• Title/Summary/Keyword: Crack Retardation

Search Result 101, Processing Time 0.026 seconds

Fatigue Crack Retardation and Retardation Mechanism in Variable Loading (The Effects of Crack Tip Branching in Crack Growth Retardation)

  • Song, Sam-Hong;Kwon, Yun-Ki
    • International Journal of Precision Engineering and Manufacturing
    • /
    • v.3 no.3
    • /
    • pp.76-81
    • /
    • 2002
  • In order to study the fatigue crack and retardation mechanism in variable loading, the effects of crack tip branching in crack growth retardation were examined. The characteristics of crack tip branching behavior were considered with respect to microstructure and crack tip branching angle was examined. Crack tip branching was observed along the grain boundary of finite and pearlite structure. It was found that the branching angle ranges from 25 to 53 degrees. Using the finite element method, the variable of crack driving farce to branching angle was examined. The effective crack driving farce (K$\_$eff/) decreased as the branching angle increased. The rate of decrease was 33% for kinked type and 29% for forked one. It was confirmed that the effect of crack tip branching is a very important factor in crack growth retardation. Therefore, crack branching effect should be considered in building the hypothetical model to predict crack growth retardation.

A Stochastic Analysis for Crack Growth Retardation Behavior and Prediction of Retardation Cycle Under Single Overload (단일과대하중하에서 피로균열진전지연거동 및 지연수명의 확률론적 해석)

  • Shim, Dong-Suk;Kim, Jung-Kyu
    • Transactions of the Korean Society of Mechanical Engineers A
    • /
    • v.23 no.7 s.166
    • /
    • pp.1164-1172
    • /
    • 1999
  • In this study, to investigate the fatigue crack retardation behavior and the variability of retardation cycles, fatigue crack growth tests were conducted on 7075-T6 aluminum alloy under single tensile overload. A retardation coefficient, D was introduced to describe fatigue crack retardation behavior and a random variable, Z to describe the variability of fatigue crack growth. The retardation coefficient was separately formulated according to retardation behavior which is composed of delayed retardation part and retardation part. The random variable, Z was evaluated from experimental data which was obtained from fatigue crack growth tests under constant amplitude load. Using these variables, a probabilistic model was developed on the basis of the modified Forman's equation, and retardation behavior and cycles were predicted under certain overload condition. The predicted retardation curve well agrees with the trend of experimental crack retardation behavior. And this model well predicts the scatter of experimental retardation cycles.

A Study on Fatigue Crack Retardation and Retardation Mechanism in Variable Loading (변동하중하에서의 피로크랙 지연현상과 지연기구에 관한 연구 - 균열성장 지연현상에 미치는 균열 가지의 영향 -)

  • Song, S.H.;Kwon, Y.K.
    • Journal of the Korean Society for Precision Engineering
    • /
    • v.14 no.6
    • /
    • pp.83-89
    • /
    • 1997
  • In order to study on fatigue crack retardation and retardation mechanism in variable loading, the effects of crack tip branching in fatigue crack growth retardation were examined. The characteristics of crack tip banching behavior was considered to micro structure. It was examined that the variation of crack tip branching angle. Crack tip branching was observed along the grain boundary of ferrite and pearlite structure. It was found that the abanching angle ranges from 25 to 53 degrees. Using the finite element method, the variable of crack driving force to branching angle was examined. The effective crack driving force ( $K_{\eff}$ ) decreased as the braching angle increases. The rate of decrease was 33% for the kinked type and 29% for the forked one. It was confirmed that the effect of crack tip branching is a very important factor in fatigue crack growth retardation. Therefore, crack branching effect should be considered building the hypoth- etical model to predict crack growth retardation.

  • PDF

The Prediction of Crack Growth Retardation Behavior by Crack Tip Branching Effects (Fatigue Behavior in variable Loading Condition) (균열가지 효과를 고려한 균열 성장 지연 거동 예측 (변동하중하에서의 피로거동))

  • 권윤기
    • Journal of the Korean Society of Manufacturing Technology Engineers
    • /
    • v.8 no.2
    • /
    • pp.126-136
    • /
    • 1999
  • We studied on crack growth retardation in single overloading condition. Crack tip branching which as the second mechanism on crack growth retardation was examined. Crack tip branching was observed to kinked type and forked type. It was found that the branching angle range was from 25 to 53 degree. The variations of crack driving force with branching angle were calculated with finite element method The variation of {{{{ KAPPA _I}}}}, {{{{ KAPPA _II}}}} and total crack driving force(K) were examined respectively So {{{{ KAPPA _I}}}}, {{{{ KAPPA _II}}}} and K mean to mode I, II and total crack driving force. Present model(Willenborg's model) for crack growth retardation prediction was modified to take into consideration the effects of crack tip branching When we predicted retardation with modified model. it was confirmed that predicted and experimental results coincided with well each other.

  • PDF

Crack Retardation byt Load Reduction During Fatigue Crack Propagation (피로균열전파 동안 하중감소에 의한 균열지연)

  • Kim, Hyun-Soo;Nam, Ki-Woo;Ahn, Seok-Hwan;Do, Jae-Yoon
    • Transactions of the Korean Society of Mechanical Engineers A
    • /
    • v.27 no.12
    • /
    • pp.2004-2010
    • /
    • 2003
  • Fracture life and crack retardation behavior were examined experimentally using CT specimens of aluminum alloy 5083. Crack retardation life and fracture life were a wide difference. between 0.8 and 0.6 in proportion to ratio of load reduction. The wheeler model retardation parameter was used successfully to predict crack growth behavior. By using a crack propagation rule, prediction of fracture life can be evaluated quantitatively. A statistical approach based on Weibull distribution was applied to the test data to evaluate the dispersion in the retardation life and fracture life by the change of load reduction.

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
    • /
    • v.6 no.1
    • /
    • pp.96-104
    • /
    • 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.

  • PDF

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
    • /
    • 2003.05a
    • /
    • pp.306-311
    • /
    • 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.

  • PDF

Fatigue Crack Growth Retardation after Single Overload Cycle in High Strengh Aluminium Weldments (고강도 알루미늄 합금 용접부에 있어서의 피로균열전파에 미치는 과하중 효과)

  • 이택순;김상태;김인식
    • Journal of Welding and Joining
    • /
    • v.6 no.1
    • /
    • pp.46-52
    • /
    • 1988
  • Retardation or delay in fatigue crack growth due to overloads are important for the accurate prediction of fatigue lives of structural materials. In this study, retardation of fatigue crack growth in Al 6061-T6 weldments and heat affected zones (HAZ) after single overload cycle had been investigated. Retardation in both weldments and HAZ was observed. It was concluded that retardation in both weldment and HAZ was greater than in base metal due to microstructural change and crack branching and crack closure were major governing factor in retardation.

  • PDF

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
    • /
    • v.19 no.2
    • /
    • pp.451-462
    • /
    • 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.

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
    • /
    • 1996.11a
    • /
    • pp.928-932
    • /
    • 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.

  • PDF