The focus of lightweight design activities is increasingly including safety components using thermoplastic fibre composites. Normally steel has been the material used for brake pedals but in a project concerning the brake pedal of the new 918 Porsche Spyder, the suitability of a plastic brake pedal for volume production has been demonstrated for the first time. The use of plastic has also led to a weight reduction of about 30% to 50% in comparison with steel versions. Dipl.-Ing. Volker Freitag, Dr Ing. h.c. F. Porsche, Weissach, Germany, and Dipl.-Ing. Thorsten Kamphaus, ZF Friedrichshafen, Damme, Germany, describe the challenges and achievements of this project.
Fig. 1: Brake pedal with pedal bearing block (photo: ZF Friedrichshafen)
Fig 2: In the new 918 Porsche Spyder, the sauitability of a plastic brake pedal for colume production has been demonstrated for the first time (photo: Porsche)
In the course of the design process for the new 918 Spyder, a sub-project was carried out to develop a brake pedal in fibre composite technology with the aim of reducing the weight of the steel pedals still used in all standard production vehicles (fig 1). This was part of an initiative to investigate the possibility of allowing new materials and manufacturing processes to be used and their suitability for volume production to be tested.
Since this component is understandably a safety component, it was necessary to pay attention not only to the weight-reduction objective but also to the engineering strength required under all operating conditions. It was also vital for a clear and traceable damage chain to be established in the event of failure.
To be able to meet these challenging requirements in the time available, a close collaboration was necessary right from the start between the supplier, ZF Friedrichshafen, the two materials vendors Lanxess and Bond Laminate, and the machine manufacturers Engel as well as technical departments at Porsche.
Development of component and materials
Component development was based on the product design. Specifications which Porsche had formulated for the component were taken over unchanged from the product design specifications of the steel version. In addition, influencing variables specific to plastic, such as temperature and moisture content, naturally had to be taken into consideration as well. The central requirements were:
- full functional capability within a temperature range of -35 to +80°C
- no fractures or cracks below a load of 3,000N
- stamp-down braking test (2.3kN/0.7s) with at least 3,000 load cycles
- operationally solid after 320,000 load cycles in the temperature range from -35 to +80°C and with different load collectives
The basic suitability of FRP hybrid technology had already been demonstrated by ZF Friedrichshafen as part of a pre-development project (fig 3 a-c).
Here, the base body of the pedal was made of a thermoformed continuous-strand glass fibre reinforced semi-finished product – known as an organic sheet (OS). This consisted of polyamide (PA) onto which the necessary ribs had been injected, as well as the connecting and mounting features.
Fig 3a: Organic sheet / Tepex Dynalite; weight: 168g (photo: ZF Friedrichshafen)
Fig 3b: Injection moulding / Durethan BKV60; weight: 332g (photo: ZF Friedrichshafen)
Fig 3c: Hybrid component; weight: 500g (photo: ZF Friedrichshafen)
On the basis of these findings and taking into consideration the installation space available the first material-independent topological optimisations were carried out and the relevant loads simulated in FEM analyses.
The application of load perpendicular to the pedal pad has a decisive influence on the design. However, the cranked bend in the brake pedal means that under load there will also be some torsion. To cope with this combination of stresses a multiaxial fabric (0°/90° and +/-45°) is required (fig 4).
The ply structure for the organic sheet was defined on the basis of the results of this analysis, as also was the optimum rib geometry of the injection-moulded component.
Even at this stage account was taken of the anisotropy of the materials used: the results from the filling simulation for the injection moulding process and from the draping simulation for the advanced composite insert were input into the modelling.
Fig. 4: Multiaxial fabric of the insert (photo: Lanxess)
Attention had also to be given in the design to the known effects of temperature and moisture on the mechanical characteristics of the PA matrix in order to be sure of delivering – even under extreme conditions – the required values laid down in the product design specifications. Since only material charts based on the standard climate were available for simulation purposes at the beginning of the project, the extensive practical experience of the raw material suppliers was an important success factor in finding the right layering structure with respect to ply number and fibre orientation.
The material used for injection moulding was a reinforced PA6 GF60 which could deliver an optimal firm bonding and thus friction-lock with the organic sheet insert.
The organic sheet itself consists of several layers of continuous-strand glass fibre reinforced plastic with varying fibre orientation which have been compressed to form a semi-finished sheet. The reinforcing fibre reaches a weight per unit area of 600g/m² and is fully wetted with PA6 and consolidated.
Process development
Making the organic sheet insert separately and then inserting and encapsulating it results on the one hand in less complexity in the tooling and in process control and on the other hand improves the quality of consolidation in the semi-finished product as well as robustness in the injection moulding process (fig 5).
The downside – higher manufacturing costs – were accepted on account of the low production quantities.
Fig 5: Manufacturing process for the 918 Spyder brake pedal (photo: ZF Friedrichshafen)
Flat blanks are cut to the required size out of the consolidated semi-finished sheets by water-jet cutting and are then clamped in a frame. The blank is heated up to the required forming temperature by means of infrared radiation and, once the defined surface temperature is reached, inserted automatically into the forming mould. Draping simulations were carried out at an early stage in order to obtain an optimal design for the forming mould. Particularly important in the forming process is an exact compliance with the specified temperature window not only during heating but also in the forming mould itself. Hold-down systems were used for a selective influence on draping behaviour with a view to securing a defined and reproducible fibre orientation. The final contours of the insert are obtained after the forming stage by trimming all round with water-jet cutters.
After trimming, the insert is in its finished state. Before overmoulding, the inserts must be heated up in a circulating air oven so as to ensure the best possible adhesion between the organic sheet and the injection-moulding material.
Once the insert is at the target temperature a fully-automatic handling unit brings it into the mould and fixes it in place. In the next step the insert is overmoulded with short glass fibre reinforced PA6 GF 60.
Component testing and validation
On account of its classification as a safety component, comprehensive testing and validation of the component was required. Particular attention was given here to the material-related influence of temperature and moisture content on component behaviour. But the braking forces applied by the driver under racing conditions also had to be taken into consideration. In addition to the pure application of load along the longitudinal axis of the vehicle the additional transverse loading on the component caused by right-foot or left-foot brakers also had to be validated.
The static and dynamic tests were carried out not only on the separate component but also on the complete module, in each case in the defined climate and temperature ranges. As expected there was a drop in the maximum breaking stress at a test temperature of +80°C and also as a result of the effects of moisture content. The required product design specification values of 3,000N (static breaking stress) and the requirements relating to engineering strength were safely fulfilled.
Fig 6: Test rig for the dynamic ‘stamp-down braking’ test and for static strength and rigidity tests (photo: PAG, ZF Friedrichshafen)
Summary and outlook
The project objective of producing a safety-related, life-critical but at the same time lightweight component in FRP hybrid technology has been achieved.
However, the limits of lightweight design became evident. In this case they were to be found primarily in the dependence of the matrix system on moisture content and temperature and during the course of the project these led to design improvements at the cost of weight. There is no doubt that this situation could be improved by using matrix materials for the organic sheets which are more stable with respect to moisture content and temperature. There is further potential for weight reduction in using carbon fibre but as is well known this still has to be ruled out in many cases due to high cost.
Integration of the forming process into the injection mould with further development towards the one-shot process will, with the associated possible reduction in manufacturing and investment costs, open the door to large-scale production or open up further possible applications in the field of safety-related components.
www.porsche.com
www.zf.com
