Towards high-volume, primary bonded, lightweight automotive structures

Increasing pressures upon high-volume automotive manufacturers has led to the requirement for lightweight body structures. These increasingly require the introduction of lightweight materials, such as fibre reinforced polymer composites. As a result, effective joining technologies are a key automotive industry requirement to enable the introduction of optimal, lightweight material combinations to automotive body structures. Following an extensive literature review, adhesive bonding techniques were identified as a potentially suitable joining solution. However, two key barriers to the adoption of adhesive only, primary bonded, structural joints within the high-volume automotive industry were identified. These were the requirement for optimal surface treatment methods for composites, applied prior to adhesive bonding and a technique to achieve rapid joint handling strength during manufacture. These industrial requirements formed the primary areas of research for this project. An extensive investigation was performed to identify suitable surface pre-treatment methods, for high-volume adhesive bonding of composites. A particular focus was placed on the use of atmospheric pressure plasma surface treatment. It was identified that this process can effectively increase the surface free energy of both a thermoset and thermoplastic composite through modification of the surface chemistry. No significant increase in adhesive bond strength was found upon a thermoset substrate. The process can however effectively influence the adhesive joint failure mode upon a thermoplastic FRP surface, with joint durability implications. Limitations of the process were identified with respect to surface contamination removal. Electromagnetic induction heating was identified as a method to rapidly heat composite and metallic substrates. As such the process was potentially suitable for accelerating the cure and strength development of structural adhesives. Equipment was specified and a bespoke testing methodology developed to identify the rate of joint strength development following optimised induction heating cycles. A full sized CFRP floor/sill joint sub-assembly was used to demonstrate the achievable joint handling strength within a sub one-minute cure cycle for a primary bonded composite intensive joint. It was demonstrated that manufacturing cycle time limitations are achievable with a load well in excess of a typical body in white weight applied prior to joint failure. As a result, the work has identified and developed, a potential high-volume joining solution to a present challenge facing the automotive industry