• 대한기계학회논문집
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• 제11권3호
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• pp.355-361
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• 1987

본 연구에서는 고온기기의 부재로 널리 사용되는 304스테인리스강을 사용하여 기기의 사용조건을 고려한 538.deg. C(1000.deg. F), 593.deg. C(1100.deg. F)및 650.deg. C(1200.deg. F)의 세가지 온도레벨하의 대기중에서 크리이프-피로시험을 실시하여 작은 인공표면결함으로부터 발생.성장하는 표면균열의 성장거동을 조사 연구하였다. 또한 주파수의 영향을 조사 하기 위하여 538.deg. C에서 고온피로시험을 실시하여 이 결과를 크리이프 및 크리이프피로 시험의 결과와 비교.검토하였다.

• Journal of Welding and Joining
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• 제18권6호
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• pp.89-95
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• 2000

Creep-fatigue crack growth behavior at the heat affected zone of 1Cr-0.5Mo steel weldment has been experimentally studied. Load hold times of the tests for trapezoidal fatigue waveshapes were varied among 0, 30, 300 and 3,600 seconds. Time-dependent crack growth rates were characterized by the {TEX}$C_{t}${/TEX} estimated with the equation proposed by the previous finite element analysis work. It was concluded that the {TEX}$C_{t}${/TEX} values calculated from the properties of parent metal were quite comparable to the accurate {TEX}$C_{t}${/TEX} values calculated from both of weld and parent metals. Scatter of data was claimed due to the difference of exact location of the cracks in HAZ. The cracks have a tendency to change their path from the original location eventually to the relatively soft HAZ(ie, near-FGHAZ region, fine grained heat affected zone).

• 대한기계학회논문집
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• 제18권3호
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• pp.548-554
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• 1994

The high temperature fatigue crack growth behavior of SUS 304 stainless steel at $550^{\circ}C$ and $650^{\circ}C$ was investigated under various kinds of stress ratio and frequency in sinusoidal waveform on the basis of the non-linear fracture mechanics. The result arranging crack growth rate by modified J-integral J' showed influence of stress ratio and frequency. All the data obtained under the test at $550^{\circ}C$ were plotted within data band of da/dN-${\triangle}J_f$ relationship for cycle-dependent crack growth. On the basis of static creep and cycle-dependent data band; both time- and cycle-dependent crack growth behavior was observed under loading conditions at $650^{\circ}C$, but cycle-dependent crack growth behavior predominantly appeared and time-dependent crack growth behaviour was little observed under loading conditions at $550^{\circ}C$. Fractographic examinations for fracture surface indicated that the fracture mode was generally transgranular. The stripes were found on fracture surface and each stripe was accompanied by a crack tip blunting and an abrupt increase in the load-point displacement. The $J'_{an}$ had a validity in case of $650^{\circ}C, but scarcely had it in case of $550^{\circ}C$.

• 대한기계학회논문집A
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• 제39권1호
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• pp.27-35
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• 2015

본 연구에서는 피로 하중 및 크리프 하중을 받는 Mod.9Cr-1Mo (ASME Grade 91)강 시편에 대한 일련의 실험결과로부터 재료물성치인 고온 균열진전 모델을 개발하였다. 이 균열진전 모델은 크리프-피로하중을 받는 균열체의 결함평가에 사용되는 물성치이다. 한국원자력연구원이 수행한 일련의 피로 균열진전(FCG) 속도 실험 및 크리프 균열진전(CCG) 속도 실험 결과로부터 균열진전 모델을 결정하고, 이를 프랑스의 고온 설계 기술기준인 RCC-MRx 와 비교함으로써 설계 물성치의 보수성에 대해 검토하였다. RCC-MRx 는 FCG 모델 및 CCG 모델을 Section III Tome 6 에서 제공하고 있는데, 실험으로부터 결정한 균열진전 모델과 비교한 결과 RCC-MRx 의 FCG 모델은 보수적인 것으로 나타난 반면 CCG 모델은 비보수적인 것으로 나타나 동 물성치에 대한 검증이 필요한 것으로 나타났다. 또한 본 연구에서는 기계적 강도 및 크리프 시험결과에 대해서도 RCC-MRx 의 물성치와 비교 및 분석하였다.

• 한국해양공학회지
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• 제2권2호
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• pp.104-111
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• 1988

This paper deals with the micro-surface-cracks behavior on the unnotched smooth specimens of Type 304 stainless steel at $593^{\circ}C$ in air under creep and creep-fatigue conditions that have 10 mim and 1 min load holding times respectively. The behaviors of the micro-surface-cracks have been visualized by means of surface replica method and optical micro-photography. The quantitative characteristics of initiation, growth and coalescence of micro-surface-cracks have been investigated by observing and measuring the crack growth behaviors. some of the important results are as follows: Main crack initiates at grain boundary in the early stage(10 to 20%)of its life time and grows through coalescence and finally leads to fracture. The distribution of micro-surface-crack length, 2a, can be plotted against the composite Weibull distribution. The growth rate of the main crack can be plotted against the stress intensity factor, crack tip opering displacement and J integral.

• 한국정밀공학회지
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• 제16권6호
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• pp.100-107
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• 1999

This paper describes a probabilistic remaining life assessment program for the creep crack growth. The probabilistic life assessment program is developed to increase the reliability of life assessment. The probabilistic life assessment involves some uncertainties, such as, initial crack size, material properties, and loading condition, and a triangle distribution function is used for random variable generation. The resulting information provides the engineer with an assessment of the probability of structural failure as a function of operating time given the uncertainties in the input data. This study forms basis of the probabilistic life assessment technique and will be extended to other damage mechanisms.

• 대한금속재료학회지
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• 제47권5호
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• pp.283-289
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• 2009

This paper dealt with a statistical analysis for evaluating the creep crack growth rate (CCGR) for Modified 9Cr-1Mo (ASTM Grade 91) steel. The CCGR data was obtained by the creep crack growth (CCG) tests conducted under various applied loads at $600^{\circ}C$. To obtain logically the B and q values used in the CCGR equation, three methods such as the least square fitting method (LSFM), the mean value method (MVM) and the probabilistic distribution method (PDM) were adopted and their CCGR lines were compared, respectively. In addition, a number of random variables were generated by using the Monte Carlo simulation (MCS), and the CCGR lines were predicted probabilistically. It was found that both the B and q coefficients followed a 2-parameter Weibull distribution well. In the case of the ranges of 10-90% for the probability variables, P(B, q), the CCGR lines were predicted. Fractographic study was conducted from the specimen after the CCG tests.

• 한국해양공학회지
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• 제5권2호
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• pp.207-207
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• 1991

The crack length measurements by electrical potential(EP) method for 1% Cr-Mo-V and 12%Cr steel of 0.5T-CT specimen were performed at $500^{\circ}C, 600^{\circ}C 700^{\circ}C$, and an applicability of stress intensity factor($K_I$), net section stress($\sigma_{net}$), $C^*$-ingegral and $C_t$ parameter was studied to measure creep crack growth rate(da/dt) with side groove and without side groove under static and cyclic loading condition. The experimental result could be summarized as follows: 1) Crack measurement by EP method was available and coincided with the Johnson,s analytical equation. 2) da/dt by $K_I$ and $\sigma_{net}$ was not adequate because of the wide scatter band according to load and temperature, but $C^*$-integral, except for transition region, was adequate. 3) $C_t$ parameter showed the best fitted line through total creep region without relating with both temperature and load condition. 4) Under the cyclic loading condition, $C_t$ parameter was proper to extimate da/dt. And it was shown that da/dt for 1% Cr-Mo V steel under the static condition(R=1) was 1.16 times faster than the case under cyclic loading(R=0), and for 12% Cr steel, 1.43 times.

• 한국해양공학회지
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• 제5권2호
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• pp.67-75
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• 1991

The crack length measurements by electrical potential(EP) method for 1% Cr-Mo-V and 12%Cr steel of 0.5T-CT specimen were performed at $500^{\circ}C, 600^{\circ}C 700^{\circ}C$, and an applicability of stress intensity factor($K_I$), net section stress($\sigma_{net}$), $C^*$-ingegral and $C_t$ parameter was studied to measure creep crack growth rate(da/dt) with side groove and without side groove under static and cyclic loading condition. The experimental result could be summarized as follows: 1) Crack measurement by EP method was available and coincided with the Johnson, s analytical equation. 2) da/dt by $K_I$ and $\sigma_{net}$ was not adequate because of the wide scatter band according to load and temperature, but $C^*$-integral, except for transition region, was adequate. 3) $C_t$ parameter showed the best fitted line through total creep region without relating with both temperature and load condition. 4) Under the cyclic loading condition, $C_t$ parameter was proper to extimate da/dt. And it was shown that da/dt for 1% Cr-Mo V steel under the static condition(R=1) was 1.16 times faster than the case under cyclic loading(R=0), and for 12% Cr steel, 1.43 times.

• 대한기계학회논문집A
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• 제26권2호
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• pp.365-373
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• 2002

For gas turbine engines, the safe life methodology has historically been used fur fatigue life management of failure critical engine components. The safe retirement limit is necessarily determined by a conservative life evaluation procedure, thereby many components which have a long residual life are discarded. The objective of this study is to introduce the damage tolerant design concept into the life management for aircraft engine component instead of conservative fatigue life methodology which has been used for both design and maintenance. Crack growth data were collected on a nickel base superalloy which have been subjected to combined static and cyclic loading at elevated temperatures. Stress analysis fur turbine disk was carried out. The program for computing creep-fatigue crack growth was developed. The residual lifes of turbine disk component under various temperatures and conditions using creep-fatigue crack growth data were estimated. As the result of analysis, it was confirmed that retirement fur cause concept was applicable to the evaluation of residual life of retired turbine disk which had been designed based on the conventional fatigue life methodology.