Introducing composite material in car bonnet

Abstract

The cars of today tend to be quite heavy in the front meanwhile light in the rear, mainly due to the location of the engine compartment and powertrain in the front of the car. If the front weight of the car could be reduced, the car could be more balanced and also the most discussed parameter for cars, namely emission of carbon dioxide, can be reduced,. This study aims to investigate how to introduce a composite materials in the bonnet, with the focus on reducing the weight, while meeting important demands on the bonnet. In this study it was chosen to consider the demands related to pedestrian safety and the stiffness of the bonnet. Pedestrian safety is a complex demand saying that the bonnet should be able to absorb a certain amount of energy from a head impact without getting the pedestrian injured. From an extensive literature study three potential material structures were found to be appropriate regarding impact resistance and flexural stiffness. Carbon fibre reinforced epoxy (CFRP)/Polyvinylchloride (PVC) foam sandwich material, aluminium/polycarbonate sandwich material and Hybrix micro-sandwich material are the materials explored in the literature study to be appropriate candidates for the demands chosen in this study. As seen in recent research based on simulations of impact resistance, some materials are especially interesting. Considering those more interesting materials, it was decided to purchase and build beam sections of each material sharing the same geometry to perform impact tests in the drop tower facility at Volvo Cars Corporation (VCC) in Gothenburg. Of those three materials selected only two materials could be compiled due to the time limit of the project. The material excluded from testing was the Al/PC sandwich material. In additional to the potential stiff and impact resistant materials, also an aluminium beam was built sharing the same geometry as the other materials and the impact resistance and stiffness of samples were measured and compared with the aluminium material used in the current bonnets at VCC. Additional to the impact tests, also a three point bending test was performed on the different materials samples according to VCC and ASTM standards, in order to determine the stiffness of the structures studied. The results showed that the CFRP/PVC foam sandwich material absorbed least amount of energy at the impact test. The amount of energy absorbed was only in the elastic region of the material, but the structure did not have the ability to deform plastically. It was interesting to note that this kind of structure indicated a possibility to reduce weight, by approximately 24 % lighter than the current aluminium material. The Hybrix material combined with steel absorbed most amount of energy but was 27 % heavier than the current aluminium material. Finally, the three point bending showed that the aluminium structure had the highest stiffness of the samples studied.