Small Crack Propagation Behavior of 316FR Stainless Steel under Thermo-Mechanical Fatigue Conditions
Yasuhiro YAMAZAKI, Wataru FUJIKI, Osamu ABE and Yutaro HARA
Abstract:Low-carbon/medium-nitrogen 316 stainless steel (316FR) is a principal candidate for the high-temperature structural materials of a demonstration fast reactor plant. Thermo-mechanical fatigue damage is one of critical issues to be known for the design and reliability of the high-temperature materials subjected to thermal cycles. Early growth of small cracks in order of micron-meters in size can provide some essential information for life and the remaining life prediction to these failures. Thus, many efforts have been made; however, there are a lot of matters to be understood. This paper is dealing with how cracks propagate under thermo-mechanical fatigue conditions. Special attentions are paid to the roles of strain rate and thermal cycles on small crack propagation behaviors in 316FR stainless steel. The experimental results revealed a difference in crack growth rates between the small and long cracks under the creep-fatigue condition: the small cracks exhibited growth rates remarkably higher than long cracks at a given fatigue J integral range. The results also indicated that the small crack propagation rate under the isothermal low cycle fatigue increased with the decreasing of the strain rate due to creep effect. It was also shown from the results that the role of irreversible creep strain, as well as that of irreversible plastic strain, was essential in small crack propagation process under the in-phase type thermo-mechanical loading. Key Words:Naturally initiated small crack, Crack propagation, Thermo-mechanical fatigue, Creep-fatigue, Low-carbon/medium-nitrogen 316 stainless steel