• Title/Summary/Keyword: fatigue limit(

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Fatigue Limit According to Microcracks in 0.57 wt.% Carbon Steel with Different HV (HV가 다른 0.57 wt.% 탄소강의 미소 균열에 따르는 피로 한도)

  • Oh, Chang-Seok;Gu, Kyoung-Hee;Lee, Gum-Hwa;Nam, Ki-Woo
    • Journal of the Korean Society of Industry Convergence
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    • v.25 no.2_2
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    • pp.233-238
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    • 2022
  • This study evaluated the fatigue limit depending on microcracks using 0.57 wt.% carbon steel with different Vickers hardness. The fatigue limit was almost constant up to a certain limit regardless of the carbon content. However, the fatigue limit decreased rapidly as the size of the crack increased. As the crack aspect ratio was smaller, the fatigue limit of the depth (point A) a lot decreased. The fatigue limit ratio of the depth decreased significantly because the crack propagation in the depth direction was fast as the crack aspect ratio became smaller. On the other hand, the fatigue limit ratio of surface cracks increased as the crack aspect ratio decreased.

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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Effect of Ion-nitriding on Fatigue Strength (피로강도에 미치는 이온실화처리의 영향)

  • Kang, Moo-Jin;Lee, Chung-Oh
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.3 no.3
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    • pp.98-103
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    • 1979
  • The fatigue limit of an ion-nitrided steel was investigated experimentally and analytically. It is found that fatigue limit can singificantly be increased by ion-nitriding, and that the case depth is the most important parameter which determines the fatigue limit. The data indicate that fatigue limit increases with the case depth as well as the surface hardness of the nitrided steel. The fracrographs of the fracture surfaces taken by a scanning electron microscope show that the fisch-eye is located at the subsurface of failed specimens. Assuming that crack propagates from the subsurface inclusions, an analytical model is proposed to predict the fatigue limit. Taking into account the stress distrbution of a nitrided specimen, fatigue limit is predicted as a function of the case depth. The proposed semiemprical formula agrees satisfactorily with the experimental data obtained from rotating beam fatigue testing.

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 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.