• Proceedings of the Korean Society Of Semiconductor Equipment Technology
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• 2004.05a
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• pp.59-62
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• 2004

The defect formation energy of boron nitride (BN) nanotubes is investigated using molecular-dynamics simulation. Although the defect with tetragon-octagon pairs (4-88-4) is favored in the flat cap of BN nanotubes, BN clusters, and the growth of BN nanotubes, the formation energy of the 4-88-4 defect is significantly higher than that of the pentagon-heptagon pairs (5-77-5) defect in BN nanotubes. The 5-77-5 defect reduces the effect of the structural distortion caused by the 4-88-4 defect, in spite of homoelemental bonds. The instability of the 4-88-4 defect generates the structural transformation into BNNTs with no defect at about 1500 K.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.8
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• pp.807-812
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• 2009

Most structure engineers give the casting components over-estimated factor of safety without any reasonable foundation due to the worries about the unavoidable defects such as shrinkages and porosity in castings; the engineers have little knowledge on the relation between the defect and structural behavior. And the workers in casting field also do not know how to control the defects by manufacturing; they do not know to where the defects move or until how size they reduce the defects. In this study, shrinkage defect was scanned by industrial computerized tomography instrument (CT), and subsequently was modeled to a spheroid primitive for structural analysis. Using these simplified models of shrinkage, we observed the effects of the defect on the results of the structural analysis. A commercial structural analysis code was used to do the analysis works. Considering the conclusions, it is possible to manage the shrinkages effectively in casting process and to design the products with more reliable

• Smart Structures and Systems
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• v.2 no.3
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• pp.253-274
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• 2006

The structural monitoring of multi-wire strands is of importance to prestressed concrete structures and cable-stayed or suspension bridges. This paper addresses the monitoring of strands by ultrasonic guided waves with emphasis on the signal processing and automatic defect classification. The detection of notch-like defects in the strands is based on the reflections of guided waves that are excited and detected by magnetostrictive ultrasonic transducers. The Discrete Wavelet Transform was used to extract damage-sensitive features from the detected signals and to construct a multi-dimensional Damage Index vector. The Damage Index vector was then fed to an Artificial Neural Network to provide the automatic classification of (a) the size of the notch and (b) the location of the notch from the receiving sensor. Following an optimization study of the network, it was determined that five damage-sensitive features provided the best defect classification performance with an overall success rate of 90.8%. It was thus demonstrated that the wavelet-based multidimensional analysis can provide excellent classification performance for notch-type defects in strands.

• Nuclear Engineering and Technology
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• v.54 no.8
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• pp.2783-2791
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• 2022

Molecular dynamics simulations were performed to predict the behavior of graphite atoms under neutron irradiation using large-scale atomic/molecular massively parallel simulator (LAMMPS) package with adaptive intermolecular reactive empirical bond order (AIREBOM) potential. Defect structures of graphite were compared with results from previous studies by means of density functional theory (DFT) calculations. The quantitative relation between primary knock-on atom (PKA) energy and irradiation damage on graphite was calculated. and the effect of PKA direction on the amount of defects is estimated by counting displaced atoms. Defects are classified into four groups: structural defects, energy defects, vacancies, and near-defect structures, where a structural defect is further subdivided into six types by decision tree method which is one of the supervised machine learning techniques.

• Journal of the Korea Institute of Military Science and Technology
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• v.24 no.5
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• pp.467-474
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• 2021

A study on optimal shape selection of a mechanical fastening for the repair of crack defect of ROK Air Force F-5 fighter wing was conducted. The crack defect occurred in the spar of the wing, and the technical manual does not specify the repair method. However, ROK Air Force decided to develop a repair technology for this defect in consideration of various logistic conditions. Three repair shapes for the proper repair were devised and the finite element analysis was performed to examine the structural safety of these three connection members. As a result of the structural safety review, two connection members except one were structurally safe with safety margins over zero because the calculated stress values were at or below the yield strength level. Therefore, two connection members were determined to be able to use for repair under the condition that the aircraft operated within the design limit load. The results of this study would be very useful if the same defect occurs in long-term aircraft operated by the ROK Air Force.

• Journal of Korean Society for Quality Management
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• v.50 no.3
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• pp.533-543
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• 2022

Purpose: The purpose of this study is to identify and resolve the causes of defects in the unmanned aerial vehicle launch system(propulsion wire fracture, rear rail deformation) and to prevent recurrence. Methods: The causes of the two defects were derived through fault tree analysis for each of the two defects and fault reproduction tests. In the case of propulsion wire, the installation of a high speed camera to check the behavior of wire was the driving force behind the defect resolution. Results: The results of this study are as follows; It was determined that the thickness of the washer was less than the maximum tolerance of the pulley was the cause of the propulsion wire fracture defect. Failure to comply with the launch procedure and insufficient safety margin were judged as the cause of the rear rail deformation defect. Accordingly, the configuration was changed to remove each defect. Conclusion: The case of this study was conducted to eliminate defects in the launch system for UAV. The causes of defects were estimated through fault tree analysis. After the configuration change, Structural analysis and launch tests were performed to demonstrate the safety and effectiveness of the modified configuration. As a result, the effect of the modified configuration was verified.

• Structural Engineering and Mechanics
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• v.60 no.4
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• pp.667-691
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• 2016

This study focuses on the damage detection of defect types in plate structures based on wavelet transform (WT) and curvelet transform (CT). In particular, for damage detection of structures these transforms have been developed since the last few years. In recent years, the CT approach has been also introduced in an attempt to overcome inherent limitations of traditional multi-scale representations such as wavelets. In this study, the performance of CT is compared with WT in order to demonstrate the capability of WT and CT in detection of defect types in plate structures. To achieve this purpose, the damage detection of defect types through defect shape in rectangular plate is investigated. By using the first mode shape of plate structure and the distribution of the coefficients of the transforms, the damage existence, the defect location and the approximate shape of defect are detected. Moreover, the accuracy and performance generality of the transforms are verified through using experimental modal data of a plate.

• Magazine of the Korea Institute for Structural Maintenance and Inspection
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• v.17 no.4
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• pp.84-88
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• 2013

• Journal of the Korea institute for structural maintenance and inspection
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• v.1 no.2
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• pp.107-113
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• 1997

Nondesructive test and structural analysis have been conducted to assess the safety of weld defective steel bridges in service. In the nondestructive test, using the radioactive ray and ultrasonic, the defective welding patterns in the steel bridges are identified. A major defective welding pattern is identified as the lack of welding area due to the insufficient welding penetration. By considering the welding defect in the above, structural analysis is conducted to evaluate the influence of welding defect on the safety of steel bridges. The results indicate that, due to the insufficient welding penetration, the stress obtained in the analysis is over the allowable fatigue stress level, and its influence on safety of the bridges is significant.

• Journal of Korea Foundry Society
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• v.30 no.5
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• pp.196-200
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• 2010

The structural stress and fatigue behavior of tensile specimen containing internal shrinkage defect were modeled. Real shrinkage defect in casting was scanned by industrial CT (computed tomography), and subsequently its shape was simplified by ellipsoidal primitives for the structural analysis (S.S.M., shape simplification method). The analysis results were compared with the results by real shrinkage shape without any simplification process. It was possible to consider real shrinkage of casting in stress analysis and the method to predict fatigue life of casting with defect was proposed.