Zum Delaminationsverhalten asymmetrischer Sandwichsstrukturen mit dünnwandiger, metallischer Deckschicht

As fiber-reinforced plastics are sensitive for interlaminar delaminations in stability-driven prob-lems the use of those in aircraft structures also leads to a higher sensitivity of these structures for stability. Consequently, such airframes are designed with relatively high safeties, e.g. in wall thicknesses or stiffeners. Sandwich structures may contribute to a solution of this circumstance. Their large bending stiffness and the elastic foundation of the skins can improve the global buckling stability as well as local skin buckling. On the other hand, they are characterized by a complex damage tolerance behavior regarding skin delaminations. Usually, their layup is relatively thick and composed of multiple layers. This leads to an impeded detection capability of skin delaminations and empha-sizes the importance of understanding the damage tolerance behavior of sandwich structures. In this work, the delamination behavior of sandwich structures is analyzed using a sandwich concept for a transport aircraft fuselage. It consists of an inner, main-load-carrying carbon-fiber-reinforced plastic skin, a polymer foam core and an outer thin-walled aluminium skin. A special focus of the work is put on the plastic deformation of the thin-walled metallic skin during its delamination and buckling processes as well as on the influence of the adhesive layer. Both effects have not been considered sufficiently by the current state of science so far. Initially, the used sandwich composite is characterized regarding its fracture mechanics by numerical, analytical and experimental investigations of DCB-specimens with mode-I-dominated loading. On the one hand, relatively high variances of the deformation and failure behavior as well as the fracture toughnesses can be examined. On the other hand, a significant influence of the material plasticities on the delamination growth becomes obvious: While the static delamination growth is impeded by plastic deformation, the cyclically loaded growth is accelerated. The adhesive layer leads to higher fracture toughnesses in some cases. Furthermore, circular delaminated sandwich specimens are analyzed under in-plane-compression loading. The growth of their skin delaminations mostly occurs suddenly and instable. It reduces the bearable load with increasing delamination size. Contrary, the influence of plastic skin deformation is more pronounced for smaller delaminations. For the numerical analy¬ses of the delaminated sandwich specimens a parametric FE-model is created. It may be implemented in existing FE-structures regardless of the FE-environment and may be used for the prediction of delamination growth. It can be shown that the deformation and buckling behavior of the aluminium skin is reproduced correctly and the experimentally determined fracture toughnesses deliver a decent failure criterion. Finally, non-destructive testing is used for the qualitative and quantitative detection and eval-uation of the skin delaminations. Ultrasonic testing as well as thermography are characterized by a fast test rate but do not produce sufficiently exact results.