Objective: To identify the cause of failure and suggest remedial actions.
Situation
- The photographs show a failed stud from the cylinder head of a cruise liner engine.
- Fracture had occurred in the first engaged thread and ran transverse to the axis of the stud.
- The studs are torqued to 900 bar. Washers are not fitted. The seating area is shown above.
What we did
- The chemical composition and mechanical properties of the studs were all within specification.
- Visual inspection revealed beach marks which on the fracture surface indicated that the failure was by fatigue.
- Areas of polishing were observed towards the edges of the fracture surface. This suggests movement between the seating face on the nut and its mating face.
- It was thought that either the pre-load was not adequate or the studs had loosened during service.
- Separation of the flange members results in the studs carrying the entire fluctuating load from combustion which is above their design load.
Impact
- Separation of the flange assembly caused the overloading and subsequent fatigue failure of the studs either by insufficient pre-load or loosening of the studs during service.
- Future fitting recommendations were:
- To clean the upper seating area to remove carbon deposits so that they could not affect the pre-load.
- To secure the nuts with lockwire, to prevent them unscrewing and thus releasing the pre-load.
- The above recommendations were written into the head fitting procedure and not further problems were encountered.