Fatigue is based on the initiation and propagation of microcracks. This period can occupy up to 90% of the lifetime of a cyclic loaded structure. It is well known that short fatigue cracks propagate faster than long cracks at the same level of the stress intensity factor ΔK [
]. Deshpande, Needleman and Van der Giessen showed that the behavior of short cracks is determined by the internal stresses under cyclic loading due to the dislocation structure in the vicinity of the crack tip [
]. However, in this investigation the influence of grain boundaries and phase boundaries was not included. Due to the emission and movement of dislocations during crack propagation grain boundaries influence the crack propagation rate, it is observed that short cracks may stop in the front of grain boundaries. Therefore the estimation of lifetime of a broad range of fatigued structures and components on the basis of the stress intensity factor is difficult.