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Mechanical Fatigue of a Stainless Steel Shaft

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Summary 

A failed shaft from a grinder was submitted for a failure analysis. Examination of the fracture surface revealed characteristics consistent with mechanical fatigue failure. These characteristics were typical fatigue marks associated with a uniformly loaded shaft with moderate to severe stress concentration subjected to rotating-bending fatigue. The failure occurred at a sharp change in shaft diameter. No indication of chemical interaction was present.  

Background

The submitted shaft sections had been in service for approximately 2 weeks prior to sudden failure. The previous shaft for this grinder had seen between 1½ and 2 years of service before failing in a similar manner. Material of construction was reported to be Type 316 stainless steel. 

Description of Material

The pieces received contained the matching faces of the sheared shaft.  Section1 was approximately 7” long. The shaft was approximately 4 1/8” in diameter at one end that was saw cut. The opposite end contained the fracture that occurred at a diameter transition from 2 3/8” to 2 7/8”.  A deformed aluminum ring was on the shaft near the fracture surface. Section 2  was approximately 10” long with a noticeable bend on the end (»10°) where the fracture occurred.

Findings

Approximately 75% of the fracture surface appeared relatively smooth and associated with striations of a propagating ductile fatigue crack. The remaining 25% of the surface had a rough texture associated with the final brittle fracture of the shaft. Noticeable deformation of the shaft was present in the area of final failure. 

On the fracture surface, several crack initiation sites were present, with one main crack that led to  failure, Figure 2.  Beachmarks were present on the main crack typical of a propagating fatigue crack. This was most likely caused by mechanical damage of the fracture surface that occurred due to the opening and closing of the fracture prior to final failure. The cracks initiated at a diameter transition next to a support bearing. The area of final failure is approximately at a 15° offset from the beach marks associated with the main crack. 

The inner ring of the bearing and associated spacers and lock-nut were removed from the shaft. Along the outer edges of where the inner bearing ring was in contact with the shaft mechanical damage typical of fretting was observed, Figure 3. Mechanical deformation was also observed in the shear grove between the threaded portion of the shaft and the bearing land.

The shaft was found to be within the specification of Type 316 stainless steel.  The hardness of the shaft was determined using a Rockwell Harness Tester. The hardness was measured to be in the range of 78 to 81 Hardness Rockwell B (HRB).  

Discussion

The observed fracture surfaces have characteristics consistent with mechanical fatigue failure. These characteristics are striations and beachmarks associated with a uniformly loaded shaft with moderate to severe stress concentration subjected to rotating-bending fatigue. The fretting observed is typical of damage caused due to flexing of the shaft.  

Fatigue failures require that there be a stress concentrator and a cyclic stress applied. In this failure, the stress concentration was provided by the abrupt change in shaft diameter.  A cyclic bending stress could have been applied due to improperly tightened belts, overloading of the grinder causing vibration, or due to an imbalance or misalignment of the shaft. 

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