Mechanical and fracture behavior of high strength steels under high strain rate deformation: Experiments and modelling

Abstract

By the automotive structure design, crashworthiness has become also an important issue so that a better understanding of plastic deformation of material at high velocity is necessary. The effects of strain rate on mechanical properties and fracture mechanism of ferritic-martensitic dual phase (DP) steel grades 780 and 1000 were investigated by both experiments and micromechanics based modeling. For the examined steels, quasi-static (0.001 s−1) and medium strain rate (0.5–1 s−1) tensile tests were carried out on a universal testing machine, while high strain rate (1500-2500 s−1) tests were performed by a Split-Hopkinson tensile bar. Afterwards, FE simulations using 2D representative volume elements (RVEs) were conducted for investigating microstructure effects on local deformation and damage of DP steels under varying strain rates. Flow curves of observed phase constituents at different strain rates were described by using a dislocation based theory and local chemical composition in combination with the Johnson-Cook (JC) hardening model. Furthermore, individual damage criteria based on the rate-dependent JC failure model were applied to describe the local crack mechanisms in DP microstructures. Calculated local stresses, strains and damage developments of deformed phases were studied. It was found that microstructure characteristics especially phase fraction differently affected the strain hardening and ductility of DP steels under low and high strain rate deformation. The damage initiation and propagation in microstructures of both steels at various strain rates predicted by RVE simulations were well correlated with the experimental results.