• Journal of Korean Society for Quality Management
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• v.43 no.4
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• pp.471-488
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• 2015

Purpose: Double sampling $T^2$ chart is a useful tool for detecting a relatively small shift in process mean when the process is controlled by multiple variables. This paper finds the optimal design of the double sampling $T^2$ chart in both economical and statistical sense under Weibull failure model. Methods: The expected cost function is mathematically derived using recursive equation approach. The optimal designs are found using a genetic algorithm for numerical examples and compared to those of single sampling $T^2$ chart. Sensitivity analysis is performed to see the parameter effects. Results: The proposed design outperforms the optimal design of the single sampling $T^2$ chart in terms of the expected cost per unit time and Type-I error rate for all the numerical examples considered. Conclusion: Double sampling $T^2$ chart can be designed to satisfy both economic and statistical requirements under Weibull failure model and the resulting design is better than the single sampling counterpart.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05b
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• pp.691-698
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• 1996

This paper propose a procedure to estimate the system lifetime distribution using simulation method in a parametric framework and also develop the criterion for terminating the simulation. We assume that a system is composed of many components whose lifetime and repair time distributions are general, and repair of each component is imperfect or not. General simulation algorithms can not be adopted for this case, due to the dependency of successive operating times and the discontinuity in base line intensity function of failure process. Then we propose algorithms for generating failure times subject to imperfect repair. We develop the event time tracking logic for identifying the system failure time, and also develop the criterion for terminating the simulation. Our procedure is composed of two phases. The first phase of the procedure is to generate the system failure times from the inputs. The second phase is to estimate the lifetime distribution of the system. The best model is selected by a fully automated procedure among well-known parametric families, and the required parameters are estimated. We give examples to show the accuracy of our procedure and the effect of repair effect of components to system MTTF(Mean Time To Failure).

• Nuclear Engineering and Technology
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• v.54 no.12
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• pp.4522-4533
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• 2022

For severe accident assessment of reactor pressure vessel (RPV), it is important to develop an accurate model that can predict transient thermo-mechanical behavior of the RPV lower head under the given condition. The present study revisits the lower head failure with two- and three-dimensional finite element models. In particular, we aim to give clear insight regarding the effect of the three-dimensionality present in the distribution of the thickness and thermal load of the lower head. For a rigorous validation of the result, both the OLHF-1 and the OLHF-2 tests are considered in this study. The result suggests that the three-dimensional effect is not negligible as far as the failure location is concerned. The non-uniformity of the thickness distribution is found to affect the failure location and time. The thermal load, which may not be axisymmetric in general, has the most significant effect on the failure assessment. We also observe that the creep property can affect the global deformation of the lower head, depending on the applied mechanical load.

• Journal of the Korea Concrete Institute
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• v.14 no.1
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• pp.110-117
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• 2002

In a strengthened bridge deck which received increased service loads, failure patterns of bridge deck vary depending on deck thickness, compressive strength of concrete, yielding strength of reinforcement, reinforcement ratio and additional strengthening ratio. General failure pattern that is most commonly reported as punching shear failure after the main rebar yields, followed by yielding of distributing rebar. In this paper, by Proposing a limit to the amount of strengthening material, a brittle failure can be prevented and a ductile failure mode similar to that developed in unstrengthened deck is derived. In order to calculated the limit strengthening ratio, the yield line theory and previously proposed plastic punching shear model have been used

• Nuclear Engineering and Technology
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• v.53 no.2
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• pp.357-367
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• 2021

In general, common cause failures (CCFs) have been modeled with the assumption that components within the same group are symmetric. This assumption reduces the number of parameters required for the CCF probability estimation and allows us to use a parametric model, such as the alpha factor model. Although there are various asymmetric conditions in nuclear power plants (NPPs) to be addressed, the traditional CCF models are limited to symmetric conditions. Therefore, this paper proposes the copulabased CCF model to deal with asymmetric as well as symmetric CCFs. Once a joint distribution between the components is constructed using copulas, the proposed model is able to provide the probability of common cause basic events (CCBEs) by formulating a system of equations without symmetry assumptions. In addition, Bayesian inferences for the parameters of the marginal and copula distributions are introduced and Markov Chain Monte Carlo (MCMC) algorithms are employed to sample from the posterior distribution. Three example cases using simulated data, including asymmetry conditions in total failure probabilities and/or dependencies, are illustrated. Consequently, the copula-based CCF model provides appropriate estimates of CCFs for asymmetric conditions. This paper also discusses the limitations and notes on the proposed method.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10b
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• pp.845-850
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• 2000

This paper presents the results of tests performed on the transfer girders which have been generally used between upper walls and lower frames in the hybrid structures. The 8 specimens were designed using (1) ACI method, (2) strut-tie model, and (3) X-type shear reinforcement cage. The capacities of the specimens are in general larger than the design values except the one designed according to strut-tie model. The reason for this difference seems to be due to the arbitrary allocation of transferred shear force to the path of direct compression strut and the path of indirect strut and tie. The failure modes turn out toe be (1) shear failure at critical shear zone, (2) compressive concrete crushing in the diagonal strut in the shear zone of transfer girder, and (3) compressive concrete crushing in the corner of upper wall.

• Structural Engineering and Mechanics
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• v.56 no.5
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• pp.727-743
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• 2015

According to former studies, the mechanical properties of reinforced concrete filled tubular steel (RCFT) columns differed greatly from that of concrete filled steel tubular (CFT) columns because of interaction of inserted reinforcement in RCFT. Employing an experiment-based verification policy, a general FE nonlinear analysis model was developed to analyze the mechanical behavior and failure mechanism of RCFT columns under uniaxial compression. The reasonable stress-strain relationships were suggested for confined concrete, reinforcements and steel tube in the model. The mechanism for shear failure of concrete core was found out in the numerical simulation, and a none-conventional method and equation for evaluating the confinement effect of RCFT were proposed.

• Journal of Korean Institute of Industrial Engineers
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• v.15 no.2
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• pp.65-75
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• 1989

A general software reliability model is developed, which includes the Jelinski-Moranda model, the Goel-Okumoto model, the Shanthikumar model and the Ross model as special cases. In each of above models estimators of the software failure rate and the number of remaining errors are presented and compared in terms of the expected absolute error loss and the expected squared error loss by a Monte Carlo simulation.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.1520-1527
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• 2005

In order to investigate the effect of improvement with bamboo mats on dredged fills, several tests were performed with different widthes (0.0B, 0.5B, 1.0B, 2.0B). The results indicate that the failure modes of the models without bamboo mats are close to local shear failure, and change to general shear failure as the width of a bamboo mat increases. A series of tests with four different widths (0.0B, 0.5B, 1.0B, 2.0B) were carried out on the models which have no desiccated surface layers, to find out the width that the general shear failure might occur. Finally, a series of tests with this width were performed on the models with three different desiccated surface layers (3cm, 5cm, 10cm). Throughout the tests, strength and deformation according to the thickness of desiccated surface layer was investigated.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.11
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• pp.1741-1751
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• 2004

It is commonly requested that the steam generator tubes wall-thinned in excess of 40% should be plugged. However, the plugging criterion is known to be too conservative for some locations and types of defects and its application is limited to a single crack in spite of the fact that the occurrence of multiple through-wall cracks is more common in general. The objective of this research is to propose the optimum failure prediction models for two adjacent through-wall cracks in steam generator tubes. The conservatism of the present plugging criteria was reviewed using the existing failure prediction models for a single crack, and six new failure prediction models for multiple through-wall cracks have been introduced. Then, in order to determine the optimum ones among these new local or global failure prediction models, a series of plastic collapse tests and corresponding finite element analyses for two adjacent through-wall cracks in thin plate were carried out. Thereby, the reaction force model, plastic zone contact model and COD (Crack-Opening Displacement) base model were selected as the optimum ones for assessment of steam generator tubes with multiple through-wall cracks. The selected optimum failure prediction models, finally, were used to estimate the coalescence pressure of two adjacent through-wall cracks in steam generator tubes.