• 한국압력기기공학회 논문집
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• 제13권1호
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• pp.61-68
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• 2017

Reactor pressure vessel (RPV) under severe accident conditions accompanied by core melting is exposed to direct high-temperature thermal loads. Understanding the creep behavior of the material is one of the most important factors for evaluating the structural integrity at these conditions. While damage evaluation studies have been conducted on critical structures of nuclear power plants through finite element (FE) analyses considering creep behavior, for accurate creep damage evaluation, constitutive equations considered in the FE analyses may have different results depending on the time hardening and strain hardening models as well as the tertiary creep consideration. The purpose of this study is to evaluate the creep damage under severe accident conditions by using FE method for a representative domestic RPV material, SA508 Gr.3. The effect of material hardening models and constitutive equations which are the main variables were also investigated.

• Geomechanics and Engineering
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• 제18권1호
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• pp.71-83
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• 2019

This investigation presented conventional triaxial and creep-permeability tests on sandstones considering thermally-induced damage (TID). The TID had no visible effects on rock surface color, effective porosity and permeability below $300^{\circ}C$ TID level. The permeability enlarged approximately two orders of magnitude as TID increased to $1000^{\circ}C$ level. TID of $700^{\circ}C$ level was a threshold where the influence of TID on the normalized mass and volume of the specimen can be divided into two linear phases. Moreover, no prominent variations in the deformation moduli and peak strength and strain appeared as TID< $500^{\circ}C$ level. It is interesting that the peak strength increased by 24.3% at $700^{\circ}C$ level but decreased by 11.5% at $1000^{\circ}C$ level. The time-related deformation and steady-state creep rate had positive correlations with creep loading and the TID level, whereas the instantaneous modulus showed the opposite. The strain rates under creep failure stresses raised 1-4 orders of magnitude than those at low-stress levels. The permeability was not only dependent on the TID level but also dependent on creep deformation. The TID resulted in large deformation and complexity of failure pattern for the sandstone.

• 콘크리트학회논문집
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• 제13권3호
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• pp.268-276
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• 2001

본 논문은 고온에서의 콘크리트 재료모델을 연구하였다. 콘크리트 응력-변형률 곡선은 온도가 증가함에 따라 그 형태가 변한다. 온도에 따른 콘크리트 재료거동의 변화를 나타내기 위하여 변형된 Saenz 제안식을 이용하여 응력-변형률 관계를 표시하였다. 고온에서의 급격한 변형률의 증가현상을 설명하기 위하여, 콘크리트의 변형률 성분을 순수 열팽창 변형률, 열적크리프 변형률, 과도 변형률 및 역학적 변형률로 구분하여 나타내었다. 열적크리프 변형률은 Baily-Norton의 장기크리프 곡선 식을 수정.제안하여 1축 실험 결과를 온도, 시간 및 응력의 함수로 표현하였고, 또한 유효응력 및 유효변형률 개념을 도입하여 다차원에서도 적용할 수 있는 모델을 제시하였다. 과도 변형률을 제안하여 다공탄성 거동을 가정한 콘크리트 내에 포함된 공극 및 수분의 작용을 역학적 거동의 영향을 분석하고자 하였다. 마지막으로, 본 논문에서 제시한 고온에서의 콘크리트 재료모델을 이용한 해석결과를 실제 화재실험자료와 비교하였다.

• 대한토목학회논문집
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• 제39권1호
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• pp.219-225
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• 2019

겨울철 발생하는 아스팔트 혼합물의 저온균열의 정량적 분석 및 평가를 위해서는 해당 아스팔트 혼합물의 저온응력이 반드시 계산되어야 하며, 이는 현재 대한민국, 미국 북부 및 캐나다 지역에서 포장 유지관리, 설계에 있어서 매우 중요한 사항 중 하나이다. 일반적으로 아스팔트 혼합물의 저온응력은 크리프 시험과 시간중첩이론을 바탕으로 계산되며 전통적으로 두 수학적 단계를 통해 계산된다. 우선 수학적, 수치적 변환과정(홉킨스-해밍 알고리즘)을 통해 크리프-강성응력에서 이완응력이 계산된다. 다음으로 이완응력 지배곡선을 구현한 후 회선적분의 수치해석적 접근을 통해 아스팔트 혼합물의 저온응력이 최종적으로 계산된다. 상기의 과정은 복잡하며, 시간이 오래 걸리는 단점이 있다. 이번 논문에서는 보다 간편한 라플라스 변환을 통해 해당 아스팔트 혼합물의 저온응력을 계산하였으며, 이의 결과를 전통적 계산 기법과 비교, 분석하였다. 결론적으로 새로이 제안된 라플라스 변환 기법은 보다 아스팔트 혼합물의 저온응력을 효과적, 효율적으로 계산할 수 있음이 발견되었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.516-521
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• 2004

In unmanned aerial vehicles (UAV), the high temperature results from friction among the air, combustion of fuel in engine and combustion gas of a nozzle. The high temperature may cause serious damages in UAV structure. The Functionally Graded Material (FGM) is chosen as a material of thc engine duct structure. Thermal stress analysis of FGM is performed in this paper. FGM is composed of two constituent materials that are mixed up according to the specific volume fraction distribution in order to withstand high temperature. Therefore, hoop stress, axial stress and shear stress of duct with 2 layers, 4 layers and 8 layers FGM are compared and analyzed respectively. In addition, the creep behavior of FGM used in duct structure of an engine is analyzed for better understanding of FGM characteristics.

• 한국압력기기공학회 논문집
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• 제16권1호
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• pp.74-83
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• 2020

A number of so-called 'Type IV' cracking was reported to occur at the welded joints of the P91 steel or P92 steel reheat steam piping systems in Korean supercritical thermal power plants. The reheat steam piping systems are subjected to severe thermal and pressure loading conditions of coolant higher than 570℃ and 4MPa, respectively. In this study, piping analyses and design evaluations were conducted for the piping system of a specific thermal plant in Korea and suggestions were made how structural integrity could be improved so that type IV cracks at the welded joints could be prevented. Integrity evaluations were conducted as per ASME B31.1 code with implicit consideration of creep effects which was used in original design of the piping system and as per nuclear-grade RCC-MRx code with explicit consideration of creep effects. Comparisons were made between the evaluation results from the two design rules. Another approach with modification or reduction of the redundant supports in the piping systems was investigated as a tool to mitigate thermal stresses which should essentially contribute to prevention of Type IV cracking without major modification of the existing piping systems. In addition, a post weld heat treatment method and repair weld method which could improve integrity of the welded joint of P91 steel were investigated.

• 소성∙가공
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• 제33권3호
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• pp.200-207
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• 2024

In this study, we developed a semi-analytical approach for the numerical analysis of residual stress in oxide scales formed on hot-rolled steels. The oxide scale, formed during the hot rolling process, experiences complex interactions due to thermal and mechanical influences, significantly affecting the material's integrity and performance. Our research focuses on integrating various stress components such as thermal stress, growth stress, and creep behavior to predict the residual stress within the oxide layer. The semi-analytical method combines analytical expressions for each stress component with numerical integration to account for their cumulative effects. Validation through instrumented indentation tests confirms the reliability of our model, which considers thermal expansion coefficient (CTE) differences, scale growth, and creep-induced stress relaxation. Our findings indicate that thermal stress resulting from CTE differences significantly impacts the overall residual stress, with growth stress contributing a compressive component during cooling, and creep behavior playing a minor role in stress relaxation. This comprehensive approach enhances the accuracy of residual stress prediction, facilitating the optimization of material design and processing conditions for hot-rolled steel products.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2061-2078
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• 2006

The main purpose of this paper is to establish the high temperature structural integrity evaluating procedures for the next generation reactors, which are to be operated at over 500$^{\circ}C$ and for 60 years. To do this, comparison studies of the high temperature structural design codes and assessment procedures such as the ASME-NH (USA), RCC-MR (France), DDS (Japan), and R5 (UK) are carried out in view of the accumulated inelastic strain and the creep-fatigue damage evaluations. Also the application procedures of the ASME-NH rules with the actual thermal and structural analysis results are described in detail. To overcome the complexity and the engineering costs arising from a real application of the ASME-NH rules by hand, all the procedures established in this study such as the time-dependent primary stress limits, total accumulated creep ratcheting strain limits, and the creep-fatigue damage limits are computerized and implemented into the SIE ASME-NH program. Using this program, the selected high temperature structures subjected to two cycle types are evaluated and the parametric studies for the effects of the time step size, primary load, number of cycles, normal temperature for the creep damage evaluations and the effects of the load history on the creep ratcheting strain calculations are investigated.

• 대한기계학회논문집
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• 제15권2호
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• pp.396-403
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• 1991

A solution is presented for the computation of the elastic-creep stresses in a hollow cylinder subjected to nonaxisymmetric temperature distribution. The creep problem is treated by the Maxwell creep model. Laplace transformation is used for reformation of the governing equation of elastic problem and Hooke's law in a function of .gamma. , .theta. , and creep constant. The governing equation is set up using the Airy stress function which leads to the biharmonic equation. The solution is obtained by using Fourer series method and Laplace inverse method used to obtain the stress components which include the variation of time. This solution shows excellent agreement with Lamkin's and Boley & Weiner's solution. The viscoelastic stresses are also obtained for the fuel rob tube subjecting nonaxisymmetric thermal load.

• 열처리공학회지
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• 제27권4호
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• pp.185-190
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• 2014

The influences of small amount of Sn addition on microstructure and creep resistance of AZ91-0.4%Ca alloy have been investigated. The microstructure of the AZ91-0.4%Ca alloy was characterized by ${\alpha}$-(Mg) dendrite cells surrounded by eutectic ${\beta}(Mg_{17}Al_{12})$ and $Al_2Ca$ phases. The 0.5%Sn addition resulted in the formation of rod-shaped CaMgSn particles with the extinction of $Al_2Ca$. The Sn-containing alloy exhibited better creep resistance below $175^{\circ}C$, but the tendency was reversed above $200^{\circ}C$. The reason was discussed in relation to the change in thermal stability of ${\beta}$ phase in response to the Sn addition.