Development of Lightweight Thin-Walled Aluminum Bumper Reinforcement Adhered with Unidirectional CFRP Sheet

"Since bumper reinforcements are positioned at front/rear ends of vehicles, weight reduction of the bumper reinforcements improves vehicle dynamic performance by reducing a yaw moment of inertia. CFRP (Carbon Fibre Reinforced Plastic) composites are attractive lightweight materials due to their excellent specific strength and rigidity. However, because of their relatively high cost, applications of CFRP materials to vehicle structural parts are limited. In this study, we developed a lightweight structural part which consists of a thin-walled Aluminum bumper reinforcement with a unidirectional CFRP sheet, in order not to increase part cost by reducing amount of Aluminum and by using only a little amount of CFRP. Compared to Aluminum, unidirectional CFRP sheets have even higher tensile strength and modulus. When vehicles crush, bumper reinforcements may be subjected to bending force. If a unidirectional CFRP sheet adhered on a tensile side of an Aluminum bumper reinforcement, not only Aluminum thickness on the tensile side but also thickness on the compression side can be reduced due to movement of a bending neutral axis. Bending strength of the developed parts can’t be predicted by a full plastic moment which could be used to predict metal parts’ bending strength because CFRP don’t deform plastically. In this study, based on Bernoulli-Euler theory, the bending neutral axis was decided considering elastic/plastic areas of the Aluminum bumper reinforcement, and bending strength of the part was predicted. To valid the calculation method, three-point bending tests on the parts were carried out. Experimental data of bending strength were in the range of predicted bounds. In addition, after the peak load, the load decreased gradually, like conventional all metal bumper reinforcements, without delamination of the unidirectional CFRP sheet. In order to launch the developed part, robustness of part’s performance was also evaluated. Finally, the part was adapted to a rear bumper reinforcement of LEXUS RC-F. The part weight is 11 % lighter compared to a conventional all Aluminum bumper reinforcement."