Young Cheol Lee

Influence of Partially Debonded Interface on Elasticity of Syntactic Foam: A Numerical Study

The effect of interfacial bonding of glass hollow microspheres and a polymer matrix on the elastic properties of syntactic foam was investigated using representative volume element (RVE) models, including partially debonded interfaces. Finite element analysis, with models having different debonding geometries, was performed to numerically estimate the elastic behavior of the models. The models consisted of bonded and debonded regions of interfaces; the bonded region was treated as the perfectly bonded interface, while the Coulomb friction model was used to describe the debonded region with a small friction coefficient. The changes in the tensile and compressive moduli of the foams were investigated in terms of the degree of interfacial debonding and debonding geometry.

Fluxless Recycling of Die-Cast AZ91 Magnesium Alloy Scrap

Magnesium die-casting has experienced dramatic growth over the past decade and the recycling of magnesium scrap has become increasingly important due to the generation of substantial quantities of scrap in the die-casting process. Magnesium is a readily recyclable material and the recycling of magnesium scrap is crucial in making magnesium more competitive. The main concern associated with using the secondary magnesium is the high level of Fe content and oxide inclusions that are detrimental to the corrosion and mechanical properties of the secondary alloy. In this study, the die-cast specimens were produced using the recycled class 1 scrap which is refined by means of Ar bubbling and Mn addition without using refining fluxes, and their mechanical properties and corrosion characteristics were investigated. The results showed that the tensile properties of the secondary AZ91 alloy were equivalent to those of the primary magnesium alloy after appropriate treatments. The corrosion resistance of the recycled magnesium was also found to increase by Ar bubbling and Mn addition.

Numerical investigation of thermal and residual stress of sapphire during c-axis vertical Bridgman growth process considering the solidification history effect

Sapphire single crystals have been highlighted for epitaxial of gallium nitride films in high-power laser and light emitting diode industries. In this study, the evolution of thermally induced stress in sapphire during the vertical Bridgman crystal growth process was investigated using a finite element model that simplified the real Bridgman process. A vertical Bridgman process of cylindrical sapphire crystal with a diameter of 50 mm was considered for the model. The solidification history effect during the growth was modeled by the quite element technique. The effects of temperature gradient, seeding interface shape and seeding position on the thermal stress during the process were discussed based on the finite element analysis results.

Influence of Glass Microsphere Filler on the Rheological Behavior of an Epoxy Resin

Abstract The rheological behavior of epoxy resin filled with glass hollow microspheres was studied using three types of microspheres with different particle size distributions. The shear-rate dependence on viscosity, as well as relative apparent viscosity, of the suspensions was investigated at different microsphere filler loadings. The change in relative viscosity with different types of microsphere loads was also investigated using the established semi empirical model to estimate the critical filler load. It was found that the critical filler load for all suspensions tested depends mostly on the filler size distribution, neither on the average filler size nor on the specific surface area of the filler.

Analysis of thermal stress through finite element analysis during vertical Bridgman crystal growth of 2 inch sapphire

Sapphire single crystals have been highlighted for epitaxial of gallium nitride films in high-power laser and light emitting diode industries. Among the many crystal growth methods, vertical Bridgman process is an excellent commercial method for growing high quality sapphire crystals with c-axis. In this study, the thermally induced stress in Sapphire during the vertical Bridgman crystal growth process was investigated using a finite element model. A vertical Bridgman process of 2-inch Sapphire was considered for the model. The effects of vertical and transverse temperature gradients on the thermal stress during the process were discussed based on the finite element analysis results.

Development of a Magnesium Alloy Rotor with a Pin-Point Gate Mold

The rotor is a key determinant of the performance of a compressor and many attempts have been made to improve the efficiency of compressors by optimising rotor design. Rotors are usually made of several layers of steel sheets with thin cavities through the steel sheets, and aluminium alloys are used to fill the cavities by high pressure die casting process, and so bind the steel sheets together. Because of their high fluidity and good damping ability, magnesium alloys can be a good alternative for a high efficiency rotor. In this study, magnesium alloys were used for manufacturing rotors by high pressure die casting process using pin-point gate mold. By adopting a pin-point gate system, additional machining was eliminated and casting defects were reduced due to good castability of magnesium alloys.