Mill Drive Shaft Failure
A chemical company provided two sections of a drive shaft to determine the cause of a fracture that developed in the tapered, keyway area.
The failure consisted of a transverse fracture through the shaft diameter near the keyway center, coupled with a peeling failure in the keyway that progressed around and along the shaft.
The various fracture surfaces, composition (or elemental composition) and microstructure were evaluated. SAE material specifications were met. The grain structure suggested a spheroidizing heat treatment, but not that any quenching or tempering operations were performed on the shaft after machining. This condition permitted easy machining of the shaft during its manufacture, but would not provide long fatigue life owing to the relatively low hardness and tensile strength of such a microstructure.
Examination of the drive shaft showed the transverse fracture surface was largely destroyed by repeated contact as the failure progressed. This made it difficult to determine the exact initiation site for this failure with certainty. However, beechmarks on the peel fracture surface provided insight into the probable order of events and general location of the initiation site. The fracture surface conditions indicated the failure occurred as two events.
The analysis of this failure was much more detailed, but provided here is a summary. The first event was most probably a transverse fatigue failure, and the second failure was the peeling failure. The latter was probably driven by the torsional load. Both failures are believed the result of operating conditions acting on toolmarks on the keyway. The toolmarks would have acted as stress risers and propagated to failure under fatigue.
Light microscopy, elemental analysis, metallography, and metallurgical analysis were some of the many tools and techniques available that were used in this failure analysis.
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