Improving Creep-Fatigue Design Methodology for Advanced Ferritic-Martensitic Steels

Advanced materials are a critical element in the development of advanced sodium-cooled fast reactors. High temperature design methodology of advanced materials is an enabling reactor technology. Removal of unnecessary conservatism in design rules could lead to more flexibility in construction and operation of advanced sodium-cooled fast reactors. Developing mechanistic understanding and predictive models for long-term degradation phenomena such as creep fatigue are essential to the extrapolation of accelerated laboratory data to reactor environments with high confidence, and to improve the American Society of Mechanical Engineers (ASME) code rules. This paper examines the cyclic softening and stress relaxation responses and associated plastic damage accumulation for Grade 91 ferritic-martensitic steel. Creep fatigue experiments were conducted at 550 degrees C in strain controlled mode under various types of creep-fatigue loading conditions. Constitutive models were developed to describe the creep-fatigue interaction in G91.