• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2006.06a
• /
• pp.503-504
• /
• 2006

This paper describes a low-level partial discharge(PD) testing that has been accepted as a non-destructive test method on insulation performance of electronic components. A comparative PD analysis combined with the Withstand Voltage Test (WVT) specified in IEC standards is carried out on high frequency switching transformers. The analysis shows that insulation degradation of the transformers under test progresses during the WVT. To avoid insulation degradation of the specimen, PD test has to be carried out at as low voltage as possible. In this study, the PD test on the transformers is performed in ranges from 50% to 70% of the test voltage specified in the WVT by measuring apparent charges below 1 pC. From the experimental results, it is expects that the low-level PD test is applicable for electronic components as a replacement of the WVT.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2004.11a
• /
• pp.625-632
• /
• 2004

This study presents the NDE (non-destructive evaluation) technique for detecting the loosened bolts on joint steel structures on the basis of TOF (time of flight) and amplitudes of Lamb waves. Probabilistic neural network (PNN) technique which is an effective tool for pattern classification problem was applied to the damage estimation using PZT induced Lamb waves. Two kinds of damages were introduced by dominant damages (DD) which mean loosened bolts within the Lamb waves beam width and minor damages (MD) which mean loosened bolts out of the Lamb waves beam width. They were investigated for the establishment of the optimal decision boundaries which divide each damage class's region including the intact class. In this study, the applicability of the probabilistic neural networks was identified through the test results for the damage cases within and out of wave beam path. It has been found that the present methods are very efficient and reasonable in predicting the loosened bolts on the joint steel structures probabilistically.

• Smart Structures and Systems
• /
• v.25 no.3
• /
• pp.265-277
• /
• 2020

Recently, the concept of "drive-by" bridge monitoring system using indirect measurements from a passing vehicle to extract key parameters of a bridge has been rapidly developed. As one of the most key parameters of a bridge, the natural frequency has been successfully extracted theoretically and in practice using indirect measurements. The frequency of bridge is generally calculated applying Fast Fourier Transform (FFT) directly. However, it has been demonstrated that with the increase in vehicle velocity, the estimated frequency resolution of FFT will be very low causing a great extracted error. Moreover, because of the low frequency resolution, it is hard to detect the frequency drop caused by any damages or degradation of the bridge structural integrity. This paper will introduce a new technique of bridge frequency extraction based on Hilbert Transform (HT) that is not restricted to frequency resolution and can, therefore, improve identification accuracy. In this paper, deriving from the vehicle response, the closed-form solution associated with bridge frequency removing the effect of vehicle velocity is discussed in the analytical study. Then a numerical Vehicle-Bridge Interaction (VBI) model with a quarter car model is adopted to demonstrate the proposed approach. Finally, factors that affect the proposed approach are studied, including vehicle velocity, signal noise, and road roughness profile.

• Acta Via Serica
• /
• v.5 no.2
• /
• pp.25-49
• /
• 2020

This essay, based on an oral presentation, provides the non-specialist, with an evaluation of the Mongols' influence and China and, to a lesser extent, on Russia and the Middle East. Starting in the 1980s, specialists challenged the conventional wisdom about the Mongol Empire's almost entirely destructive influence on global history. They asserted that Mongols promoted vital economic, social, and cultural exchanges among civilizations. Chinggis Khan, Khubilai Khan, and other rulers supported trade, adopted policies of toleration toward foreign religions, and served as patrons of the arts, architecture, and the theater. Eurasian history starts with the Mongols. Exhibitions at the Metropolitan Museum of Art and the Los Angeles County Museum of Art confirmed that the Mongol era witnessed extraordinary developments in painting, ceramics, manuscript illustration, and textiles. To be sure, specialists did not ignore the destruction and killings that the Mongols engendered. This reevaluation has prompted both sophisticated analyses of the Mongols' legacy in Eurasian history. The Ming dynasty, the Mongols' successor in China, adopted some of the principles of Mongol military organization and tactics and were exposed to Tibetan Buddhism and Persian astronomy and medicine. The Mongols introduced agricultural techniques, porcelain, and artistic motifs to the Middle East, and supported the writing of histories. They also promoted Sufism in the Islamic world and influenced Russian government, trade, and art, among other impacts. Europeans became aware, via Marco Polo who traveled through the Mongols' domains, of Asian products, as well as technological, scientific, and philosophical innovations in the East and were motivated to find sea routes to South and East Asia.

• Smart Structures and Systems
• /
• v.30 no.1
• /
• pp.17-34
• /
• 2022

For steel structures, fatigue cracks are critical damage induced by long-term cycle loading and distortion effects. Vision-based crack detection can be a solution to ensure structural integrity and performance by continuous monitoring and non-destructive assessment. A critical issue is to distinguish cracks from other features in captured images which possibly consist of complex backgrounds such as handwritings and marks, which were made to record crack patterns and lengths during periodic visual inspections. This study presents a parametric study on image-based crack identification for orthotropic steel bridge decks using captured images with complicated backgrounds. Firstly, a framework for vision-based crack segmentation using the atrous convolution-based Deeplapv3+ network (ACDN) is designed. Secondly, features on crack images are labeled to build three databanks by consideration of objects in the backgrounds. Thirdly, evaluation metrics computed from the trained ACDN models are utilized to evaluate the effects of obstacles on crack detection results. Finally, various training parameters, including image sizes, hyper-parameters, and the number of training images, are optimized for the ACDN model of crack detection. The result demonstrated that fatigue cracks could be identified by the trained ACDN models, and the accuracy of the crack-detection result was improved by optimizing the training parameters. It enables the applicability of the vision-based technique for early detecting tiny fatigue cracks in steel structures.

• Progress in Superconductivity and Cryogenics
• /
• v.25 no.1
• /
• pp.16-20
• /
• 2023

The International Electrotechnical Commission (IEC) 61788-26:2020 provides guidelines for measuring the critical current of Rare-earth barium copper oxide (REBCO) tapes using two methods: linear ramp and step-hold methods. The critical current measurement criterion, 1 or 0.1 μV/cm of electric field from IEC 61788-26 has been normally applied to REBCO coils or magnets. No-insulation (NI) winding technique has many advantages in aspects of electrical and thermal stability and mechanical integrity. However, the leak current from the NI REBCO coil can cause distortion in critical current measurement due to the characteristic resistance which causes the radial current flow paths. In this paper, we simulated the NI REBCO coil by applying both linear ramp and step-hold methods based on a simplified equivalent circuit model. Using the circuit analysis, we analyzed and evaluated both methods. By using the equivalent circuit model, we can evaluate the critical current of the NI REBCO coil, resulting in an estimation error within 0.1%. We also evaluate the accuracy of critical current measurement using both the linear ramp and step-hold methods. The accuracy of the linear ramp method is influenced by the inductive voltage, whereas the accuracy of the step-hold method depends on the duration of the hold-time. An adequate hold time, typically 5 to 10 times the time constant (τ), makes the step-hold method more accurate than the linear ramp method.

• Smart Structures and Systems
• /
• v.32 no.2
• /
• pp.123-133
• /
• 2023

This paper proposes a thermography-based coating thickness estimation method for steel structures using model-agnostic meta-learning. In the proposed method, a halogen lamp generates heat energy on the coating surface of a steel structure, and the resulting heat responses are measured using an infrared (IR) camera. The measured heat responses are then analyzed using model-agnostic meta-learning to estimate the coating thickness, which is visualized throughout the inspection surface of the steel structure. Current coating thickness estimation methods rely on point measurement and their inspection area is limited to a single point, whereas the proposed method can inspect a larger area with higher accuracy. In contrast to previous ANN-based methods, which require a large amount of data for training and validation, the proposed method can estimate the coating thickness using only 10- pixel points for each material. In addition, the proposed model has broader applicability than previous methods, allowing it to be applied to various materials after meta-training. The performance of the proposed method was validated using laboratory-scale and field tests with different coating materials; the results demonstrated that the error of the proposed method was less than 5% when estimating coating thicknesses ranging from 40 to 500 ㎛.

• Nuclear Engineering and Technology
• /
• v.55 no.1
• /
• pp.270-277
• /
• 2023

Exposure to ionizing radiation induces free radicals in human nails. These free radicals generate a radiation-induced signal (RIS) in electron paramagnetic resonance (EPR) spectroscopy. Compared with the RIS of tooth enamel samples, that in human nails is more affected by moisture and heat, but has the advantages of being sensitive to radiation and easy to collect. The fingernail as a biological sample is applicable in retrospective dosimetry in cases of localized hand exposure accidents. In this study, the dosimetric characteristics of fingernails were analyzed in fingernail clippings collected from Korean donors. The dose response, fading of radiation-induced and mechanically induced signals, treatment method for evaluation of background signal, minimum detectable dose, and minimum detectable mass were investigated to propose a fingernail-EPR dosimetry protocol. In addition, to validate the practicality of the protocol, blind and field experiments were performed in the laboratory and a non-destructive testing facility. The relative biases in the dose assessment result of the blind and field experiments were 8.43% and 21.68% on average between the reference and reconstructed doses. The results of this study suggest that fingernail-EPR dosimetry can be a useful method for the application of retrospective dosimetry in cases of radiological accidents.

• Journal of the Korean Society of Industry Convergence
• /
• v.26 no.5
• /
• pp.725-731
• /
• 2023

In this study, the harmless crack size was evaluated using carburized, quenched-tempered SCM822H steel. The possibility of detecting cracks that reduce the fatigue limit by non-destructive inspection was evaluated. The conclusions obtained are as follows. The retained austenite of surface was reduced by SP. About 35% and 65% of the retained austenite on the surface were transformed into strain-induced martensite, increasing the hardness by 79HV and 122HV over the as-received material. The maximum compressive residual stresses introduced on the surfaces were -695 MPa and -688 MPa, respectively. The fatigue limit increased by 1.48 times and 1.67 times, respectively, compared to the as-received material. The harmless crack size of SP specimen was determined differently depending on the shot ball size.

• Journal of Korean Association for Spatial Structures
• /
• v.24 no.2
• /
• pp.53-63
• /
• 2024

The cultural heritage of fortresses is often exposed to external elements, leading to significant damage from stone weathering and natural disasters. However, due to the nature of cultural heritage, dismantling and restoration are often impractical. Therefore, the stability of fortress cultural heritage was evaluated through non-destructive testing. The durability of masonry cultural heritages is greatly influenced by the physical characteristics of the back-fille material. Dynamic characteristics were assessed, and endoscopy was used to inspect internal fillings. Additionally, a finite element analysis model was developed considering the surrounding ground through elastic wave exploration. The analysis showed that the loss of internal fillings in the target cultural heritage site could lead to further deformation in the future, emphasizing the need for careful observation.