• Title, Summary, Keyword: Crack Growth Rate

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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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    • 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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Fatigue Crack Growth Behavior of Austenite Stainless Steel in PWR Water Conditions (모사원전환경에서 오스테나이트 스테인리스강의 피로균열성장 평가)

  • Min, Ki-Deuk;Lee, Bong-Sang;Kim, Seon-Jin
    • Korean Journal of Materials Research
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    • v.25 no.4
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    • pp.183-190
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    • 2015
  • Fatigue crack growth rate tests were conducted as a function of temperature, dissolved hydrogen (DH) level, and frequency in a simulated PWR environment. Fatigue crack growth rates increased slightly with increasing temperature in air. However, the fatigue crack growth rate did not change with increasing temperature in PWR water conditions. The DH levels did not affect the measured crack growth rate under the given test conditions. At $316^{\circ}C$, oxides were observed on the fatigue crack surface, where the size of the oxide particles was about $0.2{\mu}m$ at 5 ppb. Fatigue crack growth rate increased slightly with decreasing frequency within the frequency range of 0.1 Hz and 10 Hz in PWR water conditions; however, crack growth rate increased considerably at 0.01 Hz. The decrease of the fatigue crack growth rate in PWR water condition is attributed to crack closure resulting from the formation of oxides near the crack tips at a rather fast loading frequency of 10 Hz.

Characteristics of Fatigue Crack Growth in SM570, POSTEN60, 80 Steel (SM570, POSTEN60 및 80 강재의 피로균열성장특성)

  • Jeong, Young-Wha;Kim, lk-Gyeom;Kang, Sung-Lib;Nam, Wang-Hyone;Kim, Eun-Sung
    • Journal of Industrial Technology
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    • v.21 no.A
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    • pp.329-336
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    • 2001
  • In this study, a series of fatigue tests are carried out in order to estimate quantitatively the characteristics of fatigue crack growth rate for high strength steels of SM570, POSTEN60, and POSTEN80 steel, that is, the influence on fatigue crack growth rate according to the welding line, the characteristics of fatigue crack growth according to the welding method and the kinds of steel, and the characteristics of fatigue crack growth for base metal, heat affected zone and weld metal. From the test results, in case that the notch if parallel to welding line, it knows that the retardations of fatigue crack growth rate in crack tip at early stage increase remarkably than in case that the notch is perpendicular to welding line due to compressive residual stress. And the characteristics of fatigue crack growth rate according to welding method are that the dispersion of fatigue crack growth rate in case of FCAW method is smaller than that of SAW method. Also, it knows that the fatigue crack growth rate converges in high stress intensity factor range.

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Fatigue Crack Growth Characteristics of the Pressure Vessel Steel SA 508 Cl. 3 in Various Environments

  • Lee, S. G.;Kim, I. S.;Park, Y. S.;Kim, J. W.;Park, C. Y.
    • Nuclear Engineering and Technology
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    • v.33 no.5
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    • pp.526-538
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    • 2001
  • Fatigue tests in air and in room temperature water were performed to obtain comparable data and stable crack measuring conditions. In air environment, fatigue crack growth rate was increased with increasing temperature due to an increase in crack tip oxidation rate. In room temperature water, the fatigue crack growth rate was faster than in air and crack path varied on loading conditions. In simulated light water reactor (LWR) conditions, there was little environmental effect on the fatigue crack growth rate (FCGR) at low dissolved oxygen or at high loading frequency conditions. While the FCGR was enhanced at high oxygen condition, and the enhancement of crack growth rate increased as loading frequency decreased to a critical value. In fractography, environmentally assisted cracks, such as semi-cleavage and secondary intergranular crack, were found near sulfide inclusions only at high dissolved oxygen and low loading frequency condition. The high crack growth rate was related to environmentally assisted crack. These results indicated that environmentally assisted crack could be formed by the Electrochemical effect in specific loading condition.

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An Analysis of Crack Growth Rate Due to Variation of Fatigue Crack Growth Resistance (피로균열전파저항의 변동성에 의한 균열전파율의 해석)

  • Kim, Seon-Jin
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.23 no.7
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    • pp.1139-1146
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    • 1999
  • Reliability analysis of structures based on fracture mechanics requires knowledge on statistical characteristics of the parameter C and m in the fatigue crack growth law, $da/dN=C({\Delta}K)^m$. The purpose of the present study is to investigate if it is possible to predict fatigue crack growth rate by only the fluctuation of the parameter C. In this study, Paris-Erdogan law is adopted, where the author treat the parameter C as random and m as constant. The fluctuation of crack growth rate is assumed only due to the parameter C. The growth resistance coefficient of material to fatigue crack growth (Z=1/C) was treated as a spatial stochastic process, which varies randomly on the crack path. The theoretical crack growth rates at various stress intensity factor range are discussed. Constant ${\Delta}K$ fatigue crack growth tests were performed on the structural steel, SM45C. The experimental data were analyzed to determine the autocorrelation function and Weibull distributions of the fatigue crack growth resistance. And also, the effect of the parameter m of Paris' law due to variation of fatigue crack growth resistance was discussed.

Influence of overload on the fatigue crack growth retardation and the statistical variation (강의 피로균열지연거동에 미치는 과대하중의 영향과 통계적 변동에 관한 연구)

  • 김선진;남기우;김종훈;이창용;박은희;서상하
    • Journal of Ocean Engineering and Technology
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    • v.11 no.3
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    • pp.76-88
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    • 1997
  • Constant .DELTA.K fatigue crack growth rate experiments were performed by applying an intermediate single and multiple overload for structural steel, SM45C. The purpose of the present study is to investigate the influence of multiple overloads at various stress intensity factor ranges and the effect of statistical variability of crack retardation behavior. The normalized delayed load cycle, delayed crack length and the minimum crack growth rate are increased with increasing baseline stress intensity factor range when the overload ratio and the number of overload application were constant. The crack retardation under low baseline stress intensity factor range increases by increasing the number of overload application, but the minimum crack growth rate decreases by increasing the number of overload application. A strong linear correlation exists between the minimum crack growth rate and the number of overload applications. And, it was observed that the variability in the crack growth retardation behavior are presented, the probability distribution functions of delayed load cycle, delayed crack length and crack growth life are 2-parameter Weibull. The coefficient of variation of delayed load cycle and delayed crack length for the number of 10 overload applications data are 14.8 and 9.2%, respectively.

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Influence of Evaluation Methods to Fatigue Crack Propagation Rate (피로크랙 진전속도에 대한 평가방법의 영향)

  • 최병기;최남식;윤한용
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.17 no.10
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    • pp.2391-2397
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    • 1993
  • The distribution of fatigue crack growth rate is influenced by the measuring interval and methods of calculation of crack growth rate. The purpose of this paper is to suggest a method for the evaluation of crack growth rate distribution without the influence. The constant ${\Delta}P$ test and the constant .DELTA.K test are executed by using the CT specimen. The measuring interval of (${\Delta}a$/W=0.0067~0.014) crack length is not affected by methods of the calculation of the fatigue crack growth rate is suggested.

Finite Element Analysis of Creep Crack Growth Behavior Including Primary Creep Rate (1차 크리프 속도를 고려한 크리프 균열 진전의 유한요소 해석)

  • Choi, Hyeon-Chang
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.23 no.7
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    • pp.1120-1128
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    • 1999
  • An elastic-viscoplastic finite element analysis is performed to investigate detailed growth behavior of creep cracks and the numerical results are compared with experimental results. In Cr-Mo steel stress fields obtained from the crack growth method by mesh translation were compared with both cases that the secondary creep rate is only used as creep material property and the primary creep rate is included. Analytical stress fields, Riedel-Rice(RR) field, Hart-Hui-Riedel(HR) field and Prime(named in here) field, and the results obtained by numerical method were evaluated in details. Time vs. stress at crack tip was showed and crack tip stress fields were plotted. These results were compared with analytical stress fields. There is no difference of stress distribution at remote region between the case of 1st creep rate+2nd creep rate and the case of 2nd creep rate only. In case of slow velocity of crack growth, the effect of 1st creep rate is larger than the one of fast crack growth rate. Stress fields at crack tip region we, in order, Prime field, HR field and RR field from crack tip.

AE Count Rate and Crack Growth Rate under Low Cycle Fatigue Fracture Loading (저주기 피로 파괴 하중하에서 AE수 변화율과 균열성장율에 관한 연구)

  • 이강용
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.13 no.2
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    • pp.252-256
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    • 1989
  • In the low cycle fatigue fracture testing with KS(or JIS) SS41, crack growth rate, AE count rate and J-integral range are measured to get empirical relations between crack growth rate and J-integral range, AE count rate and J-integral range as well as AE count rate and crack growth rate. All the relations are shown to be linear on the log-log graphs. It is also shown that the linear relations can be formulated by using Dunegan's assumption and elastic-plastic fracture mechanics along with the well-known relation of crack growth rate and J-integral range. It is concluded that the differences between experimental and theoretical values are due to Dunegan's assumption.

Influence of Evaluation Methods for Fatigue Crack Growth Process (피로균열 성장과정에 대한 평가방법의 영향)

  • 안철봉
    • Journal of The Korean Society of Manufacturing Technology Engineers
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    • v.8 no.1
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    • pp.119-125
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    • 1999
  • The distribution of fatigue crack growth rate is subjected to the measuring interval and calculated method of growth rate. In this paper, in order to establish the method of determining the distribution of fatigue crack growth rate, which ignores those influences, a series of fatigue crack growth experiments and measuring intervals of crack length calculated reasonable are presented. The main conclusions obtained are summarized as follows: 1) As a result of the ΔP constant test and ΔK constant test, it is thought that an approximate measuring interval of 0.3~0.7mm is reasonable, which allows for few errors and is little subjected to the calculated method of crack growth rate. 2) After generally comparing the error estimation by using the experimental data of CCT specimen with the error rating of the CT specimens, it is possible that the fatigue test has few errors within the measuring interval, ξ(Δa/W)=0.0067~0.017, regardless of the dimension of specimen geometry.

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