Session: CS-07-04 Recent Developments in ASME Codes and Standards - 4 (with CS-08)

Paper Number: 180249

180249 - Technical Basis for Code Case of Alternative Design Fatigue Curves Using Tensile Strength as a Variable 

Abstract: 

The US NRC and the Argonne National Laboratory developed new design fatigue curves in NUREG/CR-6909 and design factors of 2 on stress and 12 on cycles were proposed. The design fatigue curve for austenitic stainless steels from NUREG/CR-6909 was incorporated into the 2009 Addenda of the ASME B&PV Code Section III, and those for carbon steels and low alloy steels were incorporated into the 2025 Edition of the ASME B&PV Code Section III.
The design factor of 2 on stress was proposed at the original ASME Section III and VIII Division 2, and it is believed to be overly-conservative. Also, the design factor of 12 from NUREG/CR-6909 can be considered as overly-conservative because this factor is uniformly applied even if one or more of the parameters considered in the statistical analysis can be ignored. On the other hand, EN 13445 follows a more rational approach, explicitly treating each correction factor as necessary. 
The necessity of a new fatigue analysis method to obtain accurate fatigue life with an appropriate level of conservatism was recognized in Japan and the Design Fatigue Curve (DFC) Subcommittee was established in the Atomic Energy Research Committee in the JWES from August 2011 to March 2013 (Phase 1) and the DFC Subcommittee Phase 2 was followed from April 2013 to March 2016. That work proposed new design fatigue curves based upon best-fit curves using tensile strength as a variable, improving the predictive nature and reducing remaining scatter. Then, following the principle of explicitly treating all necessary correction factors, design fatigue curves are developed based on the statistical analysis of the fatigue database, assessment of the effects of surface finish, mean stress correction by the Smith-Watson-Topper approach, and separately accounting for the variable amplitude loading effect. After that, this outcome of the DFC/DFC2 committees was provided to the JSME Committee on Power Generation Facility Codes, and the JSME Committee approved the incorporation of the new design fatigue curve methodology with some modifications into the JSME Environmental Fatigue Evaluation 2022 Edition. 
The new design fatigue curve methodology of the JSME Environmental Fatigue Evaluation were proposed to Working Group on Fatigue Strength of the ASME BPV III, incorporating a development to rectify a historic issue with how mean stress effects are accounted for in a pseudostress-vs-life curve, which had previously been mitigated by over-conservatism elsewhere in the construction. The resulting code case has been approved by BPV III. The typical features of the code case are as follows.
- The best-fit curves using tensile strength as a variable developed based on large databases
- The design factors developed based on large databases and 95% probability of survival
- Surface finish effects on stress for carbon & low alloy steels and on cycles for austenitic stainless steels
- The Smith-Watson-Topper approach for mean stress correction appropriately applied to a pseudostress-vs-life curve validated by testing.
- Variable amplitude loading effect
 

Presenting Author: Seiji Asada Mitsubishi Heavy Industires, Ltd.

Presenting Author Biography: - Graduated from Kobe University in March 1987
- Master’s degree from Kobe University in March 1989
- PhD from Kobe University in March 2009
- Joined Mitsubishi Heavy Industries in April 1989.
- Since joined in Mitsubishi heavy Industries, He has been in charge of designing and development of Reactor Pressure Vessels. Especially He has made a lot of effort at stress analysis, fatigue analysis, irradiation embrittlement issue of RPVs, fracture mechanics, and so on.
- Chair of SDO (Standard Development Organization) Convergence Board
- Member: [ASME Code Committee] BPV III, SG on Component Design, WG on Environmental Fatigue Evaluation Method
[JSME] Subcommittee on Nuclear, SG on Design and Construction, SG on Fatigue Evaluation, and so on.

Authors:

Seiji Asada Mitsubishi Heavy Industires, Ltd.
Andrew Morley Rolls-Royce
Chris Currie Rolls-Royce