Life assessment of multiaxially cyclic loaded turbine components

Abstract

ABSTRACTRotors of turbines are exposed to high thermo‐mechanical loads, especially during start‐up and shut‐down procedures. Especially in the notched region of the blade‐connection to the rotor a complex multiaxial elastic‐plastic stress state is present. The reason is the multi‐surface contact in combination with fatigue and creep‐fatigue load. Within the scope of this work the blade‐disc‐connection of a gas turbine made of the nickel‐based alloys IN792 (blades) and IN718 (disc) was investigated at typical local service temperature of 550 °C. The aim was the investigation of mutual influence of adjoining blades as well as the effect on cycles to failure by variation of friction behaviour due to different coatings on the surface of the blade root. An already existing life assessment method based on a local strain approach could be confirmed for this more complex loading situation.In contrast to this, a blade‐shaft‐connection of a steam turbine made of X12CrMoWVNbN10‐1‐1 was investigated at its typical service temperature of 600 °C. During cyclic loading, the martensitic 10Cr steel reacts with ongoing cyclic softening. The fundamental result of the work discussed here is the development of a new life assessment method based on a damage parameter, which is a function of accumulated plastic strain. This damage parameter is included in a complex material model considering static, cyclic and viscoplastic materials behaviour. Thus, the life assessment method could be verified with inelastic finite element calculations of the model specimens, which were especially designed for the low‐cycle fatigue tests of blade‐shaft‐connection presented in this paper.