• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1996년도 춘계학술대회논문집
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• pp.48-54
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• 1996

The continuous deformation , multistage deformation and stress relaxation were carried out to investigate the strain induced procipitation by torsion tests in the range of 1000∼800$^{\circ}C$, 0.05/sec∼5/sec for V-microalloyed steel. The starting temperature and time for the initiation of precipitation were determined by stress relaxation tests and the distribution of percipitates increased at higher strain rate and the mean size of precipitates was found to be about 50nm. The precipitation starting time decreased with increasing strain rate from 0.05/sec to 5 /sec and pre-strain. The effect of deformation conditions on the no-recrystallization temperature(Tnr) was determined in the multistage deformation with declining temerature. The Tnr decreased with increasing strain and strain rae. In the controlled rolling, grain refinement and precpitation hardening effects could be achieved by the alternative large pass strain at the latter half pass stage under the condition of low temperature and high strain rate.

• Journal of Welding and Joining
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• 제19권3호
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• pp.298-304
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• 2001

Turbine blade is subject to force of three types ; the torsional force by torsional mount, the centrifugal force by the rotation of rotor and the cyclic bending force by steam pressure. The cyclic bending force was a main factor on fatigue strength. SEM fractography in root of turbine blade showed micro-clack width was not dependent on stress intensity factor range. Especially, fatigue did not exist on SEM photograph in root of turbine blade. To clear out the fracture mechanism of turbine blade, nanofractography was needed on 3-dimensional crack initiation and crack growth with high magnification. Fatigue striation partially existed on AFM photograph in root of turbine blade. Therefore, to find a fracture mechanism of the torsion-mounted blade in nuclear power plant, the relation between stress intensity factor range and surface roughness measured by AFM was estimated, and then the load amplitude ΔP applied to turbine blade was predicted exactly by root mean square roughness.

• 한국자동차공학회논문집
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• 제9권6호
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• pp.170-177
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• 2001

Turbine blade is subject to torsional load by torsion-mount, centrifugal load by rotation of rotor and repeated bending load by steam pressure. Turbine with partially cracked blade has normal working condition at initial repair time but vibratory working condition at middle repair time due to crack growth. Finite element analysis on turbine blade indicates that repeated bending load out of all loads is the most important factor on fatigue strength of turbine blade. Therefore, this study shows root mean square roughness has linear relation with stress intensity factor range in 12% Cr steel and can predict loading condition of fractured turbine blade.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1997년도 추계학술대회논문집
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• pp.216-219
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• 1997

Hot workability of SiCp/AA2024 composites reinforced with different vol. % of SiCp reinforcements (0, 5, 10, 15, 20, and 30 vol. %) was investigated by hot torsion tests. Hot restoration of the composites was studied from the flow curves and deformed microstructures. Dynamic recrystallization (DRX) was occurred in all the composites during the hot deformation at 370-43$0^{\circ}C$ under a strain rate of 1.0/sec. Also, the flow stress of the composites increased with increasing the SiCp reinforcement vol. % and the difference of flow stress between the composites decreased with increasing the deformation temperature.

• 소성∙가공
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• 제8권6호
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• pp.576-582
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• 1999

The high temperature behavior of Al 6061 alloy was characterized by the hot torsion test in the temperature ranges of 400∼550℃ and the strain rate ranges of 0.05∼5/sec. To decide optimum deformation condition, three types of deformation maps were individually made from the critical strain (εc). deformation resistance(σp) and deformation efficiency (η). The critical strain(εc) for dynamic recrystallization (DRX) which was decided from the inflection point of strain hardening rate(θ) - effective stress (σ) curve was about 0.65 times of peak strain (εp). The relationship among deformation resistance (peak stress, σp), strain rate (ε), and temperature (T) could be expressed by ε=2.9×1013[sinh(0.0256σp]7.3exp (-216,000/RT). The deformation efficiency (η)which was calculated on the basis of the dynamic materials model (DMM) showed high values at the condition of 500∼550℃, 5/sec for 100% strain. The results from three deformation maps were compared with microstructures. The best condition of plastic deformation could be determined as 500℃ and 5/sec.

• 한국자동차공학회논문집
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• 제8권3호
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• pp.151-162
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• 2000

This study is concerned with computerized multi-level substructuring methods and stress analysis of coil springs. The purpose of substructuring methods is to reduce computing time and capacity of computer memory by multiple level reduction of the degrees of freedom in large size problems that are modeled by three dimensional continuum finite elements. In this paper, the spring super element developed is investigated with tension, torsion, and bending of a cylindrical bar in order to verify its accuracy and efficiency for the multi-level substructuring method. And then the algorithm is applied to finite element analysis of coil springs. The result demonstrates the validity of the multi-level substructuring method and the efficiency in computing time and memory by providing good computational results in coil spring analysis.

• 한국방재학회 논문집
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• 제8권2호
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• pp.39-43
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• 2008

Determination of strain associated with shear modulus and damping ratio during torsional test is complicated. This is due to nonuniform stress-strain variation occurring linearly with radius in a soil specimen in torsion. A conventional equivalent radius approach proposed by Chen and Stokoe appears to be adequate for evaluating strain associated with shear modulus at low to intermediate strain levels. This approach is less accurate for damping measurement, particularly at high strain. Modified equivalent radius approach was used to account for the nonuniform stress-strain effect more precisely. The modified equivalent radius approach was applied for hyperbolic, modified hyperbolic, and Ramberg-Osgood models. The results illustrate the usefulness of the modified equivalent radius approach and suggest that using a single value of equivalent radius ratio to calculate strains is not appropriate.

• 소성∙가공
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• 제6권2호
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• pp.102-109
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• 1997

The multistage deformation and stress relaxation were carried out to investigate the strain induced precipitation by torsion tests in the range of 1000~80$0^{\circ}C$, 0.05~5/sec for V-microalloyed steel. The starting temperature and time for the initiation of precipitation were determined by stress relaxation tests. The distribution of precipitates increased, as the strain rate increased and the mean size of precipitates was found to be about 10~30nm. The precipitation starting time$(P_s)$ decreased with increasing strain rate and the amount of pre-strain. The effect of deformation conditions on the no-recrystallization temperature$(T_nr)$ was also determined in the multistage deformation. $T_nr$ Tnr decreased with increasing the strain and strain rate. In the controlled rolling simulation, grain refinement and precipitation hardening effects could be achieved by the alternative large pass strain at the latter half pass stage under the condition of low temperature and high strain rate.

• 지구물리와물리탐사
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• 제12권1호
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• pp.163-171
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• 2009

임계하 균열성장(subcritical crack growth)은 암석 내에서 오랜 시간 동안 일어나는 균열시스템 발달의 주된 원인으로 그 중요성이 매우 크다. 본 연구에서는 증류수(pH = 5-7)와 수산화나트륨(NaOHaq, pH = 12)용액으로 포화된 암석과 건조암석에서의 임계하 균열성장에 대해 고찰하였다. 아울러 공극유체의 수소이온농도(pH)의 영향도 함께 살펴보았으며, 안산암과 화강암이 연구에 사용되었다. 연구방법은 온도가 조절된 환경에서 이중-비틀림 실험(double-torsion test)을 실시하여 균열성장속도와 응력확장계수와의 상관관계를 파악하였다. 연구의 결과 수성환경에서 균열성장속도가 건조한 경우보다 크게 나타났으며, 이는 기존 연구에서 물이 첨가되었을 때 균열성장속도가 빨라지는 결과와 잘 일치한다. 증류수와 수산화나트륨 사이의 결과에서는 수소이온농도의 차이에도 불구하고 균열성장속도의 차이가 나타나지 않았으며 이는 수산화이온이 균열성장을 가속한다는 기존의 결과와는 차이를 보였다. 따라서 균열의 첨단부(crack-tip)에서의 수소이온농도의 차이는 균열의 성장에 영향을 미치지만, 공극유체의 전체적인 수소이온농도(bulk pH)의 영향은 매우 적다는 결론을 얻을 수 있었다.

• 대한의용생체공학회:의공학회지
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• 제29권6호
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• pp.451-456
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• 2008

Total knee arthroplasty(TKA) using computer-assisted navigation has been increased in order to improve the accuracy of femoral and tibial components implantation. Recently, a few clinical studies have reported on the femoral stress fracture after TKA using computer-assisted navigation. The purpose of this study is to investigate the stress concentration around the femoral pin-hole for different pin-hole diameter, the modes of pin penetration by finite element analysis to understand the effects of pin-hole parameters on femoral stress fracture risk. A three-dimensional finite element model of a male femur was reconstructed from 1 mm thick computed tomography(CT) images. The bone was rigidly fixed to a 25 mm above the distal end and 1500 N of axial compressive force and 12 Nm of axial torsion were applied at the femoral head. For all cases, transcortical pin penetration mode showed the highest stress fracture risk and unicortical pin penetration mode showed the lowest stress concentration. Pin-hole diameter increased the stress concentration, but pin number did not increase the stress dramatically. The results of this study provided a biomechanical guideline for pin-hole fracture risk of the computer navigated TKA.