Abstract
Enhanced life assessment methods contribute to the long-term operation of high-temperature components by reducing technical risks and increasing economic benefits. This study investigates creep-fatigue behavior under multi-stage loading, including cold start, warm start, and hot start cycles, as seen in medium-loaded power plants. During hold times, creep and stress relaxation accelerate crack initiation. Creep-fatigue life can be estimated using a modified damage accumulation rule that incorporates the fatigue fraction rule for fatigue damage and the life fraction rule for creep damage while accounting for mean stress effects, internal stress, and creep-fatigue interaction. In addition to generating advanced creep, fatigue, and creep-fatigue data, scatter band analyses are necessary to establish design curves and lower-bound properties. To improve life prediction methods, further advancements in deformation and lifetime modeling are essential. Verification requires complex experiments under variable creep conditions and multi-stage creep-fatigue interactions. A key challenge remains the development of methods to translate uniaxial material properties to multiaxial loading scenarios. Additionally, this study introduces a constitutive material model, implemented as a user subroutine for finite element applications, to simulate start-up and shut-down phases of components. Material parameter identification has been achieved using neural networks.
