Papers by Author: Do Won Seo

Abstract: Porous SiC/Si3N4 composites were fabricated by reaction between Si3N4 and C powder in 0.5 MPa nitrogen atmosphere at 1700oC, 1800oC and 1900oC for 0.5h, 1h and 2h, respectively. The characteristics of in situ porous Si3N4/SiC composites were further investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Results show that the SiC particles are very fine. And the transformation from α-Si3N4 to β-Si3N4 is obvious at 1800oC for 2h and at 1900oC. Otherwise, at 1700oC, the formation of rod-like β-Si3N4 is inhibited. The compressive test was conducted at a cross-head speed of 0.5 mm/min to get the basic mechanical performance of the porous ceramics. The highest value of strength of these composites was obtained when sintered at 1800oC for 1h.

Abstract: The mechanism of apatite formation on the surface of novel PDMS modified SiO2-CaO- P2O5 hybrids is presented. The bioactive hybrids with different P2O5 content were synthesized by sol-gel method and soaked in the simulated body fluid (SBF) for different periods. The surface and the cross-section of the specimens were examined by use of Fourier transform infrared reflection spectroscopy, thin-film X-ray diffraction technique and scanning electron microscopy. The PDMS-SiO2-CaO-P2O5 hybrid containing 0.03 mole ratio of TEP(triethyl phosphate) / TEOS (tetraethoxysilane) exhibited faster apatite formation compared to P2O5-free hybrids. This composition resulted in the thickest apatite layer (~6µm) and this might be attributed to faster formation of the silica hydrogel on the surface of the P2O5-containing hybrids than on the surface of the P2O5-free hybrids. Resultingly, the apatite nucleation on the surface of the P2O5-containing hybrids is effectively induced by the hydrated silica at a lower degree of supersaturation. Changes in concentrations of the calcium, silicon and phosphorus in the SBF and the SEM-EDS line analysis depicting the variation of Si, Ca and P elements for the bulk, the interface and the apatite are reported. These were found to be quite correlative with each other. The rate of the concentration changes for the P2O5-containing hybrids is higher than that of the P2O5-free hybrids. The increases in the calcium and silicon concentrations are attributed to dissolution of the calcium and silicate ions from the hybrids, and the decrease in the phosphorus concentration is ascribed to formation of the amorphous calcium phosphate and crystalline apatite on the surface of hybrids by consuming the phosphate ion from the fluid.

Abstract: Ductile fracture occurs due to micro-void nucleation, growth and finally coalescence into micro-crack. In this study a new ductile fracture condition that based on the microscopic phenomena of void nucleation, growth and coalescence was proposed. Using this condition and combining with finite element model to predict the fracture locations in bulk metal forming. The macroscopic behavior of the material is described according to the flow rules of Levy-Mises. An idealized spherical void within an finite matrix is assumed. The void volume is calculated by taking the increasing volume of the continuum, caused by plastic straining, incorporated in the yield functions. In the model there includes the strain-hardening coefficient of the Ludwik-Holomom stress-strain relationship and concentration of stress. The accumulated damage value is a phenomenon in this model. The results show that it is in close accordance with observations of some experimental specimens. However, in order to obtaining the high trustiness many experiments have to be carried out.

Abstract: Two-layered Si3N4/SiC nano-composites with 50vol.% SiC have been fabricated by two-step sintering of a powder mixture of a-Si3N4 and carbon powder with a mean size of 13nm, and 5wt.% Y2O3. Nano-sized SiC particles were formed through reactions: carbon and surface SiO2 on the Si3N4 particles, and carbon and Si3N4 particles. To combine the specific advantages of nanoscale ceramics with that of metals, they are often used together within one composite component. In this study, the fabrication and mechanical properties of a nanoscale SiC layer brazed with an Inconel alloy were investigated. It was shown that, with a variation of strain rate, the joints have a bending strength of 330-380MPa, and the deflection of the interlayer increases with increasing strain rate. The fracture types are classified into three groups; cracks grow into the metal-brazing filler layer, the ceramicbrazing filler layer or inside the ceramic.

Abstract: Composite materials have drawn a lot of attention in recent years due to their being in expensive, lightweight and biodegradable. The polyetherimide has been successfully used as an effective toughening modifier for polyfuctional epoxy resins for aerospace use. The material used in this study was solvent-impregnated carbon-fiber fabric reinforced polyetherimide. In this study, effects of water absorption cycles on fracture toughness in carbon fiber reinforced composites (CFRP) have been investigated according to various absorption cycles. As a result, strength of CFRP composite was decreased according to increasing water absorption cycles.

Abstract: Using natural fibers that are inexpensive, lightweight and biodegradable, as the reinforcement for composites is difficult due to their poor interfacial properties between hydrophilic fiber and hydrophobic polymer matrices. It is necessary to evaluate fracture toughness of natural fiber reinforced composites according to water absorption rates to improve mechanical performance of those. In this study, compact tension fracture test was conducted to evaluate fracture toughness with the various specimens. The value of fracture toughness has the tendency to decrease as water absorption rate increases. And different surface treatment methods and different polymer matrices have influence on the value of fracture toughness.

Abstract: Steam generator began having severe problems 20 years ago. The main causes were wastage, wear, loose parts, denting. These problems were solved by changing the water chemistry treatment method. Therefore these damage types were replaced by stress corrosion cracking on the inner surface of the tubes and most recently by intergranular attack and stress corrosion cracking on the external surface of the tube. Steam generator has experienced various forms of damage, most involving some kind of corrosion. Some of these problems have been virtually eliminated, others are successfully managed, but a few have not yet been brought under control. This study is to evaluate the environmental effects on the susceptibility to stress corrosion cracking in Cr-Mo alloy under simulated crevice condition. The test is simulated by producing drop, wet and evaporation process. The XRF measurement is performed to investigate the amount and distribution of chromium carbide of grain boundaries. The tests are performed in the alkalic and acidic solution under various strain rates.