• Nuclear Engineering and Technology
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• v.47 no.6
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• pp.712-718
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• 2015

Cast duplex stainless steels are susceptible to thermal aging during long-term service at temperatures ranging from $280^{\circ}C$ to $450^{\circ}C$. To analyze the effect of thermal aging on leak-before-break (LBB) behavior, three-dimensional finite element analysis models were built for circumferentially cracked pipes. Based on the elasticeplastic fracture mechanics theory, the detectable leakage crack length calculation and J-integral stability assessment diagram approach were carried out under different bending moments. The LBB curves and LBB assessment diagrams for unaged and thermally aged pipes were constructed. The results show that the detectable leakage crack length for thermally aged pipes increases with increasing bending moments, whereas the critical crack length decreases. The ligament instability line and critical crack length line for thermally aged pipes move downward and to the left, respectively, and unsafe LBB assessment results will be produced if thermal aging is not considered. If the applied bending moment is increased, the degree of safety decreases in the LBB assessment.

• Journal of Welding and Joining
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• v.28 no.3
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• pp.79-85
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• 2010

Hardfacing is one of the frequently applying method to increase surface hardness in hot forging die. Recently, hardfacing receives great attention due to it's repair availability and low cost. In hot forging die, crack resistance and thermal shock resistance have been considered as major properties, However there are few studies for the assessment of these properties. So, it is necessary to establish the assessment method for crack resistance and thermal shock resistance in hardfacing for hot forging die. In this study, flux cored arc welding was applied to make hardfacing welds. Three point bending test was carried out to assess hardfacing weld's crack resistance, and high temperature bending test using salt bath was developed for thermal shock resistance. Consequently, it was possible to assess crack resistance and thermal shock resistance of hardfacing welds for hot forging die quantitatively.

• Journal of the Society of Naval Architects of Korea
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• v.54 no.5
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• pp.430-438
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• 2017

This study presents the reliability-based flaw assessment for the crack in the mooring chain of a floating type offshore structure. BSI(British Standard Institution) flaw assessment procedure BS7910 was combined with first- and second-order reliability method (FORM, SORM) so that the acceptance of a given flaw can be assessed considering the uncertainties of parameters that play important role in the flaw assessment. Considering the probabilistic nature of the crack size and long-term distribution of the stresses acting on the crack in mooring chain, the failure probability was calculated using FORM and SORM. To check the validity of the FORM and SORM, Monte Carlo simulation was also carried out to derive the true limit state function and compared with the results of FORM and SORM.

• Proceedings of the Korea Concrete Institute Conference
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• 1995.04a
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• pp.286-291
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• 1995

The deterioration of PC box girder may cause serious effect on the durability of PC structure compared to that of RC structures. In the durability assessment of PC box girder bridges, a quantitive model on crack width is considered as a measure of durability. This study suggests a durability limit state model for PC box girder bridges. This durability limit state model in formulated based on the conventional models on the cracks in concrete. And the allowable crack width is taken as an assumed value established by the design specification or provided by the maintenance authority of the structure.

• Nuclear Engineering and Technology
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• v.47 no.3
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• pp.332-339
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• 2015

The leak before break (LBB) concept is widely used in designing pipe lines in nuclear power plants. According to the concept, the amount of leaking liquid from a pipe should be more than the minimum detectable leak rate of a leak detection system before catastrophic failure occurs. Therefore, accurate estimation of the leak rate is important to evaluate the validity of the LBB concept in pipe line design. In this paper, a program was developed to estimate the leak rate through circumferential cracks in pipes in nuclear power plants using the Henry-Fauske flow model and modified Henry-Fauske flow model. By using the developed program, the leak rate was calculated for a circumferential crack in a sample pipe, and the effect of the flow model on the leak rate was examined. Treating the crack morphology parameters as random variables, the statistical behavior of the leak rate was also examined. As a result, it was found that the crack morphology parameters have a strong effect on the leak rate and the statistical behavior of the leak rate can be simulated using normally distributed crack morphology parameters.

• Journal of the Society of Naval Architects of Korea
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• v.30 no.4
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• pp.74-82
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• 1993

The present paper deals with the structural safety assessment techniques for independent type B spherical LNG tank against fatique crack initiation and propagation, which contains fellowing 3 steps. 1) Prediction of long term distribution of wave induced stresses and fatique crack intiation life using cumulative damage theory which were described at the 1st report. 2) Surface crack propagation analysis to verify that initial defects cannot penetrate tank plate. 3) Passing-through fatigue crack propagation analysis was performed based on LBF(Leak Before Failure ) theory.

• International journal of steel structures
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• v.18 no.4
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• pp.1242-1251
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• 2018

Not all loads contribute to fatigue crack propagation in the welded detail of steel bridges when they are subjected to variable amplitude loading. For fatigue assessment, therefore, non-contributing stress cycles should be truncated. However, stress range truncation is not considered during typical fatigue reliability assessment. When applying the first order reliability method, stress range truncation occurs mismatch between the expected number of cycles to failure and the number of cycles obtained at the time of evaluation, because the expected number of cycles only counts the stress cycles that contribute to fatigue crack growth. Herein, we introduce a calibration factor to coordinate the expected number of cycles to failure to the equivalent value which includes both contributing and non-contributing stress cycles. The effectiveness of stress range truncation and the proposed calibration factor was validated via case studies.

• Smart Structures and Systems
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• v.30 no.4
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• pp.385-395
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• 2022

Detecting cracks on a concrete structure is crucial for structural maintenance, a crack being an indicator of possible damage. Conventional crack detection methods which include visual inspection and non-destructive equipment, are typically limited to a small region and require time-consuming processes. Recently, to reduce the human intervention in the inspections, various researchers have sought computer vision-based crack analyses: One class is filter-based methods, which effectively transforms the image to detect crack edges. The other class is using deep-learning algorithms. For example, convolutional neural networks have shown high precision in identifying cracks in an image. However, when the objective is to classify not only the existence of crack but also the types of cracks, only a few studies have been reported, limiting their practical use. Thus, the presented study develops an image processing procedure that detects cracks and classifies crack types; whether the image contains a crazing-type, single crack, or multiple cracks. The properties and steps in the algorithm have been developed using field-obtained images. Subsequently, the algorithm is validated from additional 227 images obtained from an open database. For test datasets, the proposed algorithm showed accuracy of 92.8% in average. In summary, the developed algorithm can precisely classify crazing-type images, while some single crack images may misclassify into multiple cracks, yielding conservative results. As a result, the successful results of the presented study show potentials of using vision-based technologies for providing crack information with reduced human intervention.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.4
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• pp.432-440
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• 2007

In this study, we have investigated the closure behavior of fatigue cracks in SAPH440 steel plates under a mixed-mode I+II loading. A crack image capturing system as a direct measuring method was used to measure the closure levels at a crack tip. The crack closure levels in the fluctuation and stable sections were increased with the increase of the mode mixture ratio. The mode mixture ratio independent fatigue crack propagation rates equation was calculated by considering mixed-mode crack closure levels. The equation was examined according to the application method of crack opening ratio. The fracture surface analysis by C-scan method was also performed in order to investigate the closure mechanism and propagation mode of crack under the mixed-mode I+II loading. The crack closure under the mixed mode I+II is confirmed as a surface roughness closure by the quantitative analysis of fracture surface using the proposed surface roughness parameter.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.11 s.170
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• pp.2022-2032
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• 1999

In recent years, the subject of remaining life assessment has drawn considerable attention in the power generation industry. In power generation systems a variety of structural components, such as steam pipes, turbine rotors, and superheater headers, typically operate at high temperatures and high pressures. Thus a life prediction methodology accounting for fracture and rupture is increasingly needed for these components. For accurate failure assessment, in addition to the single parameter such as K or J-integral used in traditional fracture mechanics, the second parameter like T-stress describing the constraint is needed. The most critical defects in such structures are generally found in the form of semi-elliptical surface cracks in the welded piping-joints. In this work, selecting the structures of surface-cracked plate and straight pipe, we first perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework for including constraint effects (T-stress effects) in the R6 failure assessment diagram approach for fracture assessment.