Hybridization of Carbon Fiber Reinforced Polymers with Steel Fibers to Improve the Mechanical and Electrical Properties

Besides a high lightweight grade, structural components made of carbon fiber reinforced polymer composites (CFRPC) are also often required to provide multifunctionality in industrial applications in order to exploit additional lightweight potentials through functional integration.
Such potentials result, for example, in lightning protection for aerospace applications. In order to ensure adequate electrical conductivity for the lightning protection, carbon fiber reinforced polymers (CFRPC) are nowadays modified by additional surface materials such as copper or bronze mesh (Figure 1). However, such structures cannot be used for the mechanical function since they are neither strong nor rigid. The additional weight reduces the lightweight potential of the modified components. Against this background, research work is being carried out within the project "FUTURE" on hybrid CFRPC consisting of carbon fibers and finest steel fibers. The steel fibers not only contribute to the improvement of the electrical conductivity, but are also supposed to improve the load bearing capability by their high strength on the one hand and the energy absorption and structural integrity by the steel fibers inherent ductility on the other hand (Figure 2). Therefore, the steel fibers are no longer a "parasitic" material, but an integral part of a multifunctional hybrid composite concept, in which every material is optimally used according to its properties.
For the implementation of the project objectives, innovative textiles made of finest steel fibers (Ø 8 μm) and carbon fibers were produced. The scientific focus was on the optimization of the steel fiber content, the search for the best possible laminate architecture and the laminate-appropriate integration of stainless steel filament bundles into carbon fiber non-crimp fabrics and braids. For this purpose, the influence of the steel fiber content was evaluated by means of appropriate material formulations and material models. Thereby, the corresponding laminate architectures were developed. The concepts have been implemented on modified systems for the production of hybrid non-crimp fabrics (Figure 3) and braids, which have been subsequently evaluated and optimized in terms of their preforming behaviour (friction, bending, compacting behaviour) (Figure 4).
Compared to the pure CF non-crimp fabrics, an increased compaction resistance of the non-crimp fabrics hybridized with steel fibers could have been measured, which must be considered in the applied preforming processes. Furthermore, special preforming processes, such as Dry Fiber Placement (DFP), have also been applied and examined for the production of hybrid preforms. In the DFP, dry steel fiber rovings can be layed-up to binder preforms and further processed. Furthermore, this process allows a local load-related or homogeneous deposit of steel fiber rovings on semi-finished products (woven and non-crimp fabrics) and therefore an individual hybridisation of existing semi-finished products (Figure 5). The processes have been researched and optimized with regard to the processability of steel fiber rovings, whereby individual processing guidelines could have been derived. In a further step, test specimes have been produced from the preforms by resin-transfer-molding, which have been characterized in detail with regard to the mechanical and physical properties. Thus, electrical and mechanical investigations of the SFRPC/CFRPC hybrids have shown an electrical resistance lower by a factor of 8 as well as a 300% increase in energy absorption of the new hybrid materials. The results were finally transferred to demonstrator components (c-profile) where the technology and the improvement of the material properties could be verified.
Overall, it could be shown that the hybridization of CFRPCs or non-crimp fabrics, with steel fibers is proven to offer a high potential for functional integration and to increase the lightweight properties of the corresponding components. High electrical conductivity and high energy absorption have been demonstrated experimentally. For the hybridization of multiaxial carbon fabrics suitable preforming processes, such as the DFP, have been investigated and optimized with regard to the processability of steel fiber rovings.

The research project "FUTURE" is funded by the German Federal Ministry of Education and Research, reference number 03X3042D.

Project partners:
Airbus Group Innovations
Quickstep GmbH
Karl Mayer Technische Textilien GmbH
Institut für Verbundwerkstoffe GmbH

Further Information:
Dipl.-Ing. Florian Kühn
Manufacturing Science
Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Str. 58
67663 Kaiserslautern
Telephone: +49 (0) 631/2017 153
E-Mail: florian.kuehn@ivw.uni-kl.de