13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- results, it can be noticed that specimens with a hold time of 6 min and 60 min have similar crack growth rate as samples without holding time, regardless of which of the two parameters describing the crack behaviour is used. Contrary to the influence of hold time on crack initiation almost no influence is observed on crack propagation. Hence, cyclic loading has only an influence on the crack behaviour in the initial phase. In the holding period creep loading is dominant. Metallographic analyses show that the crack propagates inter-granularly under creep load and a large amount of cavities are found on the prior austenite grain boundaries near to the crack tip. Under fatigue loading trans-granular cracks without cavities were observed. For the samples tested under creep-fatigue loading the crack behaviour was found to be independent of the holding time. Around the crack tip an area of high cavity density was identified and the crack path under creep-fatigue loading is comparable to the crack path under creep condition. The crack propagation under creep-fatigue loading is influenced by two failure mechanisms, and therefore depends on the load level, the mean stress and the temperature. With decreasing frequency the mechanisms changes from cycle dependent at high frequencies to time dependent at lower frequencies. In literature, different relationships exist to describe the crack growth under creep-fatigue loading, which were investigated in this study. It has been shown that the accumulation rule based on stress intensity parameter KI better describes the crack growth behaviour of P91 under creep-fatigue loading. If the creep portion is described by parameter C*, the crack growth behaviour is overestimated. This may be the effect of the relatively large scatter of experiments data, or may result from uncertainties in the determination of C*. Acknowledgements The authors gratefully acknowledge the financial support provided by EPRI, Charlotte, USA. References [1] DIN EN10216: 09 Seamless steel tubes for pressure purposes - Technical delivery conditions - Part 1: Non-alloy steel tubes with specified room temperature properties; German version prEN 10216-1:2009. [2] H. Theofel, K. Maile, Untersuchung einer artgleichen Schweißverbindung für 9%Cr1%Mo-Stähle unter besonderer Berücksichtigung des Langzeitkriechverhaltens, AiF Report Nr. 9300, 1997. [3] ASTM E 1457-07, Standard Test Method for Measurement of Creep Crack Growth Rates in Metals, 2007. [4] ASTM E 2760-10, Standard Test Method for Creep-Fatigue Crack Growth Testing, 2010. [4] H. Riedel, Fracture at High Temperatures, Springer-Verlag, 1987. [6] P.C. Paris, M.P. Gomez, W.E. Anderson, A Rational Analytic Theory of Fatigue, The Trend in Engineering, 13 (1961) 9-14. [7] C. Berger, E. Roos, et al., Rissverhalten typischer warmfester Kraftwerksbaustähle im Kriechermüdungsbereich, Final report of AiF-Project Nr. 10395, 1999. [8] E. Roos, C. Berger et al., Kriech- und Kriechermüdungsrissverhalten moderner Kraftwerkstähle im Langzeitbereich, Final report of AVIF-Project Nr. A178, 2006. [9] Validation, Expansion and Standardisation of Procedures for High Temperature Defect Assessment (HIDA), Brite/Euram Project: BE1704, Final Report 1999. [10]M. Pfaffelhuber, M. Rödig, F. Schubert, H. Nickel, Risswachstum unter überlagerter Kriech- und Ermüdungsbelastung in X10NiAlTi 32 20 (Alloy 800), Report of Kernforschungsanlage Jülich Nr. 2303, 1989. [11] R. Viswanathan, Damage Mechanisms and Life Assessment of High-Temperature Components, AMS International, 1989. [12]J.M. Larson, Th. Nicholas, Cumulative-Damage Modelling of Fatigue Crack Growth in Turbine Engine Materials, Engineering Fracture Mechanics, 22 (1985) 713-730.
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