OPTIONS FOR DEFINING THE UPPER SHELF TRANSITION TEMPERATURE (T_c) FOR FERRITIC PRESSURE VESSEL STEELS

摘要

This paper evaluates current guidance concerning conditions under which the analyst is advised to transition from a linear-elastic fracture mechanics (LEFM) based analysis to an elastic-plastic fracture mechanics (EPFM) based analysis of pressure vessel steels. Current guidance concerning the upper-temperature (T_c) for LEFM-based analysis can be found in ASME Section Ⅺ Code Case N-749. Also, while not explicitly stated, an upper-limit on the K_(Ic) value that may be used in LEFM-based evaluations is sometimes taken to be 220 MPam~(1/2) (a value herein referred to as K_(LIM)). Evaluations of T_c and K_(LIM) were performed using a recently compiled collection of toughness models that are being considered for incorporation into a revision to ASME Section Ⅺ Code Case N-830; those models provide a complete definition of all toughness metrics needed to characterize ferritic steel behavior from lower shelf to upper shelf. Based on these evaluations, new definitions of T_c and K_(LIM) are proposed that are fully consistent with the proposed revisions to Code Case N-830 and, thereby, with the underlying fracture toughness data. Formulas that quantify the following values over the ranges of RT_(To) and RT_(NDT) characteristic of ferritic RPV steels are proposed: 1. For T_c, two values, T_(c(LPWER)) and T_(c(UPPER)), are defined that bound the temperature range over which the fracture behavior of ferritic RPV steels transitions from brittle to ductile. Below T_(c(LPWER)). LEFM analysis is acceptable while above T_(c(UPPER)) EPFM analysis is recommended. Between T_(c(LPWER)) and T_(c(UPPER)), the analyst is encouraged to consider EPFM analysis because within this temperature range the competition of the fracture mode combined with the details of a particular analysis suggest that the decision concerning the type of analysis is best made on a case-by-case basis. 2. For K_(LIM), two values, K_(LIM(LOWER)) and K_(LIM(UPPER)), are defined that bound the range of applied-K over which ductile tearing will begin to occur. At applied-K values below K_(LIM(LOWER)), ductile tearing is highly unlikely, so the use of the K_(Ic) curve is appropriate. At applied-K values above K_(LIM(UPPER)) considerable ductile tearing is expected, so the use of the K_(Ic) curve is not appropriate. At applied-K values in between K_(LIM(LOWER)) and K_(LIM(UPPER)) some ductile tearing can be expected, so it is recommended to give consideration to the possible effects of ductile tearing as they may impact the situation being analyzed. These definitions of T_c and K_(LIM) better communicate important information concerning the underlying material and structural behavior to the analyst than do current definitions.