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FATIGUE OF A PIPE NIPPLE

Background: 

A pipe nipple connecting a valve to a tank had experienced two sequential failures. The client noted that it was possible that the pipe nipple was over-torqued during installation, and that there was some vibration in the system. The client wanted to determine the cause of the pipe nipple crack formation and failures.

Figure 1: Macrophotograph and SEM Micrograph of the fracture surface of the pipe nipple with fracture features labeled

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Summary:

The failure of the pipe nipple was associated with vibration induced fatigue crack formation and propagation. The presence of ratchet marks around the majority of the circumference of the pipe indicate that fatigue cracks initiated in a variety of locations and propagated through the materials. The multiple crack initiation regions indicate that the forces acting on the pipe were multidirectional as would be expected due to vibration. The fatigue cracks all originated at the thread roots, which is expected in a vibrational failure.

The initiation and progression of the fatigue cracks was likely accelerated by the fact that the valve was only supported on one end, which led to a cantilever-like structure and imparted significant stress on at the thread roots where the fatigue cracks initiated.


Take Away: 

Vibrational fatigue can lead to rapid failure of materials, especially those with stress risers such as threaded sections. The risk of vibrational fatigue is increased for components that are only supported on one end.



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