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

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 MPa root m (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 RTTO and RTNDT characteristic of ferritic RPV steels are proposed: For T-c, two values, T-c(LOWER) 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(LOWER), LEFM analysis is acceptable while above T-c(UPPER) EPFM analysis is recommended. Between T-c(LOWER) 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. 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.