• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.5
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• pp.397-401
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• 2000

The corrosion-fatigue characteristics of the 12Cr steel, which is widely used in fossil power plants as a turbine blade material, are evaluated nondestructively by use of the Rayleigh surface wave. In this study, the frequency dependency of the Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in the aged specimens, and then compared to the corrosion-fatigue characteristics. The width of the backward radiation profile decreases as the increase of the aging temperature, which seems to result from the increase of the effective degrading layer thickness. This parameter also shows an inversely proportionality to the exponent, m, in the Paris law which predicts the crack size increasement due to fatigue. The result observed in this study demonstrates high potential of the backward radiated ultrasound as a tool for the nondestructive evaluation of the corrosion-fatigue characteristics of the aged materials.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.1
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• pp.26-33
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• 2013

BMI nozzles in bottom head of the nuclear vessel are one of major components in nuclear power plants. The BMI nozzles have high possibility to generate PWSCC(primary water stress corrosion crack) according to recent foreign case although operation temperature is lower then the upper head of the nuclear vessel. Thus, nondestructive inspection of the BMI nozzles is required. But, inspection of BMI nozzles is not easy since the BMI nozzles placed in high radiated area and inside the nozzles filled with boric acid. Thus, in this study, a TOFD transducer for inspection of BMI and automated scanner system with water were developed. Also, validation of performance of the developed transducer and system are performed using specimens with artificial defects.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.236-242
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• 1989

In order to investigate the effects of hot corrosion on the creep rupture properties and creep life of 304 stainless steel being used as tube materials of heavy oil fired boiler, the creep rupture tests were carried out at temperature 630.deg.C, 690.deg.C and 750.deg.C in static air for the specimens with or without coating of double layer corrosives according to the new hot corrosion test method simulating the situation commonly observed on superheater tubes of the actual boiler. The double layer corrosives are 85% V$_{2}$O$_{5}$ + 10% Na$_{2}$So$_{4}$ + 5% Fe$_{2}$O$_{3}$ as the inner layer corrosive being once melted at 900.deg. C and crushed to powder, and 10% V$_{2}$O$_{5}$ + 85% Na$_{2}$SO$_{4}$ +5% Fe$_{2}$O$_{3}$ as the outer layer corrosive. As results, in the specimen coated with the double layer corrosives, the rupture strength was extremely lowered and showed a large difference each other. The rupture ductility also lowered remarkably as a result of the brittle fracture mode due to hot corrosion. These results indicate that hot corrosion could essentially alter the creep fracture mechanism. From the metallographic observation, it was clarified that the rupture life of 304 stainless steel subjected to hot corrosion was chiefly determined by the behavior of the aggressive intergranular penetration of sulfides.des.

• Applied Chemistry for Engineering
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• v.20 no.5
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• pp.511-516
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• 2009

Photodegradation characteristics of polyolefin composites were studied. Thermogravimetric analysis results suggest that the polyolefin blends used in this study have different amounts of talc. The mechanical behaviors of polyolefin blends, which experienced UV-irradiation in accordance with SAE J1960, are investigated using tensile and Izod impact tests. These results show that as the UV-exposure time increases, a significant drop in the elongation at break and impact strength at a low temperature are observed. This may be explained by the decreases in elastic energy derived from the scission of polymer molecular chains and the low density of entanglement after UV- photodegradation. Scanning electron microscopy observations indicate that no crack and surface damage are observed, while the additional talc particles are exposed, on the UV-exposed surfaces. The exposure of talc particles may be responsible for the discoloration of UV-exposed polyolefin blend surface. Observation using Fourier transform infrared spectroscopy (FT-IR) confirms the presence of photodegradation on the surface of UV-exposed polyolefin blend.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.247-250
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• 2005

The need for improved fuel efficiency, weight reduction has motivated the automotive industry to focus on aluminum alloys as a replacement for steel-based alloy. To cope with the needs for high structural rigidity with low weight, it is forecasted that substantial amount of cast components will be replaced by tubular parts which are mainly manufactured by the extruded aluminum tubes. The extrusion process is utilized to produce tubes and hollow sections. Because there is no weld seam, the circumferential mechanical properties may be uniform and advantageous for hydroforming. However the possibility of the occurrence of a surface defect is very high, especially due to the temperature increase from forming at high pressure when it comes out of the bearing and the roughness of the bearing, which cause the surface defects such as the dies line and pick-up. And when forming a extruded aluminum tube, the free surface of the tube becomes rough with increasing plastic strain. This is well known as orange peel phenomena and has a great effect not only on the surface quality of a product but also on the forming limit. In an attempt to increase the forming limit of the tubular specimen, in the present paper, surface asperities generated during the hydroforming process are polished to eliminate the weak positions of the tube which lead to a localized necking. It is shown that the forming limit of the tube can be considerably improved by simple method of polishing the surface roughness during hydroforming. And also the extent of the crack propagation caused by dies lines generated during the extrusion process is evaluated according to the deformed shape of the tube.

• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.3
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• pp.72-78
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• 2007

A diesel particulate filter causes progressive increase in back pressure of an exhaust system due to the loading of soot particles. To maintain the pressure drop caused by DPF under proper level, a regeneration process is mandatory when excessive loading of soot is detected in the filter. It is a major reason why the relation between the amount of soot and the pressure drop in a DPF becomes crucial. On the other hand, pressure drop varies with not only the soot loading but also conditions of exhaust gas such as mass flow rate. Therefore, the relation among them becomes complicated. Furthermore, the characteristics of heat transfer in a DPF is another crucial parameter in order for the filter to avoid thermal crack during regeneration period. This study presents characteristics of pressure drop under various conditions of soot loading and mass flow rate in catalyzed diesel particulate filter. This study also shows characteristics of heat transfer in DPF when high temperature gas flows into the filter. Experiments reveal that the soot loading and mass flow rate affect characteristics pressure drop independently. Experiments also indicate that the amount of coating material has little influence on pressure drop with changes in soot loading and mass flow rate. However, increased catalyst coating may lead to the improved heat transfer which is efficiency to reduce thermal stress of the filter.

• Restorative Dentistry and Endodontics
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• v.18 no.2
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• pp.277-290
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• 1993

Dental caries is considered to be caused by demineralization by organic acid produced by microorganism. But the formation of subsurface lesion in initial caries make it diffcult to explain by simple demineralization. This study is carried out on the basis of thermodynamic concept proposed by Margolis and Moreno. The purpose of this study is to evaluate the effects of acid concentration and pH of lactate buffer system on the artificial caries lesion progress. 160 teeth without any crack, defect or opaque enamel were used and coated with nail varnish except the window ($2{\times}3$ mm). Under the constant degree of saturation(D.S.). The teeth were divided into 8 groups according to acid concentration(10mM, 25mM, 50mM, 100mM) and pH(4.3, 5.0, 6.0). Each group was immersed in buffer solution for 3, 6, 9, 18 days under controlled temperature($25^{\circ}C$). After cutting through the window and grinding, the specimens, 100-150 um in thickness, were imbibed in water or air and examined using polarilizing microscope. The depth of the surface and subsurface surface lesion were measured. 1. In the constant pH and D. S. value, the subsurface lesion progresses more rapidly as the concentration of lactic acid increases. (0.01, 0.025, 0.05, 0.1) 2. In the constant acid concentration and DS value, the subsurface lesion progresses more slowly as the pH increases. (4.3, 5.0, 5.5, 6.0) 3. The width of surface lesion seems to be constant independant of pH and acid concentration.

• Macromolecular Research
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• v.12 no.4
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• pp.399-406
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• 2004

Glass-like carbon precursors shrink significantly during curing and carbonization, which leads to crack formation and bending. Cured furan resin powder and ethanol were added to furan resin to diminish the weight loss, to suppress the shrinkage and bending, and to readily release the gases evolved during polymerization and curing. Curing and carbonization were controlled by pressure and slow heating to avoid damage to the samples. The effect of the filler and ethanol on the fabrication process was examined by measuring the properties of the glass-like carbon, such as the specific gravity, bending strength, electrical resistivity, and microstructural change. The specific gravities of the filler-added glass-like carbons were higher than those of the ethanol-added samples because of the formation of macropores from the vaporization of ethanol during the curing and polymerization processes. Although the ethanol-added glass-like carbons exhibited lower bending strengths after carbonization than did the filler-added samples, the opposite result was observed after aging at 2,600$^{\circ}C$. We found that the macropores created from ethanol were contracted and removed upon heat treatment. The electrical resistivity of the glass-like carbon aged at 2,600$^{\circ}C$ was lower than those of the samples carbonized at 1,000$^{\circ}C$. We attribute this phenomenon to the fact that aging at high temperature led to well-developed microstructures, the removal of macropores, and the reduction of the surface area.

• Conservation Science in Museum
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• v.7
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• pp.43-51
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• 2006

From the low swamp remains in Sinchang-dong, Gwangju, a number of wooden lacquerware along with various wooden artifacts were excavated. These excavated lacquerware have deteriorated and weak wooden parts and the lacquer layer come off from the wooden parts; they are very likely to peel off and crack. Therefore, we impregnated the lacquerware in PEG#4000 40% solution that was effective for vacuum freeze drying and finished freeze-drying below 0°…. We compared the weight of wood and lacquerware right after the freeze-drying with the weight after leaving them in a airtight space with 60% RH (relative humidity). The comparison results showed no change in weight; thereby we confirmed controlling the finishing temperature during freeze-drying could control the moisture in wood after drying and it could stabilize wood against the change in moisture in the atmosphere. according to the analysis of the lacquer fragment, the base layer was pasted on the wooden surface with mixed black pigment and the upper layer was pasted three or four times with the mixture of lacquer and black pigment; or it was pasted without the black base coating.

• Journal of Welding and Joining
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• v.3 no.1
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• pp.22-31
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• 1985

Hot straining embrittlement is one of the most important factors which cause the brittle fracture initiation even in the service temperature in the case of mild steel and high tensile steel. Therefore it is necessary to analyze thoroughly the hot straining embrittlement occurred in weld HAZ of the structural steels. The behaviors of plastic deformation and fracture toughness at the notch tip of the hot strained weld HAZ in structural steels (SB 41 KS, SA 588-Grade A ASTM) have been studied by the recrystallization technique and crack opening displacement (COD) test method. The obtained results are summarized as follows; 1. The plastic zone is formed at the notch tip of weld HAZ owing to nomotonic and cyclic hot stran, and the maximum plastic strain increases with the accumulated hot straining amounts. 2. The distribution of the effective strain at the plastic deformed zone in HAZ can be determined as follows; (.epsilon. over bar $_{p}$ )$_{\chi}$=.epsilon. over bar $_{cr}$ ( $R_{/chi}$/.chi.)$^{m}$ where, .epsilon. over bar $_{cr}$ : (SB 41; .epsilon. over bar $_{cr}$ = 0.2, SA 588; .epsilon. over bar $_{cr}$ = 0.1) 3. The embrittlement of weld HAZ in SB 41 and SA 588 is influenced by hot strain, and the degree of embrittlement becomes deeper with hot straining amounts. 4. The embrittlement of weld HAZ of SB 41 is not influenced by the hot straining amounts until .epsilon. over bar $_{max}$ = 0.36, $R_{\chi}$ = 0.065mm, however the embrittlement of structure in SA 588 is considerably influenced even by a small quantity of the hot straining amounts.s.