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May 31, 2016
Understanding Fatigue Crack Growth

Posted by: Casey Whalen 3:00 minute read

Fatigue crack growth (FCG) can occur when improper welds are used or a weak spot in the pipe is exposed to repeated fatigue. Typically, a crack will start at a seam weld, girth weld or spiral weld due to small imperfections that grow through cyclic fatigue. This fatigue is typically brought on through pressure cycling or temperature cycling.

In the below diagram, a defect or pre-existing crack is present but is at static rest under the system pressure. When the pressure is increased, hoop or axial forces will pull in the direction shown in the second picture. When the pressure returns to its original state, the crack has grown in length.

 

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1) Defect with no stresses. 2) Defect being pulled in tension. This causes a peeling force at the crack tip. 3) When pressure decreases, a new defect length can be measured – the crack has grown.

This growth occurs due to stress behavior at the crack tip, and continues until a critical length is reached where an increased pressurization causes a burst failure. At the crack location, the substrate cannot absorb the stress of a pressure increase. Stress is redistributed so that tensile forces are significantly intensified at the crack tip.

Increased stress will cause localized yielding until a small tensile failure occurs (seen as the crack growth). When the pressure reduces, the yielded metal can cause more stress. As this cycle continues, the crack will continue to grow.

Want to know more? Our engineering team would be happy to answer questions about how Pipe Wrap can help solve FCG. Click or call 855.655.6750.

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Category: Feature Articles, Pipe Wrap

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