Description

ASME STP-NU-041 – Update and Improve Subsection NH – Alternative Simplified Creep-Fatigue Design Methods

STP-NU-018 [3] investigated existing creep-fatigue rules to improve the provisions on creep-fatigueevaluation of Mod.9Cr-1Mo steel in Subsection NH of ASME Boiler and Pressure Vessel CodeSection III [1]. All the rules investigated were based on the time fraction rules.This report investigates newly proposed creep-fatigue evaluation methods that have not beenemployed in exiting codes and procedures, using the data on Mod.9Cr-1Mo steel collected in STPNU-018 [3]. This report selected the following five promising methods, all of which are more or lessdifferent from the conventional time fraction method, for the investigation:

Modified Ductility Exhaustion Method (MDEM)
Strain Range Separation Method (SRSM)
Approach for Pressure Vessel Applications (APVA)
Hybrid Method of Time Fraction and Ductility Exhaustion (Hybrid)
Simplified Model Approach (SMT)

This report describes the outline and the predictability of experimental results of the above methods,potential to deploying the methods to NH is investigated from the viewpoints of required database,extrapolation strategy and applicability to structural design which is more complex than predictingmaterial test results. Also mentioned is the impact to the provisions of Subsection NH when thesemethods were to be implemented replacing the current provisions.

This report deals with Mod.9Cr-1Mo steel only. However, it is to be noted that other materials suchas 316 stainless steels including low-carbon nitrogen added versions and Alloy 800H are also ofinterest for the development of New Generation Nuclear Plants.