Definition:
Fatique: The weakening or failure of a material, such as metal or wood, resulting from prolonged stress.

WHAT IS FATIGUE LOADING?
There are different types of fatigue loading. One type is zero-to-max-to zero, where a part which is carrying no load is then subjected to a load, and later, the load is removed, so the part goes back to the no-load condition. An example of this type of loading is a chain used to haul logs behind a tractor.

Another type of fatigue loading is a varying load superimposed on a constant load. The suspension wires in a railroad bridge are an example of this type. The wires have a constant static tensile load from the weight of the bridge, and an additional tensile load when a train is on the bridge.

The worst case of fatigue loading is the case known as fully-reversing load. One cycle of this type of fatigue loading occurs when a tensile stress of some value is applied to an unloaded part and then released, then a compressive stress of the same value is applied and released.