Abstract The mechanical properties of constructional steels at different strain rates were the basis for the dynamic analysis of the progressive collapse of steel structures. The effect of strain rate on the hardening behavior and ductile fracture for the high-strength steel Q690 is investigated in this paper. Quasistatic and intermediate strain-rate experiments were performed. A smooth sheet and a V-shaped notched specimen were carefully designed to cover intermediate and high-stress triaxialities. The experimental results revealed that the effect of strain rate on the hardening behavior is sensitive to stress state. A novel dynamic isotropic hardening law was developed, taking advantage of the Cowper–Symonds and Johnson–Cook hardening models. Also, negative and positive strain-rate effects on the ductility of Q690 were observed in the smooth and notched specimens, respectively. An attempt was made to introduce a new strain-rate term into an uncoupled ductile fracture model to describe the differences of the strain-rate effects at various stress states. The new hardening law and the strain-rate-dependent ductile fracture model were validated with satisfactory accuracy.
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