What Is Fatigue?

Fatigue is a special kind of failure of material in which fracture occurs not because of an instantaneous load that is applied, causing a crack to grow. Rather, it is because a stress is applied for some period of time in which the cracks gradually grow until they finally reach a critical level. This concept is especially important when dealing with metals because it is the single most common cause of failure in metallic structures. There has been much study done on the concept, and much has been learned since the beginning of the study of fatigue. Since much has been learned about fatigue, much has been done in the way of learning how to prevent it. For instance it is almost universally accepted that the cracks in fatigue always start on the surface of a material, so therefore, in order to prevent fatigue from occurring, one should strengthen the surface of the material, making it more difficult to fatigue.

(Sean Grealis)

The concept of fatigue is very simple, when a motion is repeated, the object that is doing the work becomes weak. For example, when you run, your leg and other muscles of your body become weak, not always to the point where you can't move them anymore, but there is a noticeable decrease in quality output. This same principle is seen in materials. Fatigue occurs when a material is subject to alternating stresses, over a long period of time. Examples of where Fatigue may occur are: springs, turbine blades, airplane wings, bridges and bones.

(Shawn M. Kelly )