Characterisation and computational modelling of high temperature cyclic viscoplastic behaviour of turbine steel

The aim of the research was to investigate the cyclic plasticity behaviour of a turbine rotor steel through experiments and to simulate low cycle creep-fatigue using a finite element (FE) modelling framework of materials and structures. The cyclic behaviour of FV566 12% Cr martensitic stainless steel was studied by performing a series of experimental characterisations. Both mechanical results and microscopic studies were explored. Mechanical tests have been facilitated by conducting low cycle fatigue, creep-fatigue interaction, creep-recovery and anhysteretic tests at high temperature. The effect of low cycle fatigue and creep, and their interactions during combined cycling, have been investigated to improve the understanding of the mechanisms responsible for the mechanical responses and material degradation. The microstructure changes, and deformation mechanisms in particular, were investigated through a series of characterisations by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Investigation of the microstructural evolution and the crack development within the steel was done at different life fractions. The TEM images revealed sub-grain coarsening and reduction of dislocation density during the cyclic tests. The material model used in the FE study is a previously developed improved and unified multi-axial temperature and rate-dependent visco-plastic Chaboche-Lemaitre model which takes into account non-linear kinematic and isotropic hardening. The computational methodology is a three-dimensional framework following an implicit formulation and based on a radial return mapping algorithm. The model was implemented into the Abaqus FE code through a user defined subroutine (UMAT). The UMAT was calibrated and validated across isothermal hold-time cyclic tests. The multi-axial form of the constitutive model developed was demonstrated by analysing the thermomechanical responses of an industrial gas turbine rotor subjected to in-service conditions. The damage development within the turbine rotor was demonstrated. Future work of predicting lifetime of components based on the damage mechanisms is discussed