Effects of Pre-Strain on Properties of Low-Carbon Sheet Steels Tested over a Wide Range of Strain Rates

Knowledge of high strain-rate deformation behavior of automotive body structural materials is of importance for design of new vehicles with improved crash-energy management characteristics. Since a large range of plastic strains is encountered during the forming process prior to assembly, the mechanical behavior of sheet steels under high strain rate deformation conditions must be understood after pre-straining, in addition to the as-produced condition. This paper presents the compression testing methodology employed to examine these properties, and focuses on the effects of quasi-static pre-strains (from 0 to 20%) on the subsequent behavior of a low carbon interstitial free steel tested over a broad range of strain rates (from 10⁻² to 10³s⁻¹). The results suggest that the increase in yield stress associated with increasing strain rate is not substantially influenced by prior cold work. The work hardening rate is strongly influenced by strain rate, however, and this occurrence is attributed to a combination of 1) thermal softening due to adiabatic heating, and 2) a likely change in dislocation interactions resulting from high rate deformation. The combined effects of forming strains and adiabatic heating diminish work hardening rate to a greater extent at high strain rates. The implications of these results are also discussed.