• 한국해양공학회지
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• 제23권2호
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• pp.69-73
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• 2009

The objective of this study was to obtain a better understanding of the delayed hydride cracking (DHC) of Zr-2.5Nb alloy. The DHC model has some defects: first, it cannot explain why the DHC velocity (DHCV) becomes constant regardless of an applied stress intensity factor, even though the stress gradient is affected by the applied stress intensity factor at the notch tip. Second, it cannot explain why the DHCV has a strong dependence on the method of approaching the test temperature by a cool-down or a heating-up, even under the same stress gradient, and third, it cannot predict any hydride size effect on the DHC velocity. The DHC tests were conducted on Zr-2.5Nb compact tension specimens with the test temperatures reached by a heating-up method and a cool-down method. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by being furnace-cooled, water-quenched, and liquid nitrogen-quenched. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates. This different DHC behavior of the Zr-2.5Nb tube with the cooling rate after a homogenization treatment is due to the precipitation of the $\gamma$-hydrides only in the water-quenched Zr-2.5Nb tube. This experiment will provide supporting evidence that the terminal solid solubility of a dissolution (TSSD) of $\gamma$-hydrides is higher than that of $\delta$-hydrides.

• 대한기계학회논문집A
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• 제24권4호
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• pp.1055-1064
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• 2000

First, various specimen geometries, namely, coupon type, waisted type and dog-bone type, were examined to determine appropriate fatigue specimen of thermoplastic composite material AS4/PEEK and the n, fatigue strength of smooth and notched specimens of AS4/PEEK [-45/0/+45/90]2s was investigated. Fatigue tests were performed under load controlled condition at a stress ratio of 0. 1 at a frequency of 5Hz. Stiffness degradation of specimens with fatigue cycling was monitored using an automated unloading compliance technique. The waisted type specimen is found appropriate for smooth fatigue specimen geometry of AS4/PEEK. As for the effect of stress concentration, it is found that fatigue strength is higher for a 2mm-diameter hole notched specimen than a 5mm-diameter one. Fatigue notch factor decreases with the increase of fatigue life. These results are far different from the trend of fatigue strength of metallic materials. The stiffness variation of smooth specimen was only 4% at maximum until final fracture. On the other hand, the stiffness of hole notched specimen was reduced by 45% at maximum. Notched fatigue strength was compared between thermoplastic composite AS4/PEEK and thermosetting composite Graphite/Epoxy. In long-life fatigue (>104), the AS4/PEEK composite shows superior fatigue strength, but in short-life fatigue, the fatigue strength of the Graphite/Epoxy composite is nearly equal or somewhat higher than that of the AS4/PEEK composite.

• International Journal of Concrete Structures and Materials
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• 제10권2호
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• pp.163-175
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• 2016

This paper presents a revised procedure for computation of double-K fracture parameters of concrete split-tension cube specimen using weight function of the centrally cracked plate of finite strip with a finite width. This is an improvement over the previous work of the authors in which the determination of double-K fracture parameters of concrete for split-tension cube test using weight function of the centrally cracked plate of infinite strip with a finite width was presented. In a recent research, it was pointed out that there are great differences between a finite strip and an infinite strip regarding their weight function and the solution of infinite strip can be utilized in the split-tension specimens when the notch size is very small. In the present work, improved version of LEFM formulas for stress intensity factor, crack mouth opening displacement and crack opening displacement profile presented in the recent research work are incorporated. The results of the double-K fracture parameters obtained using revised procedure and the previous work of the authors is compared. The double-K fracture parameters of split-tension cube specimen are also compared with those obtained for standard three point bend test specimen. The input data required for determining double-K fracture parameters for both the specimen geometries for laboratory size specimens are obtained using well known version of the Fictitious Crack Model.

• 한국안전학회지
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• 제32권3호
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• pp.1-7
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• 2017

Particle reinforced composites are materials that have enhanced physical properties by adding particle reinforcements to polymer materials and have been applied to a wide range of fields such as the aerospace, bio-technology and automative industry. In this study, particle reinforced composites were prepared by mixing $SiC/Al_2O_3$ to the vinyl ester as the thermoset resin. The purpose of this study is to evaluate mechanical properties and fracture behavior by the tensile test and single edge notch specimen according to the addition ratio of reinforcement. Addition of 1 and 2 wt% of the particle reinforcement to the vinyl-ester resin was effective for the strength improvement. However, when it was more than 3 wt%, its strength was decreased. Also the highest elastic modulus obtained as 3.19 GPa was found at the 2 wt% addition of reinforcement. Futhermore the fracture toughness was evaluated by the energy release rate and the maximum critical energy release rate was obtained when 1 wt% reinforcement. The results show that the limit of adding of $SiC/Al_2O_3$ for improvement of the mechanical and fracture performance is 2 wt% reinforcement particles.

• Journal of Welding and Joining
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• 제33권6호
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• pp.6-12
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• 2015

Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of $200^{\circ}C$ to $900^{\circ}C$ due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, $M_6C$ with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.

• Corrosion Science and Technology
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• 제6권3호
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• pp.103-111
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• 2007

The effect of stress factors on the growing process of stress corrosion cracking (SCC) of the sensitized 18-8 stainless steel in high temperature water was investigated using equations of crack growth rate derived from applying electric circuits to SCC corrosion paths. Three kinds of cross sections have to be considered when electric circuit is constructed using total current. The first is ion flow passage area, $S_{sol}$, of solution in crack, the second is total dissolving surface area, $S_{dis}$, of metal on electrode of crack tip and the third is dissolving cross section, $S_{met}$, of metal on grain boundary or in base metal or in welding metal. Stress may affect each area. $S_{sol}$ may depend on applied stress, $\sigma_{\infty}$, related with crack depth. $S_{dis}$ is expressed using a factor of $\varepsilon(K)$ and may depend on stress intensity factor, K only. SCC crack growth rate is ordinarily estimated using a variable of K only as stress factor. However it may be expected that SCC crack growth rate depends on both applied stress $\sigma_{\infty}$ and K or both crack depth and K from this consideration.$\varepsilon(K)$ is expressed as ${\varepsilon}(K)=h_2{\cdot}K^2+h_3{\cdot}K^3$ when $h_{2}$ and $h_{3}$ are coefficients. Also, relationships between SCC crack growth rate, da/dt and K were simulated and compared with the literature data of JBWR-VIP-04, NRC NUREG-0313 Rev.2 and SKIFS Draft. It was pointed out in CT test that the difference of distance between a point of application of force and the end of starter notch (starting point of fatigue crack) may be important to estimate SCC crack growth rate. An anode dissolution current density was quantitatively evaluated using a derived equation.

• 열처리공학회지
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• 제23권4호
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• pp.183-190
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• 2010

There are many methods to reduce the weight and the cost of the automobile body, among them, Tailor Welded Blank (TWB) is new welding method applied to body structure. It is necessary to evaluate mechanical properties of TWB structures or sheets for the application to automobile body parts. In this study, the stiffness of T-type and L-type joint structures, composite of TWB panel, which simplified two portions of side structure in automobile body were investigated. Additionally, the fatigue properties of TWB panels were obtained. Two types of welding technologies, laser and mash seam welding, were used to join mild panels with different thickness. This results are compared with conventional structures. The results are as follows: 1) The stiffness of joint structures, composite of TWB panel, is approximately 17% higher than that of conventional ones. 2) The location of welding line in TWB had a effect on the in plane bending stiffness, but not on the out of plane bending stiffness. 3) In terms of welding technology type, the mash seam welding show higher stiffness than the laser welding for in plane bending stiffness. But minimal differences in both types are revealed for out of plane bending stiffness. 4) The fatigue strength, composite of TWB panel, is lower than that of base steel. It is thought that defects in the welding zone had the action of notch in the fatigue test.

• Restorative Dentistry and Endodontics
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• 제23권2호
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• pp.690-700
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• 1998

Rubber-toughened particles which are used in the field of chemical engineering are used to increase the fracture toughness of thermoset resin. The application of Core-Shell particles, one of rubber-toughened particles, as a filler for dental composite or restoration have not been examined. The purpose of this study was to evaluate possible use of Core-Shell particles for dental composite, and the hypothesis was that additional toughening mechanisms are activated by the addition of Core-Shell particles. After blending 50vol% quartz with Bis-GMA/TEGDMA resin matrix, the experimental resins were made by the addition of Core-Shell particles with varied content level as 0, 2.5, 5, 7.5, 10, 12.5, 15, and 20wt%. Fracture toughness was determined on three-point bending specimen with single-edge notch according to ASTM-E 399. Also, flexural properties, that is, strength and modulus were measured by three-point bending testing. Fractogragh of fracture toughness specimen was observed using SEM (JEOL 6400 SEM, MA). The following results from this study were obtained ; 1. Fracture toughness of composite resin added 2.5wt% Core-Shell particles was significantly higher than control group ($p{\leq}0.05$). 2. Flexural properties were decreased with increasing Core-Shell particle content, which showed a correlation statistically ($p{\leq}0.05$). 3. A toughening mechanism such as lamination and microcrack was observed in specimen determined high fracture toughness. 4. The dispersion of Core-Shell itself and quartz filler particles was limited present high content of Core-Shell particles, which decreased a resulting mechanical properties of composites. These results suggest that adequate Core-Shell particles can be used to enhance mechanical properties included toughening for dental composites.

• 한국자동차공학회논문집
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• 제10권6호
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• pp.158-165
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• 2002

This study was to evaluate mechanical properties and toughness of the Al5083-O aluminum alloy welding zone according to the mixing shield gas ratio and heat input change. The GMA(Gas Metal Arc) welding of the base metal was carried out with four different mixing shield gas ratios(Ar100%+He0%, Ar67%+He33%, Ar50%+He50%, and Ar33%+He67%) and three different heat inputs(low, medium, and high). To investigate the Charpy absorbed energy of the weld zone, the specimens were divided base metal, weld metal, fusion line, and HAZ notched specimen according to the worked notch position. The different gas ratio and heat input had little effect upon the tensile strength. But Ar33%+He67% mixture had the greatest mechanical properties considering that the more He gas ratio concentrations, the higher yield strength and elongation. The maximum load and displacement of the weld metal notche specimen was so much low more than that of the base metal, but fusion line and HAZ notched specimens showed almost same regardless of the mixing shield gas ratio and heat input. The Charpy absorbed energy was lowest in weld metal notched specimen, and increased in the fusion line, and HAZ notche specimen in order. Ar33%+He67% mixture had the greatest toughness considering that the more He gas ratio, the higher absorption energy.

• Design & Manufacturing
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• 제14권3호
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• pp.23-30
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• 2020

Laser Assisted Thermo-Compression Bonding (LATCB) has been proposed to improve the "chip tilt due to the difference in solder bump height" that occurs during the conventional semiconductor chip bonding process. The bonding module of the LATCB system has used a piezoelectric actuator to control the inclination of the compression jig on a micro scale, and the piezoelectric actuator has been directly coupled to the compression jig to minimize the assembly tolerance of the compression jig. However, this structure generates a lateral force in the piezoelectric actuator when the compression jig is tilted, and the stacked piezoelectric element vulnerable to the lateral force has a risk of failure. In this paper, the optimal design of the flexure hinge was performed to minimize the lateral force generated in the piezoelectric actuator when the compression jig is tilted by using the displacement difference of the piezoelectric actuator in the bonding module for LATCB. The design variables of the flexure hinge were defined as the hinge height, the minimum diameter, and the notch radius. And the effect of the change of each variable on the stress generated in the flexible hinge and the lateral force acting on the piezoelectric actuator was analyzed. Also, optimization was carried out using commercial structural analysis software. As a result, when the displacement difference between the piezoelectric actuators is the maximum (90um), the maximum stress generated in the flexible hinge is 11.5% of the elastic limit of the hinge material, and the lateral force acting on the piezoelectric actuator is less than 1N.