Damage-tolerance-based life prediction of aeroengine compressor discs: II. A probabilistic fracture mechanics approach

Abstract

This paper reports the results of a demonstration computer program carried out to determine the influence of the sensitivity and reliability of non-destructive inspection (NDI) techniques on the damage-tolerance-based life assessment of aeroengine compressor discs. The program was carried out on AM-355 martensitic stainless-steel compressor discs from an aeroengine. The safe inspection intervals (SII) for the compressor discs are calculated using probabilistic fracture mechanics (PFM) principles. These calculations involve the use of the NDI data, finite-element analysis and the experimental fatigue crack growth rate (FCGR) data base generated on compact tension specimens machined from safe-life-expired discs. The PFM analysis, which randomly uses the distribution of undetected crack sizes for a given NDI technique to choose an a i value for fracture mechanics calculations, demonstrates that the worst-case combinations of a i and FCGR do not occur during 7000 disc simulations. The PFM results also indicate that it may be possible to obtain cost effective SII if the sensitive eddy current and the automated ultrasonic leaky surface wave techniques are used to inspect the discs. However, the behaviour of short cracks would have to be characterized prior to obtaining usable PFM-based SII.