Papers by Author: Sung Mo Yang

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.

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.

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.

Abstract: This paper describes a primary study for a new method of enhancing measurement with using an impedance analysis method that is applied to structural health monitoring. The goal of this research is to show basic information that is used to evaluate the structural health condition with using piezoelectric sensors. We present a study for the development of a practical and quantitative technique for the assessment of the structural health condition with using a piezoelectric impedance-based technique that is associated with longitudinal wave propagation. The natural frequency has a tendency of frequency shifting with respect to the hole size that corresponds to the real structure’s crack size. To numerically estimate the damage condition, we suggest the Cov (Covariance) and CC (Correlation Coefficient) evaluation methods.

Abstract: Pieces of large-scale equipment such as a ball mill system are subjected to heavy and alternate loads under the worst working conditions, which result in higher fault rates. However, since such equipment exerts important functions to the economy and industry, it isn’t recommendable to halt production in order to detect the defects under unconfirmed faults. We explore a reliable and practical fault diagnosis scheme for the ball mill system that is widely used in the building material industry. With utilizing the signal acquisition and process system of vibration, the field testing and analysis are performed based on the violent vibration of an edge-transmission Φ3×11m ball mill system. The primary diagnosis that is based on the configuration of the transmission system and foundation stiffness is found, and a feasible resolution scheme is obtained, so that the optimal and economic reform scheme is determined. The detailed scheme has been adopted by the production industry. This study that is based on the fault diagnosis of the edgetransmission ball mill system has a comprehensive significance of theory and reality. This provides a larger basis for vibration inspection and fault diagnosis in building material industries.

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.

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.

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.

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.