In Doctoral Thesis the researches of dependence of fatigue damages’ (cracks) accumulation are carried out on samples with numerous concentrators. The supposal that the fatigue damages’ accumulation happen dependent, the Hypothesis of Local Zones (HLZ) is proposed. On the full (large)-size Designs it allows to consider an area of damages’ concentration as a pool of some Local Zone, which defines durability of total Design. On the basis of double exponential probability distribution of fatigue durability with logarithm of loading cycles (DEDF), the four-parametrical mathematical model of Local Zone’s fatigue strength is developed. The model unites parameters of fatigue durability’s distribution function (CDF), CDF of endurance limits, S-N fatigue curve and a sensitivity threshold on cycles. It is supposed that piecewise and linear description of S-N fatigue curve with various degree “m” is connected with various mechanisms of fatigue damages’ accumulation (FDA) in a material (design) when loading level is changing. With this provision, the example of a Safe Life Time (SLT) evaluation is given for a keel beam of the Mi-26T helicopter. The existence of HLZ postulates is shown on examples of the Bench Tests and Operation Work of full-size Designs. It is shown that the best method of nondestructive test (NDT) for monitoring of Local Zones’ technical condition is the method of acoustic emission (AE) and thermo vision. Advancing controlled destruction of the critical Local Zone, presented as indicator of FDA on total Design, will allow developing operation methods by the principle of safe destruction of a Design (Fail Safe Concept).