Failure analysis of a marine intermediate shaft

A bulk-ship was not working normally when anchoring to wait for entering port. Inspection in site found that the intermediate shaft fractured. The intermediate shaft had accumulatively serviced for 43,000 h before fractured. Fractographic investigation indicated that the multiple origins fatigue fracture was the dominant failure mechanism of the shaft. The cracking propagated on two fracture planes at approximately +/- 45 degrees with respect to the shaft axis. The material of failed shaft was found to be a low carbon steel of type ASTM 1020, rather than the specified the medium carbon steel of type ASTM 1035. The average values of yield strength and ultimate strength of shaft material were respectively 16% and 20% lower than the lower limit of technical specification due to low carbon content, as a result of the decreased fatigue strength of shaft material. The fracture occurred at the fillet root of flange connected to the propeller-shaft, where high stress concentration was caused by geometry design. Deep circumferential machining grooves were present on the fillet root of flange for additive stress concentration. The inhomogeneities acted as potential sites for fatigue crack initiation. The low fatigue strength of shaft material promoted initiation and propagation of fatigue cracks in the weaker region under the reversed torsion loading. The static strength analysis of intermediate shaft was performed to assess the design implications. The measured average ultimate strength of failed intermediate shaft was 12% lower than the lower limit in Rule for Classification of Sea-going Steel Ships issued by China Classification Society. The safety factor of the failed intermediate shaft is lower than the permitted one.