• Interaction and multiscale mechanics
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• v.1 no.4
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• pp.411-422
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• 2008

Atomistic simulation of nanoindentation with spherical indenters was carried out to study dislocation structures, mean contact pressure, and nanohardness of Au and Al thin films. Slip vectors and atomic stresses were used to characterize the dislocation processes. Two different characteristics were found in the induced dislocation structures: wide-spread slip activities in Al, and confined and intact structures in Au. For both samples, the mean contact pressure varied significantly during the early stages of indentation but reached a steady value soon after the first apparent load drop. This indicates that the nanohardness of Al and Au is not affected by the indentation depth for spherical indenters, even at the atomistic scale.

• Nuclear Engineering and Technology
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• v.41 no.1
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• pp.11-20
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• 2009

Radiation effects on materials are inherently multiscale phenomena in view of the fact that various processes spanning a broad range of time and length scales are involved. A multiscale modeling approach to embrittlement of pressure vessel steels is presented here. The approach includes an investigation of the mechanisms of defect accumulation, microstructure evolution and the corresponding effects on mechanical properties. An understanding of these phenomena is required to predict the behavior of structural materials under irradiation. We used molecular dynamics (MD) simulations at an atomic scale to study the evolution of high-energy displacement cascade reactions. The MD simulations yield quantitative information on primary damage. Using a database of displacement cascades generated by the MD simulations, we can estimate the accumulation of defects over diffusional length and time scales by applying kinetic Monte Carlo simulations. The evolution of the local microstructure under irradiation is responsible for changes in the physical and mechanical properties of materials. Mechanical property changes in irradiated materials are modeled by dislocation dynamics simulations, which simulate a collective motion of dislocations that interact with the defects. In this paper, we present a multi scale modeling methodology that describes reactor pressure vessel embrittlement in a light water reactor environment.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.35 no.6
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• pp.325-331
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• 2022

Hydrogen embrittlement in metals has been a serious issue with regard to structural safety. In this study, molecular dynamics simulations revealed that the aggregation of hydrogen atoms at the crack tips suppresses the dislocation emission and thus results in cleavage fracture. A series of molecular dynamics simulations were performed considering factors such as the concentration of hydrogen atoms, loading rate, and diffusion coefficient. We investigated the conditions that minimize hydrogen embrittlement. The simulation results were consistent with the experimental results and used to quantify hydrogen embrittlement.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2004.11a
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• pp.215-220
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• 2004

Molecular Dynamics(MD) method was used to investigate the change of friction force due to interaction between dislocations and a grain boundary when a Ni tip was scratched on a Cu bicrystal. The substrate comprised a Cu bicrystal containing a vertical$\Sigma=5(210)$ grain boundary. The moving tip for scratching simulation was consisted of fixed Ni atoms emulating a rigid tip. The indentation depth was $3.6\AA$ and the scratching was performed along <110>direction in the first grain. As the scratching was continued, nucleation and propagation of dislocations were observed. In the early stage, the grain boundary played as a barrier to moving dislocations and interrupting further dislocation movement with no dislocation resulting in no propagation across the grain boundary. As the Ni tip approached the grain boundary, dislocations were nucleated at the grain boundary and propagated to the second grain. However, stick-slip phenomena that were observed on a single crystal scratching were not observed in the bicrystal. And, instead, irregular oscillation of friction force was observed during the scratching due to the presence of a grain boundary.

• Korean Journal of Materials Research
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• v.4 no.5
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• pp.590-599
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• 1994

A work on the brittle to ductile transition (BDT) in single crystal alumina has been performed to understand and assess the dynamics of dislocation mobility around crack tip of brittle material. The critical stress intensity factor and yield strengths were obtained from bending test using precracked specimens at elevated temperatures. It was found that the BDT temperature was dependent on strain rate and orientation of specimen : for (1120) fracture surface, $1034^{\circ}C$, $1150^{\circ}C$ for $4.2 \times 10^{-6}$, $4.2 \times 10^{-7}s^{-1}$ respectively. Under a 4 point bending test, the moving distance of dislocation generated near crack front in ductile range is determined by an etch pits method. The velocity of dislocation in sapphire obtained from the double etching method was applied to modelling study.

• Composites Research
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• v.22 no.3
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• pp.9-17
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• 2009

Nanoindentation behaviours on the films of softer Ag film/harder Cu substrate structure were studied by the molecular dynamics method. As a result, it was shown that the stiffness and hardness of films were strongly dependent on the thickness of films. The stiffness and hardness increased with the thickness of film within a critical range as an inverse Hall-Petch relation. The stiffness and hardness of Cu substrate with Ag film less than 5 nm were observed to be lower than those of bulk silver. In particular, the flower-like dislocation loop was created on the interface by the interaction between dislocation pile-up and misfit dislocation during the indentation of Ag film/Cu substrate with film thickness less than 4 nm, which seemed to be associated with the drop of load in the indentation load versus displacement curve.

• Korean System Dynamics Review
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• v.3 no.2
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• pp.5-27
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• 2002

This paper evaluate and compare effectiveness of urban growth management measures in Korea using system dynamics model. Simple urban dynamics model was used to compare urban growth management measures. Since the late 1960s, Korean government has been implementing various urban growth management measures without much success. In the 1960s, factories, universities, and public agencies were strongly encouraged to move out to local areas. During the 1970s, regulations on greenbelt area was adopted to prevent urban sprawl. Besides, regulations to prevent location of population inducing facilities, and promoting dislocation of those facilities were implemented simultaneously. During the 1990, regulations on total number of factories in the metropolitan area, development fees were adopted. These various method of urban management were compared. Simulation results shows that promoting decentralization of population, preventing population immigration, expanding greenbelt area are effectiveness ones compared to controlling total number of population inducing facilities, and preventing construction of new industries. Some implications of the findings were discussed.

• Korean Journal of Radiology
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• v.23 no.6
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• pp.674-687
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• 2022

Patellofemoral instability (PFI) is common in pediatric knee injuries. PFI results from loss of balance in the dynamic relationship of the patella in the femoral trochlear groove. Patellar lateral dislocation, which is at the extreme of the PFI, results from medial stabilizer injury and leads to the patella hitting the lateral femoral condyle. Multiple contributing factors to PFI have been described, including anatomical variants and altered biomechanics. Femoral condyle dysplasia is a major risk factor for PFI. Medial stabilizer injury contributes to PFI by creating an imbalance in dynamic vectors of the patella. Increased Q angle, femoral anteversion, and lateral insertion of the patellar tendon are additional contributing factors that affect dynamic vectors on the patella. An imbalance in the dynamics results in patellofemoral malalignment, which can be recognized by the presence of patella alta, patellar lateral tilt, and lateral subluxation. Dynamic cross-sectional images are useful for in vivo tracking of the patella in patients with PFI. Therapeutic approaches aim to restore normal patellofemoral dynamics and prevent persistent PFI. In this article, the imaging findings of PFI, including risk factors and characteristic findings of acute lateral patellar dislocation, are reviewed. Non-surgical and surgical approaches to PFI in pediatric patients are discussed.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.428-429
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• 2011

Recently, we performed ab initio total energy calculation and tight-binding molecular dynamics (TBMD) simulation to study structures and the reconstruction of native defects in graphene. In the previous study, we predicted by TBMD simulation that a double vacancy in graphene is reconstructed into a 555-777 composed of triple pentagons and triple heptagons [1]. The structural change from pentagon-octagon-pentagon (5-8-5) to 555-777 has been confirmed by recent experiments [2,3] and the detail of the reconstruction process is carefully studied by ab initio calculation. Pentagon-heptagon (5-7) pairs are also found to play an important role in the reconstruction of vacancy in graphene and single wall carbon nanotube [4]. In the TBMD simulation of graphene nanoribbon (GNR), we found the evaporation of carbon atoms from both the zigzag and armchair edges is preceded by the formation of heptagon rings, which serve as a gateway for carbon atoms to escape. In the simulation for a GNR armchair-zigzag-armchair junction, carbon atoms are evaporated row-by-row from the outermost row of the zigzag edge [5], which is in excellent agreement with recent experiments [2, 6]. We also present the recent results on the formation and development of dislocation in graphene. It is found that the coalescence of 5-7 pairs with vacancy defects develops dislocation in graphene and induces the separation of two 5-7 pairs. Our TBMD simulations also show that adatoms are ejected and evaporated from graphene surface due to large strain around 5-7 pairs. It is observed that an adatom wanders on the graphene surface and helps non-hexagonal rings change into stable hexagonal rings before its evaporation.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1784-1787
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• 2008

Molecular dynamics (MD) simulations are used to analyze behavior of copper nanowires under cyclic loading. The embedded atom method (EAM) potential is employed to represent atomic interaction. Cyclic load is applied in two ways (Forward Tension / Reverse Compression and Forward Compression / Reverse Tension). The results show that dislocations are piled up as a result of plastic deformation during alternate tensile and compressive loading. After cyclic loading with a change of direction, yield stress decreases in consequence of the effect by the dislocation pileups. On the other hand, under FC/RT cyclic load, phase transformation represent associated with mechanical twinning. And copper nanowire can return to almost former undeformed condition during tensile loading at 300K.