자동차 부품용 내열 알루미늄 합금의 고온 피로 변형 거동

박종수 ( Jong Soo Park ),성시영 ( Si Young Sung ),한범석 ( Bum Suck Han ),정창렬 ( Chang Yeol Jung ),이기안 ( Kee Ahn Lee ) 대한금속·재료학회 2010 대한금속·재료학회지 Vol.48 No.1

High temperature high cycle and low cycle fatigue deformation behavior of automotive heat resistant aluminum alloys (A356 and A319 based) were investigated in this study. The microstructures of both alloys were composed of primary Al-Si dendrite and eutectic Si phase. However, the size and distribution for eutectic Si phase varied: a coarse and inhomogeneous distributed was observed in alloy B (A319 based). A brittle intermetallic phase of α-Fe Al12(Fe,Mn)3Si2 was detected only in B alloy. Alloy B exhibited high fatigue life only under a high stress amplitude condition in the high cycle fatigue results, whereas alloy A showed high fatigue life when stress was lowered. With regard to the low-cycle fatigue result (250℃) showing higher fatigue life as ductility increased. alloy A demonstrated higher fatigue life under all of the strain amplitude conditions. Fractographic observations showed that large porosities and pores near the outside surface could he the main factor in the formation of fatigue cracks. In alloy n. micro-cracks were formed in both the brittle intermetallic and coarse Si phases. These micro-cracks then coalesced together and provided a path for fatigue crack propagation. From the observation of the differences in microstructure and fractography of these two automotive alloys. the authors attempt to explain the high-temperature fatigue deformation behavior of heat resistant aluminum alloys.

Low- and High-cycle Fatigue Behavior of Load-carrying Cruciform Joints with Incomplete Penetration and Strength Under-match

Takeshi Hanji,Kawin Saiprasertkit,Chitoshi Miki 한국강구조학회 2011 International Journal of Steel Structures Vol.11 No.4

The fatigue strengths of load-carrying cruciform joints with incomplete penetration and a strength mismatch between the base metal and the deposit metal were studied. Low- and high-cycle fatigue tests were performed on specimens with five matching conditions and two sizes of incomplete penetration. The test results revealed that the failure life was governed by crack propagation in both the low-cycle and high-cycle fatigue regions, and the crack propagation paths differed according to the matching and loading conditions. In addition, the fatigue strengths of the joints were compared for the degree of strength matching and the size of incomplete penetration in the low- and high-cycle fatigue regions. It was found that the effect of strength matching on the fatigue strength is negligible in the high-cycle fatigue region, but it becomes large in the low-cycle fatigue region and significantly reduces the fatigue life of the specimen in the under-matched joints.

High-cycle and very high-cycle bending fatigue strength of shot peened spring steel

NohJun Myung,Liang Wang,Nak-Sam Choi 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.11

This paper examined bending fatigue strength of shot-peened spring steel under the high cycle fatigue (HCF) and very high cycle fatigue (VHCF), and analyzed the effects of shot-peening on the high strength spring steel through service life. Hourglass shape specimens made of spring steel (Si-Cr alloys) were prepared for the rotary bending fatigue test. Actual local stresses were quantitatively calculated to compensate for the applied stress amplitudes, through which it was clarified that the shot-peening had strongly positive effects on the HCF strength, but rather negative effect on the VHCF strength. The fracturing process examination confirmed that most fish-eye fractures arose at sites deeper than the compressive residual stress zone, which did not prevent the creation of the fish-eye. The fatigue strength improvement in the VHCF range was scarcely expected as a result of the shot-peening treatment.

High-cycle fatigue characteristics of quasi-isotropic CFRP laminates

Hosoi, Atsushi,Arao, Yoshihiko,Karasawa, Hirokazu,Kawada, Hiroyuki The Korean Society for Composite Materials 2007 Advanced composite materials Vol.16 No.2

High-cycle fatigue characteristics of quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates [-45/0/45/90]s up to $10^8$ cycles were investigated. To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz, since it is difficult to investigate the fatigue characteristics in high-cycle at 5 Hz. Then, the damage behavior of the specimen was observed with a microscope, soft X-ray photography and a 3D ultrasonic inspection system. In this study, to evaluate quantitative characteristics of both transverse crack propagation and delamination growth in the high-cycle region, the energy release rate associated with damage growth in the width direction was calculated. Transverse crack propagation and delamination growth in the width direction were evaluated based on a modified Paris law approach. The results revealed that transverse crack propagation delayed under the test conditions of less than ${\sigma}_{max}/{\sigma}_b$ = 0.3 of the applied stress level.

STB2강의 고온피로강도 특성에 미치는 열처리의 영향

오세규(Sae-Kyoo Oh),김연호(Yeon-Ho Kim),이상국(Sang-Guk Lee),이종두(Jong-Doo Lee) 한국해양공학회 1989 韓國海洋工學會誌 Vol.3 No.2

It has been very important in various industry fields to improve the fatigue strength characteristics of bearings such as bearing life, fatigue limit, etc., because such poor properties could result in shortening the machinery life as well as in decreasing the accuracy. However, no successful heat treatment criterion seems to be available at present.<br/> In this study, the effect of the 170℃×120min tempering cycles repeated after 830℃×30min oil quenching for 800℃ spheroidizing-annealed bearing steel (STB2) as base metal on the 120℃ high temperature rotary bending fatigue strength characteristics were investigated, including the effects on hardness, Charpy impact value and micro-structure, in order to seek for the best heat treatment condition finally. The important results obtained are as follows :<br/> 1) The optimal cycle of tempering so that the fatigue strength σ could become the highest was the 4th cycle. And it is confirmed that this σF is about 6 times more increased than that of base metal, and about 1.3 times more increased than the case of the 1 cycle tempered.<br/> 2) As a result of the investigation for the effects of tempering cycles on hardness, the hardness at the tempering number of 2 thru 5 cycles was not decreased severely ; only about 10% decrease from those of the quenched and 1 cycle tempered case. Such hardness is equivalent to HRC61~62 with no bad effect on anti-abrasion of bearing steel.<br/> 3) In the case of 2 thru 5 cycle tempering as well as 1 cycle tempering, the impact value was not so improved comparing with the case of quenching, but an increase of 5 to 10% could be expected at least.<br/> 4) It was experimentally confirmed that the control of the mechanical properties improvement such as fatigue strength and fatigue life for bearing steels could be possible by the number of tempering cycles.

Effect of Microstructural Features on the High-Cycle Fatigue Behavior of CoCrFeMnNi High-Entropy Alloys Deformed at Room and Cryogenic Temperatures

Gyung Tae Lee,Jong Woo Won,Ka Ram Lim,Minju Kang,Heoun Jun Kwon,Young Sang Na,최윤석 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.4

In this study, we examined the effect of deformation twins and dislocation cell structures on the fatigue properties of theCoCrFeMnNi high-entropy alloy using rotational bending fatigue tests. The dislocation cell structures and deformationtwins were generated by prestraining the CoCrFeMnNi high-entropy alloy at room temperature (27 °C) and a cryogenictemperature (− 196 °C), respectively. To eliminate the effect of different material strengths on fatigue behavior, the tensilestrengths of the specimens evaluated in the fatigue tests were kept similar by controlling the prestraining under room andcryogenic temperatures. The results of the rotational bending fatigue tests revealed that the CoCrFeMnNi high-entropyalloy prestrained at room temperature exhibited higher fatigue resistance and fatigue limit than the specimen prestrained ata cryogenic temperature. A small quantity of large micro-voids was formed at the triple junction of the grain boundaries inthe specimen prestrained at room temperature, whereas a large quantity of small micro-voids was formed in the region wherethe deformation twins intersected the grain boundaries in the specimen prestrained at a cryogenic temperature. Therefore,it is concluded that the different aspects of micro-void formation affected the crack initiation and, consequently, the fatigueproperties of the room and cryogenic temperature-prestrained alloys.

Multiaxial Fatigue Life Prediction Based on High-Cycle Uniaxial Fatigue Test of Steel Pipe Weldments with Welding Defects

Hui Liu,Xiu-wen Lv,Shi-chao Chen,Qiang Zhou,Piao Zhou,Wei-lian Qu 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.1

Welding defects are unavoidable for welded structures, which can lead to fatigue damage even under the random wind load with small amplitude. It is therefore necessary to explore the effect of welding defect on the fatigue properties of welded steel pipes. Three groups of welded steel pipe specimens were designed according to welding defect conditions, i.e. specimens without welding defect (Group I), specimens with incomplete fusion (Group II), and specimens with welding porosity (Group III). Uniaxial tension–compression and torsion high-cycle fatigue tests were carried out. S–N curves of uniaxial tension–compression and torsion tests were obtained by cyclic loading with equal stress amplitude. The test results show that the high-cycle fatigue strength of weldments is obviously lower than that of base metal with the same strength under uniaxial tension–compression and torsion loading. In addition, the welding defects result in a decrease in fatigue strength, while the decrease extent by welding porosity is greater than that by incomplete fusion. Finally, because of the inherent multiaxial loading characteristics of welded structures, the high-cycle multiaxial fatigue life of steel pipe weldments was also predicted by using the modified Wöhler curve method based on the uniaxial fatigue test results. It can be found that when the stress amplitude is constant, the fatigue life of welded steel pipe decreases and the modified Wöhler curves move downward more quickly with the increase of damage parameter defined as the ratio of normal stress amplitude to shear stress amplitude on the critical plane, which means that normal stress amplitude will accelerate the cracks growth and result in faster failure of the weld materials.

Mg–5Bi–3Al 마그네슘 고속 압출재의 미세조직과 고주기피로 특성

차재원,진상철,박성혁 한국소성∙가공학회 2022 소성가공 : 한국소성가공학회지 Vol.31 No.5

In this study, the microstructural characteristics of a high-speed-extruded Mg–5Bi–3Al (BA53) alloy and its tensile, compressive, and high-cycle fatigue properties are investigated. The BA53 alloy is successfully extruded at a die-exit speed of 16.6 m/min without any hot cracking using a large-scale extruder for mass production. The homogenized BA53 billet has a large grain size of ~900 μm and it contains fine and coarse Mg3Bi2 particles. The extruded BA53 alloy has a fully recrystallized microstructure with an average grain size of 33.8 μm owing to the occurrence of complete dynamic recrystallization during high-speed extrusion. In addition, the extruded BA53 alloy contains numerous fine lath-type Mg3Bi2 particles, which are formed through static precipitation during air cooling after exiting the extrusion die. The extruded BA53 alloy has a high tensile yield strength of 175.1 MPa and ultimate tensile strength of 244.4 MPa, which are mainly attributed to the relative fine grain size and numerous fine particles. The compressive yield strength (93.4 MPa) of the extruded BA53 alloy is lower than its tensile yield strength, resulting in a tension-compression yield asymmetry of 0.53. High-cycle fatigue test results reveal that the extruded BA53 alloy has a fatigue strength of 110 MPa and fatigue cracks initiate at the surface of fatigue test specimens, indicating that the Mg3Bi2 particles do not act as fatigue crack initiation sites. Furthermore, the extruded BA53 alloy exhibits a higher fatigue ratio of 0.45 than other commercial extruded Mg–Al–Zn-based alloys.

압출재 6061 알루미늄 합금의 피로특성에 미치는 열처리의 영향

김선호(S. H. Kim),김규식(K. S Kim),어광준(K. J. Euh),조규상(K. S. Cho),이기안(K. A. Lee) 한국소성가공학회 2012 한국소성가공학회 학술대회 논문집 Vol.2012 No.5

The effect of heat treatment on the high-cycle fatigue behavior of 6061 extruded alloys was investigated in this study. Microstructure examination, tensile and high-cycle fatigue test were conducted on both AI-Si-Mg extruded (F) and heat treated (T6) conditions. From the tensile tests, yield and tensile strengths increased and elongation decreased by T6 heat treatment. The high-cycle fatigue results indicated that the fatigue strength of the 6061-T6 alloy was significantly higher than that of the 6061-F alloys. Fatigue cracks were initiated near the surface of specimens regardless of heat treatment. In the case of6061-T6 alloy, 2<SUP>nd</SUP> cracks easily appeared when the fatigue crack met Mg₂Si phase during fatigue propagation. Fine Mg₂Si phases could be formed and homogeneously distributed by T6 heat treatment. The Mg₂Si was believed to act an effective role of retardation of fatigue crack initiation and propagation. The fatigue deformation mechanism of extruded 6061 aluminum alloy was also discussed.

Al 7003 합금의 열처리(T5, T6)에 따른 인장 및 고주기 피로 변형 거동

김민종(M. J. Kim),김관영(K. Y. Kim),어광준(K. J. Euh),이기안(K. A. Lee) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.10

Tensile and high-cycle fatigue defomation behavior due to heat treatment (T5, T6) on Al 7003 alloy were investigated. Aluminum 7003 alloy was produced through casting and extruding, and then heat treatments of T5(90 ℃, 5hrs. + 150 ℃, 16hrs.) and T6 (470 ℃, 3hrs., 80 ℃ and WQ solution processing, 14hrs + 120 ℃, 42hrs.) followed respectively. Using the two kinds of Al 7003 alloys to which different heat treatments were applied, microstructure analysis, tensile tests, room-temperature high-cycle tests, etc., were performed. The observation of microstructure showed that the average grain size was 3.8 μm for T5 alloy and 4.1 μm for T6 alloy. Both kinds of heat-treated alloys were analyzed to be Al-matrix and to have MgZn₂ and Fe-intermetallic phase; MgZn₂ with T6 condition had finer and more regular distribution compared to the one with T5 condition. The tensile results showed the following properties under the T5 heat treatment condition: tensile strength of 359.0MPa; yield strength of 326.1MPa, and; ductility of 28.0%. On the other hand, under the T6 condition, tensile strength was 411.6MPa, yield strength was 373.6MPa, and ductility was 26.3%. In other words, under the T6 heat treatment condition, relatively higher strength and minor decrease in ductility were obtained compared to those under the T5 condition. The high-cycle fatigue results showed that the fatigue limit (10<SUP>7</SUP>cycle) under the T5 heat treatment condition was 290MPa, and the fatigue limit (10<SUP>7</SUP>cycle) under the T6 condition was 320WPa. Under all stress conditions, the fatigue life of T6 alloy was verified to be longer than that of T5 alloy. Based on the observation on tensile and fatigue fracture surfaces, the excellent properties of the T6 heat-treated alloy were confirmed to cone from the effective reinforcement phases with small and uniform distribution generated by the heat treatment under such condition. Based on these results, Al 7003 alloy`s tensile and fatigue deformation mechanisms were also discussed.