Impeller cracks are detectable in certain circumstances. A technique which has been successful is 'Peak Vue'; a very haigh frequency signal enveloping system (similar to ESP) but which typically finds 'impact' damage frequencies (often emitted by cracks).
You can find more info by typing 'Peak...
I have recently analysed the 'cyclic and loud' resonance emanating form a Rolls royse gas turbine engine. I eventually diagnosed this as 'stepping'; a well known but rarely found characteristic which occurs at high turbine spool speed. Can anyone tell the precise cause and give me a 'text book'...
