Pre-stressing – Joining and pre-stressing of hybrid sheet metal structures by collar drawing
In the project, an approach for joining and prestressing loops of fiber-reinforced plastic (FRP) composites and sheet metal during forming is investigated. In this process, the FRP loop is placed around two collars drawn into the sheet. A punch subsequently expands the collar plastically, stretching the loop purely elastically. After unloading, the plastically expanded collar prevents the loop from fully springing back, resulting in beneficial preloads. In the project, this mechanism is verified and parameters affecting the prestress are identified by experimental and numerical investigations and implemented in analytical design models, efficient processes and defined structures.
Although the principle of prestressing has been used for thousands of years and also plays an important role in the construction industry, its use in other areas has so far been sluggish. The principle of operation is based on the compensation of load-induced stresses by means of an oppositely impressed prestressing of the structure. There is great application potential for prestressing in lightweight construction-related sectors such as automotive engineering, where there are increasing requirements in terms of crash safety and ride comfort, while the weight of the load-bearing components is to be reduced without compromising performance. In this project, the usability of the principle of prestressing in load-bearing structures is to be made visible through a profound understanding.
The determination of the technological fundamentals for the stiffening and reinforcement of load-bearing structures is achieved in this project in several steps. A numerical model of the expansion of FRP-wrapped collars is built up in order to determine the main parameters influencing the prestressing mechanism. Based on the findings obtained, a process-optimized tool is developed and experimental investigations are carried out. The relationships determined are used to develop an analytical model of the joining and prestressing process, which is intended to represent the existing process phenomena in the best possible way so that a profound understanding of the process can be obtained. In addition to validating the numerical and analytical results, the experimental investigations also serve to identify the existing process limits and error patterns. In summary of the findings, it should be possible to develop simplified design methods for the prestressing of sheet metal structures. Figure 1 shows a simplified specimen geometry used in the project and the process used for prestressing. In order to enable a transfer to safety-relevant components, a setup for measuring the pre-stress during the process is also being developed, which, extended by a process control, can be used to set tightly toleranced pre-stress states and, consequently, tightly toleranced mechanical structural properties.
Funded by the German Research Foundation (DFG) – project number 453074727.