Joining and prestressing of hybrid sheet metal structures by collar forming

• Applicant: Professor Dr.-Ing. Peter Groche
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Production Automation and Assembly Technology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 453074727

Hybrid structures made of metal and fiber reinforced plastic (FRP) offer many advantages, as they combine the properties of two fundamentally different types of material. The joint formation between FRP and metal, however, is challenging. Conventional joining techniques have disadvantages regarding the optimal utilization of the FRP or the processing times. While the very different material behaviour of FRPs and metals often poses a challenge, it can also be the key to enable force closed joints by forming processes and improvements of the structural behaviour.In the present research project, the different material behaviour during a forming process is used to establish force and form closed joint. For this purpose, a FRP strap is placed around two collars drawn into a flat area of a sheet metal structure. In a subsequent forming step, the collars are expanded by means of a conical punch. After the punch is removed, the resulting difference in springback between the elastic-plastically deformed metal component and the purely elastically elongated FRP component results in a force which connects the components and produces a favourable stress distribution in the hybrid structure. In the case of load-bearing structures, which are subjected to a load in a preferential direction, this stress distribution can also be used to counteract an external load and delay the components failure. Due to the direct relationship between the punch movement in the forming operation and the resulting prestressing force, the proposed process is expected to allow for a targeted adjustment of the load-bearing behavior.The projects objective is the determination of the technological basis of the proposed process. In detail, the mechanisms and effects as well as the influencing parameters on the joint and prestress formation are to be identified. The process-limits and complex, asymmetric deformations are to be determined and minimized by numerical, analytical and measurement and control approaches.The project starts with the development of a numerical process model based on the finite element method in order to detect the fundamental joining and prestressing mechanisms as well as the determination of influencing parameters on the prestress generation. Based on the knowledge gained, a process-optimized tool and an analytical model for the prediction of the prestressing force are subsequently developed. Experimental investigations serve for the validation of the analytical and numerical models as well as the determination of occurring failure modes and process limits. In order to control the generated prestressing force, measurement and control concepts for the forming process have to be designed, validated and evaluated in experiments. The project concludes with the transfer of the developed processes and models to an extended application spectrum.

• DFG Programme: Research Grants