Constitutive model of Q345 steel at different intermediate strain rates

Structural elements of steel frame experience very high strain rates in the progressive collapse, and hence their constitutive properties do not remain constant but change significantly with time. Quasi-static and dynamic tensile tests of Q345 steel were performed to study the dynamic tensile behavior within the range of 0.001 to 330/s strain rates by INSTRON and Zwick/Roell HTM5020 testing machine. A three dimensional finite element model is developed using LS-DYNA to extrapolate the true stress-strain relationship after necking of Q345 steel. The hardening behavior of Q345 steel after the onset of necking is predicted with the Ludwik constitutive equation at quasi-static strain rates and with the Voce constitutive equation at higher strain rates, in which the strain hardening parameters are obtained by trial and error until the numerical results agree well with the experimental results. The linear combination of Hollomon and Voce (H/V-R) model is proved to be capable of predicting the transition of hardening rate with the increasing strain rate for Q345 steel although there is a little deviation between the experimental and fitting results. In this study, an empirical constitutive model is developed by introducing the Wagoner rate law into the H/V-R model to improve its precision in predicting the dynamic behavior of Q345 steel.