• Title, Summary, Keyword: 크리프

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The Effects of Hot Corrosion on the Creep Rupture Properties of Boiler Tube Material (보일러 管材料의 크리프破斷特性에 미치는 고온부식의 影響)

  • 오세욱;박인석;강상훈
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.13 no.2
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    • pp.236-242
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    • 1989
  • In order to investigate the effects of hot corrosion on the creep rupture properties and creep life of 304 stainless steel being used as tube materials of heavy oil fired boiler, the creep rupture tests were carried out at temperature 630.deg.C, 690.deg.C and 750.deg.C in static air for the specimens with or without coating of double layer corrosives according to the new hot corrosion test method simulating the situation commonly observed on superheater tubes of the actual boiler. The double layer corrosives are 85% V$_{2}$O$_{5}$ + 10% Na$_{2}$So$_{4}$ + 5% Fe$_{2}$O$_{3}$ as the inner layer corrosive being once melted at 900.deg. C and crushed to powder, and 10% V$_{2}$O$_{5}$ + 85% Na$_{2}$SO$_{4}$ +5% Fe$_{2}$O$_{3}$ as the outer layer corrosive. As results, in the specimen coated with the double layer corrosives, the rupture strength was extremely lowered and showed a large difference each other. The rupture ductility also lowered remarkably as a result of the brittle fracture mode due to hot corrosion. These results indicate that hot corrosion could essentially alter the creep fracture mechanism. From the metallographic observation, it was clarified that the rupture life of 304 stainless steel subjected to hot corrosion was chiefly determined by the behavior of the aggressive intergranular penetration of sulfides.des.

A study on the Creep Behavieo of Rene 80 Superalloy (Rene 80초내열 합금의 크리프 거동에 관한 연구)

  • Han, Seong-Ho;Lee, Gi-Ryong;Jo, Chang-Yong;Kim, In-Bae;Kim, Hak-Min
    • Korean Journal of Materials Research
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    • v.3 no.6
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    • pp.575-584
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    • 1993
  • The effects of atmosphere on the creep rupture properties of Ni-base superalloy Ren6 80 were investigated. Creep rupture tests were performed at $760^{\circ}C$, 657MPa and 982%, 157MPa under Ar gas atmospheres. Creep rupture mode and rupture properties (rupture life and elongation) were similar in two different atmospheres under the condition of $760^{\circ}C$, 657MPa. However, the results at $982^{\circ}C$, 157MPa showed different creep rupture mode and life between air and Ar gas atmospheres. In air, it was shown that creep cracks were initiated at surface-exposed grain boundaries and propagated along grain boundaries. In Ar gas atmosphere, the cracks were initiated at inner grain boundaries and coalesced to surface cracks which cause cup-and-cone type fracture. Due to protective oxide layer formation, air test showed longer rupture life than Ar test.

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An Experimental Study on the Creep Behavior of Frozen Sand (동결 사질토의 크리프 거동에 관한 실험적 연구)

  • Chae, Deokho;Kim, Youngseok;Lee, Jangguen;Cho, Wanjei
    • Journal of the Korean Geoenvironmental Society
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    • v.15 no.2
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    • pp.27-36
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    • 2014
  • Due to the latitudinal location of Korea, the seasonally frozen ground has been focused on as research topics such as the frost heaving under the asphalt road rather than the permafrost ground. However, the recent construction of the second Korean Antarctic research station, the Jangbogo station and the participation on the development of the natural gas pipeline in Russia arouse the research interests on the behavior of the permafrost ground. At the design process of the geotechnical structures on the permafrost ground, the evaluation of the creep characteristics of the frozen soil is very crucial. Since the domestic specification on the frozen soil testing does not exist currently, it is necessary to evaluate the creep characteristics of frozen soils systematically with regard to the affecting factors. Therefore, the creep characteristics of the frozen specimens of dense Jumoonjin sand were evaluated under various loads at -5 and $-10^{\circ}C$. Based on the test results, as the load became close to the strength and the temperature became lower, the duration of the secondary creep became shorter and more distinct tertiary creep responses were observed.

Analysis of the Critical Speed and Hunting Phenomenon of a High Speed Train (고속전철의 임계속도와 헌팅현상 해석)

  • Song, Ki-Seok;Koo, Ja-Choon;Choi, Yeon-Sun
    • Journal of the Korean Society for Railway
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    • v.17 no.5
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    • pp.342-348
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    • 2014
  • Contact between wheel and rail leads to the creep phenomenon. Linear creep theory, assuming linear increase in the creep force vs creep, results in a critical speed at which the vibration of a railway vehicle goes to infinity. However, the actual creep force converges to a limited value, so that the vibration of a railway vehicle cannot increase indefinitely. In this study, the dynamics of a railway vehicle is investigated with a 6 DOF bogie model includingthe nonlinear creep curves of Vermeulen, Polach, and a newly calculated creep curve with strip theory. Strip theory considers the profiles of the wheel and rail. The results show that the vibration of a railway vehicle results in a limit-cycle over a specific running speed, and this limit-cycle becomes smaller as the slope of the creep-curve steepens. Moreover, a hunting phenomenon is caused due to flange contact, which restricts the magnitude of the limit-cycle.

Study on Creep Damage Model of 1Cr1Mo1/4V Steel for Turbine Rotor (1Cr1Mo1/4V 터빈 로터강의 크리프 손상 모델에 관한 연구)

  • Choi, Woo-Sung;Fleury, Eric;Song, Gee-Wook;Kim, Bum-Shin;Chang, Sung-Ho
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.35 no.4
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    • pp.447-452
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    • 2011
  • It is well known that the dominant damage mechanisms in high-temperature steam turbine facilities such as rotor and casing are creep and fatigue damages. Even though coupling of creep and fatigue should be considered while predicting the life of turbine facilities, the remaining life of large steam turbine facilities is generally determined on the basis of creep damage because the turbines must generate stable base-load power and because they are operated at a high temperature and pressure for a long time. Almost every large steam turbine in Korea has been operated for more than 20 years and is made of steel containing various amounts of principal alloying elements nickel, chromium, molybdenum, and vanadium. In this study, creep damage model of 1Cr1Mo1/4V steel for turbine rotor is proposed and that can assess the high temperature creep life of large steam turbine facilities is proposed.

Method to Determine Elastic Follow-Up Factors to Predict C(t) for Elevated Temperature Structures (이차하중을 받는 고온 구조물의 C(t) 예측을 위한 탄성추종 계수 결정법)

  • Lee, Kuk-Hee;Kim, Yun-Jae
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.36 no.7
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    • pp.759-768
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    • 2012
  • This paper proposes a method to determine the elastic follow-up factors for the $C(t)$-integral under secondary stress. The rate of creep crack growth for transient creep is correlated with the $C(t)$-integral. Elastic follow-up behavior, which occurs in structures under secondary loading, prevents a relaxation of stress during transient creep. Thus, both the values of $C(t)$ and creep crack growth increase as increasing elastic follow-up. An estimation solution for $C(t)$ was proposed by Ainsworth and Dean based on the reference stress method. To predict the value of $C(t)$ using this solution, an independent method to determine the elastic follow-up factors for cracked bodies is needed. This paper proposed that the elastic follow-up factors for $C(t)$ can be determined by elastic-plastic analyses using the plastic-creep analogy. Finite element analyses were performed to verify this method.

Bending Creep Performances of Hybrid Laminated Woods Composed of Wood-Wood Based Boards (목재와 목질보드 복합적층재의 휨 크리프 성능)

  • Park, Han-Min;Kang, Dong-Hyun;Choi, Yoon-Eun;Ahn, Sang-Yeol;Ryu, Hyun-Su;Byeon, Hee-Seop
    • Journal of the Korean Wood Science and Technology
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    • v.38 no.1
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    • pp.1-10
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    • 2010
  • In this study, to study an effective use and improve strength performances of woods and wood-based materials, three-ply hybrid laminated woods which are composed of spruce in the face and three kinds of wood-based boards (MDF, PB, OSB) in the core were manufactured, and the effect of constitution elements used for the core laminae on bending creep performances was investigated. The shape of creep curves showed exponential function plots which the upper right side was increased, and differed among the kinds of wood-based boards used for the core laminae of hybrid laminated wood. The creep deformation perpendicular to the grain of faces of hybrid laminated woods was in order $C_{\perp}$(P) > $C_{\perp}$(M) > $C_{\perp}$(O) with PB, MDF and OSB in the core, respectively. It was found that the creep deformation arranged with OSB in the core had 2 times smaller than those arranged with MDF and PB in the core. By hybrid laminating, the creep deformation of spruce perpendicular to the grain was markedly decreased. On the other hand, the creep deformation parallel to the grain of the faces ($C_{\parallel}$ type) of hybrid laminated woods was in order $C_{\parallel}$(P) > $C_{\parallel}$(O) > $C_{\parallel}$(M) with PB, OSB and MDF in the core. The ratios among three hybrid laminated woods were considerably decreased, especially the difference between $C_{\parallel}$(P) and $C_{\parallel}$(O) hybrid laminated woods arranged with PB and OSB in the core was very small. These values showed 0.108~0.464 times smaller than creep deformation of three wood-based boards and it was found that creep deformation of three wood-based boards was considerably decreased by hybrid laminating. Creep anisotropy of hybrid laminated woods was greater in creep deformation than in initial deformation, whereas it was found that the values was much smaller than that of spruce parallel laminated woods.

Stress Relaxation Coefficient Method for Concrete Creep Analysis of Composite Sections (합성단면의 콘크리트 크리프 해석을 위한 이완계수법)

  • Yon, Jung-Heum;Kyung, Tae-Hyun;Kim, Da-Na
    • Journal of the Korea Concrete Institute
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    • v.23 no.1
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    • pp.77-86
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    • 2011
  • The concrete creep deformation of a hybrid composite section can cause additional deformation of the composite section and the stress relaxation of pre-compressive stress on the concrete section due to partial restraint of the deformation. In this study, the stress relaxation coefficient method (SRCM) is derived for simple analysis of complicate hybrid or composite sections for engineering purpose. Also, an equation of the stress relaxation coefficient (SRC) required for the SRCM is proposed. The SRCM is derived with the parameters of a creep coefficient, section and loading properties using the same method as the constant-creep step-by-step method (CC-SSM). The errors of the SRCM is improved by using the proposed SRC equation than the average SRC's which were estimated from the CC-SSM. The root mean square error (RMSE) of the SRCM with the proposed SRC equation for concrete with creep coefficient less than 3 was less than 1.2% to the creep deformation at the free condition and was 3.3% for the 99% reliability. The proposed SRC equation reflects the internal restraint of composite sections, and the effective modulus of elasticity computed with the proposed SRC can be used effectively to estimate the rigidity of a composite section in a numerical analysis which can be applied in analysis of the external restrain effect of boundary conditions.

Creep Crack Propagation Properties of Rotor Steel under Constant Load and Constant Ct Condition (일정하중 및 일정Ct에서 로터강의 크리프 귤열전파 특성)

  • Jeong, Soon-Uk
    • Journal of the Korean Society for Precision Engineering
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    • v.18 no.5
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    • pp.105-111
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    • 2001
  • The creep crack growth properties in 3.3NiCrMoV steel were investigated at 55$0^{\circ}C$ by using CT specimen under constant load and constant Ct condition that was held during crack growth of Imm distance. Ct lelied on load line displacement rate, C*usually increased with crack length though load is reduced in order to maintain constant Ct value as crack growth and appeared scatter band. At constant load and Ct region, crack growth slope was 0.900 and 0.844 each, on the other hand C* slope was 0.480. Fully coalesced area(FCA) ahead of crack tip increased as Ct increase to the critical value, and after that value FCA decreased. For the tertiary creep stage of crack growth test, the most of displacement was due to the steady state creep, except only small part due to the primary creep and other effects. Therefore, tests were mainly interrupted in the tertiary stage to obtain high value of Ct.

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Moment Magnifier Method for Long-Term Behavior of Flat Plate Subjected to In-Plane Compressive and Transverse Loads (바닥하중과 압축력을 받는 플랫 플레이트의 장기거동을 고려한 모멘트 증대법)

  • 최경규;박홍근
    • Journal of the Korea Concrete Institute
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    • v.13 no.1
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    • pp.38-45
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    • 2001
  • Numerical studies were carried out to develop the moment magnifier method for long-term behavior of flat plates, subjected to combined in-plane compressive and transverse loads. Nonlinear finite element analyses were performed for the numerical studies. Through the numerical studies, the long term behavior of the flat plate subjected to uniform or nonuniform floor load was investigated, and creep effects on the degradation of strength and stiffness of the slabs were examined. As a result, the creep factor was implemented to describe the creep effect on the flat plate. The moment magnifier method using the creep factor was developed for long-term behavior of flat plates. Also, the design examples were shown for the verification of the proposed design method.