Influence of Pre‐Straining and Bake Hardening on the Strain Rate Sensitivity of Automotive Sheet Steels

Abstract

AbstractThe present investigation deals with the influence of pre‐straining with or without bake hardening on the strain rate sensitivity of automotive sheet steels in typical crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 0.005‐1000 s−1 and in the temperature range 233‐373K. A bake hardening heat treatment at 170 °C for 20 min without pre‐straining does not influence the m‐value in comparison to the base material condition. A small pre‐straining near plane strain condition, as commonly found in outer door panels, or a 10% uniaxial, plane strain and biaxial pre‐straining, as typically used in formed automotive crash components, without bake hardening does not affect the m‐value of sheet steels in comparison to the base material condition. Uniaxial 2% to 10% pre‐straining, longitudinal or transverse to rolling direction with subsequent bake hardening, does not clearly change the m‐value in comparison to the base material condition either. Small differences in the strain rate sensitivity behaviour are rather attributed to experimental scattering without real physical background.