KraSchwing

The main idea of this project is to optimize the mechanical properties of CFRP-metal joints with regard to quasi-static and oscillating stresses. In multi-material design, pure adhesive bonding is often the best option due to the requirements profile for the joining technology, which consequently plays a strategically important role across all industries. However, screw connections are also indispensable in terms of releasability with regard to repair concepts. Consequently, adhesive bonding and bolting concepts are considered and compared in parallel.

In adhesive bonding, the bonded joint represents the overall system consisting of two joining partners made of the same or different materials and the adhesive layer (including boundary layers). A special issue to be illuminated in the collaboration is the influence of the individual yielding properties of the joint on the mechanical properties of the joint under quasi-static and oscillatory loading. The immediate question is how the mechanical characteristics of the joint can be positively influenced by the choice of the adhesive system as well as the joining partners. For metal-to-metal bonding, the influences are largely known, but there is still a need for research when using fiber-reinforced plastics (FRP).
A novel adaptation of the joining part materials to the joining task can be achieved, for example, by using compliant layers on the surface of FV structures. This corresponds to a gradation of the material properties over their thicknesses. As a result, the delamination tendency of the joined fiber composite structures tends to decrease, which helps to ensure structural integrity. The aim of the project is to investigate and validate this effect experimentally. The simulative mapping of the real material behavior will enable numerical optimization of the bonded joint.

A TFP-reinforced bushing is also being considered as an innovative bolted connection in the project. TFP rosettes increase the load application area. The aim of this project is to determine the dependence of the mechanical properties of the connection on the hole reveal reinforcement, in particular on the diameter of the rosette and the roving thickness used. The aim is to optimize the joining system so that the material utilization in the rosette is similar to that in the substrate, thus maximizing the load transfer from the screw to the substrate.
Preliminary investigations at DLR show potential for improving the bonding concept with regard to the joint properties. Under vibration stress, up to 5 times the service life in single-lap shear tension could be achieved at the same load amplitude by introducing compliant layers. These results need to be worked up and evaluated in detail. Similarly, improvements in the mechanical characteristics are expected from the introduction of TFP structures in bolted joints, which is to be investigated as part of the project and is also to be quantitatively classified.
Both novel joining concepts improve the mechanical properties of the joint by introducing or transferring loads in a more planar manner. The simultaneous consideration and comparison of several concepts, both experimentally and simulatively, generates added value for the design in hybrid lightweight construction.