• Journal of the Korean Society for Precision Engineering
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• v.10 no.3
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• pp.97-106
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• 1993

This is the study on fracture life under the interaction of creep and fatigue. It is difficult to explain the interaction of the creep and fatigue with indication of frequency but the dependency of the time should be considered. The formulation of material varieties causing by interaction of creep and fatigue is required in the accumulative damage method. The strain range partition method requires some of modification corresponding to the changes in temperature and load. All of other method also comprehended with above mentioned problems. Generally, in this field, the variety of stress-strain and suitable parameter is required and connective study between the macro and micro results seems to be insufficient. The linear damage rule is acquiring the support generally but it requires modification in the hgigh temperature instruments. The variety of stress effecting on crack and variety of stress on the metallurgical side are considered to be problems in the future days.

• Journal of Mechanical Science and Technology
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• v.20 no.12
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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.

• Nuclear Engineering and Technology
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• v.53 no.7
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• pp.2407-2417
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• 2021

A structural integrity evaluation program (STEP) was developed for the high temperature reactor design evaluation according to the ASME Boiler and Pressure Vessel Code (ASME B&PV), Section III, Rules for Construction of Nuclear Facility Components, Division 5, High Temperature Reactors, Subsection HB. The program computerized HBB-3200 (the design by analysis procedures for primary stress intensities in high temperature services) and Appendix T (HBB-T) (the evaluation procedures for strain, creep and fatigue in high temperature services). For evaluation, the material properties and isochronous curves presented in Section II, Part D and HBB-T were computerized for the candidate materials for high temperature reactors. The program computerized the evaluation procedures and the constants for the weldment. The program can generate stress/temperature time histories of various loads and superimpose them for creep damage evaluation. The program increases the efficiency of high temperature reactor design and eliminates human errors due to hand calculations. Comparisons that verified the evaluation results that used the STEP and the direct calculations that used the Excel confirmed that the STEP can perform complex evaluations in an efficient and reliable way. In particular, fatigue and creep damage assessment results are provided to validate the operating conditions with multiple types of cycles.

• Korean Journal of Materials Research
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• v.17 no.8
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• pp.425-432
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• 2007

The microstructural evolution of ferritic 9Cr-1Mo-V-Nb steel, subjected to creep-fatigue at $550^{\circ}C$, was evaluated nondestructively by measuring the ultrasonic velocity. The variation of the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region ($N/N_f$<0.2), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of $0.2N_f$ and $0.8N_f$, and then a decrease in the final region. The change of the ultrasonic velocity during creep-fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.

• 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.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.12 no.1
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• pp.70-77
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• 2016

In this paper, the structural design and integrity evaluations for a reactor vessel of PGSFR sodium-cooled fast reactor(150MWe) are carried out in compliance with ASME BPV III, Division 5 Subsection HB. The reactor vessel is designed with a direct contact of primary sodium coolant to its inner surface and has a double vessel concept enclosing by containment vessel. To assure the structural integrity for 60 years design lifetime and elevated operating temperature of $545^{\circ}C$, which can invoke creep and creep-fatigue damage, the structural integrity evaluations are carried out in compliance with the ASME code rules. The design loads considered in this evaluations are primary loads and operation thermal cycling loads of normal heat-up and cool-down. From the evaluations, the PGSFR reactor vessel satisfies the ASME code limits but it was found that there is a little design margin of creep damage for inner surface at the region of cold pool free surface.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.5
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• pp.417-425
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• 2007

The ferritic 9Cr-1Mo-V-Nb heat-resisting steel was experimentally studied in order to characterize its microstructural evolution during creep-fatigue by coercivity measurement. The creep-fatigue test was conducted at $550\;^{\circ}C$ with the tensile holding time of 60s and 600s, respectively. The coercivity decreased until the failure and the hardness monotonously decreased for the whole fatigue life. As the life fraction of creep-fatigue increased, the $M_{23}C_6$ carbide coarsened following the Ostwald ripening mechanism. However, the MX carbonitrides did not grow during creep-fatigue due to so stable at $550\;^{\circ}C$. The width of martensite lath increased because of the dislocation recovery at the lath boundaries. The magnetic coercivity has an influence on the microstructural properties such as dislocation, precipitates and martensite lath boundaries, which interpreted in relation to microstructural changes. Consequently, this study proposes a magnetic coercivity to quantify the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.10
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• pp.1251-1259
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• 2013

In this study, high temperature design and creep-fatigue damage evaluation of a decay heat exchanger (DHX) in the decay heat removal systems of a sodium-cooled fast reactor (SFR) have been performed. Detail design and 3D finite element analysis have been conducted for the DHXs to be installed in active and passive decay heat removal systems in Korean Generation IV SFR, and the DHX installed in the STELLA-1(Sodium integral effect test loop for safety simulation and assessment) at KAERI (Korea Atomic Energy Research Institute). Evaluations of creep-fatigue damage based on full 3D finite element analyses were conducted for the two Mod.9Cr-1Mo steel heat exchangers according to the elevated temperature design codes of ASME Section III Subsection NH and RCC-MR code. Code comparisons were made based on the creep-fatigue damage evaluation and issues on conservatisms of the design codes were discussed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.4
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• pp.1194-1201
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• 1996

For the case of creep-fatigue interaction, the damage zone developed in front of the growing crack-tip during creep regime is important because it can affect the damage mechanism to be occured by the following fatigue load. These are studied in theis paper through proper consideration of the cavitiy-size dependent sintering stress which is approximated by polynomials. It is shown that the inclination of reversed damage zone size with respect to the applied load parameter can be explained by considering realistic sintering stress distribution. However, the resultant stress field has $r^{1/2+\theta}$ singularity, regardliss of the profile of variable sintering stress, which is the same to that case solved for constant sintering stress.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.04a
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• pp.365-370
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• 1998

The creep strain of the compression zone of concrete beams subjected to cyclic loading should be a significant factor in increasing strain and deflections. An analytical model which is similar to a previous one is presented to predict the increase in cyclic creep strain and the damage using the properties of the constituent materials: concrete and steel. The analytical expressions are compared with our experimental data. The effect of concrete-creep is accounted by the term En, Icr,n, and Mcr,n. In this study, it is proved that cyclic creep exponents 'n' in Cyclic Creep Model, according to the parameters -strength, range of stress- have the various values. It is hoped that with the availability of more experimental data and better understanding of some of the complex behavior, the model could be further improved.