In most structural applications, members are subjected to variations in applied loads, causing fluctuations in the applied stresses these members see. If these fluctuations are sufficient enough, even though they may be considerably less than the static strength of the material, they will lead to premature failure given enough time. For example, if you bend a piece of wire back and forth enough times, it will break. Bending it once produced stress and strain, but not enough to cause failure. It was the repeated cycling of these stresses and resultant strain that leads to failure. The material is said to have fatigued.
There are two methods to calculate failure fatigue: Curve Stress - life, and curve strain - life.
There are several types of machines used for fatigue testing. These machines differ in the way in which the repetitive stress is applied: axially, flexurally, torsionally, or a combination of these.
The general procedure involves preparing several representative specimens. The first specimen is subjected to a high amount of stress so that it fails quickly. The second specimen is subjected to less stress until it fails, This type of procedure continues, using less and less stress and increased repetitions, until the applied stress falls below the endurance limit of the specimen and it will not fail.