A turbo disk can be regarded as a periodic system if all of the blades are assembled periodically.In some turbo disks, like steam turbines, the blades are assembled into periodic groups of blades and the disks can be also regarded as a periodic system.Cyclic fatigue during operation frequentlx causes cracks to appear in rotating turbo disks.Cracks near the roots of the blades may change local flexibility and introduce into so-called local structures irregularity.This leads to the mistuning of bladed-disk assemblies.The local defects may introduce a localization vibration in the periodic system.The interaction between this localized vibration and the propagation of cracks may cause the blade to fail fatally.