• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.350-352
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• 2009

The effect of porosity on the high-cycle fatigue properties of Al-Si-Mg casting aluminum alloys was investigated in this study. Microstructure examination, tensile and high-cycle fatigue test were conducted on both Al-Si-Mg casted (F) and heat-treated (T6) conditions. Porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The microstructure observation results showed that eutectic Si particles were homogeneously dispersed in the matrix of the Al-Si-Mg casting alloys, but there were porosities formed as cast defects. The high-cycle fatigue results indicated that the fatigue strength of the 356-T6 alloy was higher than that of the 356-F alloys because of the significant reduction in volume fraction of pores by heat treatment. The SEM fractography results showed that porosity affected detrimental effect on the fatigue life: 80% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. It was found that fatigue life decreased as the size of the surface pore increased. A comparison was made between surface pore and inner pore fur its effect on the fatigue behavior. The results showed that the fatigue strength with the inner pores was higher than that of the surface pore.

• 열처리공학회지
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• 제16권3호
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• pp.141-147
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• 2003

The effects of heat treatment on the fatigue life and welding residual stress of welded plates were investigated in this study. The plates were welded by flux cored arc welding process, and post weld heat treated at $600^{\circ}C$ for 1 hour. The residual stresses of welded plates before and after post weld heat treatment were measured by hole drilling method. To measure the fatigue life of welded plates, low cycle fatigue tests under strain control and high cycle fatigue tests under load control were performed respectively, by using cylindrical specimens containing weld metal and heat affected zone. The obtained result shows that the post weld heat treatment reduces the residual stress, and resultantly changes the fatigue life of welded plate. Goodman diagrammatic analysis has also been performed to study the effect of post weld heat treatment on the high cycle fatigue life.

• 한국자동차공학회논문집
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• 제5권1호
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• pp.129-135
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• 1997

Previous studies has shown that the curve of low-cycle fatigue life was not expressed with the single line subjected to Manson-Coffin's law type and bent to short life in low ${\Delta}{\varepsilon}_p$ region. The main cause of this phenomenon has been considered that the localization of plastic strain in the crack initiation process fosters the crack initiation. In this study, the low-cycle fatigue life was investigated for each specimens omitted crack initiation process and it was found that fatigue life curve in log(${\Delta}{\varepsilon}_p$)-log($N_f$)was bent in low ${\Delta}{\varepsilon}_p$ region as ever. Therefore, the main cause of appearance of knee point in fatigue life curve is not found in the crack initiation process but in the crack propagation process. In the crack propagation process, the localization of the plastic strain in the vicinity of crack tip and the influence of test environment on the crack propagation rate were observed and these inclinations were more remarkable in low ${\Delta}{\varepsilon}_p$ region. Hence, it was concluded that these two phenomena in the crack propagation process were proved to the main cause which accelerates the crack propagation in low ${\Delta}{\varepsilon}_p$ region and bent the fatigue life curve in result.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2005년도 추계학술대회 논문집
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• pp.627-631
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• 2005

In this study, low fatigue behavior of laser welded sheet metal were investigated. Before welding, the cross section of butt joint was prepared only by fine shearing without milling process. Specimens were same sheet metal and welding condition that using automobile manufacturing company at present. Butt joint of cold rolled sheet metal was welded by $CO_2$ laser. It is used that welding condition such as laser welding speed was 5.5m/sec and laser output power was 5kW for 0.8mm and 1.2mm sheet metal. The laser weldments were machined same or different thickness and same or different material. In order to mechanical properties of around welding zone, hardness test was performed. Hardness of welding bead is about 2 times greater than base material. We performed the low cycle fatigue tests for obtaining fatigue properties about thickness and the weld line direction of specimen. The results of strain controlled low cycle fatigue test indicate that all specimens occur cyclic softening, as indicated by the decrease in stress to reach a prescribed strain.

• 열처리공학회지
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• 제32권1호
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• pp.12-16
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• 2019

In this study, strain-controlled low cycle fatigue test for hot rolled STS304 steel was carried out at $400^{\circ}C$ and $600^{\circ}C$, respectively. High temperature fatigue test was done using an electric furnace attached on the hydraulic fatigue test machine. The results of this study show that STS304 hot rolled steel has excellent static strength and fatigue characteristics. The hysteresis loop at half life was obtained in order to calculate the elastic and plastic strain. Also, Relationship between strain amplitude and fatigue life was examined in order to predict the low cycle fatigue life of STS304 steel by Coffin-Manson equation.

• 열처리공학회지
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• 제31권1호
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• pp.18-23
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• 2018

In this study, low cyclic fatigue test was carried out at room temperature condition for rolled STS304 steel. The results of this study show that rolled STS304 steel has excellent static tensile strength and fatigue characteristics. The relationship between plastic strain range and fatigue life was examined using the triangular wave in order to predict the low cycle fatigue life of rolled STS304 steel by Coffin-Manson equation. Cyclic behavior of rolled STS304 steel was characterized by cyclic hardening with increasing number of cycle through the Hysteresis loop analysis and cyclic response of maximum stress versus number of cycles. It is found that the plastic deformation energy consumed per cycle is reduced by calculating the area of the hysteresis loop.

• 콘크리트학회논문집
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• 제22권2호
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• pp.263-271
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• 2010

이 연구는 국내에서 생산되고 있는 철근이 반복하중을 받는 경우의 파괴 특성에 대한 검증을 목적으로 한다. 이 논문은 철근콘크리트 하부구조(파일과 교각)에 배근된 축방향 철근에 대한 저주파 피로 거동을 다루었다. 전체 81개의 철근 실험체에 대하여 변형률 진폭에 따른 반복 축방향 변형률 제어 방식으로 저주파 피로 실험을 수행하였다. 실험 변수는 인장변형률과 압축변형률의 비율, 축방향 철근의 항복강도, 철근지름에 대한 철근길이의 비율, 철근의 크기와 변형률 진폭으로 선택하였다. 이 논문에서 실험 결과에 따른 저주파 피로 거동과 저주파 피로 수명을 분석하였다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2006년도 추계학술대회논문집
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• pp.89-94
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• 2006

In this paper, the effects of the system parameters on the high cycle fatigue life based on structural dynamic analysis of a turbine blade are investigated. Conventional studies have forcused on the fatigue life of turbine blades with specific system parameters. However, each parameter has statistical deviation because of inhomogeneity of material property, tolerance, and operating conditions. Therefore a methodology that estimates the effects of system parameter on the fatigue lift deviation is demonstrated.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2004년도 추계학술대회 논문집
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• pp.1025-1028
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• 2004

In this paper, we studied low cycle fatigue behavior of laser welded sheet metal that used automobile body panel. Specimens were manufactured as weld condition and sheet metal using automobile manufacturing company at present. For to know mechanical properties, micro Vicker's hardness test was performed of specimens. But, we can't confirm mechanical properties of weld bead and heat affected zone because laser weld makes very narrow weld bead and heat affected zone than other welding method. Therefore, we performed low cycle fatigue test with similar weldment, dissimilar weldment, similar thickness and dissimilar weldment, and dissimilar thickness and dissimilar weldment for fatigue properties of thickness and welding direction. As well, we analysis stress distribution of base metal, weld bead, and heat affected zone according to strain load using finite element method.

• 한국안전학회지
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• 제33권3호
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• pp.8-14
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• 2018

Ni-base superalloy has excellent resistance to extreme environments such as high temperatures and high stresses and are used as materials for large gas turbines. In this paper, the specimens were taken from the blade that were used for a long time, and their life span was studied by microstructure analysis and avoidance of cursing. The microstructural analysis of the specimens was carried out using a OM and SEM to observe the coarsening, carbides on gamma prime. Low-cycle fatigue tests were performed on new material and airfoil of long time-used blade. The test was conducted under various deformation conditions and temperature conditions of $760^{\circ}C$ and $870^{\circ}C$. The low cycle fatigue test was carried out using the Coffin-Manson equation and the fatigue life was predicted. After the test, crack path and fracture surface were analyzed using SEM.