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The University of Southampton
Rifi An Enterprise Consultancy Unit

Case Study: Catastrophic fracture in ship propellers

Porosity in casting
Porosity in casting

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 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.
  • Pore
    Pore
    Pore initiating fatigue
    Pore initiating fatigue

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