling of load transfer characteristics 115 B.4. Summary of appendix B 119 References 120 Abstract 134Docto

The Relationship between Democracy Inclination and Unification Perception: A Recommendation to the Unification Policy

In these days South Korea has faced some difficulties in pursuing the unification policy effectively. The civil society groups divided into left and right to the political orientation. And this causes serious south-south conflicts to the unification policy. Additionally among the Koreans, the desires and wishes toward unification have diminishing sharply. There seems no main group who devotes eagerly or sincerely to the unification problem. At this moment, I have recalled the democratization process of Korea which was once regarded as impossible or unrealistic to achieve. I assumed that the democratization could be finally accomplished owing to the existence of extensive ordinary peoples who hoped and preferred democracy. If reviewing the recent Korean history, it might be concluded that democratization activated and progressed the unification circumstances. Only after the democratization, the Korean government set up more positive declarations and plans about the unification and even reached basic agreements between North and South from the end of 1980 to the early of 1990, For the analysis, using the national survey datum which were initiated by IPUS(Institute for Peace and Unifications Studies), I classified the people along the opinions to the democracy. First, I extracted the democracy inclination variable by factor analysis. Then, I grouped the people into three according to democratic preference score. I named the three groups, as strong democracy inclination group(SDIG), medium democracy inclination group(MDIG), and weak democracy inclination group(WDIG). Next, I examined the relationship between democratic inclination and unification perception. The main findings are as follows. First, in the unification perceptions, SDIG has more desire and wish to the unification comparing to MDIG and WDIG. SDIG has positive and future-oriented perspective to the unification. SDIG might become main supporter to carrying future unification policy. Second, to the perception of North Korea, though the SDIG has different perceptions in some area, SDIG often shows similar perceptions with other groups. This means that, in contrast to left and right demarcation, groups divided by democratic inclination might not initiate south-south conflicts to the unification policy. Third, SDIG strongly supports government unification policies which shares democratic values. But government drives a policy which is not in line with democratic values, SDIG not only withdraws support but blames the policy and government. So I suggest that government should pursue democratic unification policy to form the civil society governance in the realm of unification

關係資産 및 企業間 메커니즘이 提携成果에 미치는 影響 : 尖端技術産業에서 國際 및 國內 提携를 中心으로

Thesis (doctoral)--서울대학교 대학원 :경영학과 경영학전공,2004.Docto

Korean Perception to the Unification during 2007~2015: Focused on Generation Gap and Intra-Generation Differentiation

세대를 구분하여 한국인 통일의식의 특성을 알아보고자 하였다. 세대 구분은 청소년 시기를 중시하는 만하임의 주장을 따랐다. 청소년 시기 경험한 한국사회 중요한 사건과 남북관계를 고려해서, 세대를 체제경쟁세대, 민주민족통일세대, 탈냉전통일세대, 신자유주의통일세대로 구분하였다. 분석결과 세대별로 통일인식에 차이가 있는 것으로 밝혀졌다. 그리고 박근혜 대통령의 통일대박론이 세대별로 미치는 효과가 다르다는 점을 분석하였다. 한편, 통일인식은 세대효과 뿐만 아니라 정부의 통일정책과 해당 시기 남북관계에 영향을 받고 있기 때문에, 통일의 필요성에 대한 국민인식을 제고하려면 세대별로 적합한 통일정책을 수립하고 남북관계개선이 필요하다는 점을 제안하였다. 그러나 세대를 단일집단으로 간주하는 것은 현실을 지나치게 단순화한다. 따라서 통일인식의 세대내 분화에 대해서 추가적으로 분석하였다. 현실에서는 세대별 격차 현상과 세대내 분화 현상이 통일인식 속에 혼재되어 나타난다. 세대격차는 추상적인 통일인식에 명확하게 남아있으며, 구체적인 수준의 통일인식에서는 세대내 분화현상이 두드러지게 나타난다. The main purpose of this paper is to examine generation gap in the perception on the unification. Following Mannheims argument, I attempted to demarcate generation according to critical domestic and inter-Korean experiences in the adolescents period, As a result, the author classified generations as follows: system competition generation, democratic-nationalistic unification generation, post-Cold war unification generation, neoliberal unification generation. In the analysis, I found that there are gaps in the unification perception among the generations, And President Park Gunhaes unification policy, which was declared in the 2013, has had differential effects to each generation. And the subsequent analysis showed that the unification perception is influenced not only generational differences but also unification policy and inter-Korean relations. So, the author proposed, to enhance national unification perception, it is necessary that government unification policy should be tailored according to each generation and inter-Korean relations are to be improved. But to consider each generation as a unitary group leads over-simplification of the real world, Intra-generation differentiation should be examined. The analyses revealed that there are intra-generation differentiation according to the type of unification consciousness. In reality, generation gap and intra-generational differentiation are mixed in the Korean unification perception. The generation gap remains apparently in the abstract level of unification perception and the intro-generation differentiation can be found clearly in the concrete level

3 오메가 방법을 이용한 마이크로 채널 내부의 미세 유량 측정 센서 개발

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2011.8. 이준식.Docto

국내 지진에서 거시진도와 계기진도의 관계

학위논문 (석사)-- 서울대학교 대학원 : 공과대학 건설환경공학부, 2018. 2. 김호경.It is important to assess damage by region and to communicate the information with relevant organizations in earthquake response. Timely and reliable information minimize public confusion and reduce unnecessary effort. Seismic intensity is the representative information related to earthquake damage. It is classified into macroseismic intensity and instrumental seismic intensity. Korea is in different earthquake environment from overseas. Therefore, it is required to improve seismic intensity assessment system suitable for Korea. It should be improved in two aspects. One is related to improving the description of macroseismic intensity, and the other is related to developing the macroseismic-intensity- prediction model. The former is researched separately, and the latter was researched in this thesis. Correlation analysis between macroseismic intensity(observed MMI) and 4 kinds of instrumental intensities were performed. Housner spectral intensity showed the highest correlation coefficient and was used for regression analysis to develop MMI prediction model. Proposed model provides more improved information than before, despite its limited applicability(MMI < 6 and epicentral distance 140km). Most cases will be in that condition, because Korean earthquakes are usually small-to-moderate. Stronger earthquake data of overseas, which are in intra-plate similar to Korea, would help to supplement the model in the future research.1. Introduction 1 2. Seismic intensity 5 2.1 Introduction 5 2.2 Kinds of macroseismic intensity 7 2.3 Kinds of instrumental seismic intensity 10 2.3.1 PGA 11 2.3.2 Housner spectral intensity 11 2.3.3 Arias intensity 12 2.3.4 JMA seismic intensity 13 2.3.5 Direction problem 16 3. Correlation analysis 17 3.1 Data 17 3.2 Correlation analysis 19 4. Regression analysis 24 4.1 Proposing a MMI prediction model 24 4.2 Comparison to previous research 31 5. Applications to Pohang earthquake(2017) 33 6. Discussion 37 7. Conclusion 40 Appendix 41 1. Modified Mercalli Intensity of 1931 41 2. European Marcoseismic Scale 45 3. JMA seismic intensity scale 50 4. Status of KMAs observatory 52 References 54Maste