• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2000.04a
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• pp.51-56
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• 2000

Demand for new nondestructive evaluations is growing to detect tensile crack growth behavior to predict long term performance of materials and structure in aggressive environments, especially when they are in non-visible area. Acoustic emission technique is well suited to these problems and has drawn a keen interests because of its dynamic detection ability, extreme sensitivity and location of growing defects. In this study, we analysed acoustic emission signals obtained in tensile test of high strength fire resistance steel for frame structure with time frequency analysis methods. The results obtained are summaries as follows ; In the T and TN specimen consisting of ferrite and pearlite grains, most of acoustic emission events were produced near yield point, mainly due to the dislocation activities during the deformation. However, B specimen under $600^{\circ}C$ - 10min had a two peak which was attribute to the presence of martensite phase. The first peak is before yield point the second is after yield point. The sources of second acoustic emission peak were the debonding of martensite-martensite interface and the micro-cracking of brittle martensite phase. In $600^{\circ}C$-30min to $700^{\circ}C$-60min specimens, many signals were observed from area before yield point and counts were decreased after yield point.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.2
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• pp.97-103
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• 1984

Post weld heat treatment(PWHT) of weldment of the low alloy steel is carried out to remove residual stress existing in weldment and to improve fracture toughness, but it is often observed that there occurs grain boundary failure and that fracture toughness decreases in weld heat affected zone(HAZ)because of PWHT. In this paper, the effect of heating rate and holding time of PWHT on fracture toughness were evaluated by crack opening displacement (CDD)test and micro-hardness test under the constant stress simulated residual stress in HAZ of Cr-Mo steel. The experimental results are as follow; (1)Transition temperature of weld HAZ after PWHT was dependent upon heating rate greater than holding time, and fracture toughness was decreased with an increase of the heating rate. (2)Softening ration of the notch tip was increased with holding time within one hour and saturated after one hour, but under applied stress it was increasing continuously. (3)The average hardness value in weld HAZ was increased with heating rate of PWHT.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.5
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• pp.717-724
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• 1987

Mechanical properties tests and fracture toughness tests of turbine rotors were performed in the wide range of temperatures, -150.deg.C-+150.deg.C, and fracture toughness values from above tests were compared with the estimated values from mechanical properties at lower and upper shelf temperatures and FATT. The relations between mechanical properties and $K_{IC}$ properties proposed by Rolfe and Begley were reviewed and confirmed through these experimental results. On the fracture surfaces of some specimens which were satisfied with the Ikeda's $K_{IC}$ criterion micro dimple zone was detected at the rear of fatigue crack zone and it was confirmed that these specimens were not satisfied with the thickness requirement of ASTM E 399.E 399.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.2
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• pp.296-302
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• 2010

If the fiber reinforced plastic is exposed to the moisture for a long period of time, most of moisture absorption occurs on the resin place, thus dropping cohesiveness between the molecules as the water molecules permeated between high molecular chains grant high molecular mobility and flexibility. Also as the micro crack occurs due to the permeation of moisture on the interface of glass fiber and epoxy resin, it is developed to the overall damage of interface place. Hence, the study on absorption is essential as the mechanical and physical properties of fiber reinforced composites are reduced. However, the study on absorption has the inconvenience needing to expose composite materials to fresh water or seawater for 1 month or up to 1 year. Therefore, this study has exposed fiber reinforced composites to fresh water and has developed a model with an accuracy of 98% after comparing the analysis value obtained by using ANSYS while basing on the experimental value of property decline by absorption and the basic properties of glass fiber and epoxy resin used in the experiment.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.1 s.256
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• pp.70-76
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• 2007

Elastic-plastic fracture mechanics is popularly used for integrity evaluation of major components, however, it is not easy to extract standard specimens from operating facility. This paper examines how ductile fracture toughness is characterized by a small punch testing technique in conjunction with finite element analyses incorporating a damage model. At first, micro-mechanical parameters constituting Rousselier model are calibrated for typical nuclear materials using both estimated and experimental load-displacement (P-$\delta$) curves of miniaturized specimens. Then, fracture resistance (J-R) curves of relatively larger standard CT specimens are predicted by finite element analyses employing the calibrated parameters and compared with corresponding experimental ones. It was proven that estimated results by the proposed method using small punch specimen is promising and might be used as a useful tool for ductile crack growth evaluation.

• Journal of the Korean Society for Precision Engineering
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• v.30 no.4
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• pp.403-408
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• 2013

In this paper, the characteristics of ablation processing of the eagle glass by pico-second laser are investigated. The laser ablation is used to process micro forms on materials. The ablation causes little thermal effect and little burr on the surface of eagle glass. In order to examine the characteristics of panic cracks, experiments are conducted under various cutting conditions such as a frequency of 600 kHz, laser powers, scan speeds and number of scan(NS). To minimize the panic cracks, the specimens are heated at $30^{\circ}C$, $45^{\circ}C$, and $60^{\circ}C$ for ten minutes respectively and then they are broken by hands. Laser powers, NS and scan speeds have an effect on glass cutting results. The ablation depths increase with an increase in the laser power and NS whereas the panic cracks decrease with an increase in scan speed. The high temperature on processed specimens reduces the panic cracks and makes good results of laser cutting. The optimal condition for eagle glass laser cutting is found to be at 30 W of laser power, 3 mm/s of scan speed and 500 of NS, respectively.

• Proceedings of the KWS Conference
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• 2004.05a
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• pp.324-326
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• 2004

l2wt%Cr Steel has been applied on turbine bucket and nozzle partition material of power plant. Turbine bucket and nozzle get damaged by solid particle within steam, therefore they are protected by surface treatments such as ion nitriding, boriding and chrome carbide HVOF spray coating. In this study, solid particle erosion(SPE) characteristics after these surface treatments are examined at operating temperature 540$^{\circ}C$ and 590$^{\circ}C$ of fossil power plant and the mechanism of damage was studied. Erosion of 12wt%Cr steel is originated by micro cutting and that of boriding and chrome carbide HVOF spray is originated by these mechanism - repeating collision, crack initiation and propagation. As the results of SPE test at 540$^{\circ}C$ and 30$^{\circ}$ impact angle that is the most commonly occurred in power plant, Boriding had the best SPE -resistance property, Cr$_2$C$_3$-25(Ni20Cr) HVOF spayed and ion nitrided samples were also better than bare metals(l2wt%Cr Steels). At 590$^{\circ}C$ and 30$^{\circ}$ impact angle, Boriding had also the most superior characteristic and HVOF spay sample was better than bare metal.

• Journal of the Korean Ceramic Society
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• v.42 no.2 s.273
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• pp.140-144
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• 2005

The microstructures and mechanical properties of Tetragonal Zirconia Polycrystals (TZP) sintered bodies, which made by pressureless and spark plasma sintering techniques, were investigated using XRD, SEM, and TEM techniques. In the spark plasma sintered samples, the TZP grains were equiaxed type including many sub-grain boundaries regardless of sintering conditions. The biaxial strength of TZP having an average of 80 nm grains in diameter was high in value with 1025 MPa, but fracture toughness showed a low value due to the absence of a fracture toughening mechanism such as transformation toughening. In the Pressureless Sintered (PLSed) samples, the grain size of TZP was strongly dependent on the sintering temperature; i.e., it gradually increased as the sintering temperature increased. The value of fracture toughness increased as the grain size increased by the stress-induced phase transformation and Borne crack deflection.

• Journal of the Korean Society for Precision Engineering
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• v.27 no.10
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• pp.40-44
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• 2010

Laser drilling is an enabling technology for Through Silicon Via (TSV) interconnect applications. Recent advances in picoseconds laser drilling of blind, micron sized vias in silicon is presented here highlighting some of the attractive features of this approach such as excellent sidewall quality. In this study, we dealt with comparison of heat affection around drilled hole between a picosecond laser and a nanosecond laser process under the UV wavelength. Points which special attention should be paid are that picosecond laser process lowered experimentally recast layer, surface debris and micro-crack around hole in comparison with nanosecond laser process. These finding suggests that laser TSV process has possibility to drill under $10{\mu}m$ via. Finally, the laser drilling platform was constructed successfully.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.2
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• pp.519-528
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• 1994

Degradation problem due to long term service in machine or structure is now one of important problems in whole industrial field. In this study, pressure vessel steel, Cr-Mo steel, which was used more than 60,000 hours, was surface-modified by laser beam radiation for the improvement of fatigue strength. To find out optimum radiation condition, hardness, residual stress measurement and fatigue tests were carried out with the specimen of different radiation conditions. Experimental results show that micro-hardness values on the surface of the radiated specimens were approximately 2.2 times higher than those of un-radiated ones. In the depth direction of the specimen, hardness on the surface showed maximum value and was decreased at the inside the specimen. Different hardness values are due to the energy density Q which was absorbed by the specimen. Fatigue tests show that fatigue life was improved by the compressive residual stress after laser beam radiation. However, some specimens with differednt conditions show the shorter fatigue life. It means that laser beam radiation with optimum parameter can improve thae fatigue strength.