• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2005년도 춘계 학술발표회 논문집
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• pp.1159-1166
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• 2005

In the present study, a series of laboratory tests with sands of different silt contents, are conducted and methods to assess non-linear behaviors based on in-situ test results are proposed. Modified hyperbolic stress-strain model is used to analyze non-linearity of silty sands in terms of non-linear degradation parameters f and g as a function of silt contents and relative density $D_R$. Stress-strain relationship results were obtained from a series of triaxial tests on sands containing different amounts of silt. Initial shear modulus which was applied to normalize modulus degradation of silty sands were determined based on the resonant column test results. From the laboratory test results, it was observed that, as the relative density increases, values of f decrease and those of g increase. Cone resistance $q_c$ for silty soil condition used in the triaxial tests were estimated based on the cavity expansion analysis. A suggestion to make an estimation of degradation parameters f and g as a function of fine contents is addressed in terms of cone resistance $q_c$ .

• 대한토목학회논문집
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• 제9권1호
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• pp.89-95
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• 1989

다져진 흙의 공시체에 최대 10만회 까지 반복하중(反復荷重)을 가하였을 때 잔류변형(殘溜變形) 및 탄성변형(彈性變形) 그리고 반복후(反復後) 일축압축시험을 행하여 응력(應力)-변형율(變形率) 관계등(關係等)을 검토하였으며, 여기에 영향을 미치는 함수비(含水比), 밀도(密度), 반복회수(反復回數), 반복하중등(反復荷重等)에 대해서 고찰(考察)하였다. 반복회수(反復回數)에 따른 변형(變形)의 증가율(增加率)은 소성한계(塑性限界)를 기준으로 함수비의 정도에 따라 크게 변화 하였으며, 반복하중(反復荷重)을 받은후(後) 구한 탄성변형계수(彈性變形係數)로부터 흙의 초기접선계수(初期接線係數)의 추정(推定)이 가능 하였다.

• 대한금속재료학회지
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• 제49권11호
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• pp.853-859
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• 2011

Compressive deformation behavior of Al-10Si-5Fe-1Zr (wt%) alloy preform fabricated by SPS(spark plasma sintering) of gas atomized powder was investigated in the temperature range from 380 to $480^{\circ}C$ and at strain rates from $1.0{\times}10^{-3}$ to $1.0{\times}10^{0}s^{-1}$. Stress-strain curves showed a peak stress (${\sigma}_p$) during initial stage of deformation, followed by a steady state flow at all temperatures and strain rates tested. The (${\sigma}_p$) decreased with both increase in temperature and decrease in strain rate. Nearly full densification was found to occur in the compressively deformed specimens irrespective of test condition. TEM observation revealed a restricted grain growth during steady state flow.

• Geomechanics and Engineering
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• 제19권3호
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• pp.269-282
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• 2019

Due to top-coal and immediate roof as cushion layer connecting with support and overlying strata, it can make significant influence on strata behaviors in fully mechanical top-coal caving working face (TCCWF). Taking Qingdong 828 working face as engineering background, $FLAC^{3D}$ and $UDEC^{2D}$ were adopted to explore the influence of top-coal thickness (TCT), immediate roof thickness (IRT), top-coal elastic modulus (TCEM) and immediate roof elastic modulus (IREM) on the vertical stress and vertical subsidence of roof, caving distance, and support resistance. The results show that the maximum roof subsidence increases with the increase of TCT and IRT as well as the decrease of TCEM and IREM, which is totally opposite to vertical stress in roof-control distance. Moreover, although the increase of TCEM and IREM leading to the increase of peak value of abutment pressure, the position and distribution range have no significant change. Under the condition of initial weighting occurrence, support resistance has negative and positive relationship with physical parameters (e.g., TCT and IRT) and mechanical properties (e.g., TCEM and IREM), respectively.

• 한국초지조사료학회지
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• 제21권2호
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• pp.81-88
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• 2001

To investigate C and N metabolisms in response to potassium-deficient stress during regrowth of Italian ryegrass(Lolium multiflorum L.), C and N metabolites were analyzed at day 0 (cutting date), 6, 12 and 24 days after defoliation. K-sufficient (control, +K) and K-absent (-K) nutrition solutions were applied from 7 days before defoliation, and continued for one cycle of 24 days-regrowth period. During 24 days of regrowth dry matter of regrowing shoots and remaining tissues were not significantly different between +K and -K treatment. In remaining stubble, all C compounds in both +K and -K treatment largely decreased (69% to 84% of the initial level) during the first 6 days of regrowth, and then rapidly recovered. The decline of soluble sugars and fructan in roots for the first 6 days much less in the -K medium. Amino acids, soluble and insoluble proteins in stubble also feel down during the first 6 days, thereafter actively replenished in both +K and -K treatment. The decline of nitrate in stubble prolonged to 12 days of regrowth. Initial amounts of all N compounds in roots were significantly lower in the -K medium. Higher accumulation of amino acids and soluble protein in roots in the -K medium was observed after 12 days of regrowth. In regrowing shoots, 3 all carbohydrates increased with a very similar pattern for both treatments. Nitrate was not significantly different between two treatments. Depress of soluble protein accumulation in -K medium was noteworthy after 12 days of regrowth. These results indicated that an active utilization of organic reserves occurred to support regrowth even under K deficient condition with a similar extent with K sufficient condition.

• 대한기계학회논문집A
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• 제26권2호
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• pp.349-356
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• 2002

For two kinds of 25Cr-20Ni stainless steels, STS310J1TB and STS310S with and without a small amount of Nb and N, creep behavior has been studied in a stress and temperature range from 147 to 392 MPa and from 923 to 773 K with a special reference to tertiary creep. The average creep life of STS310J1TB was about 100 times longer than that of the STS310S. The apparent activation energy for the initial creep rate was 330 kJ/mol in STS310J1TB, while that of the STS310S was 274kJ/mol in a power law creep region and 478 kJ/mol in a region of power law breakdown (PLB). The activation energy for STS310S below PLB is close to the for self-diffusion. When compensating for the temperature dependence of the Young's modulus and the omega value, it was found that the apparent activation energy for STS310J1TB was reduced to the activation energy for diffusion of chromium atom in gamma steel. The stress exponent of STS310S was about 12.3 above PLB and 5.1 in a power law creep region. Notwithstanding that the creep condition for STS310J1TB was in a power law creep region, its stress exponent was 7.9 larger than that of STS310S corresponding to the same creep conditions. This was ascribed to the presence of fine precipitates in STS310J1TB.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.117-122
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• 2001

For two kinds of 25Cr-20Ni stainless steels, SUS310J1TB TB and SUS310S with and without a small amount of Nb and N, creep behavior has been studied in a stress and temperature range from 147 to 392MPa and from 923 to 973K with a special reference to tertiary creep. The average creep life of SUS310J1TB was about 100 times longer than that of the SUS310S. The apparent activation energy for the initial creep rate was 330 kJ/mol in SUS310J1TB, while that of the SUS310S was 274 kJ/mol in a power law creep region and 478 kJ/mol in a region of power law breakdown (PLB). The activation energy for SUS310S below PLB is close to the that for self-diffusion. When compensating for the temperature dependence of the Young's modulus and the omega value, it was found that the apparent activation energy for SUS310J1TB was reduced to the activation energy for diffusion of chromium atom in a gamma steel. The stress exponent of SUS310S was about 12 above PLB and 5.1 in a power law creep region. Notwithstanding that the creep condition for SUS310J1TB was in a power law creep region, its stress exponent was 8.3 larger than that of SUS310S corresponding to the same creep conditions. This was ascribed to the presence of fine precipitates in SUS310J1TB.

• 해양환경안전학회지
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• 제10권1호
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• pp.29-33
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• 2004

지금까지의 강구조설계에서는 일반적인 탄성좌굴개념을 적용하고 있다. 왜냐하면 현재까지의 실적선의 데이터와 경험적인 방법에 의해 도출된 여러 가지 룰에 의한 데이터가 상당히 신뢰할만한 정도를 갖고 있기 때문이라고 판단하기 때문이다. 그러나, 최근들어 판두께가 박판인 고장력강재가 선체에 폭넓게 사용되어지면서 탄성좌굴발생 시점이 빨라졌으며 이에따른 탄소성거동을 정확히 예측할 필요성이 대두되고 있다. 이에 본 연구에서는 선체의 이중저 판넬구조에서 압축하중을 받을때의 실제판부재의 주변지지조건을 네가지로 이성화하여 해석하였으며, 이때 실제 필연적으로 존재하게 되는 열가공에 의한 비대칭형 초기처짐을 적용하였고, 비선형해석기법으로서는 Arc-length method를 적용하였고 해석코드는 범용유한요소법 소프트웨어로 잘 알려진 ANSYS를 사용하였다.

• Journal of Mechanical Science and Technology
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• 제17권3호
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• pp.440-448
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• 2003

In the present paper, numerical simulations of buoyant plume dispersion in a neutral and stable atmospheric boundary layer have been carride out. A Lagrangian Stochastic Model (LSM) with a Non-Linear Eddy Viscosity Model (NLEVM) for turbulence is used to generate a Reynolds stress field as an input condition of dispersion simulation. A modified plume-rise equation is included in dispersion simulation in order to consider momentum effect in an initial stage of plume rise resulting in an improved prediction by comparing with the experimental data. The LSM is validated by comparing with the prediction of an Eulerian Dispersion Model (EDM) and by the measured results of vertical profiles of mean concentration in the downstream of an elevated source in an atmospheric boundary layer. The LSM predicts accurate results especially in the vicinity of the source where the EDM underestimates the peak concentration by 40% due to inherent limitations of gradient diffusion theory. As a verification study, the LSM simulation of buoyant plume dispersions under a neutral and stable atmospheric condition is compared with a wind-tunnel experiment, which shows good qualitative agreements.

• 동력기계공학회지
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• 제15권5호
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• pp.22-29
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• 2011

In this study, the damage mechanism of large scale steam turbine due to water induction was analyzed and recovery characteristics were reviewed. A turbine consists of the rotating rotor and the stationary casing, and the clearance between them is very small for the efficiency enhancement. If water induction, while relatively cold steam or water is introduced into turbine, occurs, the considerable humping is caused at the casing near the initial water induction point and that induces the rubbing between rotor and casing. Finally, it leads to the catastrophic failure. Bowed rotor has the different characteristics in the recovery depending on damage degree. The elastic deformation due to light rubbing is recovered by turning the rotor with 3 rpm under normal operation condition, but most plastic deformation due to rubbing deforms the local microstructure and that results in permanent deformation which could not be recovered under normal operation condition. Bowed rotor has diverse characteristics depending on the recovery method, and the method is empirical and needs the cutting edge technology. Careful recovery treatment of the rotor will eliminate the risks and secure the high quality rotor similar to new rotor. If any critical error is made during the recovery, the rotor would not be recovered permanently and it should be scrapped.