• Title, Summary, Keyword: 크리프

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A Rheological Approach on Prediction of Concrete Creep (콘크리트 크리프 예측을 위한 유변학적 접근)

  • Kwon, Ki-yeon;Min, Kyung-hwan;Kim, Youl-hee;Yoon, Young-soo
    • Journal of The Korean Society of Civil Engineers
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    • v.29 no.1A
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    • pp.85-93
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    • 2009
  • The primary objective of this study is to construct more simple and reasonable rheological model and propose a methodology for predicting a phenomenon of concrete creep. Deformations of concrete under sustained stress can be expressed by the sum of immediately elastic deformation, time-dependent and time-independent short-term creep, and long-term creep according to the mechanism and time-dependency. To simulate these deformations, a rheological model having six parameters was constructed. In the composing of each parameter, the microprestress-solidification theory and design model code were incorporated together with the numerical approach for the components which can not be theoretically approached. Finally, actual test data were applied in the verification of the proposed model, and suitability of the model was confirmed by comparisons with existing predicting models and design codes.

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A Numerical Approach to Spherical Indentation Techniques for Creep Property Evaluation (크리프 물성평가를 위한 구형압입 수치접근법)

  • Lim, Dongkyu;Lee, Jin Haeng;Choi, Youngsick;Lee, Hyungyil
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.37 no.10
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    • pp.1229-1237
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    • 2013
  • In this study, the theory of spherical indentation based on incremental plasticity is extended to an indentation method for evaluating creep properties. Through finite element analysis (FEA), the point where the elastic strain effect is negligible and the creep strain gradient constant is taken as the optimum point for obtaining the equivalent strain rate and stress. Based on FE results for spherical indentation with various values of creep exponent and creep coefficient, we derive by regression an equation to calculate creep properties using two normalized variables. Finally a program is generated to calculate creep exponent and creep coefficient. With this method, we obtain from the load-depth curve creep exponents with an average error of less than 1.5 % and creep coefficients with an average error of less than 1.0 %.

Prediction of Steady-State Stresses within Heat Affected Zone Due to Creep Mismatch in Welded Straight Pipes (직관 용접부의 크리프 특성 불균일에 따른 열영향부 정상상태 응력 예측)

  • Han, Jae-Jun;Kim, Sang-Hyun;Chung, Jin-Taek;Kim, Yun-Jae
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.37 no.3
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    • pp.405-412
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    • 2013
  • This paper reports the steady-state stresses within the heat affected zone (HAZ) of a welded straight pipe subject to creep. The creep constants and exponent are varied systematically to see the effect of various mismatches in creep properties on the steady-state creep stresses, via detailed two-dimensional finite element (FE) creep analyses. The weldments consist of the base metal and weld metal with the HAZ, which are characterized using the idealized power creep laws with the same creep exponent. The internal pressure and axial loading are considered to see the effect of the loading mode. To quantify the creep stresses, a creep mismatch factor is introduced as a function of the creep constants and exponent. It is concluded that the ratio of the section-averaged stresses for a mismatched case to those for an evenmatched case are linearly dependent on the mismatch factor. The results are compared with the FE results, including the Type IV region, as well as the R5 procedure.

A Study on the Properties of Shrinkage and Creep Deformation in Superplasticized Concrete (유동화 콘크리트의 건조수축 및 크리프 변형특성에 관한 연구)

  • 박승범;임창덕
    • Computational Structural Engineering
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    • v.1 no.2
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    • pp.131-142
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    • 1988
  • This study was carried out to investigate the long-term deformation of superplasticized concrete. Compressive strength, shrinkage, creep and creep recovery of concretes with and without the superplasticizing admixture have been compared for one year. The test results on creep of superplasticized concrete were also compared with three methods of predicting creep; the ACI model, the CEB model and the BP model. According to test results, superplasticized concrete has good results in compressive strength at an age of 28 days of more than 22%, drying-shrinkage cured at air-conditioned storage less than 15%, creep deformation in air conditioned storage and loaded at an age of 28 days to 15% of compressive strength less than 11% of control concrete.

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Evaluation of Creep Models with the Consideration of Concrete's Strength Level (콘크리트 강도대별 크리프 예측모델 평가)

  • Moon, Jae-Heum
    • Journal of the Korea institute for structural maintenance and inspection
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    • v.14 no.4
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    • pp.154-159
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    • 2010
  • Recent concrete structures have been being constructed with higher strength concrete than normal strength concrete. Therefore, it is necessary to review the applicabilities of current design codes and models for the prediction of mechanical behaviors of concrete materials such as creep. To investigate the applicability of creep model with the consideration of the strength level of concrete, three current models (ACI 209R, CEB-FIP MC90 and EC2) were studied and compared with series of experimental results. It was shown that EC2 model which is the updated model of CEB-FIP MC90 better predicts creep coefficient for high strength level concretes and showed relatively good agreements with experimental data.

Rheological, Characterization of Aqueous Poly(Ethylene Oxide) Solutions - Creep and Creep Recovery - (폴리에틸렌옥사이드 수용액의 유변학적 특성 평가 - 크리프 및 크리프 회복 -)

  • 장갑식;김태훈;박영훈;송기원
    • Proceedings of the Korean Fiber Society Conference
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    • pp.175-178
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    • 2001
  • 일반적으로 점탄성 거동을 나타내는 고분자 액체의 전단유동특성(shear flow properties)을 평가하기 위하여 정상전단(steady shear), 동적전단(dynamic shear), 응력완화(stress relaxation) 그리고 크리프(creep) 및 크리프 회복(creep recovery) 실험 등이 활용되고 있다[1], 이때 영전단점도(zero shear viscosity)와 정상상태 회복 컴플라이언스(steady-state recoverable compliance)는 정상상태(steady state)에서 얻어지는 물리량으로, 각 실험방법으로부터 직접적 또는 간접적으로 측정이 가능하다. (중략)

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Estimation Method of Creep Coefficient in Concrete Structures (콘크리트 구조물에서 크리프 계수 추정 방법)

  • Park, Jong-Bum;Park, Jung-Il;Chang, Sung-Pil;Cho, Jae-Yeol
    • Journal of the Korea Concrete Institute
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    • v.21 no.5
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    • pp.619-628
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    • 2009
  • To predict the time-dependent behavior of concrete structures, the models which describe the time-dependent characteristics of concrete, i.e. creep and shrinkage are required. However, there must be significant differences between the displacements that are obtained using the given creep and shrinkage models and the measured displacements, because of the uncertainties of creep and shrinkage model itself and those of environmental condition. There are some efforts to reduce these error or uncertainties by using the model which are obtained from creep test for the concrete in construction site. Nevertheless, the predicted values from this model may be still different from the actual values due to the same reason. This study aimed to propose a method of estimating the creep coefficient from the measured displacements of concrete structure, where creep model uncertainty factor was considered as an error factor of creep model. Numerical validation for double composite steel box and concrete beam showed desirable feasibility of the presented method. Consideration of the time-dependent characteristics of creep as one of the error factors make it possible to predict long-term behaviors of concrete structures more realistically, especially long-span PSC girder bridges and concrete cable-stayed bridges of which major problem is the geometry control under construction and maintenance.

Creep Damage Evaluation of Cr-Mo Steel High-Temperature Pipeline Material for Fossil Power Plant Using Ultrasonic Test Method (초음파법을 이용한 Cr-Mo강 고온배관재료의 크리프손상 평가)

  • Lee, Sang-Guk
    • Journal of the Korean Society for Nondestructive Testing
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    • v.20 no.1
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    • pp.18-26
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    • 2000
  • Boiler high-temperature pipelines such as main steam pipe, header and steam drum in fossil power plants are degraded by creep damage due to severe operating conditions such as high temperature and high pressure for an extended period time. Conventional measurement techniques(replica method, electric resistance method, and hardness test method) for measuring creep damage have such disadvantages as complex preparation and measurement procedures, too many control parameters. And also these techniques have low practicality and applied only to component surfaces with good accessibility. In this paper, artificial creep degradation test and ultrasonic measurement for their creep degraded specimens(Cr-Mo alloy steels) were carried out for the purpose of evaluation for creep damage. Absolute measuring method of quantitative ultrasonic measurement for material degradation was established, and long term creep degradation tests using life prediction formula were carried out. As a result of ultrasonic tests for crept specimens. we conformed that both the sound velocity decreased and attenuation coefficient linearly increased in proportion to the Increase of creep life fraction($\Phi$c).

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Development of High Temperature Creep Properties Evaluation Method using Miniature Specimen (미소시험편을 이용한 고온 크리프 특성 평가법 개발)

  • Yu, Hyo-Sun;Baek, Seung-Se;Lee, Song-In;Ha, Jeong-Soo
    • Proceedings of the KSME Conference
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    • pp.43-48
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    • 2000
  • In this study, a small punch creep(SP-Creep) test using miniaturized specimen$(10{\times}10{\times}0.5mm)$ has been described for the development of the newly semi-destructive creep test method for high temperature structural components such as headers and tubes of boiler turbine casino and rotor and reactor vessel. The SP-Creep testing technique has been applied to 2.25Cr-1Mo(STBA24) steel used widely as boiler tube material and the creep test temperature are varied at $550^{\circ}C{\sim}600^{\circ}C$. The overall deformations of SP-Creep curves are definitely depended with applied load and creep test temperature and show the creep behaviors of three steps like conventional uniaxial creep curves. The steady state creep rate${\delta}_{ss}$ of SP-Creep curve for miniaturized specimen increases with increasing creep temperature, but the exponential value with creep loading is decreased. The activation energy$(Q_{spc})$ during SP-Creep deformation with various test temperatures shows 605.7kJ/mol that is g.eater than 467.4kJ/mol reported in uniaxial creep test. This may be caused by the difference of stress states during creep deformation In two creep test. But from the experimental results, e.g. SP-Creep curve behaviors, the steady state creep rate${\delta}_{ss}$ with creep temperature, and the exponential value(n) with creep loading, it can be summarized that the SP-Creep test may be a useful test method to evaluate the creep properties of the heat resisting material.

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A Study for Creep Effect of the Interfacial Adhesive Layer on the Behavior of Concrete with CFRP (탄소섬유시트로 보강된 콘크리트 구조물 경계면 재료의 크리프 영향 해석)

  • Park, Yong Deuk;Shin, Seung Kyo;Kang, Suk Hwa;Lim, Yun Mook
    • Journal of The Korean Society of Civil Engineers
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    • v.30 no.3A
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    • pp.221-228
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    • 2010
  • External bonding of carbon fiber reinforced polymer (CFRP) sheets has been widely accepted as a popular method for strengthening of deteriorated RC structures. The long-term behavior of CFRP-strengthened RC structure is often affected by that of the interface between CFRP sheets and concrete. This study aims at applying a viscoelastic model to describe the creep behaviour of the adhesive layer bonding CFRP sheet to concrete, the CFRP-concrete interface. Reviews of available models on concrete creep behavior have been first carried out and then new FE analysis model is proposed. The proposed FE analysis model based on the maxwell model has been verified by previous experimental results. It is shown that the creep effect of interfacial adhesive layer is very important on the long-term behavior of concrete structures strengthened with CFRP.