Abstract:
Diaqnostics of fatigue cracks in aero-engine compressor rotor blades Full Text
B. Zajac, Warszawa (PL)
Most dangerous to operational use of aircraft engines are fatigue cracks that nucleate at locations with difficult access for
testing. A fatigue crack of the first-stage compressor rotor blade occurred during service in one type of aircraft engine, only a
slight distance from the blade bottom surface fastened in the blade locking piece. Such a crack is extremely dangerous as it
can result in an aircraft crash.
The applied flaw detection method must allow to recognize reliably the fatigue cracks at early stages without impairing aircraft
availability.
To solve the problem ultrasonic surface waves were applied using digital flaw detector SOFRATEST and special probe
heads were developed for primary laboratory examinations. To obtain reproducible results artificial standard cuts were made
as reference reflectors in original blades basing on the estimation critical crack size from metallographic examinations. Due to
small dimensions of the reference reflector, high gain level was necessary and complicated geometrical blade form as well as
other disturbing effects created difficulties to recognize real fatigue cracks. To discriminate between fatigue cracks and those
coming from blade geometry Fast Fourier Transform of signals was applied. There is a distinct difference between both kinds
of signal spectrum. Interpretation capabilities have therefore been considerably extended and possibilities for fatigue crack
identification of echoes provided. Basing on this analysis blade inspection procedures were developed and implemented in
practice, to allow the remained safe life prediction.