Mechanical behaviour of dual-phase high-strength steel under high strain rate tensile loading

The effects of strain rate on the tensile properties of two commercial steels (DP700 and DP500) were investigated. Quasi-static tests (0.001 s−1) were performed using an electromechanical universal testing machine, whereas a split Hopkinson tensile bar apparatus was used for testing at high strain rates (~1100 s−1, ~1800 s−1, and ~3200 s−1). The two high-strength steels show significant strain rate sensitivity. The material parameters of the existing Johnson-Cook model were determined from experimental results. This model fits the experimental data well in the plastic zone. Digital image correlation was used together with high-speed photography to study the strain localisation in the tensile specimens at high strain rates. By using digital image correlation, the in-plane strain field and local fracture strain of the specimen were obtained. The contours of the strain field indicate an early localisation even before the peak load is attained. Numerical simulations of the dynamic tensile tests were performed using the non-linear explicit finite element code LS-DYNA with the Johnson-Cook model. Good correlation between the experiments and numerical predictions is achieved, in terms of the strain gauge signals, deformed geometry, and strain field.