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이은경,정유인,김상식,Lee, Eungyeong,Jeong, Yooin,Kim, Sangshik 한국재료학회 2015 한국재료학회지 Vol.25 No.10
Aging aircraft structures are inevitably exposed to environment for a long time facing many potential problems, including corrosion and wide spread fatigue damage, which in turn cause the degradation of flight safety. In this study, the environmental surface damages on aging aircraft structures induced during service were quantitatively analyzed. Additionally, S-N fatigue tests were performed with center hole specimens extracted from aging aircraft structures. From the results of quantitative analyses of the surface damages and fatigue tests, it is concluded that corrosion pits initiated during service reduce the fatigue life significantly. Finally, using the fracture mechanics and the EIFS (equivalent initial flaw size) concepts, the remaining fatigue life was predicted based on actual fatigue test results.
항공용 고강도 2xxx계 알루미늄 합금의 3.5 % 염수 환경에서의 응력부식균열 민감도
최희수,이다은,안수진,이철주,김상식,Choi, Heesoo,Lee, Daeun,Ahn, Soojin,Lee, Cheoljoo,Kim, Sangshik 한국재료학회 2018 한국재료학회지 Vol.28 No.12
For the aerospace structural application of high-strength 2xxx series aluminum alloys, stress corrosion cracking(SCC) behavior in aggressive environments needs to be well understood. In this study, the SCC sensitivities of 2024-T62, 2124-T851 and 2050-T84 alloys in a 3.5 % NaCl solution are measured using a constant load testing method without polarization and a slow strain rate test(SSRT) method at a strain rate of 10-6 /sec under a cathodic applied potential. When the specimens are exposed to a 3.5 % NaCl solution under a constant load for 10 days, the decrease in tensile ductility is negligible for 2124-T851 and 2050-T84 specimens, proving that T8 heat treatment is beneficial in improving the SCC resistance of 2xxx series aluminum alloys. The specimens are also susceptible to SCC in a hydrogen-generating environment at a slow strain rate of $10^{-6}/sec$ in a 3.5 % NaCl solution under a cathodic applied potential. Regardless of the test method, low impurity 2124-T851 and high Cu/Mg ratio 2050-T84 alloys are found to have relatively lower SCC sensitivity than 2024-T62. The SCC behavior of 2xxx series aluminum alloys in the 3.5 % NaCl solution is discussed based on fractographic and micrographic observations.
적층 제조된 H13 공구강의 미세조직과 기계적 특성간의 상관관계
안우진,박준혁,이정섭,최중호,정임두,유지훈,김상식,성효경,An, Woojin,Park, Junhyeok,Lee, Jungsub,Choe, Jungho,Jung, Im Doo,Yu, Ji-Hun,Kim, Sangshik,Sung, Hyokyung 한국재료학회 2018 한국재료학회지 Vol.28 No.11
H13 tool steels are widely used as metallic mold materials due to their high hardness and thermal stability. Recently, many studies are undertaken to satisfy the demands for manufacturing the complex shape of the mold using a 3D printing technique. It is reported that the mechanical properties of 3D printed materials are lower than those of commercial forged alloys owing to micropores. In this study, we investigate the effect of microstructures and defects on mechanical properties in the 3D printed H13 tool steels. H13 tool steel is fabricated using a selective laser melting(SLM) process with a scan speed of 200 mm/s and a layer thickness of $25{\mu}m$. Microstructures are observed and porosities are measured by optical and scanning electron microscopy in the X-, Y-, and Z-directions with various the build heights. Tiny keyhole type pores are observed with a porosity of 0.4 %, which shows the lowest porosity in the center region. The measured Vickers hardness is around 550 HV and the yield and tensile strength are 1400 and 1700 MPa, respectively. The tensile properties are predicted using two empirical equations through the measured values of the Vickers hardness. The prediction of tensile strength has high accuracy with the experimental data of the 3D printed H13 tool steel. The effects of porosities and unmelted powders on mechanical properties are also elucidated by the metallic fractography analysis to understand tensile and fracture behavior.
용접구조물의 부식피로수명에 미치는 용접부 개선처리 효과
임성우(SUNG-WOO IM),장인화(IN-HWA CHANG),김상식(SANGSHIK KIM),송하철(HA CHEOL SONG) 한국해양공학회 2008 韓國海洋工學會誌 Vol.22 No.2
The effect of weld improvement on the corroded fatigue life of welded structures was investigated. Toe grinding, TIG dressing and weld profiling were used as the geometric improvement methods. Fatigue tests under the corroded condition in artificial seawater were carried out to investigate the corrosion fatigue behavior of API 2W Gr.50T steel plate produced by POSCO. The test results in weld improved conditions were compared with those in as-welded condition. The test results were also compared with the design curves in UK DEn Class F. Corroded fatigue life of weld improved specimens was longer than that of as-welded specimen. Especially, the corroded fatigue life exceeded the mean SN curve in air of UK DEn Class F.
우남섭(NamSub Woo),박종명(JongMyoung Park),권재기(JaeKi Kwon),김영주(YoungJu Kim),김상식(SangShik Kim),정성윤(SungYoon Jung) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Subsea separation is an emerging technology that requires new and optimized solutions. Subsea separation system should be reliable to ensure successful operation in a wide range of 3-phase flow regimes, without need for maintenance. Separating gas, oil, water, and sand at the seabed has great economic potential for many subsea field developments. A subsea separator can avoid or simplify costly surface platforms or floating vessels, as well as being an efficient tool to enhance hydrocarbon production. One solution of interest is the separation and re-injection of water at the seabed to avoid bringing the water up to the surface facility. In this study, subsea separation system and its foreign and domestic development status are introduced.
극저온 배관용 고망간강의 S-N 피로 및 피로균열전파 거동
정대호(Daeho Jeong),우남섭(Namsub Woo),김영주(Youngju Kim),김상식(Sangshik Kim) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.춘계 No.-
The S-N fatigue and fatigue crack propagation behavior of Fe24Min (Fe-0.4C-24Mn) and Fe15Mn (Fe-0.7C-15Mn-3Al) austenitic steel was investigated at 298 K and 110 K, and the results were compared to those of STS304L (Fe-1 Si-2Mn-20Cr-10Ni) counterparts. The resistance to S-N fatigue of high-Mn steels was greater at 298 K, while it was lower at 110 K, than STS304L steel. High-Mn steels did not show any notable transformation induced plasticity (TRIP) and twining-induced plasticity (TWIP) effect under fatigue loading at ambient and cryogenic temperatures. The S-N fatigue behavior of high-Mn steel was largely dependent on tensile strength at both ambient and cryogenic temperatures, In the case of fatigue crack propagation, the near-threshold ΔK (ΔK<SUB>th</SUB>) values increased with decreasing temperature.