13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 2011 Bussan, Korea 1024944 2012 Hilton Head, USA EPRI Life Management Conference Proceedings (in publication) Analytical evaluation of creep crack initiation has been considered using different methods. The most widely used overall approaches are ASME, Rules for the Construction of Nuclear Facility Components, Class 1 Components in Elevated Temperature Service, Boiler and Pressure Code, Section III, Division 1— Subsection NH [1], TRD 301, Annex I—Design: Calculation for Cyclic Loading due to Pulsating Internal Pressure or Combined Changes of Internal Pressure and Temperature [2], RCC-MR, Design and Construction Rules for Mechanical Components of FBR Nuclear Islands, Section I [3] and R5, An Assessment Procedure for the High Temperature Response of Structures,[4] . Figure 2. Comparison of the results of different methods of analysis of the same data set of creep fatigue data from Grade 91 steel, performed (a) using ASME NH [1] and (b) using RCCMR [3]. The difference in the approaches leads to very different predictions for creep fatigue behaviour. For example, the recommended interaction line for Grade 91 steel originally proposed in ASME – NH was very conservative. At least some of this excessive conservatism arises because this alloy exhibits complex behavior during cyclic tests at high temperature, and use of time fraction without considering stress-relaxation and strain-softening effects yields a very conservative outcome. The level of conservatism using the standard ASME NH approach is demonstrated by consideration of the results shown in Fig 2(a). Clearly, the calculated lives are at least 10 times less than the observed behavior. Application of an improved methodology, such as that developed in the French Code RCCMR, indicates that by accounting for the metallurgical complexities, the prediction comes into reasonable agreement with the observed behavior, see Fig 2(b). 2.2. Detailed Reports The EPRI initiative has focused effort in specific agreed areas. One significant activity has been to facilitate preparation of State of Knowledge and other documents on Creep-Fatigue Damage
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