Engineering Thermoplastic Energy Absorbers for Bumpers 1999-01-1011
Automotive styling trends point to reduced bumper overhang, greater sweeps, and reduced overall package space for the bumper system. At the same time engineers are charged with improving bumper performance to reduce collision repair costs and enhance occupant safety further. Two key performance parameters for the bumper to meet these conflicting objectives are fast but controlled loading and efficient energy absorption (EA). The majority of today's North American passenger-car bumper systems consist of a reinforcing bar either of steel, aluminum, or composite construction, and an energy absorption media. The most widely used energy-absorber construction is made from an expanded-polypropylene foam (EPP). Honeycomb energy absorbers, which are made from an ethylene vinyl acetate (EVA) copolymer, are also still used on some of today's cars. This paper will address an alternative to the bumper energy absorber systems described above.
Engineering thermoplastics have been used with good results in applications such as knee bolsters, structural instrument panels, head-impact-protection pillars, automotive bumpers, and body panels, where structural integrity, crashworthiness, and energy absorption capacity are key requirements. High strength-to-weight ratios and specific energies of engineering thermoplastics provide the opportunity to achieve energy absorption characteristics. The novel design approach used with the new energy absorber prototype described in this paper takes advantage of the excellent energy-absorption properties of the material by maximizing the amount of available material actually involved in attenuating the impact energy. Results of the study and prototype program have shown fast load-up and high EA efficiency compared with EPP foam, suggesting the potential to lower rail loads and minimize the amount of intrusion to the vehicle. The design is comparable in weight to foam energy absorbers as is projected to be economically competitive. This paper will review the industry trends associated with bumper energy absorbers and explore the potential fit of this new prototype energy absorber design as an alternative to EPP foam. Also included is a review of the simulated performance of the prototype ETP energy absorber and a comparison of its actual test results for 8 km / h FMVSS Part 581 impact series to the performance of EPP foam packaged in the same environment.