Effects of Strain Rate and Identification of Material Constants for Three Automotive Steels

The main topic of this paper is an analysis of experimental results for three kinds of sheet steel: DP600, TRIP700 and H340LAD, which are used in the automotive industry. Such results were partly reported earlier in 1]. For comparison purposes the experimental results obtained at LPMM for an ES (DC05) mild steel have also been integrated in this paper. The tension tests were performed at room temperature in a relatively wide range of strain rates, that is from ~3.0·10^?4 s^?1 up to ~10³ s^?1. Since at low and high strain rates two different specimen geometries were applied, detailed numerical analyses have been performed in order to estimate the geometry effects on the final true stress versus true strain characteristics at different strain rates. A relatively new constitutive relation of the form = 0 is applied. This constitutive relation in the form of the Mechanical Equation of State (MES), called also the RK relation, has been developed by Rusinek and Klepaczko 2]. The main advantage introduced in the RK approach is the rate and temperature sensitivity of the strain hardening exponent, n , a very important improvement in comparison to other constitutive formulations. It appears that introduction of the rate and temperature sensitivity of strain hardening is very important in all BCC and FCC micro‐structures. In BCC structures the tangent modulus of may substantially decrease when strain rate increases. A special procedure was applied, according to Rusinek and Klepaczko 2], to determine the material constants for those three steels. An excellent fit to experimental data was obtained. Some FE calculations performed earlier on the energy absorbing profiles under impact with the RK constitutive relation have shown very good confirmation of experiment. In addition, two specimen geometries were analysed via an FE method.