During overhaul, a turbine engine disk is inspected using fluorescent penetrant inspection (FPI), and a linear indication is found at a bolt hole in the disk rim. Why is this finding treated with particular concern compared to a similar indication on a non-rotating case component?
- ADisks are inexpensive to replace, so any indication automatically results in scrapping the part
- BFPI indications on rotating parts are always false positives due to residual magnetism
- CThe disk is a life-limited rotating part subject to high centrifugal stress, and a crack can lead to catastrophic uncontained failure
- DThe disk material does not respond well to FPI, making the indication unreliable
Show answer & explanationAnswer & explanation
Correct answer: C. The disk is a life-limited rotating part subject to high centrifugal stress, and a crack can lead to catastrophic uncontained failure
Turbine disks are life-limited rotating components that experience very high centrifugal stress in service; a crack propagating from a stress riser such as a bolt hole can lead to sudden, uncontained disk failure, which is far more catastrophic than a crack in a static case. For this reason, any crack indication in a disk is treated as unacceptable and the part is rejected, unlike some allowable indications on static structure.
Why the other options are wrong
- A. Cost is not the reason for rejection; the safety-critical rotating nature of the part is.
- B. FPI is a dye penetrant method, not affected by magnetism; magnetic particle inspection is the one affected by residual magnetism.
- D. FPI is a standard and reliable method for detecting surface cracks in turbine disk materials.
Turbine Disk FPI Crack Indication
Fluorescent penetrant inspection is used to detect surface cracks in turbine disks; any crack indication in a rotating, life-limited disk is cause for rejection due to the risk of uncontained failure.
- Disks are life-limited rotating parts subject to high centrifugal stress
- Bolt holes and rim areas are common crack initiation (stress riser) sites
- Uncontained disk failure can penetrate the engine case and aircraft structure
Memory trick: A cracked disk is a bomb spinning at thousands of RPM