During a maintenance ground run, a turbine engine briefly exceeds its maximum N2 rotor speed limit. Inspection criteria require special attention to the turbine disc because sustained excessive centrifugal and thermal stress can cause which progressive failure mode?
- ACase hardening of the disc surface due to overheating
- BForeign object damage from ingested debris
- CCreep, a gradual plastic deformation and stretching of the disc material that can lead to disc growth or burst
- DFretting corrosion from vibration between mating surfaces
Show answer & explanationAnswer & explanation
Correct answer: C. Creep, a gradual plastic deformation and stretching of the disc material that can lead to disc growth or burst
Creep is a time- and stress-dependent plastic deformation that occurs in turbine discs exposed to high centrifugal loads and elevated temperatures. Overspeed events accelerate creep, potentially causing the disc bore or rim to stretch, which can eventually lead to reduced fatigue life or, in extreme cases, disc failure/burst. This is why overspeed events require special inspection and life-tracking of turbine discs.
Why the other options are wrong
- A. Case hardening is a surface treatment process, not a failure mode from overspeed.
- B. FOD is caused by ingested objects, unrelated to overspeed-induced stress.
- D. Fretting corrosion results from small relative motion between contacting surfaces, not overspeed stress.
Turbine Disc Creep
A time-dependent plastic deformation of turbine disc material caused by prolonged exposure to high centrifugal stress and elevated temperature, which can lead to disc growth or eventual failure.
- Accelerated by overspeed and overtemperature events
- Tracked via life-limited parts programs (cycle counting)
- Can eventually cause disc burst if undetected
Memory trick: Creep: the disc slowly 'stretches like taffy' under heat and spin.