• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.256-261
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• 2007

The purpose of this study is to investigate the effect of shot peening on the fatigue strength and fatigue life of two kinds of aluminum alloys. The fatigue strength behavior of aluminum alloys were estimated by the stress ratio and shot velocities. The fatigue life and strength increased with increasing the test shot velocity. However, at the shot velocity range between 50m/s and 70m/s, the compressive residual stress phenomena were observed in test conditions of different shot velocity. The optimal shot velocity is acquired by considering the peak values of the compressive residual stress, dislocations, brittle striation, slip, and fisheye on the fracture surface of test specimen. It was observed from the SEM observation on the deformed specimen that the brittle striation, fisheye were showed in the intergranular fracture structure boundaries at the this velocities. Therefore, fatigue strength and fatigue life would be considered that shot velocity has close relationship with the compressive residual stress.

• 한국주조공학회지
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• 제14권5호
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• pp.464-470
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• 1994

The horizontal continuous casting method with the heated mold was applied to study the solidification structures of the pure Al and Al-0.5wt%Si and Al-1.0wt%Si alloy rods. The results could be summarized as follows: 1. The S/L interface structures of pure Al represented the hexagonal cells at the casting speed of 590 and 350mm/min, respectively. However, the hexagonal cells became irregular as the casting speed and(or) Si amount increased. 2. The striation increased as the Si amount and casting speed increased and was found to result from the occurrence of growth twin crystals by XRD analysis. 3. The striation did not affect the mechanical and electrical property of the drawn wire from the casted rod. This means the striation is not a serious defect which has to consider in the production of micro-sized fine wire in the drawing process.

• 대한기계학회논문집
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• 제13권5호
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• pp.901-910
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• 1989

본 연구에서는 2상계 스테인리스강의 모재 및 용접부에 대하여 대기중의 피로시험과 인공해수중의 부식피로 시험을 모재부와 용접부의 .alpha.,.gamma.상의 상비율 변화에 따른 조직의 변화 및 용접잔유응력의 영향이 크랙전파에 미치는 영향을 검토하였다. 응력-스트레인의 히스테리스곡선에서 균열닫힘 거동을 실측하고 이들의 결과로 부터 유효응력확대계수와 균열전파속도와 관계를 정리하여 이들의 문제를 해석하였다.

• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2009년도 Korea Superconductivity Society Meeting 2009
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• pp.84-84
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• 2009

• 대한금속재료학회지
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• 제50권4호
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• pp.271-279
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• 2012

9-12% Cr steels have been used in thermal power plants which repeat start and stop operations. Major factors of fatigue life are temperature, frequency, stress ratio, holding time, microstructure, and environment. Normally, fatigue life decreases at high temperature, low frequency, high stress ratio, and long holding time conditions. A Mod.9Cr-1Mo steel, called G91, was developed at ORNL (Oak Ridge National Laboratory, USA) and was adopted as a high-temperature structural material in the ASME Code in 2004. However, its low-cycle fatigue and fatigue crack growth characteristics have been rarely studied. In this work, we have investigated the low-cycle fatigue crack growth behaviors of G91 steel under various test conditions in terms of temperature and stress ratio. As temperature and stress ratio increase, the crack growth rate becomes faster and striation distance also increases. On the other hand, the number of branch cracks decreases.

• Journal of Welding and Joining
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• 제28권5호
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• pp.45-50
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• 2010

The fatigue crack growth characteristics of base metal and weld joint of 9% Ni steel for LNG storage tank was carried out using CT specimen at room temperature and $-162^{\circ}C$. Fatigue crack growth rate of base and weld metals at RT and $-162^{\circ}C$ was coincided with a single line independent of the change of stress ratio and temperature. In the region of lower stress intensity factor range, fatigue crack growth rate at $-162^{\circ}C$ was slower than that at RT, and the slop of fatigue crack growth rate at $-162^{\circ}C$ increased sharply with propagating of fatigue crack, fatigue crack growth rate at RT and $-162^{\circ}C$ was intersected near the region of $2{\times}10-4\;mm$/cycle, and after the intersection region, fatigue crack growth rate at $-162^{\circ}C$ was faster than that at RT. The micro-fracture mechanism using SEM shows the ductile striation in the stable crack growth region. Also the defects of weld specimen after fatigue testing were detected using the A scan of ultrasonic apparatus.

• 동력기계공학회지
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• 제18권2호
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• pp.62-69
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• 2014

In this paper, in order to investigate the effects of marco and microscopic observations of fatigue crack growth in friction stir welded (FSWed) 7075-T651 aluminum alloy plates, fatigue crack growth tests were performed under constant amplitude loading condition at room temperature with three different pre-cack locations, namely base metal (BM-CL) and two kinds of pre-crack locations in welded joints, weld metal (WM-CL) and heat affected zone (HAZ-CL) specimens. The fatigue crack growth behavior of FSWed 7075-T651 aluminum alloy plates were discussed based on the marco and microscopic fractographic observations. The marcoscopic aspects of surface crack growth path for BM-CL and HAZ-CL specimens indicate relatively straight lines, however, the crack growth paths of WM-CL specimens grow first straight and by followed toward the TMAZ and HAZ. The microscopic aspects of fatigue fracture for BM-CL and HAZ-CL specimens indicate typical fatigue striation, but WM-CL showed intergranular fracture pattern by micro structural changes of FSW process.

• 한국재료학회지
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• 제12권8호
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• pp.669-675
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• 2002

CrN film coated by AIP method, improved the mechanical properties (Hardness, Roughness, wear and fatigue) of Ti-6Al-4V alloy. The properties were studied using GXRD, XPS, Hardness, Roughness, wear and fatigue testers. CrN thin film thickness was about 7.5$\mu\textrm{m}$ and grew with (111) orientation. Hardness of CrN thin film was very high (Hv 1390) and roughness of the surface layer was greatly improved (Ra=0.063$\mu\textrm{m}$) compared with matrix alloy (Ra=0.321$\mu\textrm{m}$). Such changes of hardness and roughness could be contributed to improving the wear resistance and fatigue life. Striation like pattern with dimples and voids, a typical fatigue fracture mode, was observed throughout the specimen.

• 대한기계학회논문집
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• 제10권6호
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• pp.830-836
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• 1986

본 연구에서는 이러한 사실에 착안하여 프랑스의 Granjon에 의해 이미 개발된 용접부의 재용성이 탁월한 종래의 정적임플란트(implant)시험기를 정한.고안하여 아아 크용접부의 피로특성에 영향을 미칠 수 있는 제영향인자를 재현하여 간편하게 피로시 험을 할 수 있는 동적 임플란트시험기를 개발하였다. 이러한 시험기를 이용하여 저 온.고압력용강재에 대해서 다음과 같은 항목들을 구명할 목적으로 본 연구는 수행된 것이다.

• 대한기계학회논문집
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• 제18권2호
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• pp.380-388
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• 1994

Study on the chaotic stirring has been performed numerically and experimentally for a shallow rectangular tank accompanying a vortex shedding. The model is composed of a rectangular tank with a vertical plate with a length half the width of the tank. The tank is subject to a horizontal sinusoidal oscillation. The chaotic stirring was analysed by Poincare sections, unstable manifolds and Lyapunov exponents. As Reynolds number is increased the stirring effect is decreased due to the growth of a regular regions near the lower surface of the tank. In the other hand decrease of Reynolds number gives a weaker vortex shedding resulting in the poorer stirring effect. It was also found that the Lyapunov exponent is the highest at the dimensionless period of 1.3-1.5, which seems to be the best condition for the efficient stirring. The experimental visualization for the deformation of materials exhibits the striation pattern similar to the unstable manifold obtained numerically.