Papers by Author: Sung Mo Yang

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Authors: Joon Hyuk Song, Hee Yong Kang, Chai Won Kim, Sung Mo Yang
Abstract: The inverse problem to identify the load is normally based on the measurement of frequency response transfer functions. In this paper, the dynamic response due to external load of vehicle structure is described with inverse problem in terms of strain from experimental and analytical response. The function of the practical dynamic load is a combination of the analytical and experimental method with analyzed strain by quasi-static finite element analysis under unit load and with measured strain by strain gage under driving load, respectively. The regularization technique is adopted to alleviate the ill-posedness of the inverse problems. To examine the proposed method, the external load applying on bus frame is identified. The load of bus is identified by the combination of the analytical and experimental method with analyzed strains. In order to get analytical strains, a quasi-static finite element analysis under unit load is performed. Road simulation test is conducted to get measured strains with strain gages. The sparseness and the noise in the measurements should be considered properly in the load identification. In this study, the regularization method is adopted to alleviate the ill-posedness of the inverse problems. This procedure can be used to get the transferred load to use an input data of fatigue analysis for a substructure of which the experimental response is difficult to get directly.
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.
Authors: Joon Hyuk Song, Hyu Sun Yu, Hee Yong Kang, Sung Mo Yang
Abstract: Resistance spot welding is used extensively to fasten sheet for automotive applications. In many components, these welds should maintain their integrity under severe loading conditions. However fatigue strength of the spot welded joint is considerably lower than base metal due to stress concentration at the nugget edge, and is influenced by its geometrical and mechanical factors such as welding condition and etc. In this paper, it is estimated that effect of strain rate variation on fatigue life of spot welded joint. The analytical method proposed to overcome above difficult using lethargy coefficient concept for evaluating the fatigue life cycle of spot welded joint. The reliability of the life cycle is completed by comparing with the life cycle obtained by fatigue test for the specimen with the welding current. And the above procedure is numerically extended to get the life of dynamic strain rate region.
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.
Authors: Jong Kyo An, Sung Mo Yang, Hee Yong Kang, Su Rok Sin
Abstract: HS40R and EZNCEN are extensively used as components of vehicle bodies because they satisfy environmental standards and improve fuel ratio. Because vehicle body sheets are becoming thinner and stronger, it is difficult to satisfy the design standards of spot welding joints. This research presents the optimization of the welding condition for various welding variables of a spot welding specimen by genetic algorithm (GA). After obtaining the optimum welding condition by use of the genetic algorithm, the fatigue properties of HS40R and EZNCEN spot welded joints are evaluated by using the finite element method (FEM) to determine the durability and stability of a structure.
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.
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.
Authors: Joon Hyuk Song, Su Rok Sin, Hee Yong Kang, Chai Won Kim, Hyu Sun Yu, Sung Mo Yang
Authors: Akira Shimamoto, Hiroshi Ohkawara, Sung Mo Yang
Abstract: Today, stress measurement methods by thermography and by photoelasticity are widely used to make stress distribution visible. However, it is difficult to separate principal stresses using only one of these methods because only the difference of principal stresses is measured in photoelasticity, and only the sum of the principal stresses is measured in thermograpy. Therefore, the inverse analysis problem must be solved to separate the principal stress in the thermoelastic method and the shear difference integration method must be used for the photoelastic method. Although there are some reports separation of the principal stresses under uniaxial stress by combining the two methods, little research under the biaxial stress has been reported due to the difficulty of experimentation. In this research, the principal stresses under biaxial stress are separated by a combined method. Moreover, it is verified that the thermoelastic stress measurement method is effective to evaluate the stress concentration factor.
Authors: Koung Suk Kim, In Young Yang, Ki Soo Kang, Sung Mo Yang, Seong Kyun Cheong
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