• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.6
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• pp.577-582
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• 2016

In this study, a multiscale method for solving a thermoelasticity problem for interphase in the polymeric nanocomposites is developed. Molecular dynamics simulation and finite element analysis were numerically combined to describe the geometrical boundaries and the local mechanical response of the interfacial region where the polymer networks were highly interacted with the nanoparticle surface. Also, the micrmechanical thermoelasticity equations were applied to the obtained equivalent continuum unit to compute the growth of interphase thickness according to the size of nanoparticles, as well as the thermal phase transition behavior at a wide range of temperatures. Accordingly, the equivalent continuum model obtained from the multiscale analysis provides a meaningful description of the thermoelastic behavior of interphase as well as its nanoparticle size effect on thermoelasticity at both below and above the glass transition temperature.

• Korean Chemical Engineering Research
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• v.61 no.1
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• pp.116-122
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• 2023

We study the relationship of entropy with the properties of molecules and also with the macroscopic specifications of the system, i.e., component and phase. Understanding different viewpoints of classical mechanics and quantum mechanics for the indistinguishability of molecules belonging to the same component, we discuss a few thermodynamic systems in which the properties of molecules are to be consistent with the component as a macroscopic term of classifying the molecules. With a clear definition of thermodynamic microstate, the drawback of the Boltzmann statistics caused by the distinguishability of molecules is avoided, and the Gibbs paradox of entropy consequently disappears. Corresponding to the characteristics of fluid and solid phases, we investigated the effects of the indistinguishability and the symmetry number of molecules and the number of microstates realized in time on the partition function and the entropy. In particular, we show that crystalline solid can be regarded as a system which does not satisfy the ergodic hypothesis.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.12
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• pp.1933-1943
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• 2001

A micromechanical model is presented for superplasticity in which heterogeneous microstructures are coupled with deformation behavior. The effects of initial distributions of grain size, and their evolutions on the mechanical properties can be predicted by the model. Alternative stress rate models such as Jaumann rate and rotation incremental rate have been employed to analyze uniaxial loading and simple shear problems and the appropriate modeling was studied on the basis of hypoelasticity and elasto-viscoplasticity. The model has been implemented into finite element software so that full process simulation can be carried out. Tests have been conducted on Ti-6Al-4V alloy and the microstructural features such as grain size, distributions of grain size, and volume fraction of each phase were examined for the materials that were tested at different strain rates. The experimentally observed stress-strain behavior on a range of initial grain size distributions has been shown to be correctly predicted. In addition, the effect of volume fraction of the phases and concurrent grain growth were analyzed. The dependence of failure strain on strain rate has been explained in terms of the change in mechanism of grain growth that occurs with changing strain rate.

• Journal of the Korean Geotechnical Society
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• v.23 no.3
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• pp.77-88
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• 2007

A micromechanics-based model to simulate the elastic and elastic-plastic behavior of granular soils is developed. The model accounts for the fabric anisotropy represented by the statistical parameter of the spatial distribution of contact normals, the evolution of fabric anisotropy as a function of stress ratio, the continuous change of the co-ordination number relating to the void ratio, and the elastic and elastic-plastic microscopic contact stiffness. Using the experimental data for metallic materials, the elastic-plastic contact stiffness is derived as a power function of the normal contact force as well as the contact force initiating the yielding of contact bodies. To quantitatively assess microscopic model parameters, approximate solutions of cross-anisotropic elastic moduli are derived in terms of the micromechanical parameters.

• Magazine of the Korea Concrete Institute
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• v.10 no.2
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• pp.99-107
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• 1998

콘크리트는 일반적으로 수회시멘트풀과 골재로 이루어진 이상의 복합체이지만 미시적으로는 수화시멘트풀과 골재, 그리고 천이영역으로 이루어진 삼상의 복합체이다. 수화시멘트풀과 골재 사이에서 형성되는 천이영역은 국부적으로 공극률이 높으므로 콘크리트의 강성과 강도에 많은 영향을 끼친다. 본 논문에서는 이러한 천이영역의 특성을 고려하여 콘크리트의 탄성계수를 추정하기 위해 이원 삼중 내포물 모델을 제안하였다. 제안된 모델에 의한 탄성계수의 추정결과는 실험결과와 비교하여 잘 일치하였으며 제안된 모델은 실험적으로 구하기 힘든 천이영역의 특성을 구하는데 사용될 수 있다.

• Journal of the KSME
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• v.20 no.4
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• pp.288-295
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• 1980

본고에서는 복합조직을 가진 강의 파괴기동 및 인성에 미치는 미시 crack의 형성거동의 문제에 대하여, 재료개발이라고 하는 흡금학상의 분야에 있어서 미시조지학적관점으로부터 검토하였다. 이와 같이 문제를 한정시켜도 강도, 연성 및 인성치의 평가에 대하여 통일적인 견해를 얻는 것이 대단히 어려우며, 이것은 서언에서 설명한 것과 같이 각상간의 역학적간섭효과를 충분히 검토한 후에 이해되어지리라 생각한다. 마지막으로, 이들 강의 최근의 진보 및 비노파괴기동에 대한 상 세한 상술은 다음 기회로 미루었으며, 그리고 원교의 불충분한 점이 있다고 생각되나 복합조직을 가진 강의 파괴거동 및 인성에 대하여 관심을 가지고 있는 분들에게 조금이라도 도움이 된다면 다행으로 생각하는 바이다.

• Journal of the KSME
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• v.24 no.1
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• pp.35-42
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• 1984

오늘날 전자현미경의 급속한 발전 및 이에 의한 파면해석의 수많은 연구결과에 의해 파괴의 정성적인 연구로부터 정량적인 연구로 발전하게 되었고, 또한 일부의 파면모양은 균열 또는 결합선단의 파괴역학적(fracture mechanics)연구의 진전과 함께 파괴역학의 파라메타로서 정량적으로 정리할 수 있게 되어 실험적 증거를 제공하는 것으로 중요시되고 있다. 본고에서는 전자현미경에 의한 파면해석을 중심으로 기술하고, 이것이 파괴원인 규명의 미시조직학적 및 파괴역학적 검토에 어떻게 이용되고 있는가를 설명하였다.

• Composites Research
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• v.24 no.1
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• pp.10-16
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• 2011

Many tests are required to predict the fatigue life of composite laminates made of various materials and having different layup sequences. Aiming at reducing the number of tests, a methodology was presented in this paper to predict fatigue life of composite laminates based on fatigue life prediction of constituents, i.e. the fiber, matrix and interface, using micromechanics of failure. For matrix, the equivalent stress model which is generally used for isotropic materials was employed to take care of multi-axial fatigue loading. For fiber, a maximum stress model considering only stress along fiber direction was used. The critical plane model was introduced for the interface of the fiber and matrix, but fatigue life prediction was ignored for the interface since the interface fatigue strength was presumed high enough. The modified Goodman equation was utilized to take into account the mean stress effect. To check the validity of the theory, the fatigue life of three different GFRP laminates, UDT[$90^{\circ}2$], BX[${\pm}45^{\circ}$]S and TX[$0^{\circ}/{\pm}45^{\circ}$]S was examined experimentally. The comparison between predictions and test measurements showed good agreement.

• Composites Research
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• v.30 no.4
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• pp.247-253
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• 2017

In this study, the mechanical behavior and interface properties of boron nitride nanotube-poly(methyl methacrylate) nanocomposites are predicted using the molecular dynamics simulations and the double inclusion model. After modeling nanocomposite unit cell embedding single-walled nanotube and polymer, the stiffness matrix is determined from uniaxial tension and shear tests. Through the orientation average of the transversely isotropic stiffness matrix, the effective isotropic elastic constants of randomly dispersed microstructure of nanocomposites. Compared with the double inclusion model solution with a perfect interfacial condition, it is found that the interface between boron nitride nanotube and polymer matrix is weak in nature. To characterize the interphase surrounding the nanotube, the two step domain decomposition method incorporating a linear spring model at the interface is adopted. As a result, various combinations of the interfacial compliance and the interphase elastic constants are successfully determined from an inverse analysis.

• The magazine of the Korean Society for Advanced Composite Structures
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• v.4 no.3
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• pp.17-23
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• 2013