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Effect of Superplastic Forming Exposure on Fatigue Crack Propagation Behavior of Ti-6Al-4V Alloy
Daeho Jeong,Yongnam Kwon,Masahiro Goto,Sangshik Kim 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.5
The effect of superplastic forming (SPF) exposure on the ε (strain)-N (number of cycles to failure) fatigue and fatigue crack propagation (FCP) behaviors of Ti-6Al-4V (Ti64) alloy was examined at 298 and 473 K. To simulate the thermal exposure during superplastic forming process, the mill-annealed Ti64 alloy sheet was heated in the vacuum chamber with the pre-determined temperature profile. Notable microstructural change during the SPF exposure included the shape of transformed β phase from fine and round particles in the as-received specimen to coarse angular particles in the as-exposed specimen. The effective grain size tended to increase with the exposure, enhancing the slip reversibility and the resistance to FCP. However, the crack hindering effect by fine, particle-like β phase became weak with the exposure, offseting the beneficial effect associated with the increment of effective grain size. The effect of SPF exposure on ε-N fatigue and FCP behavior of mill-annealed Ti64 alloy was therefore marginal, excluding the effect of α- case (the oxygen-enriched phase) on the surface.
Effect of Superplastic Forming Exposure on Tensile and S-N Fatigue Behavior of Ti64 Alloy
Daeho Jeong,Semi Hyun,Hyokyung Sung,Yongnam Kwon,Sangshik Kim 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.4
The effect of superplastic forming (SPF) on tensile and S (stress)-N (number of cycles to failure) fatigue properties of Ti64 alloy was examined at 298 and 473 K. For simulating the superplastic forming exposure, millannealed Ti64 alloy sheet was heated in a vacuum chamber with a pre-determined temperature profile. For some as-exposed specimens, the α-case formed on the surface during expousre was mechanically removed to understand the effect of α-case on the mechanical properties of Ti64 alloy. It was found that the presence of α-case significantly affected the tensile and the fatigue properties of Ti64 alloy at 298 and 473 K by providing an easy initiation site for both tensile and fatigue fracture. The microstructural change during the SPF exposure was marginal in affecting the S-N fatigue properties of Ti64 alloy. Different testing temperature of 298 and 473 K affected the S-N fatigue behavior of as-received and as-exposed (α-case removed) Ti64 specimens, but not that of as-exposed specimen.
Fatigue Crack Propagation Behavior of Fe24Mn Steel Weld at 298 and 110 K
Daeho Jeong,이순기,서인식,유장용,김상식 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.1
The fatigue crack propagation (FCP) tests were conducted on Fe24Mn steel in the region of base metal(BM), weld metal (WM) and fusion line (FL) at 298 and 110 K. The FCP rates of Fe24Mn specimens inthe region of BM, WM and FL were greatly decreased, while no notable difference in the fracture modewas observed, with decreasing temperature from 298 to 110 K. The FCP rates of Fe24Mn WM and FLspecimens were slightly lower than those of BM specimen at both room and cryogenic temperatures. TheSEM fractographic analyses suggested that each specimen showed the transgranular facets at both temperaturesin low and intermediate ΔK regimes. However, the morphological details varied depending on the regionof weld and testing temperature. Relatively large sized facets were observed for the WM specimen with thecolumnar grain boundary playing the same role as the grain boundary in the BM and the FL specimens inthe near-threshold ΔK regime. The FCP behavior of Fe24Mn steel in the region of BM, WM and FL isdiscussed at 298 and 110 K based on the fractographic and micrographic observation.
Jeong, Daeho,Ahn, Kwang-Soo,Lee, Won-Yong Elsevier 2017 Journal of electroanalytical chemistry Vol.806 No.-
<P><B>Abstract</B></P> <P>A label-free impedimetric glycosensor has been developed for the direct detection of cholera toxin (CT). The glycosensor has been fabricated by forming the self-assembled monolayers of β-galactose derivatives containing poly-ethyleneglycol (PEG) spacer on gold electrode surfaces. The selective binding of CT to the β-galactose receptors on the fabricated glycosensor has been directly monitored by electrochemical impedance spectroscopy in the presence of 5.0mM K<SUB>3</SUB>Fe(CN)<SUB>6</SUB>/K<SUB>4</SUB>Fe(CN)<SUB>6</SUB> (1:1, v/v) redox couple. gmonolayers on gold electrode surface has been determined to be ca. 2.53×10<SUP>10</SUP> M<SUP>−1</SUP>, indicating that the present glycosensor can selectively capture the target CT from sample solutions. Therefore, the present monosaccharide β-galactose derivative can be used as a receptor for CT as an alternative to CT-specific antibodies or pentasaccharide ganglioside GM<SUB>1</SUB>, most widely used in conventional CT assays. The present glycosensor can detect CT in the concentration range from 1.18×10<SUP>−12</SUP> M to 1.18×10<SUP>−9</SUP> M (<I>r</I> <SUP>2</SUP> =0.963) with a limit of detection (S/N=3) down to 7.83×10<SUP>−13</SUP> M, which is much lower than those obtained with other detection methods. Since the present glycosensor-based CT assay does not require complex and time-consuming signal amplification steps generally employed in sandwich-type immunoassays, it enables the simple and rapid detection of CT.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An electrochemical glycosensor was developed for the determination of CT. </LI> <LI> The self-assembled monolayers of β-galactose on gold shows good selectivity for CT. </LI> <LI> Binding of CT to the β-galactose increases the electron transfer resistance. </LI> <LI> The degree of resistance change is strongly dependent upon the amount of added CT. </LI> <LI> The present glycosensor does not require a complex signal amplification step. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Fatigue Crack Propagation Behavior of Fe25Mn and Fe16Mn2Al Steels at Room and Cryogenic Temperatures
Daeho Jeong,Hyokyung Sung,Taedong Park,Jongseop Lee,Sangshik Kim 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.4
The fatigue crack propagation (FCP) behavior of Fe25Mn and Fe16Mn2Al austenitic steels was investigated at 298 and 110 K, and the results were compared with the reported results of Fe24Mn2Cr steel. It was found that the FCP behavior of high-Mn, austenitic steels was largely influenced by the stacking fault energy (SFE) and the grain size. The resistance to FCP of high-Mn steels in this study was enhanced in the near-threshold ΔK regime with decreasing temperature from 298 to 110 K. The improvement for the Fe25Mn and the Fe16Mn2Al specimen was, however, marginal as compared to that of the Fe24Mn2Cr specimen. Other than the change in SFE, the secondary cracking at cryogenic temperature appeared to affect the FCP behavior of high-Mn steels, since the secondary cracks perpendicular to the crack propagating direction could reduce the effective stress intensity factor, decreasing the FCP rates. Sufficiently high stress concentration at grain boundary tended to occur at low temperature for relatively large grain sized Fe24Mn2Cr specimen and cause the secondary cracking, but not for the Fe25Mn and the Fe16Mn2Al specimen.
다양한 환경에서의 오스테나이트계 고망간강 용접부 피로거동
정대호(Daeho Jeong),권재기(Jaeki Kwon),우남섭(Namsub Woo),김영주(Youngju Kim),김상식(Sangshik Kim) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.5-2
In the present study, the fatigue crack propagation (FCP) behaviors of Fe24Mn (Fe-24Mn-4Cr) in various environments were examined in the different locations of base metal (BM), weld metal (WM) and fusion line (FL) at 298 and 110K and in artificial seawater under various cathodic protection potentials. The FCP rates of Fe24Mn BM specimen was substantially reduced with decreasing temperature from 298 to 110K over the entire ?K regime. The CP potential of -850 and -1,050 mVSCE suppressed the environmental effect of seawater on the FCP behavior of Fe24Mn BM and WM specimens, showing almost identical da/dN-?K curves for both air and seawater environments. The FCP behaviors of Fe24Mn in various environments were discussed based on the microstructural and the fractographic observations.
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.