Design and manufacture of fibre reinforced plastic/metal composites with low residual stress

• Applicant: Professor Dr.-Ing. Lothar Kroll
• Subject Area: Plastics Engineering
• Term: from 2012 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 231906584

The combination of fibre-reinforced plastic (FRP) laminates with metal sheets for lightweight hybrid structures allows a significant weight reduction of components and leads to significant energy savings in mobile applications. The individual composite components, however, exhibit a strongly different material behavior, so that in terms of process engineering and component-specific conditions, the fiber-plastic composite and metal have to be adapted to each other. In particular, the different thermal expansion properties of metal, plastic and reinforcement fibers and the resulting shrinkage and warpage effects often lead to critical stresses and failure modes. The main objective of this research project is the load adapted combination of endless fiber reinforced thermoplastic tapes with metal sheets in the form of hybrid composite laminates. The production strategy is based on the continuous fiber-foil technology, divided into fiber feed, pressure- and temperature-contnslled fiber-foil consolidation and compaction of fiber sheets and thin sheet metal. The induced residual stresses, resulting from the in-line manufacturing process and during operation, have to be determined and set in correlation with material and process parameters. Special experimental set-ups and analytical and numerical calculation methods are used, such as stress-strain analysis of plates/sheets and application of complexvalued stress functions in combination with conformal mapping. The determined parameters are used as input parameters in the design of hybrid FRP/metal composites. For the characterization and design of the hybrid composites both global failure due to edge effects and local failure in laminates weakened by breakthroughs is considered. For the minimization of residual stresses and critical interlaminar shear stresses in hybrid composites various technological measures are provided. On the one hand, an elastic buffer layer between the high stiff fiberreinforced plastic laminates and metal surface layers will be sought, based on a plastic foil with appropriate characteristics, which might offset the different expansion behavior. On the other hand it is planned, to adapt the thennal expansion coefficient through an optimization of the FRP layer structure and modification of the polymer matrix. The technological and theoretical results are used to produce stress-optimized FRP/metal laminates for the cluster and to provide design and calculation tools for the engineering-based application for the dimensioning of hybrid Metal/FRP structures.

• DFG Programme: Research Grants