Papers by Author: Hyo Sun Yu

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Authors: Seung Se Baek, Il Hyun Kwon, Dong Whan Lee, Sung Mo Yang, Hyo Sun Yu
Abstract: Power plant weldments are composed of various microstructures. Due to welding and PWHT processes, the microstructure of the base metal adjacent to fusion line is transformed into entirely different microstructures, collectively known as heat affected zone (HAZ). Creep, on the other hand, is considered as the most important deterioration mechanism of heat resistant components found in power plants. Therefore it is essential to evaluate creep properties of HAZ, which is considered to be very hazardous in weldment. Recently, most of the creep tests for HAZ are conducted using cross weld type specimen. However there are some problems with this type of creep test due to the results being largely varied according to the volume fraction of HAZ. In this paper, SP-Creep test, which has confirmed the availability for creep properties evaluation, has been conducted on each of the weldment microstructures. The results showed that each microstructure has a different creep behavior. The overall creep properties of HAZ are worse than those of the weld metal. Among the HAZ structures, fine grained HAZ has the worst creep properties.
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Authors: Su Rok Sin, Sung Mo Yang, Hyo Sun Yu, Chai Won Kim, Hee Yong Kang
Abstract: The welding quality of spot weldment is an important factor that significantly affects the strength, stiffness, safety, and other performance characteristics of vehicles. Therefore, quality control and fatigue life evaluation of spot weldment are necessary processes. This paper presents a method for determining the fatigue life of multi-lap spot weldment of a high strength steel sheet. In this method, the fatigue life is estimated by using the lethargy coefficient, which is the total defect coefficient according to rupture stress and time obtained by the quasi static tensile-shear test. Also, in this study, we modified the lethargy coefficient by using the welding current. And, we define a specific lapping constant, which is a characteristic constant of 2 or 3 lap weldments. The fatigue life obtained by the fatigue estimate equation, which contains a specific lapping constant was compared and verified with an experimental value. And we analyzed the relation of lap number, welding current and fatigue life. This method can save processing time and cost for predicting the life cycle of a structure.
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Authors: Seung Chul Shin, Sung Mo Yang, Hyo Sun Yu, Hee Yong Kang, Chae Won Kim
Abstract: This paper shows how the number of lap of spot welded joints in automotive steel sheets changes the life -cycle transform of itself. There has been significant developments in the life extension of automotive steel sheets, as well as in passenger safety. We verified the validity of the S-N curve of materials by QSTS test. Moreover, we used EZNCEN and HS40R, which are commonly used as automotive steel sheets because of their excellence in increasing fuel efficiency. The purpose of research was to compare the fatigue life of multi - lap of spot welded joints to there. In addition s, we used the two different kinds of sheets together in the same welding under the same condition. Through this whole process, the hypothesis on the life cycle of lethargy coefficients was found to be in good agreement with the result of the experiment. That is, when using sheets of the same materials in one welding, the fatigue life of two laps of a spot welded joint was found to be much superior by EZNCEN 5~14%, HS40R 17~30%,and when using each sheet of different materials, we found the decrease in fatigue life by 6~16%. Finally, the best value from the data was chosen based on the experiment for the analysis of the fatigue life of each layer.
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Authors: Joon Hyuk Song, Hee Yong Kang, Hyo Sun Yu, Sung Mo Yang
Abstract: In the beginning of design, exact load data are actually necessary for the fatigue strength and life analysis to minimize the cost and time of designing. The procedure of practical load determination is developed by the combination of the principal stresses of F.E.Analysis and experiment. This paper conceives new procedure for the determination of load direction and magnitude applied on mechanical structures. New procedure is the combination of the analytical and empirical method with analyzed strain by F.E. Analysis under unit load and with measured principal stress by strain gages under driving load, respectively. In this paper, we theorize the procedure of practical load determination and make the validity and the practicality of the procedure with the application to T-shape jointed structure. F.E. Analysis is conducted to get the principal stress on arbitrary points in the F.E. model of T-shape joint under unit load. Then experiment is carried out to get the principal stress on the same points of F.E. model. To demonstrate the actual driving condition, the load conditions are bending and torsion. From these two data sets, the magnitude, the direction and the position of load can be obtained.
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Authors: Gyun Lee, Hyo Sun Yu, Sung Mo Yang, Hee Yong Kang, Dong Pyo Hong, Joon Hyuk Song
Abstract: The use of Pb-free solder alloys are just newly developed materials and their properties are not well established and documented yet. So, the research to substitute the critical problem of the Pb solder alloy are increasing widely. To evaluate the shear property of solder joints according to temperature condition was predicted through the Shear-Punch test and FEM. By using the database according to this, the reliability evaluation technology of the solder joints tries to be formulated. In this paper, the solder used the Sn37Pb, Sn-4Ag and the Sn4Ag0.5Cu. When solder melted in the copper interface, the solder joints according to reflow times, each modeling required for the Shear- Punch was made in consideration of the IMC layer thickness. And the micro shear punch test was performed to obtain material properties of solder according to temperature condition. The shear properties of specimen which was modeled was predicted through FEM. It compared with the test value in which it obtains from Shear-Punch. Also, a method which can obtain shear properties of Pb-free solder joints easily and quickly using FEM was proposed.
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