• Journal of the Korean Society for Precision Engineering
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• v.14 no.7
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• pp.116-125
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• 1997

An investigation was performed to study the impact damage in CFRP laminated composites subjected to impact loading. A finite element model has been developed for predicting the impact damage in laminated composite plates resulting from the ballistic impact. The finite element model was based on the higher-order shear deformation theory and was used to predict the initial intraply matrix cracking and the shape and size of interface delamination in laminated composites. Numerical simulation was performed and then the initiation of the matrix cracking and the shape and size of impacted induced delamination were predicted, and te results were compared with those of impact experiments with the same dimension and stacking sequences. A linear relationship holds between impact velocity and length and width of delamination. As impact velocity is increased, the increase of delamination length is highger than the increase of delamination width.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.10
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• pp.1347-1353
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• 2011

Friction stir welding (FSW) method has extensively been used in manufacturing methods because of the several advantages over conventional welding methods, such as better mechanical properties, reduced occurrence of joining defects, high material saving, and low production time, etc. The aim of this paper is to review the optimal FSW conditions using the previous experimental results and is to investigate the fatigue crack growth rate in three different zones, WM, HAZ and BM for FSWed Al7075-T651 aluminum plates. As far as our experiments are concerned, the optimal conditions are obtained as rotation speed, 800rpm and travelling speed, 0.5mm/sec. The fatigue crack growth rate showed strong dependency on three different zones WM, HAZ and BM, and crack driving force.

• Composites Research
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• v.24 no.3
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• pp.25-30
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• 2011

The studies on the non-destructive testing methods of the composite materials are very important for improving their reliability and safety. AE(Acoustic Emission) can evaluate the defects by detecting the emitting strain energy when elastic waves are generated by the generation and growth of a crack, plastic deformation, fiber breakage, matrix cleavage or delamination. In this paper, the AE signals of the filament wound composite cylinder and sandwich cylinder during the pressure test were measured and analyzed. The signal characteristics of PVDF sensors were measured, and an AE signal analyzer which had the band-pass filter and L-C resonance filter were designed and fabricated. Also, the crack detection capability of the fabricated AE signal analyzer wes evaluated during the pressure tests of the filament wound composite cylinder and the sandwich cylinder.

• Journal of Welding and Joining
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• v.14 no.2
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• pp.79-89
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• 1996

일정진폭하중과 과대하중비 2.5의 단일 인장과대하중에 의한 4140강 용접부 의 피로균열성장거동을 실온과 -45.deg.C의 저온에서 피로시험과 파면관찰을 통하여 고찰하였다. 이때, 용접부 미시조직의 영향을 평가하기 위해 모재(parent metal), 열영향부(as-welded HAZ), 열처리된 열영향부(PWHT HAZ)로 나누어 응력비 0과 0.5로 CT시험편을 이용하여 피로시험을 실시하였다. 피로균열성장거동은 재료의 미시조직과 온도변화보다는 응력비에 크게 영향을 받았으며, 단일 과대하중에 의한 피로균열성장 지연효과가 모든 재료에서 상당히 크게 나타났다. 전자현미경에 의한 피로파면 관찰 결과, 실온에서는 연성의 스트라이에이숀과 -45.deg.C에서는 의벽개파면과 같은 피로 균열성장거동을 나타내고 있다.

• Composites Research
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• v.16 no.3
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• pp.58-67
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• 2003

To perform the realtime strain and fracture monitoring of the smart composite structures, two optical fiber sensor systems are proposed. The two types of the coherent sources were used for fracture signal detection - EDFA with FBG and EDFA with Fabry-Perot filter. These sources were coupled to EFPI sensors imbedded in composite specimens. To understand the characteristics of matrix crack signals, at first, we performed tensile tests using surface attached PZT sensors by changing the thickness and width of the specimens. This paper describes the implementation of time-frequency analysis such as short time Fourier transform (STFT) and wavelet transform (WT) for the quantitative evaluation of fracture signals. The experimental result shows the distinctive signal features in frequency domain due to the different specimen shapes. And, from the test of tensile load monitoring using optical fiber sensor systems, measured strain agreed with the value of electric strain gage and the fracture detection system could detect the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signal.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.11
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• pp.1715-1720
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• 2010

In this study, we analyzed the crack-healing characteristics of specimens; different crack lengths and coating methods of $Si_3N_4$ ceramic structures with long cracks were analyzed. Cracks with lengths of about $100-500\;{\mu}m$ were obtained using a Vickers indenter for a load of 24.5-98 N. In the case of a crack obtained by applying a load of 24.5 N, the crack-healed specimen with $SiO_2$ nanocolloid coating exhibited the highest bending strength, which was higher than that of a smooth specimen by 140%, but the bending strength of a crack-healed specimen that had a $SiO_2$ nanocolloid coating and originally had multiple cracks was lower than that of a smooth specimen. However, when compared to the cracked specimens, the bending strength of most specimens with multiple cracks increased slightly. On the basis of these results, the crack-healing characteristics of $Si_3N_4$ ceramic structures with multiple indentations were studied for different coating methods. The most effective coating method for long-crack specimens was hydrostatic pressure coating.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1997.04a
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• pp.41-46
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• 1997

TiN 코팅은 마모에 대한 저항성 및 고체 윤활 효과가 매우 우수하여 내마모성 및 저마찰이 요구되는 절삭공구나 피스톤 베어링 및 각종 축계의 코팅막으로 사용이 증가되고 있다. 일반적으로 재료의 경도와 인성은 서로 상반 관계를 갖고 있어 공학적으로 경도와 인성을 모두 요구하는 표면을 얻기 위해 연질 모재 위의 세라믹 코팅은 그 요구를 만족시킬 수 있는 가장 각광받는 표면처리 방법중의 하나이다. 그러나, TiN과 같은 경질 박막의 공학적 적용시 가장 요구되는 마모수명은 모재의 조도나 경도, 증착 방법, 접촉 상태, 코팅막의 두께 및 마모의 발생 기구 등에 따라 마찰 및 마모 메커니즘의 현저한 차이를 나타내기 때문에 예측이 거의 불가능한 실정이고, 아직까지 이러한 마모수명 비교 평가방법에 대한 기준 설정 및 정량적 정립이 이루어지지 않고 있다. 본 연구에서는 모재의 경도, 조도, 코팅 두께가 다른 TiN 경질 박막에 압입시험과 스크래치시험시 발생되는 균열 발생 메커니즘과 미끄럼 시험시 발생되는 마모 메커니즘의 연계성을 밝히고 압입 및 스크래치 시험시 코팅막이 손상되는 임계하중과 미끄럼 시험시 접촉하중 변화에 따른 마모수명의 정량적 연관성을 찾아보고자 한다.

• Proceedings of the KWS Conference
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• 1996.10a
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• pp.42-44
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• 1996

이종재료인 2$\frac{1}{4}$Cr-1Mo강과 12Cr-1Mo-V계 강의 용접부의 용접성에 대하여 모재계열의 용접재료와 Inconel계의 용접재료를 사용하여 야금학적인 관점으로부터 미세조직적 특성 및 기계적성질을 중심으로 검토하였다. 그 결과를 요약하면 다음과 같다. (1) 본 연구의 실험범위에 의하면, 내부결함, 균열 등의 용접결함을 발생하지 않는 양호한 용접부를 얻을 수 가 있었다. (2) 광학현미경을 이용한 미세조직의 관찰에 의하면, Inconel계의 용접재료를 제외한 모재계열의 용접재료를 사용하였을 경우, 용융경계부 근방에서 연속 또는 불연속적인 백색의 밴드상이 형성되었다. (3)백색밴드상의 형성부는 인장변형에 의하여 국부적인 수축변형을 일으키나, 파단은 2$\frac{1}{4}$Cr-1Mo강의 모재에서 발생하였으며, 거시적인 인장성질에는 영향을 미치지 않았다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.5
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• pp.555-559
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• 2011

Oil hydraulic piston pumps are extensively used worldwide because of their simple design, light weight, and cost effectiveness. However, an oil hydraulic pump is likely to have high leakage, friction, and low energy efficiency after long-term use. In oil hydraulic piston pumps the clearance between the valve block and the piston plays an important role in the volumetric and overall efficiency. We studied the wear property of the SACM645 material used in hydraulic piston pumps via experiments with different heat treatments. We prepared three different specimens. The maximum tensile strengths of the QT and QT-nitration specimens are similar (about 820 MPa), but the strains are significantly different. However, the fatigue characteristic depended on the heat treatment.

• Journal of Power System Engineering
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• v.16 no.4
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• pp.66-73
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• 2012

본 연구는 횡방향 TIG 용접된 Al6013-T4 알루미늄 합금 용접부의 피로균열전파거동에 미치는 용접후열처리(PWHT)의 영향을 조사하는 것이 주목적이다. 기초적으로 인장시험, 경도 및 미세조직이 조사되었으며, 피로균열전파거동을 고찰하기 위한 피로 시험은 모두 중앙균열인장(CCT) 시험편에 대하여 수행되었다. T82열처리에 있어서 시효시간은 피로균열전파율, 인장강도 및 경도에 대단히 민감함을 나타내었으며, 모재와 열영향부재의 경우가 용접재보다 기계적 성질이 우수하였다. 횡방향 TIG 용접한 Al6013-T4 시험재의 용접후열처리 조건에 따라서 피로균열전파 저항에는 차이가 나타났으며, 본 실험의 조건하에서 24시간 인공시효 PWHT-82 시험편이 피로균열전파 저항이 가장 우수한 결과를 나타내었다.