Mill Drive Shaft Failure – Failure Analysis Example from Touchstone Research Laboratory
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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Figure 1. Closeup of the keyway on the larger shaft specimen showing the impression of the drive keyway on the shaft OD.
Figure 2. Fracture surface on the specimen from Figure 1 as viewed in incident light. Note the clarity of the beach marks along its length.
Figure 3. Transverse fracture surface on the smaller, threaded portion.
Figure 4. Transverse fracture surface on the larger shaft specimen.
Figure 5. Same area shown in Figure 4 with the section that peeled from the shaft in its original location. The circled area indicates the only remaining visible beach marks on the transverse fracture surface.
Figure 6. Macrophotograph, at 1.5X, showing the circled area from figure 5 as viewed in incident light.
Figure 7. Macrophotograph, at 5X, of the area shown in figure 6.
Figure 8. Photograph of the keyway on the threaded section. Circled area is shown in figure 9 at greater magnification.
Figure 9. Macrophotograph, at 5X, showing tool marks in the circled area in figure 8. The center area is the radius area at the corner of the keyway and the diagonal marks near the bottom are portions of the circular marks left by the end mill used to cut the keyway.
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