Papers by Author: Kyong Ho Chang

Abstract: A novel electrically conductive adhesives (ECAs) with solder particle have been developed. The kinetics of the curing reaction were investigated using isothermal differential scanning calorimeter (DSC). The dynamic DSC scans were also conducted to investigate the temperature dependant curing degree. The temperature dependant viscosity characteristic of polymer was investigated using rheometer. The wetting characteristic of two solders on two kinds of metals were investigated using an optical microscope. It was found that developed resin material showed good fluxing capacity against Sn-Bi solder on Ni/Au and the wetting angles was about 24º.

Abstract: During the welding process to make welded joints, residual stress is inevitably generated and weld metal is used. Welding Residual stress is influenced on the behavior of welded joints under monotonic and cyclic loading. And the weld metals used in welding process have different mechanical characteristics than structural steels. Therefore, to accurately predict the hysteretic behavior of welded joints, the effect of residual stress and weld metal must be investigated. In this paper, the residual stress distribution in a welded tubular T-joint was investigated by carrying out three-dimensional non-steady heat conduction analysis and three-dimensional thermal elastic-plastic analysis. To consider a effect of base metal(SM490) and weld metal(E71T-1), a cyclic plasticity model was formulated based on monotonic and cyclic loading tests. And the formulated model was applied to three-dimensional elastic-plastic finite element analysis. The effect of residual stress and weld metal on hysteretic behavior of a welded tubular T-joint was investigated by carrying out numerical analyses considering residual stress and cyclic plasticity model of base metal and weld metal respectively.

Abstract: TMCP steels produced by thermo-mechanical control process are now spot lighted due to the excellent combinations of strength, toughness and weldability. Recently, in Korea, high strength SM570-TMCP steel whose tensile strength is 600MPa has been developed and applied to steel structures due to its excellent nature. But, for the application of the TMCP steel to steel structures, it is necessary to elucidate not only the material characteristics but the mechanical characteristics of welded joints. In this study, high temperature tensile properties of the SM570-TMCP steel were investigated through the elevated temperature tensile test and the characteristics of residual stresses in welded joints of the TMCP steel were studied through the three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis on the basis of mechanical properties at high temperatures obtained from the experiment. The results are then compared with the conventional quenched and tempered high strength SM570 steel.

Abstract: During manufacturing the welded joint of steel structures, residual stress is produced and weld metal is used inevitably. And residual stress and weld metal influence on the static and dynamic mechanical behavior of steel structures. Therefore, to predict the mechanical behavior of steel pile with a welded joint during static and dynamic deformation, the research on the influence of the welded joints on the static and dynamic behavior of steel pile is clarified. In this paper, the residual stress distribution in a welded joint of steel piles was investigated by using three-dimensional welding analysis. The static and dynamic mechanical behavior of steel piles with a welded joint is investigated by three-dimensional elastic-plastic finite element analysis using a proposed dynamic hysteresis model. Numerical analyses of the steel pile with a welded joint were compared to that without a welded joint with respect to load carrying capacity and residual stress distribution. The influence of the welded joint on the mechanical behavior of steel piles during static and dynamic deformation was clarified by comparing analytical results

Abstract: In recent years, the replacement repair welding, which repairs damaged steel girder bridges by replacement of the damaged sections with new steel plates through cutting and welding under in-service conditions, is spotlighted for its brilliant features, i.e. it can be achieved without incurring traffic dislocation. However, the mechanical behavior of the welded joints under cyclic loads due to the traffic which passes along the girder bridges is not clarified. In this paper, the fatigue strength of the replacement repair welded joints was investigated in order to improve reliability in the repair welded joints. Steel plate was extracted from the old steel girder bridge; then was welded with new steel plate to construct specimen with the replacement repair welded joints. Flux Cored Arc Welding (FCAW) process was used to fabricate the double "V" butt joints. Fatigue tests were conducted using a servo hydraulic controlled 50tonf capacity UTM with a frequency of 5Hz under constant amplitude loading. The fatigue strength of the replacement repair welded joints was analyzed in detail.

Abstract: During repair welding of steel bridge under service load, vibration occurs due to the dynamic loads like wind and vehicles, etc. However, the effect of vibration during welding on the fatigue strength of weldments is not yet clearly understood. In this paper, the effect of vibration during welding on the fatigue strength of structural steel weldments was investigated in order to improve reliability in the repair welded joints of steel bridge. The base material used in this investigation was SM 490A steel of weldable grade. Flux Cored Arc Welding (FCAW) process was used to fabricate the single ‘V’ butt joints. Welding was performed on the steel under the mechanical vibration of given frequency. The applied frequency was resonant frequency. Also, weldments under no vibration during welding were prepared. Fatigue tests were conducted using a servo hydraulic controlled 50tonf capacity UTM with a frequency of 5Hz under constant amplitude loading. Effect of vibration during welding on the fatigue strength of weldments was analyzed in detail.

Abstract: There have been diverse fatigue models and approaches to properly estimate solder joint reliability. However, it is one of the most difficult problems to determine which solder constitutive models and fatigue models can be applied best. In this paper, both viscoplastic and elastic-plastic-creep solder constitutive models could be utilized to calculate accumulated inelastic response under 208 K to 423 K(-65 °C to 150 °C) thermal cycling condition. And two different fatigue models, Darveaux and creep-fatigue model were applied to find solder joints fatigue life for flip chip assembly. Moreover, each fatigue life was compared to experimental result for the validation of finite element analysis. The actual number of cycles to failure was obtained from cross sectional view of the package with SEM.