Fractured Crankshaft - Could it have been avoided?

Metallurgy and Failure Analysis

16 May 2017

When Intertek's Asset Integrity Management (AIM) metallurgy and failure analysis team is first approached to analyse the failure of a part or structure, a common statement we hear from our clients is "nothing was done differently."

Could this fractured shaft have been avoided?  Let's take a look at the failure and show some of the analyses Intertek routinely conducts. Finally, we'll discuss what the client might have done differently to avoid the damage.

The subject of our analysis is a low-speed cast iron compressor crankshaft that failed unexpectedly. The client advised that "we followed all operating instructions and maintenance after repairs due to a lubrication failure and resulting bearing failure but it still fractured."

Failure mode

Analysis of the fracture face revealed a smooth zone and curved lines on fracture face, known as beach marks, and is characteristic of fatigue cracking.

The fracture at approximately 45o to the shaft axis is characteristic of fatigue due to torsional loading conditions.

Image 1: Fracture face with smooth propagation zone (P) typical of fatigue and a relatively large final fracture area.  The initiation area is arrowed.

Image 2: Bearing journals were crack tested which showed a series of short axial cracks outlined by the rectangle on the figure.

The crankshaft was a ductile cast iron with well-formed spheroidal graphite.

Cracks containing oxides were evident from the surface of the shaft and these contained numerous foreign particles unrelated to the cast iron.

Image 3: Microstructure near the fracture face with well-formed graphite nodules (G) and irregular cracks containing foreign material (arrowed).

X-ray analyses of the foreign material revealed them to be mixtures copper (Cu) and tin (Sn) and others to be particles of lead (Pb).  These elements are typical of journal bearings.

The crankshaft had fractured without bearing seizure yet there was evidence of bearing metal within the cracks next to the fracture consistent with bearing seizure.

The crankshaft had suffered thermal cracking due to an earlier seizure and this initiated fatigue cracking.

If, during repairs, the shaft was crack tested then it would not have been put back into service and the failure would have been avoided. Not only do we perform failure analysis, but Intertek's AIM group also provides testing and inspection, such as crack testing.

Drs. Peter Kentish and Joe Cavallaro have between them decades of experience in corrosion, failure analysis and materials examinations/testing.

They are based at Intertek-AIM, Adelaide, South Australia, Australia.

Dr. Peter Kentish is a Metallurgist with a PhD on the subject of stress corrosion cracking in high pressure gas pipelines. He has extensive experience in failure analyses, corrosion investigations and materials testing for mining, manufacturing, construction, fabrication, energy generation, insurance loss adjusting and as an expert witness in litigation.