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Inspectioneering Journal

Fatigue, The Science Of Getting Tired

By Richard Green at Accurate Metallurgical Services. This article appears in the September/October 2008 issue of Inspectioneering Journal

You’re at 32,000 feet in an airplane when you look out the window. You observe the wing of the jet moving up and down with the turbulence, like a child bending a coat hanger over and over again until the wire fractures into two pieces. You wonder how many times has this wing flexed. Such work causes fatigue. Fatigue is the science of things that grow tired. This is applied to both materials and biological systems such as the human body. A key difference between the materials and the human body is that fatigue damage in the human body can be normally be repaired by rest. Fatigue damage to materials, on the other hand is permanent and irreversible.

Fatigue failure mechanisms are responsible for over 90% of mechanical failures in the field. 1 The goal of fatigue testing is to determine when this cumulative damage becomes significant. Significant damage can be defined as the lower limits of NDT inspection techniques. It can also be defined as that point where the residual strength of the part is below the minimum required strength of the part. Fatigue cracks exhibit little deformation and because these cracks are fine and exhibit little deformation, they are difficult to detect even with NDT equipment. The ability to predict when and where a crack can occur is a powerful tool in component design and servicing.

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