Optimized Design Solutions for Roof Strength Using Advanced High Strength Steels

In August 2005, National Highway Traffic Safety Administration (NHTSA) proposed to increase the roof strength requirement under Federal Motor Vehicle Safety Standard (FMVSS) 216 from 1.5 to 2.5 times unloaded vehicle weight (UVW). To meet the new requirement with a minimum impact on vehicle weight and cost, the automotive community is working actively to develop improved roof architectures using advanced high strength steels (AHSS) and other lightweight materials such as structural foam. The objective of this study is to develop an optimized steel-only solution with low material and part-manufacturing costs. Since the new regulation will present a particular challenge to the roof architectures of large vans, pickup trucks and SUVs due to their large mass and size, a validated roof crush model on a B-Pillar-less light truck is utilized in this study. Optimized steel-only solutions are investigated with the application of CAE optimization technology and AHSS including dual phase (DP) steels up to 980 MPa tensile strength and boron steel for hot stamped parts. HyperStudy (R) optimization software and LS-DYNA (R) nonlinear finite element software are utilized together to search for the optimal design. Steel inserts are designed and added at critical locations to improve the load-carrying capacity. Material and gauge optimizations are also performed on key roof components to minimize the vehicle weight while meeting the enhanced roof crush load requirement. The optimized steel-only solutions are compared to the alternative designs using nylon inserts.