Advanced Fatigue Methodologies (AdFaM), a joint project of European research laboratories, vendors and plant operators, was launched in 2014 to build on the results from recent laboratory studies of fatigue behavior of austenitic stainless steels under NPP-relevant conditions that showed improved lifetimes compared to the best fit to test data presented in NUREG/CR-6909, and to further investigate transferability between specimen test results and the fatigue behavior of NPP components during plant operation.
In particular, AdFaM has focused on an empirical and mechanistic investigation of the effects of hold times on fatigue life. A small number of previous test results suggest an increase in fatigue life for stabilized grades of austenitic stainless steel when hold times (ranging from several hours to days) are introduced into a test between periods of strain-controlled cyclic loading. Tests incorporating hold times may be more representative of material behavior in NPPs, where temperature transients due to start-ups, shutdowns and major power changes may be separated by long periods of steady state operation.
Under AdFaM, fatigue endurance tests incorporating hold times have been completed on stabilized and non-stabilized stainless steel grades (Types 304L and 347) and the mechanisms responsible for the observed variations in fatigue life have been investigated using a range of microscopy techniques.
Results confirm a significant extension of fatigue life due to hold times in both stabilized and non-stabilized grades. Life extension appears to be linked to hold hardening observed in cyclic behavior, and this link has been investigated through microstructural characterization of fatigue specimens examined before and after holding at elevated temperature.
This project helps to improve the understanding of transferability of results from small specimen tests (without hold times) to analysis of NPP components and provides insights that will contribute towards continuing development of fatigue design curves and analysis methods in Design Codes such as ASME Code Section III and KTA 3201/3211.
The AdFaM project is now complete. The valuable results and insights gained from this work demonstrate the significant benefits of collaborative research between various industrial and academic partners in the area of fatigue of NPP materials.