Development of Load Multipliers (Z-Factors) in Elastic-Plastic Fracture Mechanics Evaluation in the JSME Rules on Fitness-for-Service for Nuclear Power Plants

Abstract

Appendix E-9, “Elastic-Plastic Fracture Mechanics Evaluation,” in the JSME Rules on Fitness-for-Service for Nuclear Power Plants (Codes for Nuclear Power Generation Facilities) uses the load multiplier Z-factor that is applied to elastic-plastic fracture mechanics evaluation for a pipe circumferential flaw of austenitic stainless steel piping and ferritic steel piping. The Z-factor is defined as the ratio of the limit load to the load at tearing instability. Here, the Z-factor of the current JSME Rules on Fitness-for-Service for Nuclear Power Plants was developed for a through-wall pipe circumferential flaw with the angle of 60°. In Code Case JSME S NA-CC-002, ”Alternative Rule of Criteria on Allowable Flaw Angle for Circumferential Crack” published in 2005, the Z-factor equations are applicable to a circumferential surface flaw including the angle of greater than 60°, but the Z-factor equations of the Code Case were conservatively formulated as upper-bounded those Z-factors for through-wall pipe circumferential cracks including the angle of greater than 60°. However, the original purpose of the Code Case was to eliminate the limitation of flaw angle and evaluate non-through-wall flaws. Accordingly, Z-factors for non-through-wall flaws should be developed to have the consistency. Therefore, Z-factor equations of austenitic stainless steel piping and ferritic steel piping have been developed for non-through-wall flaws.