• Title/Summary/Keyword: Overload Ratio

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A Study on Fatigue Crack Growth Retardation Phenomena of Al 7075--T6 Alloy under Multiple overload(I) (다중 과하중에 의한 A1 7075-T6 합금의 피로균열 성장지연현상에 관한 연구)

  • 이택순;이유태
    • Journal of Ocean Engineering and Technology
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    • v.6 no.1
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    • pp.96-104
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    • 1992
  • Aircraft structures and engineering structures are always subject to variable amplitude loads. Variable amplitude loads include some kind of loading history; for example, constant amplitude load, single peak overload and block overload etc. Crack growth under variable amplitude loading exhibits retardation effect. In this study, the 4 point bending fatigue test was performed by hydrolic servo fatigue testing machine on 7075-T6 Al-alloy. The retardation effect of overload ratio and numbers of overload cycle was quantitatively studied. 1) Change of retardation effect against increment of overload ratio is more evident when the multiple overload is applied than single overload is done. 2) The number of overload cycle is very important factor about the crack growth retardation effect when the overload ratio is more than 1.75; that is not when the overload ratio is less than 1.75. 3) Overload affected zone size increased gradually by increment of crack growth retardation effect. 4) Crack driving force is more greatly reduced when the crack tip branched off two direction than it sloped to one direction.

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Study on the Retardation Effect of Overload on the Corrosion Fatigue Crack Propagation Al-Alloy used for the Shipbuilding (과하중에 의한 선박용 알루미늄 합금재의 부식피로 파괴지연에 관한 연구)

  • Lim, Uh-Joh;Lee, Jong-Rark;Lee, Jin-Yeol
    • Journal of Ocean Engineering and Technology
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    • v.2 no.2
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    • pp.122-129
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    • 1988
  • Recently with the rapid development in marine and shipbuliding industries such as marine structures, ship and chemical plants, there occurs much interest in the study of corrosion fatigue characteristics which was closed up an important role in mechanical design. In this study, the 5086 Al-alloy was tested by use of rotary bending fatigue tester. The retardation effect of overload on the corrosion fatigue crack propagation in sea environment was quantitatively studied. 1) Retardation effect of corrosion fatigue crack propagation is most eminent when overload ratio is 1.52, overload magnitude corresponds to about 77% and 55% of yield strength and tensile strength respectively. 2) After overload ratio 1.52 was used, retardation of corrosion fatigue crack growth rate is largely retarded and quasi-threshold stress intensity factor range($\Delta\textrm{K}_{th}$) appears. 3) According to m of experimental constant, retardation effect of corrosion fatigue crack propagation corresponds to about 25% of constant stress amplitude when overload ratio is 1.52. 4) When overload ratio 1.52 was used, retardation parameter (RP) decreases to about 0.43 and corrosion sensitivity (S)decreses to about 2.1.

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단일 과대하중에의한 크랙지연 거동에 관한 연구

  • 송삼홍;권윤기
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 1993.04b
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    • pp.328-332
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    • 1993
  • Single overload tests are carried out for SS41. Specimens are used hollow type and solid type that stress condition prevail plane stress and plane strain at surface crack. According to the crack initiation and propagation period, single overload applies to three regime and overload ratios change into 1.5, 2.0. Crack retardation zones at initation range aren't different in both specimens respectively, however at propagation range come into large scale in hollow specimen. Delayed load cycles come into large scale in solid type. And the more the overload ratio increase, the larger the retardation zone increase but the magnitude doesn't exactly equal to the expected from the overload ratio.

A Study on the Effect of the Overload Ratio on the Fatigue Crack Growth Retardation (과대하중비가 균열성장지연에 미치는 영향에 관한 연구)

  • Kim, Kyung-Su;Kim, Sung-Chan;Shim, Chun-Sik;Park, Jin-Young;Cho, Hyung-Min
    • Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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    • 2003.05a
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    • pp.306-311
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    • 2003
  • A growing fatigue crack is known to be retarded on application of an overload cycle. The retardation may be characterized by the total number of cycles involved during retardation and the retarded crack length. The overload ratio plays an important role to influence the retardation behavior. The objective of the present investigation is to study the effect of different overload ratio on the retardation behavior. For DENT(double edge notched tension) specimens and ESET(eccentrically-loaded single edge crack tension) specimens, fatigue crack growth tests are conducted under cyclic constant-amplitude loading including a single tensile overloading with different overload ratios. The proposed crack retardation model predicts crack growth retardation due to a single tensile overloading. The predictions are put into comparison with the experimental results to confirm the reliability of this model.

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The Effect of Stress Ratio on Fatigue Crack Propagation Rate and Arrest Behavior in 7075-T735 Al Alloy (7075-T735 Al 합금의 피로균열 진전속도와 정류거동에 미치는 응력비의 영향)

  • 오세욱;강상훈;허정원;김태형
    • Journal of Ocean Engineering and Technology
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    • v.6 no.1
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    • pp.131-139
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    • 1992
  • The understanding and appllication of fatigue crack propagation mechanism in variable amplitude loading is very important for life prediction of the air travel structures. Particularly, the retardation and arrest behavior of fatigue crack propagation by single tension overloading is essential to the understanding and appllication of fatigue crack propagation mechanism in variable amplitude loading. Numerous studies of the retardation behavior have been performed, however investigations of the arrest behavior have not been enough yet. As for the arrest behavior, Willenborg had reported that the overload shut-off ratio $[R_{so}=(K_{OL})/K_{max})_{crack arrest}]$ had been the material constant, but recently several investigators have reported that the overload shut-off ratio depends upon the stress ratio. In this study, authors have investigated the effect of stress ratio on the threshold overload shut-off ratio to generate arrest of fatigue crack growth in high tensile aluminum alloy 7075-T735 which have used in material for air travel structures, It has been $-0.4\leqqR\leqq0.4$ till now, the region of stress ratio investigated. The threshold overload shut-off ratio has decreased as stress ratio has increased in overall region of -$-0.4\leqqR\leqq0.4$ and the linearity has been seen in this material. Moreover, the experimental equation between $R_{so}$ and R has been made; The relation has been $R_{so}=-R+2.6$.

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Effect of Single Overload on the Fatigue Crack Growth Behavior of Laser Welded Sheet Metal (단일 과대하중에 의한 레이저 용접 판재의 피로균열 전파거동)

  • 곽대순;김석환;오택열
    • Journal of the Korean Society for Precision Engineering
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    • v.21 no.4
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    • pp.161-169
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    • 2004
  • In this study, we investigated fatigue crack growth behavior of laser welded sheet metal due to a single overload. Fatigue specimens were made using butt joint of cold rolled sheet metal that was welded by $CO_2$ laser. The fatigue crack propagation tests were performed in such a way that fatigue loading was parallel to the weld line while crack propagation was perpendicular to the weld line. Single overload was applied when fatigue crack tip was arrived near the weld line. The distances between the crack tip and the weld line at which a single overload was applied were 6, 4 and 2mm. The effect of specimen thickness and overload ratio on the fatigue behavior was determined. The plastic zone size of crack tip due to the single overload was determined from the finite element analysis. For investigating fatigue crack growth behavior, we used different thickness specimen 0.9mm and 2.0mm, and variable overload ratio applied fatigue crack propagation test. Also we used finite element analysis for investigating the plastic zone size of crack tip when single overload applied

Effect of Anisotropy on Fatigue Crack Propagation Rate and Arrest Behavior with 2024-T3 Alumunum Alloy (2024-T3 A1 합금의 이방성이 피로균열진전속도와 정류거동에 미치는 영향)

  • 오세욱;김태형;오정종
    • Journal of Ocean Engineering and Technology
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    • v.7 no.1
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    • pp.124-132
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    • 1993
  • In order to examine the effect of anisotropy and stress ratio on fatigue crack propagation rate and opening-closing behavior and also arrest behavior by single tension peak overload, the fatigue tests of constant amplitude atress and single tension peak overload adding to cycle of constant amplitude were carried out in stress ratio of -0.4, -0.2, and 0.4 with materials of T-L and L-T directions in 2024-T3 aluminum alloy plate. Crack opening-closing begavior were measured by the compliance method using COD gage and strain gage. In case of the crack opening-closing behavior was measured by strain gage, the effect of stress ratio is unchangeable. But in the case of COD gage, that is remarkably decreased. Fictitious effective stress intensity factor(U sub(f)) and effective stress intensity factor ratio(U) in L-T direction was higher than those in T-L direction and also threshold arrest overload ratio incrased as stress ratio decreased and that of T-L direction was higher than that in L-T direction.

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Prediction of Crack Growth Retardation Behavior by Single Overload (단일 과대 하중에 의한 균열 성장 지연 거동 예측)

  • 송삼흥;최진호;김기석
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 1996.11a
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    • pp.928-932
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    • 1996
  • Single overload fatigue tests with overload sizes ranging from 50% and 100% have been performed to investing ate the fatigue crack growth retardation behavior. A modified and experimental method of Willenborg's model for prediction of crack growth retardation behavior has been developed, based on evaluations of equivalent plastic zone size (EPZS) changing its size along the overload plastic zone boundary. The minimum crack growth rates of each overload size are linearly decreased with overload size increasing, but fatigue lives extended by single overload are increasing much more unlike the crack growth rates. Comparisons of crack growth behavior predicted by EPZS model and Willenborg model have shown that the EPZS model accounts for overload effects better than Willenborg model. These effects include delayed retardation, large retardation region, minimum crack growth rate, and the increase rate of crack growth rate in the region crack growth rate recovered.

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A Study on Crack Retardation Behavior by Single Overload (단일 과대하중에 의한 균열지연거동에 관한 연구)

  • 송삼홍;권윤기
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.2
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    • pp.451-462
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    • 1995
  • Single overload tests performed to examine the crack retardation behavior for the specimen thickness and overload ratios. Delayed crack length was tend to increase in small thickness and big overload ratio but was difference between delayed crack length and plastic zone size that expected in specimen thickness. So retardation behavior that estimated in plastic zone size, was not sufficient. Crack tip branching and striation distribution, secondary mechanisms that effected in retardation behavior, was examined by experiment and finite element analysis. Crack tip branching was affected by micro structure, and appeared the more complicatedly according to increasing damage by overload and decreasing crack driving force in base line stress level. And crack tip branching the branching angle decreased crack driving force in the crack tip. And a characteristic of the fractography on retardation zone was that striation distribution did not appear due to decreased crack driving force.

An Experimental Study on the Fatigue Crack Propagation Behavior Under Mixed-Mode Single Overload (혼합모드 단일과대하중 하의 피로균열 전파거동에 관한 실험적 연구)

  • Song, Sam-Hong;Lee, Jeong-Moo;Hong, Suck-Pyo
    • Proceedings of the KSME Conference
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    • 2003.04a
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    • pp.119-124
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    • 2003
  • In this study, retardation behavior of fatigue crack under single overloading of the mixed mode I+II state was experimentally investigated. To produce single overload in the mixed mode I+II state, the compact tension shear (CTS) specimen and loading device were used. The propagation tests for fatigue crack were performed under mode I loading overloading afterwards. We examined the observed deformation aspects, variation of fatigue life and crack propagation rate, and the aspects of retardation behavior from tests. The retardation effect of mixed-mode single overload on fatigue crack propagation behavior was smaller than that of mode I single overload. It has been confirmed that the retardation behavior did not immediately appear and the retardation length was short when the component of mixed-mode overload was changed.

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