JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011 - 37

Fig. 4: Influence of fibre orientation in the contacted layer on the bond strength

Fig. 5: Medial forces and maximum displacements of double-Z profile samples under pressure crash loads

under crash loads. For a fundamental qualification, several combinations of double-Z profiles made of 1.5 and 2 mm DD11 steel and 9-layer CFRP reinforcements were produced. These profiles were tested on a carriage crash test facility under compression forces comparable to an automotive crash box. Figure 5 illustrates the results of the crash tests (Fig. 5). The medial force and the maximum displacement for mere steel and hybrid structures with different wall thicknesses are shown in the diagram. To analyse the lightweight potential in detail, the values were referred to the weight of the samples. As a result, the medial specific force for the 3.5 mm hybrid solution is for example twice the value of the 1.5 mm mere steel solution: 60 N/g for the hybrid compared to 30 N/g for the steel structure. As a first result, it can be stated that hybrid materials offer a significant weight saving potential. The next step is to increase the number of crash tests. This means to vary steel materials, prepreg layers and forces, for example three-point bending.

This procedure can be used to manufacture automotive structures, for example b-pillars or rocker panels, within cycle times of less than 5 minutes compared to typically 15 minutes. Using multiple tools or multiple flexible press devices allows a further reduction of cycle times (Fig. 6 b) because this concept significantly reduces down time.

Conclusions
Hybrid systems consisting of sheet metal and fibre-reinforced plastics offer a major potential for lightweight design in the automotive industry. The prepreg press technology can significantly reduce the number of process steps and process times compared to the current solutions. By using the prepreg press technology, CFRP prepregs are formed into steel structures. Bonding is achieved using the epoxy resin as an adhesive, which offers a high joint strength. Furthermore, it has been shown that the structures offer a high crash performance. n

Fig. 6: Automated prepreg-press process a) with a single press device and a double mould, and b) with multiple flexible press devices

Approaches for mass production
Substantial demands to implement hybrid structures into automotive applications are adequate approaches for mass production. The prepreg press technology can meet this demand by the use of automation tools like robots or assembly lines (Fig. 6 a). In this case, a robot inserts formed sheet metal into a heated mould. The same robot inserts a tailored prepreg. Forming and curing are carried out as described before.

More information: christian.lauter@uni-paderborn.de

Acknowledgements The authors would like to thank the EFRE fund of the EU and the state of North RhineWestphalia for supporting the research project within the scope of the Ziel 2 programme. Furthermore, we gratefully acknowledge the support by our project partners Benteler-SGL, Audi, and Johann Meier Werkzeugbau and the cooperating Chairs of the University of Paderborn, LUF, LTM and LWK.

No69 November - December 2011 / jec composites magazine
References [1] D. Lutz: “Chassis lightweight design exemplified on a suspension strut – State of the art and potential”. Proceedings of 11. Aachen Colloquium Automobile and Engine Technology, Aachen, pp. 437-455, 2002. [2] A. Horoschenkoff: “Statt Stahl und Aluminium”. Kunststoffe, No. 5, pp. 50-54, 2010. [3] S. Grasser: “Composite-MetallHybridstrukturen unter Berücksichtigung großserientauglicher Fertigungsprozesse”. Proceedings of Symposium Material Innovativ, Ansbach, 2009. [4] M. Maciej: “Faserverbundkunststoffe: Von der Kleinserienfertigung von Sichtbauteilen zur Großserienproduktion von Strukturteilen”. Proceedings of InnoMateria – Interdisziplinäre Kongressmesse für innovative Werkstoffe, Köln, 2011. [5] S. P. McEntee and C. M. O’Bradaigh: “Large deformation finite element modelling of single-curvature composite sheet forming with tool contact”. Composites, Part A26A, pp. 207213, 1998.

Feature land transportation

JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011

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