Prestressed, hybrid stringer sheet structures
An increasing demand in lightweight structures can be observed in many fields of industry. Especially in the aviation and the automotive sector lightweight components with a high stiffness and strength are needed. To meet these requirements, stringer structures as well as hybrid structures are commonly used. Hybrid components combine the advantages of two or more materials in order to create structural parts with a high strength and stiffness as well as a reduced weight. Another way to increase the strength of structural parts is the prestressing of components with a force anticipating the working load as it is known from the prestressing of concrete in the construction industry. However, the combination of these approaches as well as cost-efficient manufacturing methods are insufficiently researched.
The aim of this research project is the stiffness-optimized production of planar and tubular hybrid stringer sheet structures with an additional prestressing of the stringer sheet during a hydroforming process. Therefore, metal stringer sheets are combined with fiber reinforced plastics, which induce the prestress due to their different Young’s moduli.
The main aim of the research project, apart from the proof of feasibility, is the determination of the process window as well as typical cases of failure. Furthermore, the prestressing potential is being analyzed.
A demonstrator with prestressed stringer using a steel cable as a substitution for the fiber reinforced plastic is shown in Figure . The cable that was loose before the forming process was stressed in such a way that it could not be pushed in manually afterwards.
First, finite element analyses are used to determine the necessary process parameters for the forming process as well as the requirements for the connection of the fiber reinforced plastic and the stringer sheet. In addition, typical cases of failure are deduced. Based on these results, tools are developed, proper materials are analyzed and control curves for characteristic temperatures, pressures and holding forces are determined. The target component geometries are shown in Figure .
After the production of the necessary test specimens the determined control curves as well as the designed tools are reviewed based on forming experiments and sensitivity analyses. Furthermore, optical measurement tools are used to determine the sheet strain and geometry. Design and controlling approaches are developed. Finally, compression tests as well as burst tests and modal analyses are conducted in order to determine the realized prestress and its influence on the component strength.
The research project is funded by the German Research Foundation (DFG).