Lightweight and reliable metal-composite joints based on harmonization of strength properties of joined parts

One of the key problems of designing composite primary aircraft structures is the problem of development of lightweight and reliable joints for such structures. For conventional bolted joints used for connection of laminated composite primary structure elements, the loads are transferred via the contact of metallic and composite parts having considerably different strength and stiffness parameters. As the local strength properties of composites are defined by polymer resins, having several times less strength as compared to the metallic alloys, the maximal load that can be transferred via the joint is limited by the properties of resins. As a consequence, the metallic parts of such joints are loaded to a very low extent, that causes low weight efficiency. In the presented paper, the approach to development of metal-composite joints based on the principle of harmonization is proposed. The harmonization principle is to minimize the disbalance of strength properties of metallic and composite parts in the local zones of contact. This principle can be realized by two ways: either by removing the resin from the zone of contact to increase local stiffness of composite part ("stiff" joint concept) or by creation of metallic part with porous structure to decrease local stiffness of the metallic part ("soft" joint concept). In this paper, the evaluation of both concepts is given, based on numerical and experimental investigations carried out in this work.