• 대한기계학회논문집A
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• 제29권4호
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• pp.511-517
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• 2005

Fretting fatigue is often the root cause of the nucleation of cracks at attachments of structural components. Since fretting fatigue damage accumulation occurs over relatively small volumes, the subsurface cyclic plastic strain is expected to be rather non-uniformly distributed in polycrystalline materials. The scale of the cyclic plasticity and the damage process zones is often on the order of microstructure dimensions. Fretting damage analyses using cyclic crystal plasticity constitutive models have the potential to account for the influence of size, morphology, and crystallographic orientation of grains on fretting damage evolution. Two-dimensional plane strain simulations of fretting fatigue are performed using the cyclic properties of Ti-6Al-4V. The crystal plasticity simulations are compared to an initially isotropic $J_{2}$ theory with nonlinear kinematic hardening as well as to experiments. The influence of initially isotropic versus textured microstructure in the presence of crystallographic slip is studied.

• Korea-Australia Rheology Journal
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• 제13권3호
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• pp.141-148
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• 2001

In this research, experimental studies leave been performed on the hydrodynamic interaction between a spherical particle and a plane wall by measuring the force between the particle and wall. To approach the system as a resistance problem, a servo-driving system was set-up by assembling a microstepping motor, a ball screw and a linear motion guide for the particle motion. Glycerin and dilute solution of polyacrylamide in glycerin were used as Newtonian and non-Newtonian fluids, respectively. The polymer solution behaves like a Boger fluid when the concentration is 1,000 ppm or less. The experimental results were compared with the asymptotic solution of Stokes equation. The result shows that fluid inertia plays all important role in the particle-wall interaction in Newtonian fluid. This implies that the motion of two particles in suspension is not reversible even in Newtonian fluid. In non-Newtonian fluid, normal stress difference and viscoelasticity play important roles as expected. In the dilute solution weak shear thinning and the migration of polymer molecules in the inhomogeneous flow field also affect the physic of the problem.

• 소성∙가공
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• 제17권1호
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• pp.13-18
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• 2008

Mg and Mg alloys are promising materials for light weight high strength applications. In this paper, grain refinement of pure Mg using severe plastic deformation was tried to enhance the mechanical properties of the hard-to-deform metallic material. The microstructure and the mechanical properties of Mg processed by equal channel angular pressing(ECAP) at various processing temperatures were investigated experimentally. ECAP with channel angle of $90^{\circ}$ and corner angle of $0^{\circ}$ was successful at $300^{\circ}C$ without fracture of the samples during the processing. The hardness of the ECAP processed Mg decreased with increasing ECAP processing temperature. The effect of temperature on the hardness and microstructure of the ECAP processed Mg were explained by the dislocation glide in the basal plane and non-basal slip systems and by the dynamic recrystallization and recovery.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 1997년도 춘계 학술발표회 논문집 Proceedings of EESK Conference-Fall 1997
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• pp.15-20
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• 1997

Source parameters of the December 13, 1996 Yeongweol earthquake are estimated using the grid test technique. Thirty polarities of P waves recorded at KMA, KIGAM, KSRS and JAPAN stations are used for the event. The obtained fault plane solution shows predominantly strike-slip motion with small amount of thrust component. The orientation of the fault is 180$\pm$10$^{\circ}$in strike, 50$\pm$5$^{\circ}$in dip and 150$\pm$5$^{\circ}$in rake, or 292$\pm$3$^{\circ}$in strike, 65$\pm$5$^{\circ}$in dip and 30$\pm$10$^{\circ}$ in rake. These solutions are very similar to those of earthquakes occurred at Sagju, Pohang and offshore Gunsan. The compressional axis of stress field is trending from ENE to WSW, which is consistent with the previously defined typical regional tectonic stress orientation in and around Korean Peninsula.. From the result of this study and other source mechanisms around the Korean Peninsula, we are of opinion that tectonic stress around the Korean Peninsula may be more attributed to the collision of Indian plate with the Eurasian plate than subduction of Pacific and Philippine plates.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1998년도 하계학술대회 논문집 A
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• pp.107-109
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• 1998

This paper presents mechanical analysis of gas turbine generator (113MVA, $3{\phi}$, 2P, 0.9PF, F class, 3600rpm, 60Hz, 13.8kV, 4.72kA, Air-Cooling) field coil deformation. Rotor end coil deformation is only appeared on turbine end but collector end coil is normal. Expansion direction of end coil is tangential not axial. Deformation appears more severe at top turn. Retaining ling is expanded by centrifugal force of coil and itself. In case friction coefficient between coil top surface and retaining ring insulation inner surface is small, coil end length ${\ell}$ does not change. However, in case friction coefficient big condition, coil end is expanded ${\Delta}{\ell}$ due to start and stop. Deformation is assumed about 30mm by watching photograph inner surface of retaining ring is coated by Teflon at manufacturing condition. Usually Teflon coating insulation surface is small friction coefficient. It's value 0.08${\sim}$0.15. However it's value exceeds more than 0.297. Since top turn deformation appears. The distortion and subsequent failure have occurred because of the lack of a sufficient slip-plane between the top field coil conductors and the inside surface of the retaining ring insulation on the turbine end of the field-winding.

• 전산구조공학
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• 제10권2호
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• pp.161-169
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• 1997

차량의 동적특성, 즉 소음, 진동, 승차감 및 조종안정성능 등에 중요한 영향을 미치는 부품중에 하나로 차체의 중량을 지지하고 노면과 접촉하여 회전하면서 조타성을 지니는 타이어를 고려하지 않을 수 없다. 지금까지의 이러한 타이어 최적설계에 이용되어지고 있는 예측기법으로는 유한요소해석 방법이 널리 활용되어 지고 있으나, 이는 타이어에 공기압 주입 및 차량의 조종안정성능이 우수한 타이어 제품개발 및 개선을 위해 차량 주행시 타이어와 노면과의 접지면에 작용하는 힘과 모멘트를 예측할 수 있는 유한요소해석 적용기법을 개발하였으며,이러한 해석기법을 통한 결과와 실측치는 매우 유사한 관계를 지니고 있음을 알 수 있었다.

• 한국정밀공학회지
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• 제30권1호
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• pp.18-23
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• 2013

Omni-directional robot is a typical holonomic constraint robot that has three degrees of freedom movement in 2D plane. In this study, a new omni-directional robot whose wheels are arranged in radial directions was proposed to improve driving performance of the robot. Unlike a general omni-directional robot whose wheels were arranged in a circumferential direction, moments do not arises in the proposed robot when the robot travels in a straight line. To analyze driving performance, dynamic modeling of the omni-directional robot, which considers friction and slip, was carried out. By friction measurement experiments, the relationship between dynamic friction coefficient and relative velocity was derived. Dynamic friction coefficient according to the angle difference between robot travel direction and wheel rotation direction was also obtained. By applying these results to the dynamic model, driving performance of the robot was calculated. As a result, the proposed robot was 1.5 times faster than the general robot.

• 열처리공학회지
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• 제17권3호
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• pp.139-145
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• 2004

Equal channel angular pressing (ECAP) technique had been adapted to the Mg alloy (AZ31) for achieving effective grain refinement through severe deformation. The average grain size of $2.5{\mu}m$ could be obtained after 4 passes. The stability of the ECAPed structure at elevated temperatures was examined by annealing the ECAPed materials over a wide range of temperature between 473 and 748 K. The average activation energy, Q, for static grain growth of 1, 2 and 3 passes was 33.7 kJ/mole (=0.25QL, activation for lattice diffusion). The abnormally low Q value in the lower temperature range may indicate that grain growth occurs in the unrecrystallized microstructure where non-equilibrium grain boundaries containing a large number of extrinsic dislocations exist. The yield stresses of the ECAPed alloys decreased whereas the elongations increased after the ECAP process. These results should be related to the modification of texture for easier slip on basal plane.

• 한국지구과학회지
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• 제38권7호
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• pp.561-569
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• 2017

2015년 12월 22일 전북 익산시 북쪽 지역에서 발생한 익산지진($M_L=3.85$)의 본진 및 2개 여진에 대해 FOCMEC (FOCal MEChanism determination) 프로그램을 이용하여 단층면해를 구하였다. 본진의 단층면해는 북동-남서 또는 북서-남동 방향의 단층면을 가지는 역단층성 주향이동단층운동의 특성을 보이며, 이는 남한 내륙지진의 단층운동 특성과 거의 유사하다. 익산지진 전후 발생한 미소지진 이벤트를 검출하고자 2015년 12월 15일~2016년 01월 22일 진앙으로부터 반경 100 km 이내에 위치한 13개 관측소에 기록된 연속 지진파형 자료를 PQLII 프로그램(PASSCAL, 2017)으로 분석하고, 19개 지진의 진원지를 새로 결정하였다. 미소지진의 진앙 분포는 특정 단층 혹은 구조선에 집중되는 현상은 나타나지 않았으나, 익산지진과 3개의 여진은 한곳에 집중 분포한다. 익산지진의 진도 분포는 전화 문의 및 피해 접수 자료를 수집하여 구하였으며, 또한, 각 관측소에 기록된 PGA (Peak Ground Acceleration)를 이용하여 계기진도를 도출하였다. 그 결과 익산지진의 MMI 진도등급은 최대 V로 평가된다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2010년도 하계학술대회 논문집
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• pp.31-31
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• 2010

The growth of the high-quality GaN epilayers is of significant technological importance because of their commercializedoptoelectronic applications as high-brightness light-emitting diodes (LEDs) and laser diodes (LDs) in the visible and ultraviolet spectral range. The GaN-based heterostructural epilayers have the polar c-axis of the hexagonal structure perpendicular to the interfaces of the active layers. The Ga and N atoms in the c-GaN are alternatively stacked along the polar [0001] crystallographic direction, which leads to spontaneous polarization. In addition, in the InGaN/GaN MQWs, the stress applied along the same axis contributes topiezoelectric polarization, and thus the total polarization is determined as the sum of spontaneous and piezoelectric polarizations. The total polarization in the c-GaN heterolayers, which can generate internal fields and spatial separation of the electron and hole wave functions and consequently a decrease of efficiency and peak shift. One of the possible solutions to eliminate these undesirable effects is to grow GaN-based epilayers in nonpolar orientations. The polarization effects in the GaN are eliminated by growing the films along the nonpolar [$11\bar{2}0$] ($\alpha$-GaN) or [$1\bar{1}00$] (m-GaN) orientation. Although the use of the nonpolar epilayers in wurtzite structure clearly removes the polarization matters, however, it induces another problem related to the formation of a high density of planar defects. The large lattice mismatch between sapphiresubstrates and GaN layers leads to a high density of defects (dislocations and stacking faults). The dominant defects observed in the GaN epilayers with wurtzite structure are one-dimensional (1D) dislocations and two-dimensional (2D) stacking faults. In particular, the 1D threading dislocations in the c-GaN are generated from the film/substrate interface due to their large lattice and thermal coefficient mismatch. However, because the c-GaN epilayers were grown along the normal direction to the basal slip planes, the generation of basal stacking faults (BSFs) is localized on the c-plane and the generated BSFs did not propagate into the surface during the growth. Thus, the primary defects in the c-GaN epilayers are 1D threading dislocations. Occasionally, the particular planar defects such as prismatic stacking faults (PSFs) and inversion domain boundaries are observed. However, since the basal slip planes in the $\alpha$-GaN are parallel to the growth direction unlike c-GaN, the BSFs with lower formation energy can be easily formed along the growth direction, where the BSFs propagate straightly into the surface. Consequently, the lattice mismatch between film and substrate in $\alpha$-GaN epilayers is mainly relaxed through the formation of BSFs. These 2D planar defects are placed along only one direction in the cross-sectional view. Thus, the nonpolar $\alpha$-GaN films have different atomic arrangements along the two orthogonal directions ($[0001]_{GaN}$ and $[\bar{1}100]_{GaN}$ axes) on the $\alpha$-plane, which are expected to induce anisotropic biaxial strain. In this study, the anisotropic strain relaxation behaviors in the nonpolar $\alpha$-GaN epilayers grown on ($1\bar{1}02$) r-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVO) were investigated, and the formation mechanism of the abnormal zigzag shape PSFs was discussed using high-resolution transmission electron microscope (HRTEM).