• 열처리공학회지
• /
• 제18권4호
• /
• pp.235-241
• /
• 2005

Thermal creep properties of the advanced zirconium fuel claddings named by HANA alloys which were developed for high burn-up application were evaluated. The creep test of HANA cladding tubes was carried out by the internal pressurization method in temperature range from 350 to $400^{\circ}C$ and in the hoop stress range from 100 to 150 MPa. Creep tests were lasted up to 800 days, which showed the steady-state secondary creep rate. The creep resistance of HANA fuel claddings was affected by final annealing temperature and various factors, such as alloying element, applied stress and testing temperature. From the results the microstructure observation of the samples before and after creep test by using TEM, the dislocation density was increased in the sample of after creep test. The Sn as an alloying element was more effective in the creep resistance than other elements such as Nb, Fe, Cr and Cu due to solute hardening effect of Sn. In case of HANA fuel claddings, the improved creep resistance was obtained by the control of final heat treatment temperature as well as alloying element.

• 한국재료학회지
• /
• 제14권7호
• /
• pp.451-456
• /
• 2004

Thermal creep properties of the zirconium tube which was developed for high burnup application were evaluated. The creep test of cladding tubes after various final heat treatment was carried out by the internal pressurization method in the temperature range from $350^{\circ}C to 400^{\circ}C$ and from 100 to 150 MPa in the hoop stress. Creep tests were lasted up to 900days, which showed the steady-state secondary creep rate. The creep resistance of zirconium claddings was higher than that of Zircaloy-4. Factors that affect creep resistance, such as final annealing temperature, applied stress and alloying element were discussed. Tin as an alloying element was more effective than niobium due to solute hardening effect of tin. In case of advanced claddings, the optimization of final heat treatment temperature as well as alloying element causes a great influence on the improvement of creep resistance.

• 한국해양공학회지
• /
• 제25권6호
• /
• pp.110-115
• /
• 2011

Numerouslong-term consolidation and secondary compression settlements may occur in Busan clay, which is astructured soft clay and consists of a thick clay deposit. As a surcharge load is applied to soils, soils experience different stress paths with depth. Therefore, it is necessary to study the long-term consolidation behavior of Busan clay considering stress conditions such as OC or NC states. In this study, a series of long-term consolidation tests were performed to investigate the consolidation characteristics of Busan clay for 20 days. The undisturbed clay samples were taken from 3 sites located in the Nakdong River estuary. The results showed that the creep rate of the Busan clay gradually decreased with time, which indicated that the secondary compression settlement decreased with time. In addition, the experimental results for 3 samples showed that the ratios were about 0.0363 and 0.051, respectively.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2001년도 추계학술대회논문집A
• /
• pp.180-185
• /
• 2001

In the life assessment for plant structural component, the research on deterioration of toughness and material properties occurred in weldments has been considered as very important problems. In general, the microstructures composed in weldments are hugely classified with weld metal(W.M), fusion line(F.L), heat affected zone(HAZ), and base metal(B.M). It has been reported that the creep characteristics on weldments having variable microstructures could be unpredictably changed. Furthermore, it is also known that HAZ adjacent to F.L exhibits the decreased creep strength compared to those in base or weld metals, and promotes the occurrence of Type III and Type IV cracking due to the growth of grains and the coarsening carbides precipitated in ferritic matrix by welding and PWHT processes. However, the lots of works reported up to date on creep damage in power plant components have been mostly conducted on B.M and the creep properties on a localized microstructures in weldments have not as yet been throughly investigated. In this paper, for various microstructures such as coarse grain HAZ(CGHAZ), W.M and B.M in X20CrMoV121 steel weldment, the small punch-creep(SP-Creep) test using miniaturized specimen(t=0.5mm, 0.25mm) is performed to investigate a possibility for creep characteristics evaluation.

• 대한기계학회논문집A
• /
• 제30권11호
• /
• pp.1494-1501
• /
• 2006

Mechanical and physical properties of a copper alloy for a liquid rocket engine(LRE) combustion chamber liner application were tested at various temperatures. All test specimens were heat treated with the condition they might experience during actual fabrication process of the LRE combustion chamber. Physical properties measured include thermal conductivity, specific heat and thermal expansion data. Uniaxial tension tests were preformed to get mechanical properties at several temperatures ranging from room temperature to 600$^{\circ}C$. The result demonstrated that yield stress and ultimate tensile stress of the copper alloy decreases considerably and strain hardening increases as the result of the heat treatment. Since the LRE combustion chamber operates at higher temperature over 400$^{\circ}C$, the copper alloy can exhibit time-dependent behavior. Strain rate, creep and stress relaxation tests were performed to check the time-dependent behavior of the copper alloy. Strain rate tests revealed that strain rate effect is negligible up to 400$^{\circ}C$ while stress-strain curve is changed at 500$^{\circ}C$ as the strain rate is changed. Creep tests were conducted at 250$^{\circ}C$ and 500$^{\circ}C$ and the secondary creep rate was found to be very small at both temperatures implying that creep effect is negligible for the combustion chamber liner because its operating time is quite short.

• 터널과지하공간
• /
• 제14권1호
• /
• pp.69-77
• /
• 2004

암석의 시간 의존 변형거동은 암석의 가장 근본적인 역학적 성질 중의 하나이며, 크립거동의 특성과 메커니즘에 대한 연구는 지하 암반 구조물의 장기적 안정성을 평가하는데 필수적이라 할 수 있다. 이에 본 연구에서는 국내 화강암을 대상으로 단축압축하중 하에서 크립시험을 실시하였고, 수분과 응력 수준을 달리하여 각각의 조건에 따른 크립 특성을 비교, 분석하였다. 크립시험 결과 화강암은 1차, 2차 및 3차로 확연히 구분된 크립거동을 보였다. 1차 크립거동은 대체로 24시간 내에 종결되었으며, 2차 크립 변형률은 단축압축강도에 대한 재하응력의 비율이 증가함에 따라 증가하였다. 일정 응력하에서 화강암의 최대강도는 시간에 따라 감소하는 경향을 보였다. 완전 포화된 시료는 자연 건조된 시료보다 훨씬 큰 크립 변형량 및 변형률을 보였으며, 포화시료의 파괴강도 및 파괴시간은 급격히 감소하였다. 본 연구의 크립시험 결과를 버거모델(Burger's model) 및 이전 연구자들에 의해 제안된 두 개의 경험적 모델에 적용하여 크립의 유동상수를 구하였다. 세 개의 모델 모두 화강암의 크립거동을 잘 나타내었으나, 그 중 버거모델이 실험결과와 가장 잘 일치하였다. 각각의 모델에서 결정된 유동상수들은 함수량과 응력 수준에 영향을 받았다. 또한 본 실험결과에 기초하여 자연건조 및 완전 포화된 화강암 시료에 대하여 응력 수준과 시간의존변형의 경험적 관계식을 유도하였다.

• 한국지반공학회논문집
• /
• 제24권5호
• /
• pp.107-115
• /
• 2008

본 연구는 1차압밀 중의 크리프의 영향과 Yin이 제안한 탄-점-소성 모델에 대한 적용성을 검토하였다. 탄성모델을 이용한 일반적인 압밀이론은, 1차압밀 과정을 표현할 수 있으나 2차압밀을 표현할 수 없다. 이러한 결과는 2차압축을 표현할 수 있는 점성에 기인하며, 때로는 Ladd 등(1977)이 제안한 가정 A 및 B와 같은 스케일효과(실험실 공시체와 현장조건 사이의 점토층 두께의 차이)와 관련되어진다. 통상적으로 1차압밀 중의 크리프의 존재는 많은 연구자에 의해 확인되어졌으며, 가정 B가 잘 맞는 것으로 되어있다. 한편, 대형압밀시험을 통해 가정 A와 B의 중간적인 특성이 Aboshi(1973)에 의한 가정 C로써 제안되어졌다. 본 연구에서는 1차압밀 중의 침하-시간관계에 대한 크리프의 영향을 명백히 하기 위해, peat와 점토에 대해 분할형 압밀시험기를 이용하여 가정 B의 압밀시험을 행하였다. 그리고 Yin의 탄-점-소성 모델을 이용하여 실험결과를 해석하였다. 얻어진 결과는 다음과 같다. 1차압밀 종료시에 분할 공시체의 압축은 과잉간극수압 소산속도의 차이에 의해 동일하지 않았다. 또한 분할형 압밀시험기에 의해 측정된 평균변형률과 Yin의 EVP 모델을 이용한 해석치는 잘 일치하였다. 그러나 과잉간극수압의 소산에 대해서는 측정치가 Yin 모델에 비해 빨리 소산되었다.

• Journal of Advanced Marine Engineering and Technology
• /
• 제38권10호
• /
• pp.1244-1250
• /
• 2014

The temperature of the main engine cabin of commercial vessel is very high. The material SS-316L undergoes creep damage at temperatures exceeding $450^{\circ}C$. It is essential to maintain the highly stressed engine cabin below the creep regime. Hence, seawater is employed in this kind of maritime vehicles as cooling liquid. It obtains the thermal energy at the cooling pipe line after passing through main engine cooling system. To harness the energy in the seawater, a turbine can be installed to absorb the energy in the seawater before being released into the sea. In this study, a cooling pipe line is selected to apply the tubular type hydro turbine for transferring the energy. Numerical analysis for investigating the performance and the internal flow characteristics of the tubular turbine is conducted. The results show that the maximum efficiency of 85.8% is achieved although the efficiency drops rapidly at partial flow rate condition. The efficiency descends slowly at the condition of excess flow rate. There is a relatively wide operating range of flow rate of this turbine to keep high efficiency at the excess flow rate condition. For the internal flow of the turbine, there is uniform streamline on the suction and pressure sides of the blade at the design point. However, the secondary flow appears at the suction and pressure sidesat the excess flow rate.In addition, it appears only at pressure side at the partial flow rate condition.

• 한국지반공학회논문집
• /
• 제21권3호
• /
• pp.133-140
• /
• 2005

연약지반에서 압밀속도를 가속화시키고 설계 전단강도를 얻기 위하여 선행압밀공법과 병행된 연직배수공법이 널리 사용되어 왔다. 본 연구에서는 배수계가 설치된 연약지반에서 압밀과정 동안에 간극비의 감소로 인한 압축성과 투수계수의 변화를 고려할 뿐만 아니라 1차압밀 동안에 유발되는 2차압밀(혹은 크리프)을 고려할 수 있는 축대칭 비선형 압밀이론이 개발되었다. 측대칭 비선형 압밀해석을 위한 수치해석 프로그램, AXICON은 가정 B를 바탕으로 유한차분법을 적용하여 개발되었다. 가정 A를 바탕으로 한 Hansbo의 해석적인 해와 AXICON 해석결과를 비교$\cdot$분석하였다. 또한 Ska-Edeby 시험성토 현장지반에서 관측된 침하량과 간극수압의 계측자료와 비교하였다. 해석결과에 의하면 가정 A는 현장침하량을 일반적으로 과소 평가하며, 가정 B가 논리적으로 타당하다. 또한 제안된 해석 프로그램을 사용하면 실제 현장의 거동을 잘 예측할 수 있다.

• Corrosion Science and Technology
• /
• 제12권2호
• /
• pp.102-112
• /
• 2013

A very high-temperature gas reactor (VHTR) is one of the next generation nuclear reactors owing to its safety, high energy efficiency, and proliferation-resistance. Heat is transferred from the primary helium loop to the secondary helium loop through an intermediate heat exchanger (IHX). Under VHTR environment Alloy 617 is being considered a candidate Ni-based superalloy for the IHX of a VHTR, owing to its good creep resistance, phase stability and corrosion resistance at high temperature. In this study, high-temperature corrosion tests were carried out at 850 - $950^{\circ}C$ in air and impure helium environments. Alloy 617 specimens showed a parabolic oxidation behavior for all temperatures and environments. The activation energy for oxidation was 154 kJ/mol in helium environment, and 261 kJ/mol in an air environment. The scanning electron microscope (SEM) and energy-dispersive x-ray spectroscopy (EDS) results revealed that there were a Cr-rich surface oxide layer, Al-rich internal oxides and depletion of grain boundary carbide after corrosion test. The thickness and depths of degraded layers also showed a parabolic relationship with the time. A corrosion rate of $950^{\circ}C$ in impure helium was higher than that in an air environment, caused by difference in the outer oxide morphology.