The load transmission into thin-walled CFRP structures is often realized through a conventional bolted or adhesive connection. In case of the bolted connection, the necessary hole disturbs or damages the loadbearing fibers inside the laminate, resulting in a weakening of the laminate and thus an unused light-weight potential. While keeping the fibers intact, the adhesive connections can only introduce loads only into the interface layer, which either reduces the transmissible load or increases the necessary surface. In addition, bolted connection can be inspected while the status assessment of an adhesive connection is very hard.
In order to solve the hole bearing problematic while also increasing the available adhesive surface the Project Multilayer-Insert (MLI) as part of the Priority Program 1712 proposes a local metal-hybrid area for fiber-fare load introduction. Due to the layer-wise design, the hybrid-area can be built up during within the Automated Fiber Placement Process producing the surrounding CFRP structure. The shape and orientation of the metal sheets and the layer-wise design of the hybrid area itself increases the contact area between metal and CFRP and is able to distribution the load into every CFRP-layer.
In order to lessen the impact of the hybrid area on the shape of the structural part, the combined thickness of a metal sheet and two adhesive layers should equal one CFRP layer. The requirement leads to either a thin adhesive layers as well as thin metal sheets or thick CFRP layers ad a multitude of adhesive connections. In the field of material science the key challenges for the combination of CFRP with metals corrosion, adhesion and thermal stresses are considered. Thereby, various surface treatments and interlayers that satisfy the previously mentioned challenges are investigated. On the one hand, the surface treatment and interlayer should guarantee the adhesion between metal and CFRP and on the other hand, it should avoid the corrosion tendency between the two materials as well as a reduction of the thermal stresses which is achieved by a modification of the cure cycle. Furthermore, the surface treatment and interlayer must overcome the manufacturing process without damage to guarantee the corrosion protection after the production. Additionally, the characterization of fiber metal laminates (FML) is analyzed and a testing method especially for FML is presented.