Papers by Author: Jae Boong Choi

Abstract: Fatigue damage caused by alternating operational stresses in terms of temperature or pressure change is the one of important damage mechanisms in the nuclear power plants (NPPs). Although components important to safety were designed to withstand the fatigue damage, cumulative usage factor (CUF) at some locations can exceed the design limit beyond the design life. So, it is important to monitor the fatigue damage of major components during the long term operation. To evaluate fatigue damage, the Green’s function approach has been generally used. In this approach, thermal stresses can be directly calculated from the convolution integration on the coolant temperature history and Green’s function. And, Green’s function is defined as a stress variation at the arbitrary point when the coolant temperature is increased as a unit step. However, this approach cannot be applied to the fatigue analysis using temperature-dependent material properties because it is assumed that the system is linear. In this paper, the modified Green’s function approach considering temperature-dependent material properties is proposed by using neural network. To verify the modified Green’s function method, thermal stresses by the proposed method are compared with those by finite element analysis (FEA) at the transition wall of reactor pressure vessel and the analysis results between two methods are well agreed. Finally, it is anticipated that more precise fatigue evaluation is performed by using the proposed method.

Abstract: The purpose of this paper is to investigate fluid characteristics and flat panel display behaviours on contact-free flat panel display handler nozzles. Since air force distributions streaming bottom surface of a flat panel display highly dependent on operating as well as design conditions and configuration of contact-free handler nozzle, influences of various parameters such as flow rate, supply air pressure, floating height are examined through a series of computational fluid dynamics analyses. Moreover, dynamic finite element analyses of the flat panel display are carried out to assure that an oscillation effect caused by disturbances is not significant. Key findings from the both computational fluid dynamics and structural integrity analysis results are presented and discussed, which can be figured out an optimized operating condition of contact-free handler nozzles.

Abstract: In the ASME Code Section III ‘design by analysis’ approach, stresses are determined by numerical method and compared with corresponding stress limits. This approach provides several stress criteria for fatigue life assessment and procedures for categorizing the representative stress components. Since the stress criteria were derived from two-dimensional basis, however, it may inappropriate to delineate structural components with complex geometry. In this paper, detailed transient analyses are performed for modular pressurizer with an asymmetric geometry, which includes perforated parts to mount various piping and equipments. Also, the applicability of an effective elastic modulus to consider the perforation and the appropriateness of stress linearization method using stress classification line are assessed. Then, the cumulative usage factor as well as stress intensities at critical locations of the pressurizer are calculated and compared with corresponding allowable design stress limits. The key findings of this work can be used to make regulatory guides for evaluation and confirmation of structural intensity of components with asymmetric perforated parts.

Abstract: Fretting damage is a critical problem to prevent failure of press-fitted shaft such as the rotor of a steam turbine, railway axles or coupling. To clarify the characteristics of surface damage due to fretting in press-fitted shaft, experimental methods were applied to small-scale specimen with different bending load conditions. Fatigue tests and interrupted fatigue tests of press-fitted specimen were carried out by using a rotate bending fatigue test machine. Macroscopic and microscopic characteristics were examined using scanning electron microscope (SEM), optical microscope or profilometer. It is found that small fatigue cracks are nucleated early in life regardless of bending stress, and thus the most portion of fatigue life on press fits can be considered to be crack propagation process. Most of surface cracks are initiated near the contact edge, and multiple cracks are nucleated and interconnected. Furthermore, the fretting wear rates at the contact edge increase rapidly at the initial stage of total fatigue life. It is thus suggested that the fatigue crack nucleation and propagation process is strongly related to the evolution of surface profile by fretting wear in press fits.

Abstract: Goal setting safety legislation for high pressure systems was first introduced in 1973 and retained in the High Pressure Safety Management Law in Korea. This has enabled a move towards inspection strategies based on the risk of failure. This trend towards a risk based approach is being supported by extensive plant operating experience, improved understanding of material degradation mechanisms, and the availability of fitness-for-service assessment procedures. RBI (Risk-Based Inspection) is very effective and efficient of the inspection and maintenance techniques. In particular, RBI will be found how to apply both inspection interval as a result of RBI implementation and re-inspection interval of pressurized equipment defined by the High Pressure Safety Management Law in Korea. The objectives of this paper are to develop an enhanced KGS-RBITM program to resolve shortcoming inclusive of the above issue and to evaluate the risks of equipments in Naphtha Cracking Center (NCC) which is a typical facility of petrochemical plant using the KGS-RBITM program. The results of the risk assessment and re-inspection interval of internal opening using KGS-RBITM program are useful in determining the inspection planning.

Abstract: The purpose of this paper is to investigate principles of levitation and restitution of blowing nozzle prior to fabricating a prototype of air handling system. Since air force distributions streaming bottom surface of a flat panel display (FPD) highly dependent on operating as well as design condition and configuration of air handling system, influences of various parameters such as flow rate, supply air pressure, floating height and tilted angle are examined through a series of computational fluid dynamics (CFD) analyses. Moreover, dynamic finite element analyses of the FPD are carried out to assure that an oscillation effect caused by disturbances is not significant. Key findings from the both CFD and structural analysis results are presented and discussed, which can be utilized as technical bases for development of the practical air handling system.

Abstract: The present work deals with an applicability of the local approach to assess in-plane size effects among different sized compact tension (CT) specimens. To characterize ductile crack growth of typical nuclear materials, SA515 Gr.60 and SA516 Gr.70 carbon steels, finite element analyses employing modified GTN and Rousselier models as well as fracture toughness tests were carried out. Material damage parameters were calibrated using standard CT specimens and reflected to predict fracture resistance (J-R) curves of larger CT specimens. Since comparison results between numerically estimated J-R curves and experimentally determined ones corresponded well, it is anticipated that the local approach might be used as a promising tool for ductile fracture evaluation incorporating the in-plane size effect.

Abstract: Fracture toughness data from the cleavage resistance test of structural steels often show a large scatter. Geometry dependency as well as the scatter makes it difficult to evaluate appropriate fracture integrity of cracked components. To address these restrictions, several stochastic models have been proposed by Beremin group, Mudry and other researchers while each of them employs specific estimation scheme and micro-mechanical parameters. The purpose of this paper is to investigate applicability of the Weibull stress model in transition temperature regime and to quantify constraint effect among different-sized CT and PCVN specimens. The constituting parameters m and σu are determined at three temperatures by maximum likelihood estimate (MLE) technique in use of FE analysis results and experimental data of PCVN specimens. Also, failure probabilities of PCVN and CT specimens are calculated from the Weibull parameters, which are used for derivation of a prototype of toughness scale diagram. The diagram provides a technical basis to resolve transferability issue in the same material under different temperatures and constraint conditions.

Abstract: This study introduces a nano smart material to develop a novel sensor for Structural Health Monitoring (SHM) of mechanical and civil systems. Mechanical, civil, and environmental systems need to become self-sensing and intelligent to preserve their integrity, optimize their performance, and provide continuous safety for the users and operators. Present smart materials and structures have fundamental limitations in their sensitivity, size, cost, ruggedness, and weight. Smart materials developed using nanotechnology have the potential to improve the way we generate and measure motion in devices from the nano to the macro scale in size. Among several possible smart nanoscale materials, Carbon Nanotubes (CNT) have aroused great interest in the research community because of their remarkable mechanical, electrochemical, piezoresistive, and other physical properties. To address the need for new intelligent sensing based on CNT, this study presents piezoresistivity and electrochemical properties and preliminary experiments that can be applied for SHM. This study is anticipated to develop a new multifunctional sensor which can simultaneously monitor strain, stress and corrosion on a structure with a simple electric circuit.

Abstract: The effects of acid treatment methods on the fracture of single walled carbon nanotubes are investigated using gel electrophoresis and Raman spectroscopic techniques. The mechanism of gel electrophoresis is based on the migration of charged particles in an electric field, and the gel matrix acts as a sieve for the analyte. A comparative Raman spectroscopy analysis, performed on the migrated nanotubes in the gel, shows that small diameter tubes are cut shorter by the acid treatments.