New time-independent formulation for creep damage in polycrystalline metals and its specialisation to high alloy steel for high-temperature applications

In this work, a new time-independent, micromechanism based, creep damage formulation, derived in the framework of the continuum damage mechanics (CDM) approach to creep proposed by Bonora et al., has been developed and specialized for high chromium steel grades. One of the major features of the proposed formulation is that the kinetic law of creep damage evolution depends only on the accumulated inelastic strain and stress triaxiality and not on time needed to reach a given creep deformation level. The effects associated with the microstructure evolution during high-temperature service, such as the nucleation, growth and coarsening of secondary phases, have been also taken into account through the definition of an evolution law for damage-model parameters. A procedure for the identification of the damage-model parameters using short-duration creep-test data (<10,000 h) has been developed. Long-term creep lives, up and beyond 50,000 h, in the temperature range 550-650 degrees C have been successfully predicted for ASTM steel grade 92 with high degree of accuracy.