Reliability Engineering Snapshot TM

The Illustrated Newsletter For Failure Analysis, Non Destructive Evaluation, and Predictive Maintenance

 

Material Properties

Case Study No. 15:

Stiffening Ring 1st Failure - FractographWhen something really important breaks, it is important to know whether panic is really approprate or whether instead, the answer literally rests in the palm of our hands.

The picture to the left shows the fracture surface of a stiffening ring on a large kiln. It failed after being in service for nearly four years. It was installed in order to remedy other problems. That kind of performance is medeocre to say the least. The cross section of the ring (shown to the left) is approximately two inches in width by four inches in debth. The top of the picture is the outer diameter of the ring. When the ring cracked it cracked from the outer diameter and travelled radially inward. The crack was cut out and about two inches of the crack surface was recovered.

One of the questions a cool headed maintenance man asks himself is "Do I have time to think about how I am going to repair it?" The answer to that question was answered by reading the information on the fracture surface. Before ever recovering the crack, it was already known that if the beach mark lines were round and small at the point of initiation, then the problem more than likely took three years to happen. If the beach marks were not round, but were straight in shape and oriented across the width of the ring, then the problem was new in nature, and an immediate resolution was required. Luck prevailed and the rings were small in size and began in the upper left hand corner of the picture above. When the mechanics were locating the crack so that it could be cut out, they used a weld rod to strike an arc to mark the crack. That strike mark is visible in the picture, and by coincidence the rings start right there and radiate out and down.

This information meant that the problem could be researched since, if the crack was simply just welded up without solving the true cause for it,the ring would take approximately four years before it cracked again (being hopeful of course).

Stiffening Ring 2nd Failure - Fractograph
The repaired crack has been in service for five years. Three years after the first crack appeared, a second crack appeared. Again, the same thinking was used. Mainly, the shape of the beach marks would dictate the scope and timing of the repair. The top of the picture to the right is the outer diameter of the ring. Only one inch was able to be recovered. The beach marks begin just to the left of center at the top of the picture. They are not as pronounced as the previous picture, but they ARE there.

What all of this meant is that when the load being applied is low and well within the design limits of the material, then the beach marks will be circular half-moon shaped. As the load increases the circular shape changes into a straight line which is perpendicular to the direction of the stress. If either of the stiffening rings shown in the top and bottom pictures had been overloaded, then the beach marks would have been straight across the cross sectional area of the stiffening ring (i.e. parallel straight lines oriented from left to right and very close to the outer diameter surface).

By knowing that the magnitude of the problem was low, long term repairs could be designed, and panic could be avoided.

 

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