PSI - Issue 2_B

Patrick Mutschler et al. / Procedia Structural Integrity 2 (2016) 801–808 Author name / Structural Integrity Procedia 00 (2016) 000–000 7 where Δ K 1 is the threshold stress intensity factor range for R →1 and t +h is an empirical fit constant for positive R -ratios. The parameters Δ K 1 and t +h from the analytical threshold equation and the main influencing parameters n , C FM , p and q from the FM-equation are optimized for the individual temperatures. The resulting parameters can be described as a function of temperature. Using this functional relationship, the crack growth rate can be described analytically as a function of the R -ratio and the temperature. However, initial results show that there is no uniform trend for individual parameters. Fig. 6b shows the analytical description (dashed lines Fig. 6b) of the experimentally evaluated threshold values (circles in Fig. 6b) on the basis of equation (2). Further, a fit by a third order polynomial for the individual temperatures is presented (surface in Fig. 6b). It is noticeable that the experimentally evaluated threshold values at RT are significantly higher than those at the elevated temperatures. However, for higher R -ratios the threshold value decreases. This R -influence is significantly lower for the elevated temperatures. Further, equation (2) leads to implausible values for T = 300 °C because for higher R -ratios than R = 0.5 the function produces increasing values. The cause for this is the small R -influence at T = 300 °C. 5. Conclusion Due to the flexible energy supply and the subsequently rising number of load cycles for conventional power plants, the fatigue crack growth becomes much more important in the future. Therefore, the experimental data base must be extended with regards to fracture mechanical parameters for relevant temperatures and relevant power plant steels, such as the X20CrMoV12-1. Therefore, crack growth tests were carried out between temperatures T = 300 °C-600 °C. Beside the temperature, the R -ratio, the specimen orientation, the frequency and the normalized K -gradient were investigated. The temperature and the R -ratio have been identified as the main influencing factors. The expected trend that larger R -ratios lead to lower threshold values can be observed. On the Paris-line the R -ratio has almost no influence. At the elevated temperatures, a significant reduction of the threshold value between room temperature and T = 300 °C occurs. The crack growth tests between T = 300 °C and T = 600 °C lead for the same R ratio to almost the same threshold values. At the Paris-area increasing temperatures result in higher crack growth rates. Further, a kinking of the Paris-line and a double “s-shape” is visible for temperatures higher than T = 300 °C. The other investigated parameters have no or just a minor influence on the crack growth rate. For the analytical description of the experimental data quantil curves are determined for the investigated temperatures and R -ratios. For the mathematical description the Forman-Mettu-equation is used. The optimizing process is made with a modified material adjustment program which is programmed in matlab. The results have shown a limited suitability of the FM-equation. Acknowledgements The authors thank the Ministry for Economic Affairs and Energy for funding the joint project THERRI (English: Determination of characteristic values for estimating thermal fatigue crack growth in power plants, German: Ermittlung von Kennwerten zur Bewertung thermischen Ermüdungsrisswachstums in Kraftwerken) Further, the authors greatly acknowledge the support by the partners TÜV NORD SysTec GmbH & Co. KG, the Chair of Technical Thermodynamics at the University of Rostock, the KNG power plant Rostock and the Research Institute Jülich. References NASGRO-Fracture Mechanics and Fatigue Crack Growth Analysis Software (2010). Version 6.0: NASA and Southwest Research Institute. BS 7910, 2013: Guide to methods for assessing the acceptability of flaws in metallic structures. Test Method for Measurement of Fatigue Crack Growth Rates (2015). West Conshohocken, PA: ASTM International. Babu, M. Nani, Sasikala, G., Dutt, B. Shashank, Venugopal, S., Bhaduri, A. K., Jayakumar, T. (2014): Fatigue crack growth behavior of RAFM steel in Paris and threshold regimes at different temperatures. In: Nuclear Engineering and Design 269, S. 103–107. Chaboche, J., Gallerneau, F. (2001): An overview of the damage approach of durability modelling at elevated temperature. In: Fatigue Fract Engng Mater Struct 24, S. 405–418. Chaswal, V., Sasikala, G., Ray, S. K., Mannan, S. L., Raj, B. (2005): Fatigue crack growth mechanism in aged 9Cr–1Mo steel. Threshold and Paris regimes. In: Materials Science and Engineering: A 395 (1-2), S. 251–264. Chen, Q., Kawagoishi, N., Nisitani, H. (2000): Evaluation of fatigue crack growth rate and life prediction of Inconel 718 at room and elevated

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