• Journal of Welding and Joining
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• v.19 no.1
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• pp.58-64
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• 2001

The initiation and propagation lives of fatigue crack were studied for spot weldments composed of cold rolled steel plates(SPC$\times$SPC) and galvanized steel plates(GA$\times$GA) using DC potential drop method(DCPDM). Through the various test results, it was known that the fatigue crack initiation and propagation behaviors in all specimens could be definitely detected by DCPDM. The fatigue crack initiation life( $N_{i}$) detected by DCPDM in SPC$\times$SPC and GA$\times$GA spot weldments increased as the welding current and the nugget diameter( $N_{d}$) increased. The fatigue crack propagation life($\Delta$ $N_{f-i}$) declined as the difference of $N_{i}$ and the fatigue fracture life( $N_{f}$) also increased according to the decrease of fatigue load, $\Delta$P and the increase of nugget diameter. In the same spot weldments, the increase of nugget diameter came to increase fatigue crack propagation life owing to a decrease of stress concentration in front of nugget, especially the increasing extent for GA$\times$GA spot weldment was very high. In the welding current 6kA, $N_{f}$ for GA$\times$GA spot weldment decreased more than that of SPC$\times$SPC specimen due to zinc layer coated in steel plate and undersized nugget diameter. On the other hand, in 8kA and 10kA, the GA$\times$GA spot weldment showed higher $N_{f}$ in spite of lower $N_{i}$, than that of SPC$\times$SPC specimen except 3,000N fatigue load.ue load. load.d.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2008.05a
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• pp.57-60
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• 2008

This study investigated the results of insulation heat curing method using double layer bubble sheet in slab concrete and mass concrete in cold weather environment. First of all, when double bubble sheets are applied, it was shown that slab concrete was protected from early freezing by remaining between 6 and 15℃ even in case outside temperature drops -9℃ below zero until the 2nd day from piling, and in the case of mass concrete, with the maximum temperature difference between the center and surface less than 4℃, crack occurrence index was close to 2 and no hydration heat crack occurred by internal constraint. The insulation heat preservation curing method using the double bubble sheet applied in this field prevented early freezing owing to stable curing temperature management, deterring concrete strength development delay at low temperature, and obtained the needed strength. Also, it was proven that the method is highly effective and economic for cold weather concrete quality maintenance through curing cost reduction like construction period shortening and labor cost reduction, etc by reducing the process of temporary equipment installation and disassembling.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.801-806
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• 2001

In the latest, the construction period became the most important factor in the domestic building works because buildings is higher and larger gradually. Accordingly, cold-weathering concrete constructions are performed more and more. So, the demands of concrete quality management methods are increased in cold-weathering concrete constructions. In this study, the mock-up experiment was performed to correct input data in the kimpo H construction field. Using the results of this experiment, the method of temperature analysis was presented to prevent early aged freezing damage and to control thermal crack in this study.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.4
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• pp.161-169
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• 2004

In this study, we investigated fatigue crack growth behavior of laser welded sheet metal due to a single overload. Fatigue specimens were made using butt joint of cold rolled sheet metal that was welded by $CO_2$ laser. The fatigue crack propagation tests were performed in such a way that fatigue loading was parallel to the weld line while crack propagation was perpendicular to the weld line. Single overload was applied when fatigue crack tip was arrived near the weld line. The distances between the crack tip and the weld line at which a single overload was applied were 6, 4 and 2mm. The effect of specimen thickness and overload ratio on the fatigue behavior was determined. The plastic zone size of crack tip due to the single overload was determined from the finite element analysis. For investigating fatigue crack growth behavior, we used different thickness specimen 0.9mm and 2.0mm, and variable overload ratio applied fatigue crack propagation test. Also we used finite element analysis for investigating the plastic zone size of crack tip when single overload applied

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.12
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• pp.1619-1625
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• 2012

An electrical steel strip is commonly used as a core material in all types of electric transformers and motors. It is produced by a cold rolling process. In this paper, a damage-mechanics-based approach that predicts the edge fracture of an electrical steel strip during cold rolling is presented. We adopted the normal tensile stress criterion and the fracture energy method as a damage initiation criterion and a damage evolution scheme, respectively. We employed finite element analysis (FEA) to simulate crack initiation and propagation at the initial notch located at the edges of the strip. The material constants required in FEA were experimentally obtained by tensile tests using a standard and a notched sheet-type specimen. The results reveal that the edge crack was initiated at the entrance of the roll bite and that it rapidly evolved at the exit. The evolution length of the edge crack increased as the length of the initial notch as well as the front tension reel force of the strip increased.

• Proceedings of the KWS Conference
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• 1992.04a
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• pp.13-15
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• 1992

• Proceedings of the KWS Conference
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• 2003.11a
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• pp.242-244
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• 2003

• Journal of Welding and Joining
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• v.20 no.4
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• pp.429-436
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• 2002

• Proceedings of the KWS Conference
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• 2002.05a
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• pp.83-84
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• 2002

• Journal of Welding and Joining
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• v.17 no.1
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• pp.52-54
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• 1999