• 열처리공학회지
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• 제7권4호
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• pp.277-287
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• 1994

The effect of grain size on the tensile properties and fatigue behavior of austenitic high Mn steel has been investigated. The recrystallized austenite grain size of the cold rolled high Mn steel was increased as the annealing temperature increased from $600^{\circ}C$ to $1000^{\circ}C$. Larger austenite grain size decreased the yield strength and the tensile strength, and increased the uniform elongation due to transformation of some austenite into twins or E-martensite phase during deformation. Austenite grain refinement increased the tendency to form dislocation cells, instead. The specimen annealed at $1000^{\circ}C$ with large grain size showed lower fatigue crack propagation rate in low ${\Delta}K$ region due to rougher fracture surface caused by formation of deformation twins during fatigue at the crack tip region.

• Journal of Welding and Joining
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• 제21권1호
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• pp.72-79
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• 2003

Fatigue behaviour of partially penetrated butt-welded joints in high strength steel plates, in which crack-like structural defect, i.e. lack of penetration(LOP), is inevitably introduced during welding processes, was investigated. Fatigue lives of two types of welded joints, namely X-grooved and K-grooved joints, were experimentally determined first. Observed fatigue crack propagation behaviours of the partially penetrated butt-welds were interpreted through considering 3-dimensional semi-elliptical crack shape in front of the LOP. Based on such interpretation, a fracture mechanical method to estimate stress intensity factors at the crack tip was proposed. Since the fatigue lift of the partially penetrated butt-welds was strongly influenced by the ratio of size of the LOP to thickness, D/t, the D/t was used as a main parameter to calculate the fatigue lift by using the proposed method. Comparison of the fatigue lift obtained experimentally and analytically agreed well with each other. Hence it is suggested that the method used in this work to predict fatigue lift of the partially penetrated butt-welds can be applied to real cases with improved lift-prediction capability.

• 한국기계가공학회지
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• 제6권1호
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• pp.62-70
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• 2007

The development of new materials that are light-weight, yet high in strength has become vital to the machinery, aircraft and auto industries. However, there are a lot of problems with developing such materials that require expensive tools, and a great deal of time and effort. Therefore, the improvement of fatigue strength and fatigue life are mainly focused on by adopting residual stress. The fatigue crack growth rate of the Shot-peened material was lower than that of the Un-peened material. And in stage I, threshold stress intensity factor of the shot-peen processed material is high in critical parts unlike the Un-peened material. Also, fatigue crack growth exponent and number of cycle of the Shot-peened material was higher than that of the Un-peened material. That is concluded from effect of da/dN. And Fatigue life shows more improvement in the Shot-peened material than in the Un-peened material. And compressive residual stress of surface on the Shot-peen processed operate resistance force of fatigue crack propagation.

• 한국강구조학회 논문집
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• 제28권6호
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• pp.459-465
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• 2016

본 연구에서는 인장이음에 대한 반복하중의 영향을 파악하기 위하여 단순 인장이음을 대상으로 피로시험을 수행하였다. 인장이음의 볼트체결에 따른 축력과 작용하중에 의한 축력의 변화를 측정하였고 반복하중에 의한 볼트의 축력과 파괴양상, 이음의 피로강도를 조사하였다. 인장이음의 구조 상세 변수는 플랜지 두께와 고장력 볼트의 직경으로서 이들의 조합에 따라 볼트와 연결부의 강성이 달라지도록 하였다. 피로시험결과, 반복하중을 받는 인장이음의 파괴모드는 EC3에서 제시하고 있는 정적 파괴모드별 극한하중을 이용하여 평가될 수 있었다. 인장이음의 피로강도는 지레작용을 고려하지 않은 EC3(36)의 피로강도보다 상당히 안전측의 결과로 나타났다. 그러나, 지레작용에 의한 부가축력은 볼트 축력의 증가를 일으키기 때문에 피로강도에 대한 신중한 평가가 필요하다.

• Journal of Welding and Joining
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• 제8권2호
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• pp.40-52
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• 1990

Fatigue behavior of the AH, DH and EH grade TMCP(Thermo-Mechanical Control Process) steels was studied. High cycle and low cycle fatigue tests were carried out for the weldment and base metal of each steel. The results showed that the fatigue limit at 2 * $10^6$ cycles was 33 to 37 kg/$mm^2$ for the base metal and 30 to 34 kg/$mm^2$ for the weldment. The ratio of fatigue limit to tensile strength for TMCP steels was 0.65 to 0.71, which was a value close to the upper limit for the ordinary steels. It was also found that the high cycle fatigue behavior of TMCP steels could be affected by the microstructures of base metal. It will be necessary to have fine structure for TMCP steels to increase the fatigue resistance. In low cycle fatigue test, the fatigue lifetime of AH and DH steels accorded well with the ASME best fit curve, while that of EH steel was considerably lower than the fatigue lifetime of the other steels. Fatigue resistance of the weldment made by high heat input(180kJ/cm) welding was not lower than that made by low heat input(80kJ/cm) welding in case of high cycle fatigue, but the high heat input welding decreased the fatigue resistance in case of low cycle fatigue.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2001년도 추계학술대회 논문집
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• pp.264-268
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• 2001

In this study, CT specimens were prepared from spring steel(SUP9) processed shot peening which was room temperature, low temperature and high temperature experiment. And we got the following characteristics from fatigue crack growth test carried out in the environment of room, and high temperature at $25^{\circ}C,\; 50^{\circ}C, \;100^{\circ}C,\; 150^{\circ}C,\; and\; 180^{\circ}C$ in the range of stress ratio of 0.05 by means of opening mode displacement. The threshold stress intensity factor range $\DeltaK_{th}$ in the early stage of fatigue crack growth (Region I ) and stress intensity factor range $\Delta$K in the stable of fatigue crack growth (Region II) was decreased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature and high temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2003년도 춘계학술대회 논문집
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• pp.313-318
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• 2003

The development of new materials with light weight and high strength has become vital to the machinery, aircraft and auto industries. However, there are a lot of problems with developing such materials that require expensive tools, and a great deal of time and effort. Therefore, the improvement of fatigue strength and fatigue life are mainly focused on by adopting residual stress(in this thesis). The compressive residual stress was imposed on the surface according to each shot velocity(57, 70, 83, 96 m/sec) based on Shot-peening, which is the method of improving fatigue lift: and strength. By using the methods mentioned above, I arrived at the following conclusions 1. The fatigue crack growth rate(da/dN) of the Shot-peened material was lower than that of the Un-peened material. And in stage I, ${\Delta}K_{th}$, the threshold stress intensity factor, of the shot-peen processed material is high in critical parts unlike the Un-peened material. Also m, fatigue crack growth exponent and number of cycle of the Shot-peened material was higher than that of the Un-peened material. That is concluded from effect of da/dN. 2. Fatigue life shows more improvement in the Shot-peened material than in the Un-peened material. And compressive residual stress of surface on the Shot-peen processed operate resistance force of fatigue crack propagation.

• 한국생산제조학회지
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• 제7권5호
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• pp.66-71
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• 1998

It is required the superior materials for the parts of machines or structures, which could be endurable in severe load and environment. According to advancement of casting technology, nodular graphite cast iron is used as suitable for such condition. But nodular graphite cast iron is scattering of fatigue strength and low reliability. Therefore in this study, the effect of matrix structure and number of nodular graphite on the initiation of fatigue crack and fatigue strength. It was found that the material which has relatively high ferrite volume fraction was more easily cracked than other materials and fatigue limit was low. The material which has not found pinhole on the surface, the crack was initiated in graphite went through ferrite and propagated into through graphite, but separated graphite and ferrite grain boundary and combined with other cracks to fro large one.

• 한국자동차공학회논문집
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• 제20권5호
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• pp.53-64
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• 2012

Deep and narrow welds can be produced by laser welding at high welding speeds with a narrow heat-affected zone (HAZ) and little distortion of the workpiece. This study aims to evaluate the usefulness of laser welding at automobile component manufacture. Microstructure observation, hardness test, tensile test and fatigue life test are performed by using the fiber laser welded SPFC590 steel sheets which is used widely in the manufacture of automotive seat frame. Three kinds of specimens are only a SPFC590 steel plate, quasi-butt joint plate and lap joint plate by laser welding. The following results that will be helpful to understand the static strength, fatigue crack initiation and growth mechanism were obtained. (1) The tensile strength of quasi butt joint specimens nearly equal to base metal specimens, but lap joint specimens fractured in shear area of weld metal. (2) The fatigue strength of quasi-butt joint specimen was approximately 8 percent lower than that of the base metal specimens. Furthermore, the lap joint specimens were less than 86 percent of the base metal specimens. (3) The lap joint fatigue specimens fractured at shear area in high level stress amplitude, while fractured at normal area in low level stress amplitude. From these results, the applicability of the laser welding to the automobile component is discussed.

• 한국강구조학회 논문집
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• 제15권6호통권67호
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• pp.629-637
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• 2003

본 논문은 강교량에서 큰 응력범위가 발생하는 위치에 불가피하게 존재하는 가장 일반적인 피로 취약 상세인 면내 거셋 용접부에 대한 3단계에 걸친 피로 시험 결과를 분석 평가 하였다. 플랜지 및 부착물의 두께, 부착물의 길이와 적용강재를 변화시킨 총 57개의 피로 시험편이 제작되어 인장피로시험이 수행되었으며, 1단계 시험에서는 실물크기 및 시험체에 대한 피로 실험도 수행되었다. 실험 결과로부터 현 설계기준에 제시된 대상상세에 대한 피로강도 기준의 적정성을 평가하고, 플랜지와 용접 연결된 부착물의 두께, 길이 등의 기하학적 형상 및 적용강재의 강도가 피로강도에 미치는 영향과 균열의 발생 및 진전 거동 특성을 규명하였다. 외국의 피로실험 자료와 비교 분석한 결과 강재가 서로 달라도 유사한 피로강도를 갖는 것으로 나타났으며, 이는 강종 보다는 기하학적 형상 및 용접관련사항이 피로강도에 미치는 영향이 크다는 것을 나타내며, 형상에 따라 응력집중계수의 차이가 매우 커서 거셋 부착부의 변화부 반경, 길이 및 플랜지의 폭에 따라 상세분류를 좀더 세분화 할 필요가 있는 것으로 나타났다.