A split Hopkinson bar could be used for obtaining high strain rate material properties of sheet metals for an auto-body. In high speed tensile test of sheet metals, a new tension split Hopkinson bar is needed. From the experiments with the new apparatus, the material properties of sheet metals in the high strain rate have been acquired. The experimental results are used to obtain the constitutive relation of sheet metals. The results show that the strain rate hardening effect for sheet metals can not be described by the original Johnson-Cook constitutive relation. This relation has been modified to include a higher-order term for the hardening effect. The constitutive relations obtained are used to simulate a square tube to know the energy absorption and impact force. The analysis is performed with elasto-plastic shell element. The strain rate effect is an important factor in crashworthiness.