JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011 - 36

structures
allow a pre-curing. The final curing of the CFRP reinforcement could take place, for example, in a downstream cataphoretic painting process. Before an industrial application of this approach can be developed, basic research work on results and a suitable process design for prepreg pressing is necessary. In particular, the highly diverse material properties of the semi-finished components require an adequate process design. Accordingly, the aim of the technological research work is to develop suitable process strategies and tool concepts so that components with good performance can be produced in a short and robust process chain at minimum costs. seconds by using optimised process parameters. The final curing of the epoxy resin is achieved in a downstream cataphoretic painting process that lasts 30 minutes at 180 °C. Using the matrix resin as an adhesive to bond the sheet metal and the CFRP makes any additional joining process superfluous.

Bonding of CFRP and sheet metal

Fig. 3: Joint strength of prepreg-pressed samples and adhesive as compared with adhesive-bonded samples

Prepreg press technology
Prepregs are pre-impregnated, semifinished fibre products that are produced continuously on special machines and shipped on coils. After producing a layer structure according to the expected loads and cutting, the prepregs and the sheet metal are joined in a separate forming process. To activate the process, a robot places formed sheet metal into the heated mould. Then the tailored prepreg is placed automatically into the mould and pressed by a heated punch onto the sheet metal (Fig. 2). Under influence of high tool temperature (180 °C) and pressure (0.2 to 0.5 N/mm²), the prepreg is pre-cured in the closed mould. This step can occur within short cycle times of about 120

jec composites magazine / No69 November - December 2011

Fig. 2: Prepreg pressing process steps to produce hybrid automotive structural parts, for example a b-pillar

Bonding CFRP and sheet metal is a decisive factor for the functioning and strength of hybrid materials. In order to characterize the bonding properties and to compare them with those of adhesivebonded joints, shear-tensile samples were investigated according to DIN 1465. To find the best compromise between economical aims and joint strength, consolidation time and temperature during the prepreg-press process were varied according to the reaction-velocity temperature rule (Arrhenius equation). Temperature was varied between 120 °C and 200 °C. The highest strength was reached at 180 °C for a curing time of 210 seconds. The prepreg-pressed samples were compared with adhesive-bonded samples and the influence of different types of surface treatments was investigated (Fig. 3).

An impact-resistance-modified structural epoxy adhesive was used for the adhesivebonded samples (Dow Betamate 1620). In all samples, the orientation of the fibres closest to the boundary layer was vertical to the loading direction (90°). No significant strength variations were found between the prepreg-pressed samples and the adhesivebonded samples. For the pressed samples, failure occurred between the boundary layer and the second fibre layer of the CFRP, while the adhesive-bonded samples failed through delamination of the first fibre layer. The joint area itself remained undamaged. The fibre orientation of this layer has the greatest influence on the properties of the joint (Fig. 4). Samples with fibres vertical to the loading direction (90°) had the highest strength. The elastic matrix material dominated the mechanical properties close to the boundary layer. The fibres only bear a small part of the load. Thus, the laminate can absorb local tension peaks like a bond line. Samples with fibres in the contacted layer parallel to the loads failed at 75 % of the load applied. The inhomogeneous structure of CFRP is reflected in the properties of the joint. The specific properties of such materials need to be considered in the design of multimaterial systems in order to achieve the intended compound properties.

Crash tests of hybrid structures
Applications for hybrid structures manufactured by prepreg-pressing include crash-relevant automotive structural components. Various tests were carried out to analyse the behaviour of hybrid materials

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

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011