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Illustrated Case Studies in the Industrial World of Failure Analysis, Non Destructive Evaluation, and Predictive Maintenance

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Case Study No. 85:

Fatigue Failure at Radius of Trunnion ShaftOne should not take the fillet radius for granted. In reality, it needs to be adequately sized to handle the stresses AND it needs to be treated with respect when machining it in. When machining a stepped shaft, be careful that you don't put a groove in it when starting or ending that radius.

The shaft shown to the left, came off of a failed kiln trunnion. It was a hefty shaft, weighing in at nearly 6" diameter where the bearing sat (shown here in this sectioned piece of shaft). It stepped up to about 7.5" where the roller was located. This is what a typical fatigue failure looks like for a rotating shaft with a uni-directional applied load (i.e. the direction of the load is stationary with respect to the rotating shaft). The fracture surface is perpendicular to the axis of the shaft. The cracks start from multiple locations spread uniformly around the shaft circumference, and travel inwards. They continue until such time as the shaft can no longer support the load. If you don't catch the failure in time, as in this case, a lot of the fracture topography is destroyed. Some of it remained intact in this particular case. The bottom of the shaft in this picture is shiny in appearance. This usually happens when the crack surface is beaten smooth over time, as the crack opens and closes. The top of the shaft has a rougher surface that is more indicative of fast fracture. The discoloration is rust. Therefore, this section of the shaft failed first a long time ago. However, this fast fracture was not enough for a catastrophic failure. The shaft continued to carry the load for a long time.

This failure topography is in stark contrast to the torsional fatigue failure topography illustrated in November's MATERIAL PROPERTIES article. This shaft had at least 24 million revolutions ( 80 million revolutions, max.) before it failed.

Fatigue Failure of Trunnion Shaft at Fillet RadiusWhat started this failure was the way in which the fillet radius was cut into the shaft, as the shaft diameter increased from 6" up to 7.5". The picture at the left illustrates a remaining section of fillet radius with the crack progressing along what appears to be a single pointed tooling mark. The machinist buffed the radius with a wire brush. The brush marks can be seen in the picture to the left and the picture below. The buff marks also tend to highlight the sharp transition between the fillet radius and the shaft. This is especially noticeable between the two parallel white lines shown in the pictures for purposes of illustration.
Close Up of Shaft Fillet Radius
Surface Fracture at Radius showing Radial Chevrons at SurfaceAnother indicator that the fracture initiated along the foot of the fillet radius is the very small, but distinct, series of chevrons located along the outer circumference of the shaft. These chevrons indicate the crack initiated along many locations along the circumference and then traveled radially inwards (see arrows in picture). These cracks arrested themselves before the fast fracture region (coarse appearance).


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