Objective: To identify the cause of failure.
Situation
- Two ship propellers were supplied containing a number of cracks and one blade was missing completely.
- The propellers were manufactured by casting Grade Cu3 Ni-aluminium bronze.
- Failure was not attributed to hitting a submerged object or adverse sea conditions.
- One blade had broken off at the root on the port propeller. The remaining blades had no visual cracks apparent.
- On the starboard propeller, cracks were observed at the trailing edges of two blades for about 20-50mm.
What we did
- A full visual examination of the propellers was undertaken together with penetrant testing to reveal the extent of the cracks.
- Detailed fractographic and metallographic examination was performed.
- Mechanical and chemical testing of both propeller materials was undertaken and were found to be within specification.
- Appropriate Lloyds Register rules regarding casting and weld repairs were consulted.
Impact
- RIfI’s work successfully identified the cause of the failure:
- All five cracks had initiated and grown by a fatigue mechanism.
- Four of the cracks had initiated at porosity/lack of fusion at the edge of weld repairs.
- The fifth crack had initiated at a large inclusion/dross formed during the casting process.
- There was no evidence that the inspection routines and stress relieving post repair heat treatment called for by Lloyds Register rules had been performed.
- Based on RIfI’s findings the propellers were scrapped and the evidence proved vital in the insurance claim.