Integral structures offer intrinsic benefits in stiffness and fatigue performance. However, the damage tolerance of integral structures is a concern because of the potential loss in fail safety, for instance when a skin/stringer structure is replaced by a monolithic structure: a crack in the skin can propagate in the stiffeners.

It is, therefore, useful to develop design concepts and design tools to improve the damage tolerance of integral structures. One approach is to use skins which include crack retardation features in order to decelerate fatigue crack growth. Finite element calculation of the stress intensity factor variation when crossing rectangular padding features and subsequent computation of the fatigue crack growth rate shows a crack retardation.

Two panels made of 2027 T351 40 mm thick plates are tested in the L-T direction. Both have the same weight but one has a thinner skin with additional pads. Fatigue crack growth from an initial crack which includes a broken central stiffener shows a crack retardation by 70% due to the padded geometry.

Finally, a static test is performed with the aim to demonstrate crack bifurcation. The relative strength to toughness of the material is too low, so that overload failure of a first outer stiffener is observed

The tests performed firstly validate the modeling tools and secondly show the benefit obtained by the crenellated geometry, which can be used to reduce the damage tolerance criterion severity for integral structures.