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Mechanical Properties of Compound Extruded Aircraft Stringer Profiles Under Cyclic Loading

The worldwide competition in the field of aircraft structures leads to an increasing need for functionality and safety as well as for cost and weight reduction. For instance stringers could be directly welded on the aircraft's skin sheet. The requirements to be met are increased safety against crack initiation and crack growth as well as improved residual strength against failure after harmful impact of foreign objects. The application of continuously reinforced aluminium profiles which are manufactured by compound extrusion leads to increased strength and stiffness of the profiles by combining the aluminium matrix with high strength wires. Thus aircraft stringers of such profiles represent an innovative concept with improved properties. The characterisation of compound extrusions based on medium and high strength aircraft aluminium alloys EN AW‐6056 and EN AW‐2099 shows that a good embedding of the reinforcing high strength wires (Co‐based and Fe‐based) can be achieved. Furthermore the mechanical properties under cyclic loading of the profiles were measured and the S/N‐curves for the different compound combinations were determined. Subsequently the crack initiation and propagation was analysed by using metallographic and SEM investigations. The fatigue resistance of reinforced specimens is increased compared to unreinforced ones. The fatigue cracks originate at the surface of unreinforced specimen while the cracks in reinforced specimens are initiated at the wire–matrix interface.