Implementation of innovative technologies on the bonnet for passive safety: From marketing to safety regulation

The vehicle bonnet assembly is a key structural element in a vehicle assembly in terms of vulnerable road users (VRU) safety. As the design and development of a bonnet is becoming more difficult because of the contradictory requirements, due to the current homologation process. On one hand the bonnet must be stiff enough to withstand loads and be durable during the lifecycle of the vehicle, while it also must be soft enough to provide energy absorption in case of a pedestrian collision. The thesis project was developed starting from the concept car bonnet design made of carbon fibre, redesigning to an aluminum bonnet for low volume production to achieve different design requirements that include regulations, manufacturability, and style - as the bonnet is an exterior class A surface, the quality and aesthetic appeal of the component is important. The regulations that are considered during the development of the part are the VRU collision test that are necessary for homologation and are part of the scope of the vehicle development, as the scope for this project is a 5-star rating in euro NCAP, the bonnet’s design must reach a good score in the head form tests. Stiffness and noise, vibration, and harshness (NVH) of the bonnet are verified with Finite Element Method (FEM) analysis, using Altair Hypermesh as a tool for preprocessing and post processing of the FEM model, doing torsional and latch stiffness tests for the bonnet assembly to verify that the parts can endure the torsional and closing loads. And on the NVH side, it is verified that the frequency response of the bonnet assembly is different than the global vehicular frequency response so that resonance is avoided between the components. The design for manufacturing approach of the bonnet is done considering lightweight materials such as aluminum (Ecodal-608 and Ecodal-605) with the design constraints that the material provides as the tooling analysis shows following design guidelines for aluminium, and with the help of the form limit diagrams (FLD) verify if the part is feasible or not for stamping. Finally, the style of the outer panel is given by the design office, and it is followed through the surface design and modified according to the engineering best practices for sheet metal joining and manufacturability of the part. The resultant design of the bonnet is compared between different vehicular approaches: mass production, low volume, and prototype vehicles as each of these classes have different developments and fulfil different objectives, but all must be homologated under the same rules.