• Journal of the KSME
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• v.27 no.2
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• pp.132-136
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• 1987

최근 저자는 혼합모우드에 대한 Budiansky와 Rice의 $J_k$ 적분에 모순이 있음을 지적한 바 있다. 그러나, Kishimoto등은 파괴진행영역을 고려한 경로 독립적분J/^/을 제시하여 주목을 끌고 있다. Landes등은 Rice의 J적분을 정상상태 크리이프에까지 연장 적용하기 위한 $C^*$적분을 소개하였다. 그후 크리이프파괴에 대한 매개변수로서 Liu등에 의한 $C_{gk}{\;}^*$, Brust등에 의한 $T_k{\}^*$등이 소개되어 계속 연구중이다. 여기에서는 이러한 매개변수들에 대해 개괄적으로 서술하고자 한다.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1991.10a
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• pp.96-101
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• 1991

프리스트레스트 콘크리트 교량은 콘크리트와 PS 강재의 역학적 장점들을 십분 활용하여 경제적인 단면 구성이 가능한 반면, 서로 다른 두 재료의 복합적인 특성들 즉, 콘크리트의 크리이프 (creep), 건조수축 (shrinkage)과 PS 강재의 이완 (relaxation) 등과 같이 시간에 따라 변화하는 인자들로 인하여 복잡한 구조적 거동을 보여 해석상 어려움이 따른다. 이와같은 복합거동은 시공순서와 시공방법에 의해 시공중의 구조계와 지지조건 등이 변화하는 경우에는 더욱 복잡한 양상을 띄게 된다. 뿐만 아니라 박스 거더와 같은 박벽요소에서는 일반적인 보요소(beam element)로는 나타낼 수 없는 ？(warping)을 무시할 수 없으므로 ？ 자유도를 구현할 수 있는 특수한 기능의 구조해석용 프로그램의 개발이 절실히 요망된다. 그리고 시공단계별로 출력되는 많은 양의 수치결과들을 설계와 시공실무자에 지향된 형태로 조합, 변형시켜 그래픽 화면상에 나타내는 후처리 프로그램(Post-processor) 기능도 구조해석용 프로그램의 개발 못지않게 중요하다. 본 연구의 목적은 삼성종합건설과 서울대 토목공학과가 공동으로 ？(warping) 자유도를 포함한 7개의 자유도를 갖는 3차원 보요소를 사용하여 PS 콘크리트의 재료적 특성인 크리이프와 건조수축, 그리고 강재의 이완(relaxation)을 포함한 프리스트레싱력의 손실을 고려할 수 있고 시공 단계별 구조계의 변화 및 지지조건들의 다양한 변화를 효율적으로 모사할 수 있는 프로그램의 개발에 있다.

• Transactions of the Korean Society of Automotive Engineers
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• v.13 no.3
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• pp.186-192
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• 2005

The apparent activation energy Qc, the applied stress exponent n, and rupture life have been determined from creep test results of AZ31 Mg alloy over the temperature range of 200$^{\circ}C$ to 300$^{\circ}C$ and the stress range of 23.42 MPa to 93.59 MPa, respectively, in order to investigate the creep behavior. Constant load creep tests were carried out in the equipment including automatic temperature controller with data acquisition computer. At the temperature of $200^{\circ}C{\sim}220^{\circ}C$ and under the stress level of 62.43~93.59 MPa, and at around the temperature of $280^{\circ}C{\sim}300^{\circ}C$ and under the stress level of 23.42~39.00 MPa, the creep behavior obeyed a simple power-law relating steady state creep rate to applied stress and the activation energy fur the creep deformation was nearly equal to that of the self diffusion of Mg alloy including aluminum From the above results, at the temperature of $200^{\circ}C{\sim}220^{\circ}C$ the creep deformation for AZ31 Mg alloy seemed to be controlled by dislocation climb but controlled by dislocation glide at $280^{\circ}C{\sim}300^{\circ}C$ .And relationship beween rupture time and stress at around the temperature of $200^{\circ}C{\sim}220^{\circ}C$ and under the stress level of 62.43~93.59 MPa, and again at around the temperature of $280^{\circ}C{\sim}300^{\circ}C$ and under the stress level of 23.42~39.00 MPa, respectively, appeard as fullow; log$\sigma$=-0.18(T+460)(logtr+21)+5.92, log$\sigma$ = -0.25(T+460)(logtr+21)+8.02 Also relationship beween rupture time and steady state creep rate appears as follow; ln$\dot$ =-0.881ntr-2.45

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.3
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• pp.355-361
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• 1987

Creep and fatigue tests were carried out on crack growth properties of small surface cracks in 304 stainless steel at 538.deg.C, 593.deg. C and 650.deg. C in air, by using small plate specimens with a small artificial pit. All the data of the crack growth rate per hour obtained in the present tests were correlated with the maximum stress intensity factor, so that the applicability of linear fracture mechanics to the crack growth of surface cracks at elevated temperature was investigated. In the creep test, relatiion of .sigma.$\^$n/.t$\_$f/=C is obtained between failure time and nominal stress at each temperature level, where n has the value of 11-14 depending on the temperature level. In the creep and fatigue crack growth properties of surface cracks at the elevated temperatures, the maximum stress intensity factor, $_{4}$$\_$max/, is some extent applicable parameter to describe the surface crack growth rate under the present experimental conditions. The crack growth rate per hour increases when the holding time decreases, and creep crack growth rate per hour becomes the lowest limit of crack growth rate per hour in this tests.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.5
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• pp.539-547
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• 1985

The low-cycle fatigue crack growth behavior of SUS 304 Stainless steel was investigated at 650.deg. C by the nonlinear fracture mechanics. Crack Propagation can be separated in to cycle-dependent and time-dependent, the former is correlated with .DELTA. $J_{f}$ , J-intergral range and the latter is correlated with J', modified J integral. Transition from cycle-dependent to time-dependent crack growth was successfully predicted using the .betha. hypothesis, which was proposed by the authors on the basis of an analysis on the interaction of elastic and creep strain. To investigate the reliability of .betha.-hypothesis, experimenting by the change of stress-level, stress rate and frequency, following conclusions were obtained. (1) High temperature fatigue crack propagation was separated into cycle-dependent and time-dependent. (2) Transition of crack propagation was predicted by .DELTA. $J_{c}$/.DELTA.$_{f}$ or .betha. (3) Lower limit in cycle-dependent crack propagation was obtained..

• Journal of Power System Engineering
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• v.9 no.4
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• pp.96-101
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• 2005

Magnesium alloys have been widely used for many structural components of automobiles and aircraft because of high specific strength and good cast ability in spite of hexagonal closed-packed crystal structure of pure magnesium. In this study, uniaxial tension tests at high temperature and creep tests are done in order to investigate the characteristics of high temperature and mechanisms for creep deformation of AZ31 Mg alloy. Yield stress and ultimate tensile stress decreased with increasing temperature, but elongation increased from results of uniaxial tension test at high temperature. The apparent activation energy Qc, the applied stress exponent n and rupture life have been determined during creep of AZ31 Mg alloy over the temperature range of 473K to 573K and stress range of 23.42 MPa to 93.59 MPa, respectively, in order to investigate the creep behavior. Constant load creep tests were carried out in the equipment including automatic temperature controller, whose data are sent to computer. At around the temperature of $473K{\sim}493K$ and under the stress level of $62.43{\sim}93.59%MPa$, and again at around the temperature of $553K{\sim}573K$ and under the stress level of $23.42{\sim}39.00MPa$, the creep behavior obeyed a simple power-law relating steady state creep rate to applied stress and the activation energy for the creep deformation was nearly equal, respectively, and a little low to that of the self diffusion of Mg alloy including aluminum. Also rupture surfaces at high temperature have had bigger dimples than those at lower temperature by SEM.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.5
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• pp.73-78
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• 2006

In this paper, creep tests of Mg-Zn-Mn and Mg-Zn-Mn-Ca alloys, which were casted by mold with Mg-3%Zn-1%Mn and Mg-3%Zn-1%Mn-0.2%Ca, were done under the temperature range of 473-573K and the stress range of 23.42-78.00Mpa. The activation energies and the stress exponents were measured to investigate the creep plastic deformation of those alloys, and the rupture lifes of Mg-Zn-Mn alloy were also measured to investigate the fracture behavior. From the results, the activation energy of Mg-Zn-Mn and Mg-Zn-Mn-Ca alloys under the temperature range of 473-493K were measured as 149.87, 145.98KJ/mol, respectively, and the stress exponent were measured as 5.13, 6.06 respectively. Also the activation energies Mg-Zn-Mn and Mg-Zn-Mn-Ca alloys under the temperature range of 553-573K were obtained as 134.41, 129.22KJ/mol, respectively, and tress exponent were obtained as 3.48, 4.63, respectively. Finally stress dependence of rupture life and the activation energy of rupture life of Mg-Zn-Mn under the temperature range of 473-493K was measured as 8.05, 170.0(KJ/mol), respectively, which were a little higher than the results of steady state creep.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.1
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• pp.71-81
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• 1994

A method is presented for the analysis of curved segmentally erected prestressed concrete box girder bridges including time-dependent effects due to load history, temperature history, creep, shrinkage, aging of concrete and relaxation of prestressing steel. The segments can be either precast or cast-in-place. Thin-walled beam theory and finite element method are combined to develop a curved nonprismatic thin-walled box beam element. The element consists of three nodes and each node has eight displacement degrees of freedom, including transverse distortion and longitudinal warping of the cross section.

• Journal of Ocean Engineering and Technology
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• v.9 no.2
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• pp.98-111
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• 1995

High temperature low cycle fatigue shows that cycle-dependent crack growth owing to cyclic plastic deformation occurred simultaneosly with time-dependent crack growth owing to intergranular deformation. Consequently, to estimate crack growth rate uniquely, many to investigators have proposed various kinds of parameters and theories but these could not produce satisfactory results. Therefore the goal of this study is focused on prediction of crack growth rate using predominant damage rule, linear cumulative damage rule and transitional parameter ${\Delta}J_c/{\Delta}J_f$. On the basis of these sinusoidal loading waveform at 600$^{\circ}C$ and 700$^{\circ}C$.

• Magazine of the Korean Society of Agricultural Engineers
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• v.28 no.2
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• pp.74-86
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• 1986

유동화콘크리트의 공학적 특성에서 시간-의존거동을 확인하기 위하여,나프타렌 설폰산염 폴리머 고축합물인 Rheobuild 1000과 메라민 설폰산염 고축합물인 NP-20의 고성능감수제를 사용한 유동화콘크리트와 보통콘크리트를 제조하여 비교.고찰을 행하였으며, 고성능감수제의 종류 및 함량이 콘크리트의 공학적 특성에 미치는 영향을 구명하기 위하여 재령 3일, 14일, 28일, 60일, 90일, 180일의 압축강도를 측정, 조기 및 장기압축강도를 조사하였고, 인장강도 및 탄성 변형에 미치는 영향을 조사하였다. 또한 습윤 및 에어콘디션의 양생조건하에서 시간의 경과에 따른 건조수축 및 크리이프 변형을 조사.분석함으로써 유동화콘크리트의 시간-의존거동을 확인하였다. 실험결과, 사용 고성능 감수제의 종류에 따라 차이는 있으나, 고성능 감수제의 사용은 일반적으로 워커빌리티 성능을 개선하고 압축 및 인장도를 크게 향상시키며, 탄성계수는 보통의 콘크리트에 비하여 높게 나타났다. 또한 건조수축 및 크리프 변형의 감소에 매우 양호한 결과를 나타내어 앞으로 건설용 용도로써 효과적인 것으로 판단되었다.