• Nuclear Engineering and Technology
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• 제36권4호
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• pp.316-326
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• 2004

The $40\%$ of wall criterion, which is generally used for the plugging of steam generator tubes, is applied only to a single crack. In a previous study, a total number of 9 failure models were proposed to estimate the local failure of the ligament between cracks, and the optimum coalescence model of multiple collinear cracks was determined among these models. It is, however known that parallel axial cracks are more frequently detected than collinear axial cracks during an in-service inspection. The objective of this study is to determine the plastic collapse model that can be applied to steam generator tubes containing two parallel axial through-wall cracks. Three previously proposed local failure models were selected as the candidates. Subsequently, the interaction effects between two adjacent cracks were evaluated to screen them. Plastic collapse tests for the plate with two parallel through-wall cracks and finite element analyses were performed to determine the optimum plastic collapse model. By comparing the test results with the prediction results obtained from the candidate models, a COD base model was selected as an optimum model.

• Nuclear Engineering and Technology
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• 제36권4호
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• pp.327-337
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• 2004

It is commonly required that tubes with defects exceeding $40\%$ of wall thickness in depth should be plugged; however, this criterion is too conservative for some locations and for some types of defects. Many studies have been done with the aim of developing an alternative plugging criteria, and these studies have shown that steam generator tubes with a certain range of axial through-wall cracks could remain in service without any safety or reliability problems. However, these studies have been limited, thus far, to consideration of single cracked tubes, necessitating a study on multiple cracks, which are commonly found. A crack coalescence model applicable to steam generator tubes with two collinear axial through-wall cracks was proposed in the previous study. In this paper, the investigation is extended to the parallel axial cracks spaced in a circumferential direction, because parallel axial cracks are more frequently detected during in-service inspections than collinear axial cracks. Interaction effects between two parallel cracks are evaluated by performing elastic and elastic-plastic finite element analyses.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.170-177
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• 2003

This paper proposes a robust method for the Ramberg-Osgood (R-O) fit to accurately estimate elastic-plastic J from engineering fracture mechanics analysis based on deformation plasticity. The proposal is based on engineering stress-strain data to determine the R-O parameters, instead of true stress-strain data. Moreover, for practical applications, the method is given not only for the case when full stress-strain data are available but also for the case when only yield and tensile strengths are available. Reliability of the proposed method for the R-O fit is validated against detailed 3-D Finite Element (FE) analyses for circumferential through-wall cracked pipes under global bending using five different materials, three stainless steels and two ferritic steels. Taking the FE J results based on incremental plasticity using actual stress-strain data as reference, the FE J results based on deformation plasticity using various R-O fits are compared with reference J values. Comparisons show that the proposed R-O fit provides more accurate J values for all cases, compared to existing methods for the R-O fit. Advantages of the proposed R-O fit in practical applications are discussed, together with its accuracy.

• 한국정밀공학회지
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• 제22권10호
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• pp.137-143
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• 2005

The integrity of steam generator tubes in nuclear power plant should be maintained sufficiently during operation. For sake of this, complicated assessment procedures are required such as fracture mechanics analysis, etc. The integrity assessment of tubes has been performed by using conventional deterministic approaches while there are many uncertainties to carry out a rational evaluation. In this respect, probabilistic integrity assessment is considered as an alternative method for integrity assessment. The objectives of this study are to develop an integrity assessment system based on probabilistic fracture mechanics and to predict the failure probability of steam generator tubes containing an axial through-wall crack. The developed integrity assessment system consists of three evaluation modules, which apply first order reliability method, second order reliability method and Monte Carlo simulation method, respectively. The system has been applied to predict failure probability of steam generator tubes and the estimation results showed a promising applicability of the probabilistic integrity assessment system.

• Corrosion Science and Technology
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• 제3권6호
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• pp.233-239
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• 2004

Primary water stress corrosion cracking of steam generator tubings occur on many tubes in pressurized water reactors(PWRs), and they are repaired using sleeves or plugs. In order to develop proper repair criteria, it is necessary to know the leak behavior of the tubes, which have stress corrosion cracks. Crack development tests were carried out on the tubes at room temperature, and leak rate and burst pressure were measured on the degraded tubes at room temperature and a high temperature. No leakage was detected on the tubes where 100 % through wall crack developed, at 1560 psi, which is an operating pressure difference of pressurized water reactors(PWRs). In some tests, leak rates of the tubes increased with time at a constant internal water pressure. A test tube showed a very small amount of leakage at 2700 psi in a high temperature pressure test at $282^{\circ}C$, but it disappeared after the pressure increased slightly. Even cracks are 100 % through wall, they need to open in order to reach a certain amount of leak rate at the operating pressure difference.

• 대한기계학회논문집A
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• 제27권9호
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• pp.1571-1578
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• 2003

This paper proposes a robust method for the Ramberg-Osgood(R-O)fit to accurately estimate elastic-plastic J from engineering fracture mechanics analysis based on deformation plasticity. The proposal is based on engineering stress-strain data to determine the R-O parameters, instead of true stress-strain data. Moreover, for practical applications, the method is given not only for the case when full stress-strain data are available but also for the case when only yield and tensile strengths are available. Reliability of the proposed method for the R-O fit is validated against detailed 3-D Finite Element (FE) analyses for circumferential through-wall cracked pipes under global bending using five different materials, three stainless steels and two ferritic steels. Taking the FE J results based on incremental plasticity using actual stress-strain data as reference, the FE J results based on deformation plasticity using various R-O fits are compared with reference J values. Comparisons show that the proposed R-O fit provides more accurate J values for all cases, compared to existing methods for the R-O fit. Advantages of the proposed R-O fit in practical applications are discussed, together with its accuracy.

• 한국구조물진단유지관리공학회 논문집
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• 제5권1호
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• pp.195-205
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• 2001

An analytical finite element approach to nonlinear behavior of reinforced concrete shear walls with opening under monotonic loading was presented in this paper. In order to achieve the objectives of present paper, the orthogonal anisotropic models for cracked reinforced concrete element based on smeared crack concept were used as the nonlinear material models of biaxial state of stress. The stiffness of cracked concrete was evaluated through the combined use of tension and compression stiffness models in and parallel directions of crack, respectively and shear transfer effect due to the aggregate interlocking at crack surface. The stress and strain of reinforcement in concrete was evaluated using the average stress and average strain relation with bond effect. based on smeared crack concept. The diagonal reinforcing bar was modeled using truss element with bond effect. A special significance of diagonal reinforcement near opening was given to the shear wall with opening and an effective distribution of diagonal reinforcement was presented in order to give an ultimate strength increment as well as a crack control.

• Nuclear Engineering and Technology
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• 제54권6호
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• pp.2188-2197
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• 2022

To ensure the safety margin of a reactor pressure vessel (RPV) under normal operating conditions, it is regulated through the pressure-temperature (P-T) limit curve. The stress intensity factor (SIF) obtained by the internal pressure and thermal load should be obtained through crack analysis of the nozzle corner crack in advance to generate the P-T limit curve for the nozzle. In the ASME code Section XI, Appendix G, the SIF via the internal pressure for the nozzle corner crack is expressed as a function of the cooling or heating rate, and the wall thickness, however, the SIF via the thermal load is presented as a polynomial format based on the stress linearization analysis results. Inevitably, the SIF can only be obtained through finite element (FE) analysis. In this paper, simple prediction equations of the SIF via the thermal load under, cool-down and heat-up conditions are presented. For the Korean standard nuclear power plant, three geometric variables were set and 72 cases of RPV models were made, and then the heat transfer analysis and thermal stress analysis were performed sequentially. Based on the FE results, simple engineering solutions predicting the value of thermal SIF under cool-down and heat-up conditions are suggested.

• 한국안전학회지
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• 제15권2호
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• pp.51-58
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• 2000

Recently, the J/T analysis based on elastic-plastic finite element analysis is popularly used in the nuclear industry to assess the integrity of a cracked pipe. The objective of this paper is to evaluate the effect of stress-strain curve for weld metal, variation of crack incremental length(${\delta}a$), and crack face pressure on the J/T analysis result. For this purpose, a parametric analysis was performed and the results calculated from finite element analysis were compared with those from the piping experimental data(stainless steel weldment pipe with circumferential through-wall crack). The numerical result using base metal material property is in agreement with the experimental one and the maximum load is decreased as the ${\delta}a$ for J/T analysis is increased.

• 대한기계학회논문집A
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• 제31권4호
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• pp.496-505
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• 2007

This paper proposes closed-form plastic limit load solutions for elbows under in-plane bending, via three-dimensional (3-D), small strain FE limit analyses using elastic-perfectly plastic materials. A wide range of elbow and thinning geometries are considered. For systematic analyses of the effect of the axial thinning length on limit loads, two limiting cases are considered; a sufficiently long wall thinning, and the circumferential part-through surface crack. Closed-form plastic limit load solutions for wall thinning with intermediate longitudinal extents are then obtained from these two limiting cases. The effect of the axial extent of wall thinning on plastic limit loads for elbows is highlighted by comparing that for straight pipes. Although the proposed solutions are developed for the case when wall thinning exists in the center of elbows, it is also shown that they can be applied to the case when wall thinning exists anywhere within the elbow.