Structural integrity assessment of turbine discs in presence of potential defects: Probabilistic analysis and implementation

Rotor discs for gas turbines are heavy components usually designed following a safe-life approach, where the low-cycle fatigue design is based on life curves with suitable probabilistic margins. However, in such a component, there is the possible occurrence of undetected defects that can propagate at each startup-shutdown cycle, and therefore, defect acceptance criteria have to be defined for the different rotor regions (considering stress distribution and temperature). Moreover, the failure risk should be evaluated considering static fracture at overspeed as a potential failure mode. In order to make such an assessment for a turbine rotor disc, a semi-probabilistic approach for the integrity assessment-based current standards has been developed. The approach has then been implemented into a code able to process axis-symmetric Finite Element (FE) analyses of the disc and to produce maps for the expected life (for a given defect size) and for the acceptable defect (for a target life). The probabilistic assessment of turbine rotor discs is first discussed together with a sensitivity study about the most significant calculation assumptions. Then, details of the semi-probabilistic code are shown together with application to a turbine disc.