4. CONCLUSIONS The analysis of specimens fatigue cracks in the samples allows finding a new approach to the required changes that should be introduced into the design. For specimens with widespread fatigue damage the completely new design concepts were applied. When it was not the widespread fatigue damage case then the changes in the design concerned geometric features of the existing structure only. In paper [2] the influence of geometric design features on the fatigue durability of the objects tested were analyzed. On the analysis of the results base the changes in the designs which led to increase in its durability were introduced [6, 7]. For example, the durability of specimen 1.1.1pm has increase a b Fig. 7. The crack course in specimen 1.1.1d, a – the skin side view, b – the stringers side view. a b Fig. 8. Crack propagation in segment 1.2pm, specimen No. 5, a - seen from skin side. The crack originated at the rear spar propagates up to the oval hatch frame. The skin between the oval and circular hatches lost of its stability. The crack originated at the circular hatch ends at the front spar. b - stringers side view.
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