Determination of the probability of failure of turbogenerator rotors based on LCF experimentation

Turbogenerator rotors are a typical example of large components experiencing low cycle fatigue (LCF). Every machine switch-on and switch-off corresponds to a LCF cycle, for a total amount of 10,000-15,000 cycles in the whole machine life. The design task of rotors and coil retaining rings (CRRs) involves serious safety issues: incidents caused by unexpected failures may lead to rotor explosion with catastrophic effects. For this reason, the structural analysis must be integrated by the estimation of the probability of failure in the machine life stages, to be fulfilled by a suitable probabilistic method. However, there are very few papers in literature, tackling this issue. The object of this paper is to show a suitable methodology to quantify the safety of a rotor, starting by the knowledge of LCF experimental data and of the nominal loads in the region of the shrink-fit coupling with the CRR. The determined value for the probability of failure at the end of machine life, 5*10^-6, is acceptable with reference to the safety requirements of several structures under fatigue, even in the nuclear field.