Fatigue damage is caused due to pulsing loads that, through sliding bands, create intrusions and extrusions on the material surface. Cracks iniciate on these micro damaged areas. Loads can act on the crack in mode I – traction, mode II – longitudinal shear stress, mode III – transverse shear stress, or in a combination of them. Crack iniciation is controlled by the stress range delta sigma , whereas its propagation is controlled by the stress intensity factor range deltaK. Cracks propagate whenever deltaK is bigger than the crack propagation threshold deltaKth. Short non-propagating cracks do not behave as the long ones. If the stress gradient around the cracks tip is large enough, the crack soon reaches a point where the stress around its tip is no longer big enough to propagate it, thus the crack stops.