• Title, Summary, Keyword: Fatigue Limit

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Improvement of Fatigue Limit in Spheroidal Graphite Cast Iron with High Strength and Toughness (고강도·고인성 구상흑연주철의 피로한도의 개선)

  • Kim, M.G.;Kim, J.H.
    • Journal of the Korean Society for Heat Treatment
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    • v.12 no.1
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    • pp.40-46
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    • 1999
  • Rotary bending fatigue tests were carried out to investigate the improvement of fatigue limit in annealed and austempered spheroidal graphite cast iron. Main results obtained are as follows. (1) The tensile strength(hardness) of Series C is higher than that of Series B, and fatigue limits are 245MPa in Series C, 230MPa in Series B and 195MPa in Series A, respectively. (2) The fatigue limits of Series B and Series A are mainly governed by the resistance to fatigue crack initiation. Whereas, the fatigue limit of Series C is governed by the resistance to fatigue crack initiation and growth. The defect size and the resistance to crack initiation and growth should be considered to clarify the fatigue properties in spheroidal graphite cast iron. (3) Improvement of fatigue limit by half-austempering is more reasonable than that of full-austempering treatment in multi defective materials as spheroidal graphite cast iron.

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A Study on the Governing Factor of Fatigue Limit in Austempered Ductile Iron (오스템퍼링 구상흑연주철의 피로한도 지배인자에 관한 연구)

  • 정회원;김진학
    • Transactions of the Korean Society of Automotive Engineers
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    • v.7 no.5
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    • pp.221-229
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    • 1999
  • We examined the governing factors of fatigue limit in annealed and austempered ductile iron specimens machined micro hole(dia.<0.4mm) in rotary bending fatigue test. Also, the quantitative relationship between fatigue limit and maximum defect size in specimens was investigated. Artificial defect(micro-pit type, dia.<0.4mm) on specimen surface did not bring about an obvious reduction of fatigue limit in austempered ductile iton(ADI) as compared with annealed ductile iron. According to the investigation of ${\sqrt{area}}_c$ which is the critical defect size to crack initiation at artificial defect, ${\sqrt{area}}_c$ of ADI was larger than that of annealed ductile iron. This shows that the situation of crack initiation at artificial defect in ADI is more difficult in comparison with annealed ductile iron. Maximum defect size is one of the important parameters to predict fatigue limit. And, the quantitative relationship, between the fatigue limit ${\sigma}_{\omega}$ and the maximum defect size ${\sqrt{area}}_{max}$ can be expressed to ${\sigma}_{\omega}^n{\cdot}{\sqrt{area}}_{max}=C_2$ where, $C_2$ are constant. Moreover, it is possible to explain the difference in fatigue limit between, austempered and annealed ductile iron by introducing the parameter ${\delta}(=N_{sg}/N_{total})$in a plain spectimen.

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Effects of 2-Phase Matrix Structure on Fatigue Limit of High Strength Ductile Iron (고강도 구상흑연주철의 피로한도에 미치는 2상 기지조직의 영향)

  • Kim, Jin-Hak;Ji, Jueng-Keun;Kim, Min-Gun
    • Journal of Industrial Technology
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    • v.19
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    • pp.75-79
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    • 1999
  • Rotary bending fatigue tests were performed to investigate the effects of 2-phase matrix structure on fatigue limit with prepared specimens in high strength ductile irons. Two types of the specimens with different microstructures have been used. Series A has sorbite and series B has bainite. Fatigue limits of both specimens are improved comparing with as cast specimen. The fatigue limit is higher in series B than in series A. The reason why the fatigue limit of series A shows inferiority to that of series B is due to the transition of micro fatigue cracks to mesocrack occurs very rapidly, so increased stress intensity factor drives the fatigue crack growth. The higher fatigue limit of series B which has bainite is caused by the ${\gamma}$ layer contained in microstructure impede the rapid growth of micro fatigue crack to mesocrack and ${\alpha}$ layer around graphite has the higher capacity for the absorption of plastic deformation energy than sorbite.

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Quantitative Evaluation of Fatigue Strength using a Surface defective Low Carbon Steel (저탄소강의 표면결개 방의 영향에 의한 피로강도의 정량적 평가)

  • 윤명진
    • Journal of The Korean Society of Manufacturing Technology Engineers
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    • v.4 no.4
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    • pp.42-49
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    • 1995
  • It is not clearly known how defects or inclusions of a low carbon steel affect a fatigue strength. We study this issue using SM15C materials. The investigation is carried out by a quantitative evaluation, and experimental findings are: (1) a fatigue limit of A series smooth specimen is 205MPa, and that of B, C, D series is 245MPa, 304MPa and 245MPa, respectively. (2) the fatigue limit varies with respects to the stress distribution I the vicinity of a defects and crack. (3) the micro hole creates a half-circular shape crack, while the hole depth is not critical to the fatigue strength, (4) considering the fatigue strength, the hole diameter is more significant than the hole depth, and (5) Fatigue limit of artificially defected specimen is lower than that of a flawless one (5-10%), however, there exist allowance size and depth of defect which don't get to influence at fatigue limit.

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The Study on Estimation Fatigue Limit in Induction Surface Hardened S45C Steel (S45C강의 고주파 열처리 표면경화재 피로한도 예측에 관한 연구)

  • 이수진;전형용;성낙원
    • Transactions of the Korean Society of Automotive Engineers
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    • v.6 no.1
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    • pp.134-142
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    • 1998
  • The effects of small hole defect size and effective case depth(ECD) on the four point bending fatigue limit of induction surface hardened S45C steel were investigated the fatigue limit evaluation of hardened materials is very difficult because of relations of the hardness gradient and residual stress. In this study, it was possible to characterize fatigue limit and fatigue life of induction surface hardened S45C steel in terms of the hole defect size and effective case depth(ECD) and quantitative evaluation of the fatigue limit with hole defects use Murakami's evaluation method and the range of evaluated values is a good accuracy compared with results.

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Fatigue properties of nitrided titanium using fluidized bed furnace (유동상로를 이용한 질화처리티타늄의 피로강도 특성)

  • Kim, Min-Gun;Ji, Jueng-Keun
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.22 no.1
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    • pp.142-147
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    • 1998
  • Rotary bending fatigue tests were carried out on the nitrided titanium in order to investigate the effect of nitriding layer on fatigue limit. Main results obtained are as follows. (1) The fatigue limit of nitrided pure titanium is remarkably reduced because of enlargement of grain size at high heat treating temperature and high stress field created from the elastic interaction in the compound layer. (2) Further test using specimen which was removed nitrified layer gradually, were also conducted and it was found that by removing the compound layer the fatigue limit recovered as the level of basic material and rather increased by coming of a diffusion layer. Therefore it is concluded that the surface compound layer generated by nitriding treatment reduced the fatigue limit but diffusion layer increased it.

Behavior of Initiation and Propagation of Fatigue Crack under Periodic Overstressing (In the case of Fatigue Limit Stresses) (과대, 과소 응력하에서의 피로크랙 발생거동 (피로한도 응력을 중심으로))

  • 송남홍;원시태
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.15 no.6
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    • pp.1839-1851
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    • 1991
  • Fatigue crack behavior is studied through the two-level rotary bending test with the deep non-through radial holed notch specimens of low carbon steels(SM22C). The main factors investigated are the effects of the damage zone size around crack tip and phenomena of closing or opening of the crack tip. Obtained results are summarized as follows. Fatigue crack behavior in second level stressing slightly lower than fatigue limit is closely related to the size of damage zone produced by the first level stress higher than fatigue limit and to the phenomena of crack closing and opening for the second level stress. The non-propagating crack limit condition depends upon the crack length l$_{1}$ propagated under the first level stress and the magnitude of second level stress .sigma.$_{2}$ lower than the fatigue limit. The non-propagating crack limit condition is expressed by following eq. $\sigma_2^{6.1}{\times}l_{1}=7.35{\times}10^{6}[(kg_{f}mm^{6.1}(mm)]$

Design of the Staircase Fatigue Tests for the Random Fatigue Limit Model (확률적 피로한도모형하에서 계단형 피로시험의 설계)

  • Seo, Sun-Keun;Park, Jung-Eun;Cho, You-Hee;Song, Suh-Il
    • Journal of the Korean Society for Quality Management
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    • v.35 no.3
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    • pp.107-117
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    • 2007
  • The fatigue has been considered the most failure mode of metal, ceramic, and composite materials. In this paper, numerical experiments to asses the usefulness of two Dixon's methods(small and large samples) and 14 S-N methods on assumptions of lognormal fatigue limit distribution under RFL(Random Fatigue Limit) model are conducted for staircase(or up-and-down) test and compared by MSE(Mean Squared Error) and bias for estimates of mean log-fatigue limit. Also, guidelines for staircase test plans to choose initial stress level and step size are recommended from numerical experiments including sensitivity analyses. In addition, the parametric bootstrap method to construct a confidence interval for the mean of log-fatigue limit by the percentile method using a transition probability matrix of Markov chain is presented and illustrated with an example.

Evaluation of Fretting Fatigue Behavior of Aluminum Alloy(A17050-T7451) Under Cyclic Bending Load (알루미늄 합금(AI7050-T7451)의 반복 굽힘 하중하의 프레팅 피로거동 평가)

  • Kim, Jong-Sung;Yoon, Myung-Jin;Choi, Sung-Jong;Cho, Hyun-Deog
    • Journal of the Korean Society of Manufacturing Process Engineers
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    • v.9 no.1
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    • pp.25-34
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    • 2010
  • Fretting damage reduces fatigue life of the material due to low amplitude cyclic sliding and changes in the contact surfaces of strongly connected machine and structures such as bolt, key, fixed rivet and connected shaft, which have relative slip of repeatedly very low frequency amplitude. In this study, the fretting fatigue behavior of 7050-T7451 aluminum alloys used mainly in aircraft and automobile industry were evaluated. The plain fatigue test and fretting fatigue test under cyclic bending load carried out commercial bending fatigue tester and specially devised equipments to cause fretting damage. From these experimental work, the following results obtained: (1) The plain fatigue limit for stress ratio R=-l was about 151MPa. (2) In case of fretting fatigue, fatigue limit for stress ratio R=-l about 72MPa, the fatigue limit for R=0 about 81MPa, and the fatigue limit for R=0.3 about 93MPa. (3) The fatigue limit reduction rates by the fretting damage were about 52%(R=-1), 46%(R=0) and 38%(R=0.3) respectively. (4) The fatigue limit reduction rate decreased with stress ratio increase. In fretting bending test, as stress ratio increased, occurrence of initial oblique crack by fretting decreased or phased out, so that fracture surfaces were formed by plain fatigue crack occurrence, and such tendency was notable as stress amplitude increased. (5) Tire tracks and rubbed scars were observed in the fracture surface and contacted surface.

Fatigue Strength Evaluation of the Clinch Joints of a Cold Rolled Steel Sheet

  • Kim, Ho-Kyung
    • International Journal of Railway
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    • v.2 no.4
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    • pp.131-138
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    • 2009
  • Static tensile and fatigue tests were conducted using tensile-shear specimens to evaluate the fatigue strength of a SPCC sheet clinch joint. The maximum tensile strength of the specimen produced at the optimal punching force was 1750 kN. The fatigue endurance limit (=760 N) approached 43% of the maximum tensile load (=1750 N) at a load ratio of 0.1, suggesting that the fatigue limit is approximately half of the value of the maximum tensile strength. The FEM analysis showed that at the fatigue endurance limit, the maximum von-Mises stress of 373 MPa is very close to the ultimate tensile strength of the SPCC sheet (=382 MPa).

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