• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.5
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• pp.373-380
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• 2000

It is useful to use NDE methods to assess the mechanical properties of materials since destructive methods are time-consuming and usually require cutting of sample from the material/component. In the present research, ultrasonic characteristics have been utilized to evaluate changes of mechanical properties due to heat treatment temperature and condition. The attenuation coefficient of ultrasonic wave increased as the heat treatment temperature because the grain size increased in size as the temperature. The attenuation coefficient decreased as the heat treatment has been progressed (quenched, tempered, PWHT). In the case of ultrasonic velocity measurement, velocity difference between quenched and tempered/PWHT was 40 m/s. There was a good relationship between the attenuation coefficient and the toughness. The relationship can be used for the nondestructive evaluation of the forged reactor vessels. Moreover, the method may be effectively used in the field application.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.105.1-105.1
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• 2012

고온가압소결으로 제조된 SiCf/SiC 복합체는 부식과 침식에 강하고 우수한 열적 성질과 고온에서의 높은 기계적 강도를 유지하는 장점을 가진 복합체다. 복합체의 파괴인성은 섬유와 기지 사이에 존재하는 열분해탄소 (PyC) 계면층에 의해 큰 영향을 받는데, 고온가압소결중 첨가되는 소결조제 ($Y_2O_3$, MgO, $Al_2O_3$)와 반응하여 계면이 손상되어 복합체의 기계적 특성치가 낮아지는 결과를 보였다. 본 연구에서는 계면의 손상을 보호하고자 PyC 계면상 위에 SiC 층을 증착하였는데 계면층과 SiC 층의 증착은 화학기상 증착법(CVD)을, 기지채움 공정은 전기영동법(EPD)과 고온가압소결방법(Hot Pressing)을 이용하여 복합체를 제조하였다. Tyranno-SA 섬유에 소스가스인 메탄을 열분해 하여 200nm 두께로 PyC 계면상을 증착하고, 두께를 달리하여 보호층으로써의 SiC 층을 single 과 double layer로 증착하였다. SiC 나노분말과 소결 첨가제인 $Y_2O_3$, $Al_2O_3$, MgO를 첨가한 슬러리를 전기영동법(EPD)을 이용하여 섬유내부에 슬러리를 함침시켰고, 이러한 프리폼을 $1750^{\circ}C$/20MPa의 조건으로 고온 가압소결 하여 $SiC_f$/SiC 복합체를 제조하였다. 이렇게 single layer와 double layer로 제조된 $SiC_f$/SiC 복합체에 대해 밀도와 미세구조를 관찰하였고, 기계적 특성을 비교하여 보호층으로써의 SiC 증착효과를 고찰하고자 하였다.

• Journal of the Korean Ceramic Society
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• v.41 no.7
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• pp.508-512
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• 2004

For abutment of dental implants, (Y, Nb)-TZP/Alumina composites were prepared by addition of 10-90 vol% alumina at an interval of 10 vol％ into tetragonal zirconia solid solution which consists of 90.24 mol% Zr $O_2$, 5.31 mol% Y$_2$ $O_3$, and 4.45 mol% Nb$_2$O$\_$5/. Biaxial flexure strength and fracture toughness of composite were optimized by adding 10 vol% alumina, which resulted in 900 MPa and 8.9 MPam$\^$1/2/, respectively. The composite did not undergo low temperature degradation even after autoclave treatment at 200$^{\circ}C$ for 10 h. 65 of (Y, Nb)-TZP/Alumina composite abutments were employed into 40 patients and any adverse reaction, screw loosing, or fracture of abutments was not observed for the span of 2 years, indicating that the ceramic abutments can be safely used for restorations.

• Polymer(Korea)
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• v.26 no.6
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• pp.767-777
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• 2002

In this work, biodegradable modified aliphatic polyester (MAP) in tetrafunctional epoxy (4EP) resin was investigated in terms of cure kinetics, thermal stabilities, rheological properties, and mechanical interfacial properties. DSC results of the blends show that the cure activation energies (E$\_$a/) were increased in 10 wt% of MAP compared with neat 4EP, due to the increasing intermolecular interaction between 4EP and MAP. The decomposed activation energies (E$\_$t/) derived from Coats-Redfern method, were increased within the 10∼30 wt% composition range of MAP contents, resulting from increasing the cross-linking density of the blend system. Rheological properties of the blend system were investigated under isothermal condition using a rheometer. Cross-linking activation energies (E$\_$c/) were determined from the Arrhenius equation based on gel time and curing temperature. As a result, the E$\_$c/ showed a similar behavior with E$\_$a/. The fracture toughness (K$\_$IC/) of the mechanical interfacial properties was discussed in semi-IPN behaviors of the casting specimen.

• Polymer(Korea)
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• v.39 no.2
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• pp.219-224
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• 2015

In this work, the effect of fiber array direction including $0^{\circ}$, $0^{\circ}/90^{\circ}$, $0^{\circ}/45^{\circ}/-45^{\circ}$ was investigated for mechanical properties of basalt fiber-reinforced composites. Mechanical properties of the composites were studied using interlaminar shear strength (ILSS) and critical stress intensity factor ($K_{IC}$) measurements. The cross-section morphologies of basalt fiber-reinforced epoxy composites were observed by scanning electron microscope (SEM). Also, the surface properties of basalt fibers were determined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). From the results, it was observed that acid treated basalt fiber-reinforced composites showed significantly higher mechanical interfacial properties than those of untreated basalt fiber-reinforced composites. These results indicated that the hydroxyl functional groups of basalt fibers lead to the improvement of the mechanical interfacial properties of basalt fibers/epoxy composites in the all array direction.

• Restorative Dentistry and Endodontics
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• v.24 no.4
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• pp.604-613
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• 1999

This study was performed to evaluate the effect of resin and filler type on the fracture toughness of light-activated composites. Experimental composites were prepared using urethane tetramethacrylate(UTMA) and bisphenol glycidylmethacrylate(Bis-GMA) monomers and five different types of silica fillers. Fracture toughness was measured by a single edge V-notched beam(SEVNB) method, which was discussed from ASTM E399-78. Rectangular bars of $2.5{\times}5{\times}26mm$ were prepared with experimental composites and a notch about 2.25mm deep was carved at the center of the long axis of the specimen using a dental diamond disk driven by a dental micro engine. The flexural test was carried out at a crosshead speed of 0.05mm/min and fracture surfaces were observed under scanning electron microscope. The results obtained were summarized as follows: 1. The fracture toughness values of UTMA-based composites were relatively higher than those of Bis-GMA-based composites. 2. The highest fracture toughness value was observed in the UTMA-based composite containing the $1.5{\mu}m$-spherical fillers. 3 Aging in the distilled water at $37^{\circ}C$ for 10 days showed the increase of fracture toughness, which was severer in the Bis-GMA-based composites than those of UTMA-based composites. 4. The AE amplitude occurring during the fracture toughness tests was the highest at the point of macroscopic fracture.

• Journal of Welding and Joining
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• v.35 no.3
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• pp.82-87
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• 2017

Recent trends in shipbuilding and offshore industries are a huge increase in the ship size and the exploration and production of oil and natural gas in the arctic offshore region. High performance steel plates are required by these industrial trends. Also in IMO(International Maritime Organization) has begun to regulate of fuel of ship to environmental protection, therefore it is little bit difficult to use bunker-C oil to working ship. As the problem of environmental change such as global warming is emerged, the operation of the ship is considered to be involved in the environmental change problem, and the regulation of environmental pollution is gradually strengthened. As these environmental regulations are strengthened demand for LNG fuel ships is rapidly increasing. Currently, cryogenic steels used in LNG tanks include aluminum alloy, SUS 304, and 9%-Ni steel. Those steels are has high cost to construction of large LNG carrier. The new materials were suggested several steel mills to decrease construction cost and easy construction. The new cryogenic steel should be evaluate safety to applied real structure include LNG ship. Therefore, in this study, fracture toughness of weld joints were investigated with cryogenic steel for application of LNG tank.

• Composites Research
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• v.26 no.2
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• pp.129-134
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• 2013

The strength of adhesive joints employed in composite structures under cryogenic environments, such as LNG tanks, is affected by thermal residual stress generated from the large temperature difference between the bonding process and the operating temperature. Aramid fibers are noted for their low coefficient of thermal expansion (CTE) and have been used to control the CTE of thermosetting resins. However, aramid composites exhibit poor adhesion between the fibers and the resin because the aramid fibers are chemically inert and contain insufficient functional groups. In this work, electrospun meta-aramid nanofiber-reinforced epoxy adhesive was fabricated to improve the interfacial bonding between the adhesive and the fibers under cryogenic temperatures. The CTE of the nanofiber-reinforced adhesives were measured, and the effect on the adhesion strength was investigated at single-lap joints under cryogenic temperatures. The fracture toughness of the adhesive joints was measured using a Double Cantilever Beam (DCB) test.

• Journal of the Korea Concrete Institute
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• v.23 no.1
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• pp.31-38
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• 2011

In this study, in order to observe the behaviors of fiber reinforced polymer (FRP) strengthened and steel fiber reinforced concrete specimens for impact and static loads, flexural and punching tests were performed. For the one-way flexural and two-way punching tests, concrete specimens with the dimensions of $50{\times}100{\times}350$ mm and $50{\times}350{\times}350$ mm were fabricated, respectively. The steel fiber reinforced concrete specimens showed much enhanced resistance on two-way punching of static and impact loads. In addition the FRP strengthening system provided the outstanding performance under a punching load. Because of a large tensile strength and toughness of ultra high performance concrete (UHPC), the UHPC specimens retrofitted with FRP showed marginally enhanced strength and energy dissipating capacity.

• Journal of the Korean Ceramic Society
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• v.36 no.1
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• pp.7-14
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• 1999

TiZrB2 solid solution was synthesized using fine powders of Ti, Zr and B by SHS microwave process. The characterization of the synthesized powder and sintered bodies ws investigated. The combustion temperature and rate were increased with increasing the mole ratio of Zr in temperature profile, and showed the maximum combustion temperature and velocity values of 285$0^{\circ}C$ and 14.6mm/sec in Ti0.2Zr0.8B2 composition. Phase separation has been occured into a composite with TiB2 and ZrB2 phases from TiZrB2 solid solution, which was hot pressed sintering at 30 MPa for an hour at 190$0^{\circ}C$. At the composition of Ti0.8Zr0.2B2 the best properties has been obtained in relative density, bending strength, fracture toughness and hardness, with 99%, 680 MPa, 7.3MPa.m1/2 and 2750 Kg/$\textrm{mm}^2$ respectively.