2.7.1 Long-Term Operation
Due to the long-term operation (LTO) of nuclear power plants (NPPs), some components experience a total number of cycles greater than 105. Improvement of codes and standards could be achieved by introducing high cycle fatigue curves (105 to 109 cycles) and answering the question of endurance limit existence for stainless steel under constant or random loads (NULIFE [Keim2012)].
The effect of hold-times on the fatigue life endurance is an ongoing research field that has for example been investigated in the ADFAM (Advanced Fatigue Methodologies) project to optimize fatigue assessment of critical components launched within NUGENIA.
Different models exist to predict the synergism of creep and fatigue but the limited extent of relevant data sets make robust statistics difficult and fitting long term data is still problematic (MATTER [Utili2012]).