CREEP-FATIGUE LIFE ASSESMENT TECHNIQUE FOR STEAM TURBINE ROTORS: AN EFFICIENT CONTINUUM DAMAGE MECHANICS BASED APPROACH

Incorporation of renewable energy, in the era of energy transition, lead to enhanced creep-fatigue life consumption of conventional power plant components, e.g., steam turbine rotors. The present requirements are difficult to address with the conventional uncoupled life assessment methods due to their inherent conservatism. The use of damage coupled unified constitutive models for the analysis of steam turbine rotors in the past decade has shown promising results. High computational efforts is a major drawback of these models. The methods available for reduction in computation time are based on fixed start-up conditions repeating cyclically, however, flex operation result in arbitrary or non-fixed cycle types. In this paper, novel techniques of mapping the data generated from temperature measurements to derive inelastic strain history for arbitrary cycle types is proposed. Unlike the conventional uncoupled techniques utilizing stress/strain ranges, the derived strain history enables a path dependent progressive creep-fatigue damage prediction employing unified constitutive models. The derived strain history is used together with the concept of Representative Input Cycle using a non-iterative Asymptotic Numerical Method for highly efficient damage computation. Various transient loadings on a steam turbine rotor are studied using a unified constitutive model based on Chaboche's kinematic hardening including the damage parameter, Chaboche-Rousselier's isotropic hardening model including the damage parameter, Norton's viscoplastic flow model, Lemaitre's damage potential function and Kachanov-Rabotnov's creep damage law. First, a conventional full finite element method is used and then the proposed method is employed to study the evolution of damage and compared.