Evaluation and prediction of creep-fatigue lifetime of F82H

• Creep–fatigue tests of F82H at 550 °C clarified εpp constancy and εpc increase with holding time per cycle. • Stress relaxation behavior was independent of holding time per cycle and expressed by a unified constitutive law. • SRP constants for εpc were identified, enabling lifetime prediction within a factor of two. • A predictive model integrating SRP, relaxation law, and hysteresis geometry estimates lifetime for any arbitrary holding time per cycle. F82H, a reduced-activation ferritic/martensitic steel, is a key structural candidate for fusion breeding blankets, where components experience simultaneous high‑temperature creep and cyclic loading. This study evaluates its creep–fatigue (CF) behavior at 550 °C under a total strain range of 1.00% and proposes a predictive framework for CF lifetime estimation. CF tests with compression holding showed progressive lifetime reduction with increasing holding time per cycle. This study focused on two specific types of inelastic strains: plastic strain (εpp), which occurs due to tension and compression loads, and creep strain (εpc), which develops when a material is held under peak stress over time. It revealed that εpp remained constant, while εpc increased with dwell time. Stress relaxation behavior was independent of holding time and described by a unified constitutive relation. Material constants for εpc were identified, enabling the application of the strain‑range partitioning (SRP) method, which accurately reproduced experimental lifetimes within a factor of two. By integrating SRP, stress‑relaxation modeling, and hysteresis‑loop geometry, a master curve was constructed to predict CF lifetime for arbitrary holding times. The approach offers an efficient tool for CF assessment of fusion structural materials.