• Journal of the Korea Concrete Institute
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• v.19 no.2
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• pp.135-144
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• 2007

Recently, fiber-reinforced plastic(FRP) wraps are blown as an effective material for the enhancement and rehabilitation of aged concrete structures. The purpose of this investigation is to experimentally investigate behavior of concrete cylinder wrapped with FRP materials. Experimental parameters include compressive strength of concrete cylinder, FRP material, and confinement ratio. This paper presents the results of experimental studies on the performance of concrete cylinder specimens externally wrapped with aramid, carbon and glass fiber reinforced Polymer sheets. Test specimens were loaded in uniaxial compression. Axial load, axial and lateral strains were investigated to evaluate the stress-strain behavior, ultimate strength ultimate strain etc. Test results showed that the concrete strength and confinement ratio, defined as the ratio of transverse confinement stress and transverse strain were the most influential factors affecting the stress-strain behavior of confined concrete. More FRP layers showed the better confinement by increasing the compressive strength of test cylinders. In case of test cylinders with higher compressive strength, FRP wraps increased the compressive strength but decreased the compressive sham of concrete test cylinders, that resulted in prominent brittle failure mode. The failure of confined concrete was induced by the rupture of FRP material at the stain, being much smaller than the ultimate strain of FRP material.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.9
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• pp.1093-1098
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• 2013

A single-crystal sapphire is used as a transparent bulletproof window material; however, few studies have investigated the dynamic behavior and fracture properties under high-speed impact. High-speed and high-resolution sequential images are required to study the interaction of the bullet with the brittle ceramic materials. In this study, a device is developed to capture the sequence of high-speed impact/penetration phenomena. This system consists of a speed measurement device, a microprocessor-based camera controller, and multiple CCD cameras. By using a linear array sensor, the speed-measuring device can measure a small (diameter: up to 1 2 mm) and fast (speed: up to Mach 3) bullet. Once a bullet is launched, it passes through the speed measurement device where its time and speed is recorded, and then, the camera controller computes the exact time of arrival to the target during flight. Then, it sends the trigger signal to the cameras and flashes with a specific delay to capture the impact images sequentially. It is almost impossible to capture high-speed images without the estimation of the time of arrival. We were able to capture high-speed images using the new system with precise accuracy.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.11
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• pp.927-933
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• 2016

In a solid propulsion system, the rocket nozzle is exposed to high temperature combustion gas. Hence, choosing an appropriate material that could demonstrate adequate performance at high temperature is important. As advanced materials, carbon/silicon carbide composites (C/SiC) have been studied with the aim of using them for the rocket nozzle throat. However, when compared with typical structural materials, C/SiC composites are relatively weak in terms of both strength and toughness, owing to their quasi-brittle behavior and oxidation at high temperatures. Therefore, it is important to evaluate the thermal and mechanical properties of this material before using it in this application. This study presents an experimental method to investigate the fracture behavior of C/SiC composite material manufactured using liquid silicon infiltration (LSI) method at elevated temperatures. In particular, the effects of major parameters, such as temperature, loading, oxidation conditions, and fiber direction on strength and fracture characteristics were investigated. Fractography analysis of the fractured specimens was performed using an SEM.

• Korean Journal of Materials Research
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• v.9 no.8
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• pp.837-842
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• 1999

The Ta/TaN multilayer structure with repeating layers of a poly-crystalline Ta layer of high ductility and a TaN layer of high hardness is expected to exhibit toughness. This paper reports the results on the hardness and the adhesion strength of Ta/TaN multilayers and compositional gradient Ta/TaN layers deposited on the high speed steel substrate by reactive sputtering as a function of annealing temperature. The TaN film deposited with the $N_2$/Ar ratio of 0.4 in the reactive sputtering process exhibits the highest crystallinity, and the highest hardness and the results of scratch test of the Ta/TaN multilayers. The hardness and adhesion strength of the Ta/TaN multilayers becomes deteriorated with increasing the annealing temperature in the heat treatment right after depositing the layers. Therefore, post-annealing treatments are not desirable in the case of the Ta/TaN multilayers from the standpoint of mechanical properties. Also the hardness of Ta/TaN multilayers increases with decreasing the compositional modulation wavelength, but the adhesion property of the layers is nearly independent of the wavelength. On the other hand, the compositional gradient Ta/TaN film exhibits the highest hardness and the value of scratch test for the post-annealing temperatures of 20$0^{\circ}C$ and 40$0^{\circ}C$, respectively. This tendency of the compositional gradient Ta/TaN films differs from that of the Ta/TaN multilayers.

• Korean Journal of Materials Research
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• v.10 no.7
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• pp.464-470
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• 2000

The compacting and sintering behavior of titanium powders containing oxygen in the range of 1980~8450 ppm was examined. The powders were prepared by the hydride-dehydride (HDH) and by the deoxidation by solid state(DOSS) methods. Their compaction density ranged from 69.0% to 62.3% and decreased with the increase in the oxygen content. It was explained by the effect of oxygen on the hardness of powders. Unlike the compaction density, the oxygen content did not affect the apparent density greatly being 90.5$\pm$0.5% after sintering at $1100^{\circ}C$ for 2 hours. Their average grain size was $60\mu\textrm{m}$ and the size and distribution of pores were about the same for all cases. The hardness of sintered samples showed a linear increase with oxygen and could be expressed as VHN(sintered)= 135.5+64.3$\times$$(wt{\%}O_2)$ The exami-nation of fracture surface revealed that the ductile-brittle transition occurs at oxygen contents of 2987~5582 ppm.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.72-72
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• 2009

전자부품의 내부접속 및 파워반도체의 다이본딩과 같은 1차실장에는 고온환경에서의 사용과 2차실장에서의 재용융방지를 위해 높은 액상선온도 및 고상선온도를 필요로 하여, Pb-5wt%Sn, Pb-2.5wt%Ag로 대표되는 납성분 85%이상의 고온솔더가 널리 사용되고 있다. 생태계와 인체에 대한 납의 유해성이 보고된 이래, 무연솔더에 대한 연구가 활발히 진행되어 왔으나, Sn-Ag-Cu계로 대표되는 Sn계 합금으로 대체 중인 중온용 솔더와는 달리, 고온용 솔더에 대해서는 대체합금에 대한 연구가 미흡한 실정이다. 대체재의 부재로 인해 기존의 납을 다량함유한 솔더로 1차실장이 지속됨으로서, 2차실장의 무연화에도 불구하고 전자부품 및 기기의 재활용에 큰 어려움을 겪고 있다. 지금까지 고온용 무연솔더로서는 융점에 근거해 Au-(Sn, Ge, Si)계, Bi-Ag계, Zn-(Al, Sn)계의 극히 제한된 합금계만이 보고되어 왔다. Au계 솔더는 현재 플럭스를 사용하지 않는 광학, 디스플레이 분야 등 고부가가치 공정에 사용되고 있으나, 합금가격이 매우 비싸며 가공성이 나빠 대체재료로서는 적합하지 않다. Bi-Ag계 솔더 또한 취성합금으로 와이어 및 박판으로 가공하는데 어려움이 크며, 솔더로서 중요한 특성중 하나인 전기전도도 및 열전도도가 나쁜 편이다. 이에 비해, Zn계 합금은 비교적 낮은 합금가격, 적절한 가공성과 뛰어난 인장강도, 우수한 전기전도도 및 열전도도를 지녀, 고온용솔더 대체재료의 유력한 후보로 생각된다.이전 연구에서, 필자의 연구그룹은 Zn-Sn계 합금을 고온용 무연솔더로서 제안한 바 있다. Zn-Sn계 합금은 충분히 높은 융점과 함께, 금속간화합물이 없는 미세조직, 우수한 기계적 특성, 높은 전기전도도 및 열전도도 등의 장점을 나타내었다. 본 연구에서는 기초합금특성상 고온솔더로서 다양한 장점을 지닌 Zn-30wt%Sn합금을 고온용 솔더의 대표적인 적용의 하나인 다이본딩에 적용하여, 접합부의 강도 및 미세조직, 열피로 신뢰성에 대해 분석을 함으로서 실제 공정에의 적용가능성에 대해 검토하였다. Zn-30wt%Sn을 이용해 Au/TiN(Titanium nitride) 코팅한 Si다이를 AlN-DBC(aluminum nitride-direct bonded copper)기판에 접합한 결과, 양측에 완전히 젖은 기공이 없는 양호한 다이접합부를 얻었으며, 솔더내부에는 금속간화합물을 형성하지 않았다. Si다이와의 계면에는 TiN만이 존재하였으며, Cu와의 계면에는 Cu로부터 $Cu_5Zn_8,\;CuZn_5$의 반응층을 형성하였다. 온도사이클시험을 통한 열피로특성평가에서, Zn-30wt%Sn를 이용한 다이접합부는 1500사이클 지점에서 Cu와 Cu-Zn금속간화합물의 사이에서 피로균열이 형성되며, 접합강도가 크게 감소하였다. 열피로특성 향상을 위해 Cu표면에 TiN코팅을 하여 Zn-30wt%Sn 솔더로 다이접합한 결과, Si다이와 기판 양측에 TiN만으로 구성된 계면을 형성하였으며, TEM관찰을 통해 Zn-30wt%Sn과 극히 미세한 접합계면이 형성하고 있음을 확인하였다. Zn-wt%30Sn솔더와 TiN층의 병용으로 2000사이클까지 미세조직의 변화 및 강도저하가 없는 극히 안정된 고신뢰성의 다이접합부를 얻을 수가 있었다.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.4
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• pp.439-445
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• 2001

The destructive method is reliable and widely used lot the estimation of material degradation but, it have time-consuming and a great difficulty in preparing specimens from in-service industrial facilities. Therefore, the estimation of degraded structural materials used at high temperature by nondestructive evaluation such as electric resistance method, replica method, Barkhausen noise method, electro-chemical method and ultrasonic method are strongly desired. Ultrasonic nondestructive evaluation technique has been reported good to attain efficiency of measurement, high sensitivity of measurement, and rapidity and reliability of result interpretation. In this study, it was verified experimentally the feasibility of the evaluation of degraded 2.25Cr-1Mo steel specimens which were prepared by the isothermal aging heat treatment at $630^{\circ}C$ by high frequency longitudinal wave method investigating the change of attenuation coefficient by FFT analysis and wavelet transform. Because of carbide precipitation increase and spheroidization near grain boundary of microstructure to aging degradation, attenuation coefficient had a tendency to increase as degradation proceeded. It was identified possibly to evaluate degradation using the characteristics of high-frequency ultrasonics. Frequency dependence of ultrasonic attenuation coefficient to aging degradation appeared large, which made sure that attenuation coefficient is an important parameter for evaluation of aging degradation.

• Korean Journal of Materials Research
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• v.10 no.6
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• pp.398-402
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• 2000

Three kinds of $Al_3Ti-Cr$ alloys, namely, $Al_{67}Ti_{25}Cr_8,\;Al_{66}Ti_{24}Cr_{10}\;and\;Al_{59}Ti_{26}Cr_{15}$, were prepared by induction melting followed by the thermomechanical treatment. The corrosion behavior in 3.5% NaCl solution and the high-temperature oxidation behavior at 1000, 1100 and $1200^{\circ}C$ for the prepared alloys were investigated. Electrochemical results indicated increased resistance to localized corrosion with increasing Cr content. Cr additions were found to prevent passive film from undergoing brittle fracture. XPS results revealed the passive films of $Al_3Ti-Cr$ alloys were composed mainly of $Al_2O_3$ that coexisted with $TiO_2$ and $Cr_2O_3$. The overall oxidation resistance of the prepared alloys were excellent. Specifically, the oxidation resistance increased in the order of $Al_{59}Ti_{26}Cr_{15},\;Al_{66}Ti_{24}Cr_{10}\;and\;Al_{67}Ti_{25}Cr_8$. As the Al content in the base alloys increased, the $Al_2O_3$ formation was facilitated leading to the increased oxidation resistance.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.1
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• pp.91-99
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• 2017

The purpose of this paper is to evaluate the flexural performance of reinforced concrete (RC) beams repaired with polymer mortar. The repaired and non-repaired 13th beams which was fabricated by considering repair position, repair depth, and curing age of polymer mortar as test variables were tested under three point loading. All specimens repaired in compressive and tensile zone did not fail due to interfacial failure between polymer mortar and concrete but failed when the strain of repaired mortar exceeded the ultimate tensile strain of polymer mortar. Maximum load of specimens repaired in compressive zone was similar to that of non-repaired specimen, reference specimen. Additionally, their ductility index was higher than that of reference specimen. On the other hand, specimens repaired in tensile zone failed very brittlely and have a lower ductility index than reference specimen. Nonlinear analysis by using OpenSees was performed to predict the behavior of RC beam repaired with polymer mortar. Two dimension frame element was used to simplify an analysis model and fiber model was applied to consider the material non-linearity. It was confirmed from the analysis results that nonlinear analysis properly predicts the behavior of specimens repaired in compressive zone and overestimates the behavior of specimens repaired in tensile zone.

• Korean Journal of Materials Research
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• v.8 no.6
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• pp.538-544
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• 1998

The spallation of a thermal barrier coating layer depends on the formation of brittle spinels. thermal expansion mismatch between ceramic and metal. the phase transformation of a ceramic layer and residual stress of coating layer. In this work. the formation mechanism of oxide scale formed by oxidation treatment at 90$0^{\circ}C$ was investigated in order to verify oxidation behavior at the interface between E-beam coated $Zr0_2$-7wt.% $Y_20_3$ and plasma sprayed CoNiCrAIY. Some elements distributed in the bond coating layer were selectively oxidized after oxidation. At the initial time of oxidation. AI-depletion zone and $\alpha$-$Al_O_3$,O, were formed at the bond coating layer by the AI-outward diffusion. After layer grew until critical thickness. spinels. $Cr_20$, and $C0_2CrO_4$ by outward diffusion of Co. Cr, Ni were formed. It was found that the formation of spinels may be related to the spallation of $Zr0_2$-7wt.% $Y_20_3$ during isothermal oxidation.