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오진근(Jinkeun Oh),김낙준(Nack J. Kim),이성학(Sunghak Lee),이의휘(Eui W. Lee) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8
The effects of microstructural factors on high-cycle fatigue properties of welded regions of an investment cast Ti-6AI-4V alloy were investigated in this study. High-cycle fatigue were conducted for the welded regions, which were processed by two different welding methods, i.e. tungsten inert gas (TIG) welding and electron beam (EB) welding, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The base metal was composed of a Widmanstatten structure, which was then transformed to a basket-weave structure having thin a platelets after welding and annealing. The high-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of coarse micropores formed during welding, although it had highest yield strength.
열간압연으로 제조된 Ti Aluminide 다층판재의 미세조직 : Ⅱ. 금속간화합물의 성장거동 Part Ⅱ: Growth Behavior of Intermetallic Phases
吳振根,表成奎,李聖鶴,金洛俊 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.4
The present study is to investigate the growth behavior of intermetallic phases in multilayered Ti/TiAl_3 sheets fabricated by hot rolling. When these sheets were heat-treated at 1000℃, interfacial phases such as Ti_3Al, TiAl, and TiAl_2 were grown consuming Ti and TiAl_3. As the heat treatment proceeded, TiAl was grown consuming TiAl_2, thereby finally fabricating multilayered sheets composed of Ti_3Al and TiAl. Conventional lamellar structure composed of Ti_3Al and TiAl could also be fabricated by heat treatment of multilayered Ti/TiAl_3 sheets at 1400℃. The growth rate and activation energy of interfacial phases were obtained from their growth behavior. The results indicated that the growth of intermetallic phases was mainly controlled by volume diffusion than by grain boundary diffusion. The formation process of the multilayered sheets and the formation mechanism of interfacial phases were elucidated, and an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment was eventually suggested in this study.
정밀주조된 Ti-6Al-4V 합금 판재의 피로성질에 미치는 판재 두께의 영향
吳振根,李玎求,金洛俊,李星鶴,李義徽 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.7
Effects of plate thickness on high-cycle fatigue properties and fatigue crack propagation behavior of investment-cast Ti-6Al-4V alloy plates having Widmansta¨tten structure were investigated in this study. High-cycle fatigue test and fatigue crack propagation test were conducted on three cast plates having different thickness, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The high-cycle fatigue results indicated that fatigue strength of the three cast plates was quite similar because of their similar tensile strength. In the case of the fatigue crack propagation, the thicker cast plate composed of thinner α platelets had the slightly faster crack propagation rate than the other plates. The effective microstructural factor determining the fatigue crack propagation rate was found to be the thickness of a platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.
Vulnerability Case Analysis of Wireless Moving Vehicle
Sangyun Oh,Jinkeun Hong 한국융합학회 2018 한국융합학회논문지 Vol.9 No.8
최근 드론 관련 산업이 활성화되면서 드론에 대한 대중들의 관심이 폭발적으로 증가하였고, 드론을 활용하는 사례 가 많이 늘어나고 있다. 군사용 드론의 경우 보안문제는 항공기나 순항미사일의 방어시스템 수준으로 보안이 철저하지만 상용 소형 저가형 드론들은 여전히 보안이 취약하거나 보안을 고려하지 않은 상태에서 출시되고 활용되는 경우가 많다. 이로 인하여 공격자가 쉽게 드론의 루트 권한을 얻고 내부 파일에 접근하거나 fake 패킷을 보내는 등의 드론 탈취 공격이 가능하다. 그런데 이 드론의 탈취 문제는 또 다른 위험한 공격으로 이어질 수 있다. 이런 측면에서 본 논문에서는 특정 드론의 통신과정에서 공격 사례를 중심으로 분석함으로써 상용 드론에 내재된 취약점을 확인하였다. 본 논문에서는 취약점 을 스캐닝 공격, 중간자 공격, 인증 철회공격, 패킷 중지 명령 공격, 패킷 재전송 공격, 신호 조작 및 디 컴파일 공격 관점에 서 접근하여 분석하고 실험하였다. 본 연구는 드론 공격과 취약점을 분석하는데 참고할 수 있는 유용한 연구로 사료된다. As the industry related to drones has been activated, the public interest in drones has increased explosively, and many cases of drone-using are increasing. In the case of military drones, the security problem is the level of defense of the aircraft or cruise missiles, but commercial small and low cost drones are often released and utilized without security count-measure. This makes it possible for an attacker to easily gain access to the root of the drones, access internal files, or send fake packets. However, this droning problem can lead to another dangerous attack. In this regard, this paper has identified the vulnerabilities inherent in the commercial drones by analyzing the attack cases in the communication process of the specific drones. In this paper, we analyze and test the vulnerability in terms of scanning attack, meson attack, authentication revocation attack, packet stop command attack, packet retransmission attack, signal manipulation and de-compile attack. This study is useful for the analysis of drones attack and vulnerability.
Suppression of Coating Layer Crack by Diffusion Heat Treatment in Al-Si Coated HPF Steels
Sang‑Man Yun,Hojun Gwon,Jinkeun Oh,Sung‑Joon Kim 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.11
The effect of diffusion heat treatment (DHT) on the cracking of the coating layer in Al-Si coated hot press forming (HPF)steel was investigated. Cracking-induced micro-delamination in the coating layer has limited applications of the Al-Si coatedHPF steels. Introduction of DHT process can modify the microstructure and constituent phase in the coating layer whichcan reduce the cracking. DHT was performed at 650 °C for 360 min to drive Fe diffusion into the coating layer. Formationof band-shaped τ1-Fe3Al2Si3 in the coating layer during the DHT was observed by EBSD and SEM. After HPF process ofthe diffusion heat treated coating specimen, the phase fraction of FeAl increased and that of η-Fe2Al5 decreased comparedto the conventional Al-Si coating specimen, due to the increase of τ1-Fe3Al2Si3. Owing to the higher fracture toughness ofFeAl compared to η-Fe2Al5, application of DHT improved the toughness of the coating layer and reduced the crack densityin it. DHT did not significantly affect the mechanical properties of the base steel.
Jo, Min Cheol,Park, Jaeyeong,Sohn, Seok Su,Kim, Seongwoo,Oh, Jinkeun,Lee, Sunghak Elsevier Sequoia 2017 Materials science & engineering Structural materia Vol.707 No.-
<P><B>Abstract</B></P> <P>Hot press forming (HPF) steel sheets are austenitized, press-formed, and rapidly cooled to obtain a martensitic microstructure with an ultra-high strength. When they are insufficiently austenitized, their microstructures might contain a small amount of untransformed ferrite, which can deteriorate impact toughness as well as strength, but its causes and relevant fracture mechanisms have not been clearly verified yet. In this study, thus, 1.8-GPa-grade HPF sheets were austenitized at various temperature and time, and their tensile and Charpy impact test results were analyzed in relation with untransformed ferrite and its effect on fracture mechanisms. In the HPF sheets containing the untransformed ferrite, voids were formed mostly at ferrite/martensite interfaces, and were grown and propagated linearly to form a cleavage crack, whereas deformation bands were well developed without voids or cracks in the non-ferrite-containing sheets. The highly localized strains accommodated in the soft ferrite made ferrite/martensite interfaces or ferrite itself work as fracture initiation sites, which led to the brittle fracture and consequently to the deterioration of impact energy. This result can provide an important idea for optimization of austenitization conditions demanded for ultra-high strength and excellent impact toughness in HPF applications.</P>