|Failures, like history, can repeat themselves. However, just when you think that you know the answer before the question is even asked, you find that the answer has changed. Never assume that the fracture surface is going to look like all of the other fractures that have been collected over time, you may be fooled; and it might cost you dearly.
The failed section to the left is the recovered fracture of a critical stiffening ring on a rotary dryer. There have been two failures on its "sister" ring located a couple of feet beside it. The failures have been low stress fatigue, something that one would expect after 80 million cycles. It would have been easy enough to dismiss this third failure as being the same as the other two previous failures. However, in maintenance engineering nothing is ever "cut and dry." There is always a new twist, and that twist is what will cause an engineer grief. In this case there was a new twist, something that was foreboding.
Explaining the fracture above, the stiffening ring is approximately nine feet in diameter, two inches across, and two inches deep. The inner diameter was welded directly to the shell to give the shell additional strength. The outer diameter surface is in the foreground of the far left picture. The fracture was then rotated up in the picture next to it. From that viewpoint it was then opened up from the front of the picture. The cross sections are shown in the two pictures immediately below. It is easy to see once again the beach marks pointing to the fracture origin. At first glance the failure once again looks like low level stress, and it is; what is very different is the initiation point of the crack, the reason the crack began in the first place.
|In the picture to the lower left you can see a protruding ledge (top center of picture) on what would be the outer diameter surface of the stiffening ring. In the picture to the lower right you're looking at the mating ledge from on top looking down onto the outer surface of the stiffening ring. For all of you welders out there you can recognize the classic stringer pattern of a weld bead! For whatever reason, someone struck an arc and started the bead at the right and pulled the weld bead to the left. When the arc was struck, there was not complete fusion between the bead and the parent base metal. In the picture to the lower left, there is a small region to the left where there is a tiny void. Actually, the beach marks lead to this small void.
It was this small void that crippled a rotary dryer. The scary part is that now visual inspection was tuned to look for these haphazard weld stringers. As a result of this new revelation, five more stringers were found during the repair. What is scarier than that ...... they have yet to be inspected!
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